Arctic Climate Impact Assessment: Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation

August 21, 2012, 10:44 pm
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This is Section 7.3.3 of the Arctic Climate Impact Assessment
Lead Author: Terry V. Callaghan; Contributing Authors: Lars Olof Björn, F. Stuart Chapin III,Yuri Chernov,Torben R. Christensen, Brian Huntley, Rolf Ims, Margareta Johansson, Dyanna Jolly Riedlinger, Sven Jonasson, Nadya Matveyeva,Walter Oechel, Nicolai Panikov, Gus Shaver; Consulting Authors: Josef Elster, Heikki Henttonen, Ingibjörg S. Jónsdóttir, Kari Laine, Sibyll Schaphoff, Stephen Sitch, Erja Taulavuori, Kari Taulavuori, Christoph Zöckler

 

Species responses to climate change are complex. They respond individualistically to environmental variables such as temperature[9], and various processes within a given species (e.g., reproductive development, photosynthesis, respiration, leaf phenology in plants) respond individualistically to a given environmental change. Knowledge of species responses to changes in temperature comes from many sources including indigenous knowledge, current species distributions related to climate, and experimental manipulations of temperature in the laboratory and field.

Plants (7.3.3.1)

The information presented in this section relates to individual plant species and how they have responded to changes in various aspects of climate and UV radiation. The information is taken mainly from experiments in which climate variables or UV radiation levels were modified and the responses of the individual species determined while they were growing in natural communities. Indigenous knowledge is also included.

Responses to current changes in climate

Indigenous knowledge studies in Canada describe poor vegetation growth in eastern regions associated with warmer summers and less rain[10], but describe increased plant biomass and growth in western regions, particularly in riparian areas and of moisture-tolerant species such as shrubs[11], due to lengthening of the growing season, marked spring warming, and increased rainfall.

Inuit participating in the Tuktu and Nogak Project in the Kitikmeot region of Nunavut[12] observed that vegetation was more lush, plentiful, and diverse in the 1990s compared to earlier decades. Willows and alders were described as taller, with thicker stem diameters and producing more branches, particularly along shorelines. Other indigenous communities have also reported increases in vegetation, particularly grasses and shrubs – stating that there is grass growing in places where there used to be only gravel. On Banks Island, in the western Canadian Arctic, Inuvialuit point to observations that the ''umingmak'' (muskox) are staying in one place for longer periods of time as additional evidence that vegetation is richer[13]. In addition, Riedlinger[14] has documented Inuvialuit observations of an increase in forbs such as ''qungalik'' (Arctic sorrel – ''Oxyria digyna''), which is described as coming out earlier in the spring, and noticeably "bigger, fresher, and greener".

The Arctic Borderlands Ecological Knowledge Co-op monitors the annual quality and quantity of salmonberries locally called ''akpiks'' (cloudberry – ''Rubus chamaemorus''), and has documented recent observations of high temperatures early in the year that "burn" berry plant flowers, early spring melt that results in inadequate moisture for the plants later in the year, and intense summer sun that "cooks" the berries before they can be picked[15]. On Banks Island, local residents report years where the grass remained green into the autumn, leaving it vulnerable to freezing[16]. This corresponds to experiments that show a similar effect on Svalbard[17].

In northern Finland, marshy areas are said to be drying up. Sami reindeer herders from Kaldoaivi in Utsjoki have observed that berries such as bog whortleberry (''Vaccinium uliginosum'') have almost disappeared in some areas. Other berries such as cloudberry and lingonberry (''V. vitis-idaea'') are said to have declined in the last 30 years[18]. Indigenous peoples' observations of declining cloudberry production are supported by experiments that postulate declines in growth in warm winters[19] and provide detailed mechanisms of fruit production[20].

Indigenous knowledge also records changes in species distribution: some existing species have become more widespread and new species have been seen. In addition to increased shrub abundance, Thorpe et al.[21] documented reports of new types of lichens and flowering plants on Victoria Island in Nunavut and more individual plants of the same species[22]. The increases in shrubs in this area correspond to aerial photographic evidence of increases in shrub abundance in Alaska[23]. However, the reports of new types of plants, and lichens in particular, contrast with experimental evidence that shows a decrease in lichens and some mosses when flowering plant biomass increases[24]. A possible reason for this is that results from warming experiments cannot be extrapolated throughout the Arctic because of variations in recent and projected climate owing to both cooling and warming (sections 2.6.2.1 and 4.4.2): warming experiments in continuous vegetation show declines of lichens, whereas lichens may expand their distribution during warming in the high Arctic where vascular plant competitors are sparse[25].

In contrast to observed responses of plants to recent warming, remote sensing by satellites has shown that the start dates of birch pollen seasons have been delayed at high altitudes and in the northern boreal regions of Fennoscandia[26]. In the Faroe Islands, there has been a lowering of the alpine zone in response to a 0.25 °C cooling over the past 50 years[27].

Projected responses to future temperature changes

Warming ''per se'' is very likely to be favorable to the growth, development, and reproduction of most arctic plant species, particularly those with high phenotypic plasticity (flexible/responsive growth and development). However, other limiting factors such as nutrients and moisture or competition from immigrant species are likely to modify plant responses to warming. In some cases, the direct and indirect effects of warming are projected to generate negative responses:

  • Increased respiration relative to photosynthesis can result in negative carbon balances, particularly in clonal plants that accumulate old tissues, for example, the cushion form ecotype ([[Introduction to Arctic Tundra and Polar Desert Ecosystems|Fig. 7.3) of purple saxifrage (''Saxifraga oppositifolia''[28]) and some species of the herb ''Ranunculus''[29].

  • It is possible that cushion forms of arctic plants, including mosses, that have low atmospheric coupling and experience high temperatures will experience thermal death during warming, particularly when combined with reduced cooling by evapotranspiration under drought conditions.

  • Exposure to high levels of solar radiation and increases in temperature could possibly cause damage and death to some species, particularly those occupying shady and wet habitats, that have low thermal tolerances (as low as 42 °C in Arctic sorrel[30]).

  • During warming, arctic species with conservative nutrient-use strategies, slow growth, and particularly inflexible morphologies such as those of cushion and mat plants, are likely to be at a competitive disadvantage to more responsive, faster growing, taller species immigrating from southern latitudes. After six years of shading (simulating competition), increasing temperatures, and fertilizing a heath and a fellfield community in Swedish Lapland, shading was found to have the greatest effect on aboveground growth[31]. In another experiment, flowering of the dwarf heather-like shrub ''Cassiope tetragona'' stopped when it was shaded[32]. In contrast, a meta-analysis by Dormann and Woodin[33] found no significant effect of shading on biomass.

Populations at the most environmentally extreme boundary of their distributions (in terms of latitude, altitude, and habitat mosaics within landscapes) tend to be responsive to amelioration of physical environmental factors such as temperature that limit their distributions; these populations have the potential to expand their distribution. In contrast, populations at the most environmentally benign boundary of their distribution tend to be constrained by competition with more responsive species of more benign environments[34] and tend to be displaced by environmental amelioration.

An International Tundra Experiment (ITEX[35]) meta-analysis of arctic vascular plant species responses to simulated summer warming (1.2 to 1.8 °C mean daily near-surface and soil temperature increase) using standard open-top chambers compared key species from 13 sites over a period of one to four years[36]. The simulated temperature increase is comparable to the projected increase in mean arctic summer air temperature of 1.8 °C by 2050 (mean of the five Arctic Climate Impact Assessment (ACIA)-designated model scenarios). In ITEX and earlier experiments, phenology (bud burst and flowering) was advanced in warming treatments at some sites[37]. In Swedish Lapland, growth accelerated and the period between thawing and anther appearance advanced by two weeks[38]. In contrast, there was little change in growth cessation at the end of the season in response to higher temperatures. However, nutrient addition prolonged the growth period of polar semi-desert species on Svalbard in autumn but reduced frost hardening, leading to dramatic loss of aboveground biomass during November 1993, which was extremely warm and wet[39]. This corresponds with the indigenous observations noted above.

Experiments conducted by ITEX showed that initial increases in vegetative growth were generally, but not always, reduced in later years, probably because temperature increases stimulated the use of stored resources more than the uptake of new resources. Similarly, growth responses of subarctic dwarf shrubs to soil warming increased initially but soon returned to former levels. This response followed an initial increase in nitrogen mineralization as a result of soil warming, but increases in mineralization did not persist[40]. In contrast, reproductive success improved in later years in the ITEX experiments[41] due to the extended period between flower bud initiation and seed set in arctic flowering plants. Similarly, over an 18-year period, flowering of the widespread sedge ''Carex bigelowii'' was strongly correlated with July temperature of the previous year[42]. ''Eriophorum'' species exhibited even more dramatic interannual variation in flowering than ''Carex'' species, but there was no simple correlation with weather in the flowering year or the previous year[43].

The ITEX experiments showed that responses of growth and reproduction to temperature increases varied among vascular plant life forms. Herbaceous species responded more strongly and consistently to warming than did woody forms over a four-year period[44]. Over longer time periods, the growth form, number, and position of meristems in some woody plants such as ''Betula nana'' (Fig. 7.3) allowed a much greater response that completely changed the height and structure of the whole canopy ([45] but see [46] for a different response). In the subarctic, Graglia et al.[47] showed that initial plant responses (abundance) to temperature increases and other treatments persisted throughout a ten-year period. Graminoids were particularly responsive to fertilizer additions in the subarctic and their increased growth and litter production suppressed the growth of mosses and lichens[48]. Evergreens were more responsive to nutrient addition and temperature increases than deciduous species[49].

Mosses and lichens appear to be particularly vulnerable to climate warming, at least in areas of continuous vegetation cover. A meta-analysis of lichen responses to warming experiments across the Arctic showed that lichen biomass decreased as vascular plant biomass increased following warming[50]. This group of plants is particularly important because a large proportion of global lichen diversity is found in the Arctic, some species are important winter forage for reindeer/caribou, and some are important nitrogen fixers in strongly nitrogen-limited systems. A 22-year study of the lichen flora of the Netherlands showed changes that researchers suggest are related to an increase in temperature, although the subtropical species might be more sensitive to nitrogen[51]. Fifty percent of the arctic–alpine/boreal–montane lichen species were declining while subtropical species were invading[52]. The widespread moss ''Hylocomium splendens'' shows a complex response to warming[53]: in warming experiments growth is reduced[54], whereas growth increases in relation to increases in mean annual temperature throughout its arctic distribution range (section 7.3.5.2, Fig. 7.15; [55]). This suggests some limitation in the experimental simulation of natural/anthropogenic warming. If, however, moss growth and abundance are reduced by higher temperatures, soil thermal regimes, biogeochemical cycling, and energy and heat exchange between the biosphere and atmosphere will be significantly affected[56].

Plant species respond differently to warming according to previous temperature history related to latitude, altitude, interannual temperature variations, and interactions among species. Phenological responses to warming are greatest at cold sites in the high Arctic[57], whereas growth responses to warming are greatest at sites in the low Arctic. Growth responses of ''Cassiope tetragona'' to warming were greatest at a site in the high Arctic and a high-altitude site in the low Arctic when compared with the warmest low-altitude site in the low Arctic[58]. Over a period of five years, shoot elongation responses to warming were greatest in cold summers[59]. Laine[60] showed that the reproduction of bilberry (''Vaccinium myrtillus'') depended to some extent on the climate in the previous years (see section 14.7.3 for examples of this in trees), whereas Shevtsova et al.[61] showed no such response for co-occurring lingonberry and crowberry (''Empetrum nigrum'').

Most information on plant responses to climate warming is limited to the short term and small plot – even if the short term is two decades. Because of the great longevity and clonal growth of arctic plants, it is difficult to extrapolate plant responses from an individual plant to the population. However, the impacts of climate change (temperature, nutrients, carbon dioxide [CO2]) on demographic parameters and population growth statistics were determined for the sedge ''Carex bigelowii'' by Carlsson and Callaghan[62] and Callaghan and Carlsson[63], who showed that climate change increased tiller size, vegetative production of young tiller generations, survival of young tillers, and flowering, and reduced the age of a tiller at flowering and tiller life span. Two mathematical models showed that the changes in demographic parameters led to an increase in the population growth rate, with young tillers dominating this increase. The rate of vegetative spread more than doubled, while cyclical trends in flowering and population growth decreased substantially.

Responses to precipitation changes

Precipitation in the Arctic is extremely variable between seasons and from place to place, but the amount of snow is difficult to measure (section 2.4). Precipitation varies from over 1000 millimeters (mm) in coastal areas (e.g., Norway and Iceland) to less than 45 mm in the polar deserts, where most of the annual precipitation occurs as snow. The interaction between precipitation and temperature is extremely important for plant growth and ecosystem processes and it is difficult to separate their effects.

 

caption Fig. 7.8. Relationship between timing of the growing season and the seasonal pattern of irradiance together with an indication of where transient switches from carbon (C) sink to carbon source could occur. (Source: Modified from [1])

 

 

Observations show that precipitation has increased by up to 20% in northern latitudes within the last 40 years (section 2.6.2.2), although there has been a 10% decrease in snow-cover extent in the Northern Hemisphere in the last 20 years (section 2.4.1). The most recent climate scenarios for the North Atlantic region suggest increased mean annual temperatures and precipitation for the entire region over the next 100 years ([64]; section 4.4).

Effects of changes in snow depth, duration, and timing of the snow-free period

The interaction between snow amount and temperature determines the start and duration of the snow-free period. The duration of the snow-free period at high northern latitudes has increased by five to six days per decade and the week of the last observed snow cover in spring has advanced by three to five days per decade between 1972 and 2000[65]. Even if precipitation increases, therefore, temperature increases may still result in shorter duration of snow and less snow cover (sections 2.6 and 4.4.3). In contrast, the start of the growing season has been delayed by up to one week over the last 20 years in the high-altitude and northern boreal regions of Fennoscandia[66]. Hydrological models applied to the Tana River Basin of northernmost Finland project increases in growing-season length, from 30 days in the mountains to 70 days near the coast of the Barents Sea, by 2100[67]. This change is associated with an earlier start to the growing season of about three weeks and a delayed end of two to three weeks.

 

caption Fig. 7.9. Snow bank vegetation showing increasing vegetation development with increasing growing-season length represented as distance from the snow patch, Disko Island, West Greenland. (Source: Photograph by T.V. Callaghan)

 

The timing of the start of the snow-free period is of critical importance, and more important than the timing of autumn snowfall, because solar angles are already high when plants start growth and each extra snow-free day at the beginning of the growing season will enable plants to access high levels of photosynthetically active radiation (Fig. 7.8; see also section 7.5). In an alpine area, productivity decreased by about 3% per day that the snow release date was delayed [68]. The timing of snowmelt has also been found to have considerable effects on plant phenology (more so than temperature in some cases[69]), with a contracting of development time that is associated with a decrease in productivity and reproductive output[70]. Some plant species, such as the deciduous shrub species ''Salix pulchra'' and ''Betula nana'', can respond to changes in growing-season timing[71], but others, particularly evergreen and early-flowering species appear to be particularly vulnerable[72].

An experiment that manipulated snow conditions by using snow fences at Toolik Lake, Alaska, showed that drifts increased winter temperatures and CO2 flux[73]. Under the drifts, temperatures were more constant than in control plots. Plant growth increased despite a shorter growing season, although this was thought to be a transitory response and contrasts with the reduced growth of plants associated with late snow beds (Fig. 7.9).

Frost resistance and avoidance

Changes in snow depth and duration are likely to negatively affect the frost resistance and avoidance of many plants at high latitudes. Damage to foliage and apical meristems occurs when they are "triggered" to premature bud burst and development by an earlier onset of the growing season (resulting from early snowmelt) when the annual hardening/dehardening process is at its most sensitive phase, and when there is a risk of short periods of cold weather. Bilberry is a species whose requirement for cool temperatures to enable it to break dormancy (i.e., chilling requirement) is fulfilled early[74]. Accelerated dehardening of bilberry was found as consequence of a minor (2 to 3 °C) increase in temperature[75], suggesting that climatic warming is very likely to entail a real risk of early dehardening and subsequent frost damage of shoots. The explanation for this may be the higher but fluctuating temperatures, which increase the cryoprotectant-consuming freeze–thaw cycles[76]. In addition to frost resistance, frost avoidance is likely to be disturbed by a thin or lacking snow cover. The risk is likely to be highest at high latitudes, where plants that are genetically adapted to the presence of snow may have lost some potential for frost resistance during their evolution. Subarctic provenances of bilberry, for example, have shown reduced frost resistance compared to provenances from southern Finland[77].

Other global change factors might affect frost resistance, but few, sometimes conflicting, reports have been published of studies performed at high latitudes. Nitrogen pollutant (or fertilizer) can impair the frost resistance of plants. Such an effect was demonstrated for mountain avens (''Dryas octopetala'') on Svalbard during a warm period in early winter[78] (see previous subsection on projected temperature responses). However, recent studies with the heather ''Calluna vulgaris''[79], bilberry[80], and lingonberry[81] have demonstrated improved frost resistance caused by extra nitrogen, probably because these ericaceous species are plants adapted to low-nutrient habitats, such as those at high latitudes.

Snow depth and duration vary greatly with topography at the landscape level. High summer temperatures are very likely to decrease the abundance and size of snow beds. Changes in snow patches observed by indigenous peoples are already causing concern in Baker Lake, Clyde River, and Iqualuit; Fox[82] describes aniuvak (permanent snow patches) that are melting in the hills around those communities. Aniuvak are good areas for caching meat and provide a sanctuary for caribou to evade flying insects. Indigenous peoples' explanations for the melting snow patches relate more to changes in precipitation and mean relative humidity than to temperature increases. The specialized plants characteristic of late snow beds[83] are very likely to be at particular risk as temperatures increase.

Summer precipitation

Altered timing and rate of snowmelt are very likely to differentially alter the availability of water in different facies of the tundra landscape mosaic, which are very likely to in turn significantly affect the predominant vegetation type and its growth dynamic through the active season[84]. Artificial increases in summer precipitation produced few responses in arctic plants compared with manipulations of other environmental variables[85]. However, mosses benefited from moderate summer watering[86] and nitrogen fixation rates by blue-green algae associated with the moss ''Hylocomium splendens'' increased[87]. Addition of water to a polar semi-desert community in summer produced surprisingly few responses[88]. Also in the high Arctic, comparisons were made between sites with high and low plant densities. Although there was little difference in soil moisture and plant water relations, and water availability did not constrain the adult vascular plants, surface water flow in snow-flush areas (accumulations of snow that provide water during summer) allowed greater development of cyanobacterial soil crusts, prolonged their nitrogen fixing activity, and resulted in greater soil nitrogen concentrations[89]. Because of their importance in facilitating vascular plant community development, Gold and Bliss[90] projected that the effects of climate change on non-vascular species are very likely to be of great consequence for high-arctic ecosystems.

Responses to increased atmospheric carbon dioxide concentrations

There are very few arctic experiments that have manipulated atmospheric CO2 concentrations in the field[91], but more laboratory experiments have been conducted on arctic vascular plants[92], mosses, and lichens[93].

The first experiment that manipulated CO2 in the Arctic concluded that elevated CO2 concentrations had no long-term effects because photosynthetic acclimation (i.e., down-regulation, the physiological adjustment of photosynthetic rate so that no differences are found between plants grown at ambient and elevated CO2 levels) in cottongrass (''Eriophorum vaginatum'') was apparent within three weeks and biomass did not increase. However, there was prolonged photosynthetic activity in autumn and more biomass was allocated to roots[94]. The lack of response and enhanced root biomass were attributed to nutrient limitation[95]. Although increases in tiller production of cottongrass were not considered to be an important response, this can lead to long-term increases in population growth[96].

Longer-term CO2 enrichment experiments in the subarctic also show that growth responses are dominated by early, transient responses[97]. Four dwarf shrubs were studied over the first three years of the experiment; one, the deciduous bilberry, showed increased annual stem growth (length) in the first year whereas two other evergreen dwarf shrubs (mountain crowberry – ''Empetrum hermaphroditum'' and lingonberry) showed reduced growth. In the seventh year, increased CO2 concentrations significantly increased the leaf ice nucleation temperature (i.e., reduced frost resistance, which can be harmful during the growing season) in three of the four species tested[98]. Bog whortleberry (or bog bilberry – ''Vaccinium uliginosum''), lingonberry, and mountain crowberry showed increases in leaf ice nucleation temperature exceeding 2.5 °C whereas bilberry showed no significant effect, as in another study[99]. Increased CO2 concentrations interacted with high UV-B radiation levels to increase leaf ice nucleation temperature by 5 °C in bog whortleberry. This effect coincides with indigenous knowledge and other experiments that show increased frost sensitivity of some arctic plants to changes in climate and UV-B radiation levels (see responses to cloudiness and photoperiod in this section).

An expected (and subsequently observed) response to increased atmospheric CO2 concentrations was a change in leaf chemistry (e.g., an increase in the carbon to nitrogen ratio) that was expected to affect herbivory[100] and decomposition[101]. Surprisingly, herbivory was not affected. However, increased CO2 concentrations were found to play a role in nutrient cycling by altering the composition of microbial communities after five years[102] (see Microorganisms below), suggesting that chemical changes are occurring in plants exposed to high CO2 concentrations but the changes have not yet been identified.

In laboratory studies, the moss ''Hylocomium splendens'' that naturally experiences high CO2 levels in the birch woodlands of the Swedish subarctic was shown to have photosynthetic rates that were limited by light, temperature, and water for most of the growing season[103]. Enhanced CO2 concentrations for five months decreased photosynthetic efficiency, light compensation point, maximum net photosynthesis, and surprisingly, growth[104]. Similar experiments with three lichen species, ''Cladonia arbuscula'', ''Cetraria islandica'', and ''Stereocaulon paschale'', failed to show any response of fluorescence yield to enhanced CO2 concentration (1000 ppm), although there was an interaction between CO2 and UV-B radiation levels[105]. Perhaps the lack of response in moss and lichens reflects their adaptation to the currently high levels of CO2 that they experience close to the ground surface[106] via the process of down-regulation.

In contrast to some views that responses of plants (mainly growth) to increased CO2 concentrations are relatively small and by inference insignificant[107], recent results show that in the long term, increased CO2 concentrations can have the wide-ranging and important effects discussed previously[108].

Responses to increased ultraviolet-B radiation levels

One common method for simulating the effects of ozone depletion has been to irradiate organisms and ecosystems with artificial UV-B radiation. Results are often reported in relation to the equivalent percentage of ozone depletion. It should be noted, however, that the radiation spectrum from the lamps used in experiments differs from the spectrum of the radiation increase that would ensue from real ozone depletion. Therefore, the degree of simulated ozone depletion depends on the "weighting function" applied in the calculations, and the knowledge of the appropriate weighting function is very incomplete. Weighting functions are also species-specific: a certain amount of applied artificial radiation does not correspond to the same degree of ozone depletion for a plant and a tadpole, for example. The information in the following sections should be read with this in mind.

 

caption Table 7.6. Summary of UV-B radiation effects on subarctic dwarf shrubs (based on Phoenix [2] and other sources referred to in the text).

 

Relatively little is known about plant responses to changes in UV-B radiation levels. Field experiments on subarctic (Table 7.6) and high-arctic ecosystems show species-specific responses to ambient UV-B radiation levels and to enhanced UV-B radiation levels equivalent to a 15% decrease in stratospheric ozone from 1990 levels. (The 15% decrease is equivalent to losses of ozone projected to occur throughout much of the Arctic by 2015. However, the values do not apply to Beringia for April and October 2015[109].) On the whole, the effects of increased UV-B radiation levels are relatively few compared with effects of increased temperature and nitrogen[110].

A global meta-analysis of plant responses to increased UV-B radiation levels showed that there was a small but significant reduction in biomass and plant height[111]. In the subarctic, measurements of stem length, branching, leaf thickness, flowering, berry production, phenology, and total UV-B radiation absorbing compounds were affected significantly by ambient UV-B radiation levels in only two of three dwarf shrubs (i.e., bog whortleberry and lingonberry[112]). Mountain crowberry and lingonberry showed no responses to enhanced UV-B radiation levels after seven years of exposure whereas bog whortleberry and bilberry showed few responses (Table 7.6). Enhanced UV-B radiation levels have been shown to reduce the height growth, but not biomass, of the mosses ''Sphagnum fuscum'' and ''Hylocomium splendens'' in the subarctic[113].

 

caption Fig. 7.10. Dwarf shrub distributions in relation to latitude and solar UV-B radiation incident at the surface of the earth[3].

 

The UV-B radiation studies (Table 7.6) showed that arctic species were more tolerant of enhanced UV-B radiation levels than previously thought, and that the production of UV-B radiation absorbing compounds did not show the simple relationship with UV-B radiation dose expected from laboratory studies. Another surprise effect was the responsiveness of frost hardiness in some arctic dwarf shrubs to increased UV-B radiation levels. Dunning et al.[114] pioneered investigation of the relationship between UV-B radiation levels and frost resistance in a ''Rhododendron'' species and concluded that increased exposure to UV-B radiation increases (although only marginally) cold resistance. In contrast, K. Taulavuori et al.[115], found decreased frost resistance in bilberry in response to elevated UV-B radiation levels and Beerling et al.[116] showed decreased frost resistance in bog whortleberry, lingonberry, and mountain crowberry. A combination of elevated CO2 and UV-B radiation levels increased late-season frost sensitivity of leaves of bog whortleberry from -11.5 to -6 °C. Increased frost sensitivity at the beginning and/or end of the short arctic growing season is likely to curtail the season even further. As some models of vegetation redistribution related to temperature change use the critical freezing temperatures for leaf damage in temperate trees and shrubs[117], modeled past and future northward migration of temperate vegetation should be reconsidered in relation to changing CO2 and UV-B radiation levels.

The resilience of the subarctic dwarf shrubs to enhanced UV-B radiation levels probably reflects pre-adaptation to higher levels than are currently experienced in the Arctic[118]. These species currently extend southward to about 40° N and they probably existed even further south in a higher UV-B radiation regime during the early Holocene. The increased UV-B radiation levels currently applied in experiments are equivalent to the difference in ambient UV-B radiation levels between the site of the experiment (68° N) and Helsinki (59° N) (Fig. 7.10). In addition, many arctic plants have thick leaves that might attenuate UV-B radiation entering leaf tissues. However, one particular climate–UV radiation interaction that could possibly increase the damage experienced by plants is the combination of possible earlier snow-free periods[119] with higher spring UV-B radiation levels at the surface of the earth[120]. Such a combination of effects would expose young, potentially sensitive plant shoots and flower buds to particularly high UV irradiation[121].

Responses to changes in cloudiness and photoperiod

An important characteristic of the arctic environment is the daily and seasonal patterns of the light period or photoperiod. At midwinter, intermediate latitudes (40° to 50° N) have about an eight-hour day length, whereas a polar night without sunrise prevails north of the Arctic Circle (66.5° N). Consequently, day-length change during spring and autumn occurs much faster at high latitudes.

Frost-resistance patterns change seasonally and are environmentally controlled, mainly by temperature and day length; which of these factors predominate depends on the seasonal growth cycle[122]. The development of frost resistance by almost all woody plants at high latitudes is characterized by strong dependency on the photoperiod for growth cessation and cold hardening. Scots pine (''Pinus sylvestris'') seedlings from the northern boreal forest develop a high degree of frost resistance during the late summer as a consequence of the shortening days[123]. The frost-hardening process is initiated even at high temperatures (20 °C) in experimental conditions that mimic the ambient photoperiod[124]. Given the marked photoperiodic control of the frost-hardening process in woody species at high latitudes, it is understandable that they harden more extensively compared to populations at lower latitudes under similar temperatures. For example, the lowest survival temperature of bilberry in the central Alps (~50° N) at midwinter is around -35 °C ([125] and references therein), while the same level of frost resistance is already achieved at the end of September in northern Finland (65° N)[126].

In a changing climate, photoperiod will not change, but species that are migrating will experience changes in photoperiod. It is unlikely however, that this will constrain species initially. Many northern boreal species, for example, experienced arctic photoperiods earlier in the Holocene before they were displaced southward by climate cooling (section 7.2). If and when species with a more southerly distribution migrate into the Arctic, photoperiod constraints could possibly affect growth and flowering but this is largely unknown. However, experiments with transplanting herbs between the Austrian Alps, Abisko, and Svalbard showed that allocation of biomass in some species such as glacier buttercup (''Ranunculus glacialis'') was affected by photoperiod and this constrained any potential increases of vigor that might have occurred due to climate warming[127]. In contrast, herbs such as ''Geum''[128] and some grasses[129] that are not sensitive to photoperiod could possibly benefit from climate warming.

It has been suggested that increased UV-B radiation effects might be small in the future because of increased cloudiness (section 4.4.4) that is likely to counteract to some extent the effect of decreasing ozone levels[130]. However, projections of increased cloudiness, and particularly future cloud types, are uncertain. It is more likely that UV-B radiation effects will be reduced by decreases in albedo as snow and ice distribution and seasonal duration decline, and as the boreal forest displaces part of the current tundra.

Arctic plants differ in the degree to which they gain or lose carbon in photosynthesis at "night" (22:00 to 04:00 hours when light intensity is low but it is not necessarily dark). Under conditions of cloudy nights, those species that gain carbon at night, for example, Arctic dryad (''Dryas integrifolia''), alpine foxtail (''Alopecurus alpinus''), glaucous willow (''Salix glauca''), and arctic willow (''Salix arctica'') (25–30% of diurnal carbon gain[131]), are likely to have reduced competitive ability compared with species that do not. In contrast, increased cloudiness during the day probably favors those species that gain carbon at night. Those species that lose carbon at night (e.g., ''Eriophorum angustifolium''[132]) are likely to be disadvantaged by warming.

Responses to potential changes in pollinator abundance and activity

The rapid phenological changes that have been observed in response to simulated climate change have the potential to disrupt the relationships that plants have with animal, fungal, and bacterial species that act as pollinators, seed dispersers, herbivores, seed predators, and pathogens[133]. These disruptions are likely to have the strongest impact if the interacting species are influenced by different abiotic factors or if their relative responses to the same factors (e.g., elevated temperatures) are different. However, wind and self-pollination are more widespread among arctic flowering plants, so any mismatch between pollinator activity and flowering phenology is likely to be of greater significance to any plants immigrating to the Arctic as temperatures increase. Little appears to be known about these processes.

Summary

Species responses to changes in temperature and other environmental variables are complex. Species respond individualistically to each environmental variable. Plant species also respond differently to warming according to previous temperature history related to latitude, altitude, interannual temperature variations, and interactions among species. Some species are already responding to recent environmental changes. Indigenous knowledge, aerial photographs, and satellite images show that some arctic vegetation is becoming more shrubby and productive.

Summer warming experiments showed that initial increases in the growth of vascular species were generally reduced with time, whereas reproductive success improved in later years. Over short periods (four years), herbaceous plants responded more than woody plants, but over longer periods, woody plant responses were dominant and could change the canopy height and structure. Mosses and lichens were generally disadvantaged by higher-plant responses to warming.

Responses to warming are critically controlled by moisture availability and snow cover. Already, indigenous observations from North America and Lapland show a drying trend with reduced growth of economically important berries. However, experimental increases in summer precipitation produced few responses in arctic plants, except for mosses, which showed increased growth. An experiment that manipulated snow conditions showed that drifts increased winter temperatures and CO2 flux and, surprisingly, that plant growth increased despite a shorter growing season. In general, however, any earlier onset of the snow-free period is likely to stimulate increased plant growth because of high solar angles, whereas an increase in the snow-free period in autumn, when solar angles are low, will probably have little impact.

Carbon dioxide enrichment experiments show that plant growth responses are dominated by early, transient responses. Surprisingly, enhanced CO2 did not affect levels of herbivory, but significantly increased the leaf ice nucleation temperature (i.e., increased frost sensitivity) of three of four dwarf shrub species, and altered the composition of microbial communities after five years. A general lack of responses of mosses and lichens reflects their adaptation to the currently high levels of CO2 that they experience close to the ground surface.

Ambient and supplemental UV-B radiation levels produced complex, individualistic, and somewhat small responses in species. Overall, arctic species were far more tolerant of enhanced UV-B radiation levels than previously thought, and the production of UV-B absorbing compounds did not show the simple relationship with UV-B radiation dose expected from laboratory studies. Some arctic dwarf shrubs exhibited increased frost sensitivity under increased UV-B radiation levels. The arctic photoperiod is unlikely to be a general constraint to species migrations from the south, as trees and southern species previously occurred further north than at present.

Animals (7.3.3.1)

In contrast to plants, there are relatively few experiments that have addressed how animal populations respond to simulated climate change and UV-B radiation levels in the Arctic. The few experiments have focused on invertebrates (e.g., insects and soil animals) for which the microclimate can be manipulated on small experimental plots. Experiments on free-ranging vertebrate populations may not be feasible for logistical reasons. On the other hand, more time series of population data are available for conspicuous vertebrates such as reindeer/caribou and lemmings than, for example, soil invertebrates. Time series can be analyzed with respect to the influence of current climate variability (including recent changes).

Responses to current changes in climate and ultraviolet radiation levels

 

caption Fig. 7.11. (a) Population dynamics of Svalbard reindeer at Brøggerhalvøya and sibling voles at Fuglefjella on Svalbard[4]; (b) observed and projected changes in vegetation[5].

 

Ice-crust formation on the tundra as a result of freeze–thaw events during the winter affects most terrestrial arctic animals. Dense snow and ice severely limit forage availability for large ungulates such as reindeer/caribou and muskox[134]. Dramatic reindeer population crashes resulting from periodic ice crusting have been reported from the western coastal part of the Russian Arctic, Svalbard, and Fennoscandia[135]. Similar events have been reported for muskox in the southern parts of their range in Greenland[136]. Inuit in Nunavut report that caribou numbers decrease in years when there are many freeze–thaw cycles[137] and the probability of such freeze–thaw events is said to have increased as a result of more short-term fluctuations in temperature. In central Siberia, where winter climate is colder and more stable, reindeer population dynamics are less climate-driven[138]. Swedish Saami note that over the last decade, autumn snow lies on unfrozen ground rather than on frozen ground in the summer grazing areas and this results in poor-quality spring vegetation that has rotted[139]; certain microfungi seem to be responsible for this[140].

Long and accurate time series data on Svalbard reindeer populations[141] show that the amount of precipitation during the winter, which is highly variable and is well described by the Arctic Oscillation index[142], provides the most important check on the reindeer population growth rate in concert with population density. Winters with freezing rain were associated with severe population crashes both in one reindeer population (although the natural dynamics of an introduced herd may have contributed to this) and in an introduced population of sibling voles (''Microtus rossiaemeridionalis''; Fig. 7.11).

 

caption Fig. 7.12. Yearly winter survival rate (with 95% confidence intervals) of experimental tundra vole populations plotted against the number of days with temperatures above 0 °C during December through February. Mean winter temperature and the year are denoted above the survival rate estimates[6].

 

Episodes of mild weather and wet snow lead to a collapse of the subnivean space and subsequent frost encapsulates food plants in ice, making them unavailable to small mammal herbivores, and even killing plants in some cases[143]. Accordingly, the survival rate of tundra voles (''Microtus oeconomus'') decreases dramatically in winters with many freeze–thaw cycles[144] (Fig. 7.12). For example, the lemming increases observed at Kilpisjärvi (northwest Finnish Lapland) in 1997 and 2001 were probably curtailed by warm spells and rain in January that resulted in freezing of the ground layer[145]. Inuit residents of the western Canadian Arctic are also concerned with the impacts of thaw slumping on lemming populations and their predators (owls). Thaw slumps at lake edges have been occurring more extensively and at a faster rate in recent years, linked to warmer temperatures and an increase in wind activity and rain; thawing of ice-bound soil destroys lemming burrows[146].

There has been speculation about whether the recent dampened amplitude of population cycles and more spatially asynchronous dynamics of voles and lemmings in northern Fennoscandia may be the result of occasionally unfavorable winters disrupting the normal population dynamics[147]. Figure 7.13 illustrates changes since the beginning of the 1990s in populations of the formerly cyclic and numerically dominant grey-sided vole (''Clethrionomys rufocanus'') and other vole species. In long qualitative time series (up to 100 years), periods with loss of cyclicity and synchrony are evident[148], but it is unclear whether this is related to fluctuations in climate. There is a correlation between sunspot activity and snowshoe hare cycles in North America[149], but no such relationship has been found for the mountain hare (''Lepus timidus'') in northern Finland[150]. There are no relationships between sunspot activity and outbreak years in the autumnal moth in Fennoscandia[151], although there are few studies of the role of climatic variability in arctic insect and soil arthropod populations because of the lack of long quantitative time series.

 

caption Fig. 7.13. Population dynamics of the grey-sided vole and other vole species (combined) at Kilpisjärvi, northern Finland between 1950 and 2000[7].

 

Arctic indigenous peoples are rich sources of information about recent changes in animal health and behavior, in particular reindeer/caribou. Increases in vegetation (longer grass, riparian areas with denser vegetation) are linked to increased forage availability and more mosquitoes and flies, resulting in increased insect harassment of reindeer/caribou[152]. Changes in "the warmth of the sun", day length, and the timing of the growing season may trigger reindeer/caribou to cross a frozen lake or river when the ice is no longer thick enough to support their weight[153]. However, some of the environmental changes may be beneficial. Stronger and more frequent winds are said to provide reindeer/caribou with relief from insect harassment, meaning they can spend more time inland and not in coastal areas[154]. Qitirmiut in Nunavut know that caribou adapt to the heat by staying near coastal areas and shorelines, lying on patches of snow, drinking water, standing in the water, eating moist plants, and sucking mushrooms[155]. However, increases in the number of extremely hot days, combined with changing water levels and vegetation patterns, are likely to affect the ability of reindeer/caribou to respond in these ways.

Climatic cooling has to some extent caused habitat degradation in some coastal areas as a result of grubbing by snow geese on their staging ground. The lesser snow goose (''Anser caerulescens caerulescens'') breeds in coastal areas of the Hudson Bay region, which has experienced climatic cooling since the mid-1970s. This has delayed migration of the breeding populations[156]. Huge aggregations of staging and local geese in the coastal marshes have led to intense grubbing and degradation of salt-marsh sward[157]. Long-term observations and modeling have shown that goose reproductive variables are directly and indirectly dependent on selected climatic variables, particularly those relating to spring[158]. Nest initiation date, hatching date, and clutch size were associated with the date of the last snow on the ground and mean daily temperature between 6 and 20 May. Early snowmelt allows geese to forage and females to build up nutrient stores before nest initiation. Goslings that hatch earlier in the spring have a higher probability of survival than those hatching later. Inclement weather, such as cumulative snowfall, freezing rain, and northerly and easterly winds can result in nest abandonment by females and even adult starvation while incubating eggs.

==Responses to projected changes in climate==

Despite adaptations to low temperatures, warming experiments have shown that temperatures higher than normal do not present any physiological problem for arctic arthropods provided that water is available[159]. Arctic aphids were more successful in terms of the number of completed generations through the summer when temperature was experimentally elevated[160]. The effects of experimental warming were more pronounced in the high Arctic at Svalbard than in subarctic Abisko[161]. The combination of high temperatures and drought seem to be very problematic for terrestrial invertebrates[162], but the hydrological aspect of climate change in tundra habitats is an important issue that has rarely been addressed in studies of arctic animals[163].

Some of the most important effects of higher summer temperatures on arctic terrestrial animals are likely to be mediated through intensified interspecific interactions (parasitism, predation, and competition). Higher temperatures in the Arctic are very likely to lead to invasions of species with more southerly distributions. Such range expansions are projected to be particularly rapid in those species for which food resources (e.g., host plants) are already present[164]. For example, the mountain birch ''Betula pubescens'' ssp. ''czerepanovii'', the main food plant of the autumnal moth, occurs in the continental parts of the Fennoscandian forest tundra where winter temperatures are occasionally lower than the tolerance limit for over-wintering eggs[165]; however, warmer winters are likely to lead to the exploitation of this existing food source. Many insects belonging to the boreal forest already invade the low-arctic tundra in quite large quantities every summer[166] and the Arctic is subject to a "steady rain" of wind-dispersed small invertebrates[167] that are likely to rapidly become established when environmental conditions are adequate. Due to the lack of long-term monitoring programs, there are presently no arctic equivalents of the detailed and quantitative documentation of the northward spread of insects in Europe (e.g., [168]). Several generalist predators not yet present in the Arctic are likely to spread northward with increased ecosystem productivity due to warming. The red fox has already expanded into the Arctic, probably at the expense of the Arctic fox[169].

Winter warming will alter snow cover, texture, and thickness. A deeper snow cover is likely to restrict reindeer/caribou access to winter pastures, their ability to flee from predators, and energy expenditure traveling across snow. Changes in snow depth and texture are very likely to also determine whether warm-blooded small vertebrates will find thermal refuges for resting in snow dens (ptarmigan and hares) or for being active in the subnivean space[170]. Ice-crust formation reduces the insulating properties of the snowpack[171] and makes vegetation inaccessible to herbivores. There is ample observational evidence that the current incidence and degree of winter ice crusting clearly affects the population dynamics patterns of both large and small mammal herbivore species (see previous subsection). Moreover, there is experimental evidence that population densities of numerically dominant tundra Collembola (springtail) species such as ''Folsomia quadrioculata'' and ''Hypogastrura tullbergi'' can be halved following an episode of freezing rain on Spitzbergen[172]. The projected winter temperature increase of 6.3 °C by 2080 (mean of the five ACIA-designated model scenarios) is very likely to result in an increase of alternating periods of melting and freezing (section 6.4.4). Putkonen and Roe[173] found that episodes with rain-on-snow in the winter presently occur over an area of 8.4 x 106 square kilometers (km2) in the Arctic and they projected that this area would increase 40% by 2080–2089. The projected increase in the frequency of winter warming is very likely to severely suppress population densities; distort the cyclic dynamics and degree of geographic synchrony in lemmings, voles, and geometrid moths; and in some cases even lead to population extinctions.

Responses to projected increases in ultraviolet-B radiation levels

The extent to which animals are adapted to incident UV-B radiation levels must be inferred in most cases. Hairs and feathers necessary for insulation against low temperatures also presumably protect the skins of mammals and birds from UV-B radiation, while white winter hair and feathers reflect UV-B radiation to some extent. The eyes of non-migratory animals must be extremely well adapted to UV-B radiation in order to be effective in the dark arctic winter yet also cope with high UV-B radiation levels in the bright, snowy spring. Invertebrates have coloring that may serve many functions. Melanic forms of invertebrates might have advantages in thermoregulation and UV-B radiation protection[174]. If white coloration, insulation, and melanistic thermal regulation decrease due to reduced snow cover and higher temperatures, sensitivity to increased UV-B radiation levels is likely to increase. Four species of Collembola on Svalbard were investigated by Leinaas[175] with respect to UV-B radiation tolerance: ''Hypogastrura viatica'', ''Folsomia sexoculata'', ''Onychiurus groenlandicus'', and ''O. arcticus''. The first three species coexist in wet shore habitats, with the very heavily pigmented ''H. viatica'' on the surface and ''F. sexoculata'', which as an adult is also very heavily pigmented, lower down. ''O. groenlandicus'' is a soil-dwelling, unpigmented species. Although ''O. arcticus'' is most commonly found under small stones and in rock crevices, and is thus rather unexposed, it has some pigmentation. In an experiment with enhanced UV-B radiation levels (0.5 Watts/square meter (W/m2) in the 300 to 320 nanometer (nm) band for 12 to 14 hours per day, approximately equivalent to clear sky summer conditions in southern Norway) the unpigmented ''O. groenlandicus'' experienced 100% mortality within one week, while the heavily pigmented ''H. viatica'' was not affected.

Caterpillars of subarctic moths have skins that absorb UV-B radiation to varying extents and the degree of absorption may depend on previous exposure to high UV-B radiation levels[176]. However, UV-B radiation levels affect animals indirectly via the quality and quantity of food that is available to them as a result of UV-B radiation impacts on plant growth and secondary metabolite production (Ecosystem structure|section 7.4.1.4).

It is possible to infer some responses of animals to future increases in UV-B radiation levels by comparing them to the effects of natural UV-B radiation levels on animals along latitudinal gradients. Along a south-to-north gradient starting at 55.7° N, ambient UV-B radiation levels reduced hatchling size in frogs at sites up to 66° N, with no latitudinal gradient in UV-B radiation tolerance[177]. Surprisingly, for a given time of the year, although UV-B radiation levels decrease with increasing latitudes, the frogs were exposed to higher UV-B radiation levels during the sensitive stages of their life cycles (egg and tadpole) at high latitudes than at low latitudes[178]. These studies suggest that an increase in UV-B radiation levels due to anthropogenic ozone depletion is likely to reduce the population of those amphibians that have distribution ranges extending into the Arctic.

Enhanced UV-B radiation levels are thought to improve the immune system of the autumnal moth in the subarctic and to destroy the polyhedrosis virus. As this virus and the parasitoid wasp ''Cotesia jucunda'' are both important controllers of the survival of moth caterpillars, increased UV-B radiation levels could possibly lead to increased moth populations and birch forest defoliation. However, no direct effects of enhanced UV-B radiation levels on moth fecundity or survival have been detected[179].

Summary

Evidence for animal responses to climate change is scarcer than for plants because field experiments are less feasible for mobile animals, especially vertebrates. In many cases inferences are made based on time-series analyses of population abundance data for a few conspicuous species such as ungulates and lemmings.

Winter climate impacts, especially those events that affect properties of snow and ice, are particularly important. Freeze–thaw cycles leading to ice-crust formation have been shown to severely reduce the winter survival rate of a variety of species, ranging from soil-dwelling springtails (Collembola) to small mammals (lemmings and voles) to ungulates (in particular reindeer/caribou). Such icing induces conditions of anoxia that affect invertebrates, creates unfavorable thermal conditions for animals under the snow, and renders vegetation unavailable for herbivores. A deeper snow cover is likely to restrict reindeer/caribou access to winter pastures and their ability to flee from predators. The projected increase in the frequency of freeze–thaw cycles is very likely to disrupt the population dynamics of many terrestrial animals, and indications that this is already happening to some extent are apparent in the recent loss of the typical three-to-four year population cycles of voles and lemmings in subarctic Europe.

Experimental elevation of summer temperature has shown that many invertebrates respond positively to higher temperatures in terms of population growth, as long as desiccation is not induced. Many invertebrates, such as insects, are very likely to rapidly expand their ranges northward into the Arctic if climate warming occurs, because they have vast capacities to become passively or actively dispersed and host species (both plants and animals) are already present north of their present range borders.

Little is known about the responses in arctic animals to expected increases in UV-B radiation levels. However, there are some indications that arctic animals are likely to be more exposed and susceptible to such changes than their southern counterparts. The effects of increased UV-B radiation levels on animals are likely to be subtle and indirect, such as reduced food quality for herbivores and increased disease resistance in insect pest species.

Microorganisms (7.3.3.1)

Recent experiments that manipulate the environment (e.g., soil heating, changing the water table, atmospheric CO2 enrichment, and UV-B radiation supplementation and attenuation) have added new information about the effects of environmental change on the soil microbial community at the species level. In general, climate change is likely to alter microbial community composition and substrate utilization[180]. Tundra soil heating, atmospheric CO2 enrichment, and amendment with mineral nutrients generally accelerate microbial activity (leading to a higher growth rate). Higher CO2 concentrations tend to intensify root exudation, which is the main source of available carbon for soil and rhizosphere bacteria. Much less is known about the transient changes in the species composition of soil microorganisms induced by manipulation of UV radiation levels, although supplementation of UV-B radiation in the field resulted in changes in the composition of microbial communities[181]. Laboratory incubation of tundra soils from Barrow, Alaska, at different temperatures had strong effects on community composition assessed from a molecular biology approach, but only after a temperature shift of more than 10 °C[182].

 

caption Fig. 7.14. Simulation of changes in a tundra microbial community (Barrow, Alaska) induced by climate warming: (a) population dynamics of dominant soil bacteria (right axis applies to ''Bacillus''); (b) carbon budget including gross primary production (GPP), soil respiration, and litter dynamics. The simulation assumed that average air temperature instantly increased by 10 °C in year two of the simulation, indicated by the vertical arrows in each panel[8].

 

A mathematical simulation of the changes in tundra microbial community structure[183] showed, surprisingly, that the effects of temperature on the soil microbial community were less significant compared with effects on the plant community. Probable reasons for this include strong metabolic interactions between individual populations within the microbial community (in which the product of one organism is used as a nutrient substrate by other organisms) that stabilize community structure in a wide range of environmental conditions; the wide temperature tolerance of microbial species; and the lower resolution power of microbial taxonomy as compared with plant taxonomy.

The model[184] generated realistic patterns of mass and energy flow (primary productivity, decomposition rates, and soil respiration) under present-day conditions and in response to warming, pollution, fertilization, drying/rewetting of soil, etc. (Fig. 7.14). Figure 7.14a shows that L-selected species (''Bacillus'') display only sporadic occurrence under normal cold tundra conditions, in agreement with observations, and attain high population density after soil warming. Simulated soil warming accelerated both primary productivity and organic matter decomposition, but the latter was more affected. Soil warming also led to a negative carbon balance in the soil, as respiration exceeded photosynthesis leading to a decline in accumulated organic carbon (Fig. 7.14b; sections 7.4.2.1, 7.4.2.2, 7.5.1.1, and 7.5.4).

Conidia (spores) of the fungus ''Metarhizium'' are sensitive to UV-B radiation. There are great differences between strains, but strains from high latitudes are less tolerant than those from lower latitudes[185]. In one species (''M. anisopliae'') it was shown that UV-A also had a negative effect and, when comparing strains, the sensitivity to UV-A radiation did not correlate with the sensitivity to UV-B radiation[186]. Several groups have studied the effects of UV-B radiation on phylloplane (leaf surface-dwelling) fungi and litter-decomposing fungi. Moody et al.[187] found that five of the investigated species were sensitive and seven relatively insensitive to UV-B radiation. The spore production in the litter decomposers was generally inhibited by UV-B radiation (except for one species), while that in phylloplane species was unaffected. However, the sensitivity of spores is not equivalent to the sensitivity of the metabolic machinery of the vegetative body of a fungus (i.e., the thallus or mycelium) that produces the spores.

In the subarctic (Abisko), a study of the decomposition rates of a standard litter type showed that there was a change in the composition of fungal species resulting from elevated UV-B radiation levels[188]. These results to some extent resemble those from an earlier experiment studying the decomposition of dwarf-shrub litter at the same site[189].

The arctic periglacial environment represents a unique mosaic of unstable habitats (gradation between terrestrial and shallow wetland environments) where large variations in cyanobacterial and algal diversity, productivity, and life strategy exist[190]. Prokaryotic cyanobacteria and eukaryotic algae have different life strategies with respect to their susceptibility to severe and unstable conditions[191]. Cyanobacteria are well adapted to changeable conditions involving low and high radiation levels (including UV-B radiation), and cycles of desiccation and rehydration, increasing and decreasing salinity, and freezing and thawing. This gives them a great ecological advantage and allows them to be perennial. In contrast, eukaryotic algae have higher rates of photosynthesis and lower resistance to changes in irradiation, desiccation, rehydration, and freeze–thaw cycles. These features predetermine their annual character. If the arctic terrestrial environment becomes colder, the cyanobacteria are very likely to become the dominant community. In contrast, if temperatures become warmer, the eukaryotic algae are very likely to start to predominate. In addition, the ongoing temperature increase in the Arctic is very likely to influence cyanobacteria and algal production, as well as the balance between cyanobacteria and algae and invertebrate herbivore activity. Invertebrate grazing pressure is likely to increase and much of the visible cyanobacteria and algae biomass could possibly disappear from arctic locales[192].

Summary

Tundra soil heating, CO2 enrichment, and amendment with mineral nutrients generally accelerate microbial activity. Higher CO2 concentrations tend to intensify root exudation, which is the main source of available carbon for soil and rhizosphere bacteria. Supplementation of UV-B radiation in the field resulted in changes in the composition of microbial communities. Laboratory incubation of tundra soils had strong effects on community composition after a temperature shift of more than 10 °C. Surprisingly, the effects of many factors on the soil microbial community were less significant compared with effects on the plant community. However, a mathematical simulation of the changes in microbial community structure in the tundra showed that soil warming resulted in stimulation of bacterial growth.

The effects of increased UV-B radiation levels on microorganisms include damage to high-latitude strains of fungal spores, and damage to some species of leaf-dwelling fungi and soil-dwelling decomposer fungi that resulted in a change in the composition of the fungal communities.

Cyanobacteria are better adapted to changeable and harsh conditions than algae, and in milder climates are likely to be dominated by algae. However, herbivory of both cyanobacteria and algal biomass is likely to increase in a warmer climate.

Chapter 7: Arctic Tundra and Polar Desert Ecosystems 
             7.1 Introduction 
             7.2 Late-Quaternary changes in arctic terrestrial ecosystems, climate, and ultraviolet radiation levels 
             7.3 Species responses to changes in climate and ultraviolet-B radiation in the Arctic
                          7.3.1 Implications of current species distributions for future biotic change
                          7.3.2 General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels
                          7.3.3 Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation
                          7.3.4 Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels
                          7.3.5 Recent and projected changes in arctic species distributions and potential ranges
             7.4 Effects of changes in climate and UV radiation levels on structure and function of arctic ecosystems in the short and long term
                          7.4.1 Ecosystem structure
                          7.4.2 Ecosystem function
             7.5 Effects of climate change on landscape and regional processes and feedbacks to the climate system 
             7.6 Synthesis: Scenarios of projected changes in the four ACIA regions for 2020, 2050, and 2080 
             7.7 Uncertainties and recommendations

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Glossary

Citation

Committee, I. (2012). Arctic Climate Impact Assessment: Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation. Retrieved from http://www.eoearth.org/view/article/155211

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