Biomes

Effects of ultraviolet-B on forest vegetation in the Arctic

This is Section 14.12 of the Arctic Climate Impact Assessment.

Lead Author: Glenn P. Juday; Contributing Authors: Valerie Barber, Paul Duffy, Hans Linderholm, Scott Rupp, Steve Sparrow, Eugene Vaganov, John Yarie; Consulting Authors: Edward Berg, Rosanne D’Arrigo, Olafur Eggertsson,V.V. Furyaev, Edward H. Hogg, Satu Huttunen, Gordon Jacoby, V.Ya. Kaplunov, Seppo Kellomaki, A.V. Kirdyanov, Carol E. Lewis, Sune Linder, M.M. Naurzbaev, F.I. Pleshikov, Ulf T. Runesson,Yu.V. Savva, O.V. Sidorova,V.D. Stakanov, N.M.Tchebakova, E.N.Valendik, E.F.Vedrova, Martin Wilmking. 

During summer in the Arctic, levels of UV-B radiation are generally not much different than at mid-latitudes. However, the Arctic and Antarctic are uniquely susceptible to short-term, intense stratospheric ozone depletion, especially in their respective spring seasons. For general background on ozone depletion and UV-B radiation in the Arctic, see Chapter 5.

Exposure to increased UV-B (280–315 nanometer [nm]) radiation is known to inhibit plant growth, development, and physiological processes. Long-lived, slow-growing plants such as trees may show cumulative effects of increasing UV-B radiation levels[1]. Effects of elevated UV-B radiation levels on plant processes vary in severity and direction and among species, varieties, and clones, as well as among plant parts and developmental stages. To avoid UV-B radiation, plants have developed screening mechanisms including increased leaf thickness or cuticle[2], optical structures to scatter and reflect UV-B radiation[3], and the accumulation of UV-B screening phenolics and flavonoids in the epidermal cells of the leaves[4].

Until the 1980s, relatively few plant species had been screened to determine the effect of enhanced levels of UV-B radiation. Some species show sensitivity to present levels of UV-B radiation while others are apparently unaffected by very large enhancements in UV radiation levels. Dicotyledonous crop plants such as peas[5] and canola seem to be more susceptible to increased UV radiation levels than cereals such as wheat[6] and barley, although many other factors play an important role in sensitivity. Even varieties of the same species can exhibit substantial variability in UV-B radiation sensitivity[7]. About two-thirds of the few hundred species and cultivars tested appear to be susceptible to damage from increased UV-B radiation levels. Crop damage caused by UV-B radiation under laboratory conditions generally has been attributed to impairment of the photosynthetic process[8]. In addition to gross disruption of photosynthesis, UV-B radiation may inhibit plant cell division as a physiological change, causing reduced growth and yields[9]. Accurately determining the magnitude of the effect of elevated UV-B radiation levels in the field is difficult, because interactions with other environmental factors, such as temperature and water supply, affect the reaction and overall growth of the crop[10].

Sensitivity to UV-B radiation differs among forest species and populations and is influenced by environmental conditions[11]. The composition of flavonoids varies according to altitude and latitude. High-latitude, low-altitude species and populations are more sensitive (fewer protective mechanisms) to enhanced levels of UV-B radiation than low-latitude, high-altitude species and populations (more developed defense mechanisms), reflecting natural levels of exposure to UV-B radiation[12]. Scots pine populations growing at high latitudes are rich in prodelphinidin[13] and significant differences in the characteristics of UV-absorbing compounds occur among species of pine[14] and birch[15].

Ultraviolet-B radiation has many direct and indirect effects on plant growth and development[16]. The direct effects are most damaging because the photons of UV-B radiation cause lesions in important UV-B absorbing biomolecules such as nucleic acids and proteins[17]. Photoproducts of DNA formed by UV-B radiation are all toxic and mutagenic[18] and altered DNA or RNA structures may interfere with transcription and replication causing slower protein synthesis as a result of UV-B radiation stress[19]. The indirect effects of UV-B radiation on plants can also cause damage by the formation of free radicals and peroxides[20]. Excessive UV-B radiation levels can also alter patterns of gene activity[21].

Field studies in both crop plants and trees suggest that the primary effects of increased UV-B irradiance are subtle, light-induced morphological responses that alter carbon allocation[22]. Recent UV radiation acclimation studies of subarctic and arctic plants have emphasized a multitude of responses ranging from avoidance and protective mechanisms to inhibition and accumulation of effects[23]. Enhanced UV-B radiation levels affect litter decomposition directly (photodegradation and mineral nutrient cycling) and indirectly (chemical changes in plant tissues) and also affect the biochemical cycling of carbon[24]. Both direct and indirect effects include physical, chemical, and biological components. The indirect effects of UV-B radiation are likely to be more significant than direct effects in subarctic and arctic forest ecosystems.

Subarctic conifers are long-lived with long generation times; damage from UV-B radiation is likely to accumulate over the years. Winter injuries to evergreens are caused by the interaction between freezing, desiccation, and photo-oxidation[25]. In subarctic and arctic conditions, late-winter cold temperatures, enhanced UV radiation levels from intense solar radiation, and water deficiency are the major environmental risks. Xeromorphic leaves and small leaf size of subarctic and arctic trees reduce evapotranspiration. However, late spring in high-latitude subarctic and arctic ecosystems is characterized by high levels of solar radiation and fluctuations between freezing and thawing temperatures. The UV radiation dose received by evergreens in the late winter, early spring, and at the beginning of the short growing season is high due to reflectance from persistent snow cover. Some earlier studies considered evergreens to be tolerant, but other studies have revealed evergreens to be sensitive to enhanced UV-B radiation levels[26].

Experimental field evidence indicates that enhanced UV-B radiation levels mainly increase the amount of soluble UV-B-absorbing compounds in summer-green (deciduous) plants[27], but the protective functions of wall-bound phenolic compounds and epicuticular waxes in evergreens are more complicated. The responses are species-specific among pine species[28].

In needles from mature subarctic Scots pine, enhanced UV-B radiation levels induced xeromorphic (change in plant anatomy to enhance drought resistance) characteristics, including smaller epidermal area and enhanced development of the cuticle layer[29]. Ultraviolet responses increased the concentration of UV-B radiation absorbing compounds in the epidermal cells and induced high and accumulating proportions of oxidized glutathione[30]. The cumulative stress has been measured as gradually decreased total glutathione and an increased proportion of oxidized glutathione levels in one- to three-year-old needles[31].

The most consistent field response to enhanced UV-B radiation levels[32] is an increase in concentrations of soluble or wall-bound UV-B radiation absorbing compounds in leaves, complicated by great seasonal, daily, and developmental variation, both in epicuticular and internal compounds. It is important to remember that not all soluble flavonoids are UV-inducible[33] or acid-methanol extracted (e.g., cell-wall bound UV-B absorbing phenylpropanoids[34]).

Increased amounts of epicuticular waxes and UV-absorbing compounds, such as flavonoids, and smaller leaf/ needle surface area are plant defense mechanisms against UV-B radiation. In young Scots pine seedlings, UV-B radiation induces flavonoid biosynthesis[35]. However, an increase in UV-B absorbing compounds may result in a decrease in cell expansion and cell-wall growth[36]. Diacylated flavonol glucosides provide protection from UV-B radiation[37] and biosynthesis of these compounds and the development of waxy cuticles appears to provide effective UV radiation protection in young needles[38]. The protective effects come about through rapid development of epicuticular waxes, an increase in cutinization, and an initial increase followed by inhibition of UV-screening compounds[39].

Plant-surface wax morphology, chemistry, and quantity respond to environmental changes. Many diols[40] are important as reflectors to avoid harmful UV radiation, but they also absorb in the UV spectrum. In many tree species, phenolics and other UV-protective substances are situated in the cuticle. The role of epicuticular waxes has been considered mainly to provide reflectance of UV radiation, but some wax components, for example, secondary alcohols (e.g., nonacosan-10-ol) and β-diketones, absorb UV radiation[41]. Naturally established treeline Scots pines in a study site at Pallas-Ounastunturi National Park, Finland, were exposed to enhanced UV-B radiation levels during the full period of arctic summer daytime. The average needle dry weight increased and the wax content decreased in the UV-treated trees. The responses were observed both in the previous and current needle year[42].

A long-term study of silver birch conducted over three growing seasons showed changes in growth (i.e., shorter and thinner stems), biomass allocation, and chemical protection, while the effects on secondary metabolites in the bark were minor[43]. The changes did not occur until the third growing season, demonstrating the importance of long-term studies and the cumulative effects of UV-B radiation. A three-year study of Scots pine showed how varying defense mechanisms within the season, needle age, and developmental stage protected the Scots pine needles against increased UV-B radiation levels[44]. However, protective pigment decreased during the third year of exposure, suggesting that cumulative UV-B radiation exposure affects defense mechanisms and possibly makes these defenses insufficient for long-term exposure.

Quantitative changes were detected in secondary metabolites (plant chemicals) in leaves of dark-leaved willow (Salix myrsinifolia) exposed to enhanced UV-B radiation levels[45]. The changes in amount of secondary compounds are likely to have indirect effects at the ecosystem level on willow-eating insects and their predators, and on the decomposition process. Both dark-leaved and tea-leaved willow (S. phylicifolia) showed chemical adaptations to increasing UV radiation levels. However, the chemical adaptations were based more on clone-specific than on species-specific responses[46]. Several types of phenolic compounds in seedlings of nutrient-limited silver birch respond to UV-B irradiance, and seedlings are less susceptible to UV radiation when grown in nutrient-limiting conditions[47]. However, changes in secondary metabolites of birch exposed to increased UV-B radiation levels might increase its herbivore resistance[48]. A lower level of animal browsing on birch because of this chemical change induced by increased UV-B radiation levels could possibly improve the performance of birch over its woody plant competitors. Silver birch exposed to UV-B radiation with nutrient addition also displays an efficient defense mechanism through production of secondary metabolites, and demonstrates the additive effects of nutrient addition[49]. These results clearly establish that UV-B radiation plays an important role in the formation of secondary chemical characteristics in birch trees at higher latitudes[50].

 Chapter 14: Forests, Land Management, and Agriculture
14.1. Introduction
14.2. The boreal forest: importance and relationship to climate
14.3. Land tenure and management in the boreal region
14.4. Use and evaluation of the ACIA scenarios
14.5. Agriculture
14.6. Tree rings and past climate
14.7. Direct climate effects on tree growth
14.8. Climate change and insects as a forest disturbance
14.9. Climate change and fire
14.10. Climate change in relation to carbon uptake and carbon storage
14.11. Climate change and forest distribution
14.12. Effects of ultraviolet-B on forest vegetation
14.13. Critical research needs

 

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Citation

Committee, I. (2012). Effects of ultraviolet-B on forest vegetation in the Arctic. Retrieved from http://www.eoearth.org/view/article/152373

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