Arctic environments north of the treeline
This is Section 10.2.3 of the Arctic Climate Impact Assessment
Lead Author: Michael B. Usher; Contributing Authors:Terry V. Callaghan, Grant Gilchrist, Bill Heal, Glenn P. Juday, Harald Loeng, Magdalena A. K. Muir, Pål Prestrud
Arctic organisms must either survive or avoid the long, cold winters. Adaptations range from avoidance behavior (long-distance migration, migration from tundra to forest, migration down the soil profile) to specific physiological, morphological, and life history traits in both plants and animals. Species with specific adaptations to cold conditions often lack the flexibility to adapt to new conditions, particularly interactions with immigrant, competitive species from the south. For example the displacement of Arctic fox (Alopex lagopus) by red fox (Vulpes vulpes), and many arctic plant species that are shade intolerant.
In addition to the constraints of low temperatures on biodiversity, the contrast between summer and winter conditions is also important. The photoperiod is likely to constrain budburst, frost hardening, and reproduction in some potentially immigrant shrubs and trees. It is also likely to affect the endocrinology of mammals leading to constraints on reproduction and the onset of appetite. Short growing seasons select for plants that are perennials and have long development periods, for example three to four years from flower bud initiation to seed set. Marked temperature differences between summer and winter conditions currently select for plants that accumulate and store resources: up to 98% of biomass can be below ground. Such storage organs are likely to become a respiratory burden with warmer winters, and slow-growing plant species with multi-year development are eventually likely to be displaced by faster growing species, including annuals.
Overall, species richness in the Arctic north of the treeline is low. About 3% of the species making up the global flora occur in the Arctic. However, lower taxonomic groups are better represented than higher orders: only 0.7% of the flowering plant species occur in the Arctic compared with 1.6% of the conebearing plants. At a scale of 100 m2, however, the diversity of the flora of some arctic communities can equal that of temperate or boreal latitudes owing to the generally small size of arctic plants. Within the Arctic, the diversity of animals (about 6000 species) is twice that of plants. Again, with lower taxonomic groups better represented. Springtails, at 6% of the global total, are better represented than advanced invertebrate groups such as beetles with 0.1% of the global total. Climatic warming is very likely to increase the total number of species in the Arctic as species with more southern ranges shift northward, but the overall composition of the flora and fauna is vulnerable to the loss of arctic species at lower taxonomic orders. Some taxonomic groups are particularly species rich in a global context: any impact of climate warming on such species, for example, willows (Salix spp.), sawflies, stoneflies, wading birds, and salmonid fish, is likely to affect their diversity at the global level.
An important consequence of the decline in numbers of species with increasing latitude is a corresponding increase in dominance. For example, one species of collembolan, Folsomia regularis, may constitute 60% of the total collembolan density in polar deserts. Examples for plants include the cotton-grass Eriophorum vaginatum, and Dryas species. These “super-dominants” are generally highly adaptable, occupy a wide range of habitats, and have significant effects on ecosystem processes. Lemmings (Lemmus spp. and Dicrostonyx spp.) are super-dominant species during peak years in their population cycles.
Trophic structure is less complex in the Arctic than further south. In all taxonomic groups, the Arctic has an unusually large proportion of carnivorous species and a low proportion of herbivores. As herbivores are strongly dependent on the response of vegetation to climate variability, warming is likely to alter the trophic structure substantially as well as the dynamics of arctic ecosystems. The herbivore-based system in most tundra habitats is dominated by one or two lemming species, while the abundance of phytophagous (plant-eating) insects relative to plant biomass is small on arctic tundra. Large predators such as wolves, wolverines, and bears are less numerous in the tundra than the boreal forest and predation impacts on tundra ungulates are usually low. Thus, the dynamics and assemblages of vertebrate predators in arctic tundra are almost entirely based on lemmings and other small rodent species (Microtus spp. and Clethrionomys spp.), while lemmings and small rodents consume more plant biomass than other herbivores. Climate has direct and indirect impacts on the interactions among trophic levels, but there is greater uncertainty about the responses to climate change of animals at higher trophic levels.
Mechanical disturbance to plants and soils (animals can avoid or respond to such problems) occurs at various scales. Large-scale slope failures, such as active layer detachment, destroy plant communities but open niches for colonization by new generations of existing species or immigrant species with ruderal characteristics (fast growth, short life span, large reproductive capacity, and widespread dispersal of seeds). Such disturbances can also lead to recruitment of old genotypes of species producing long-lived seed that has been buried for hundreds of years. Sorting of stones and sediments in the active layer from daily to seasonal freeze–thaw cycles causes patterning of the ground and the creation of a mosaic of habitats at the landscape scale and a range of niches at the centimeter to meter scale. Such sorting, together with longer term permafrost degradation, movement of soils on slopes, and displacement by moving compacted snow and ice, exerts strong forces on plant roots. Above ground, wind-blasted ice crystals can erode plant tissues that extend above the protective snow cover. Mechanical impacts in the soil select for species without roots (mosses, lichens, algae), species with very shallow and simple root systems (e.g., Pinguicula spp.), and species with mechanically elastic roots (e.g., Phippsia algida and Tofieldia pusilla). Amelioration of the mechanical impacts is likely to lead to displacement of specialized species by more competitive neighboring species.
Super-dominant species such as lemmings have large effects on ecosystem processes. Lemming peak densities exceed 200 individuals per hectare in the most productive Lemmus habitats of Siberia and North America and the standing crop may approach 2.6 kg dry weight per hectare. Lemmings have a high metabolic rate and Lemmus spp. in particular has low digestive efficiency (about 30%, compared to 50% in other small rodents). Consequently, their consumption rate and impact on the vegetation exceeds that of all other herbivores combined (with the exception of the local effects of geese near breeding colonies). Also, lemmings destroy more vegetation than they ingest and after population peaks typically 50% of the above-ground biomass has been removed by the time of snow melt. In unproductive snowbeds, which are favored winter habitats of the lemming Lemmus lemmus, between 90 and 100% of the moss and graminoids present during winter may have been removed.
In forest near the treeline, insect defoliators can have devastating impacts on the ecosystem. The autumnal moth (Epirrita autumnata) shows cyclicity in its populations and outbreak proportions occur approximately every 10 to 11 years. Many thousands of hectares of forests are defoliated in outbreak years and defoliated forests require about 70 years to attain their former leaf area. However, insect outbreaks in sub-arctic Finland, followed by heavy reindeer browsing of regenerating birch shoots, have led to more or less permanent tundra.
These outbreaks are important for predators, such as snowy owl (Nyctea scandiaca) and arctic fox, which both prey on lemmings, and parasitoids such as the wasp Cotesia sp., which lays its eggs in caterpillars of the autumn moth. Changes to the populations and population trends of species such as lemmings and forest insect pests are very likely to have far reaching consequences for the biodiversity of the vegetation they consume, and for their predators and parasitoids, as well as for ecosystem processes like nutrient cycling.
The geography of the Arctic forces a range of constraints on the ability of vegetation zones and species to shift northward. In mainland Fennoscandia and many parts of the Russian Arctic, apart from Taymir and the western Siberian lowland, the strip of tundra between the boreal forest and the ocean is relatively narrow. Trees already occur close to the Arctic Ocean at Prudhoe Bay and Khatanga. Any northward movement of the forest will completely displace the tundra zone, and hence its biodiversity, from these areas. On the western Siberian plain, extensive bog ecosystems limit the northward expansion of forest and in arctic Canada, the high Arctic archipelago presents a natural barrier to dispersal of plants and range extensions of animals, while the barrens (polar desert and prostrate dwarf shrub tundra with less than 50% of the ground covered by vegetation) consist of soils that will constrain forest development for perhaps hundreds of years.
Continuous and discontinuous permafrost are characteristic of the Arctic. Permafrost, particularly its effect on the thickness of the active layer, limits the depth and volume of biologically available soil and reduces summer soil temperatures. These constraints limit plant rooting, the activity of soil flora, fauna, and microbes, and ecosystem process such as decomposition. Thawing of permafrost can have dramatic effects on biodiversity, depending upon drainage, precipitation changes, and, consequently, soil moisture. Permafrost thawing associated with waterlogging can prevent the northward advance of the treeline and even initiate a southward retreat. In other areas, such as the North Slope of Alaska, where precipitation is only about 125 mm/yr, permafrost thawing is likely to lead to drying and in some areas novel communities, reminiscent of the tundra-steppe, could form.
In addition to the effects of permafrost on biodiversity, biodiversity can also affect permafrost. A complete cover of vegetation, particularly highly insulative mosses, buffers soil temperatures from climate warming. In extreme cases, vegetation can lead to permafrost growth and a thinning of the active layer.
Arctic terrestrial ecosystems have the same types of feedback to the climate system as many other ecosystems, but the magnitude of these feedbacks is greater than most others. Per square meter, the tundra stores about half as much carbon as the boreal forests (about 9,750 g/m2 and 20,500 g/m2, respectively, 15,900 g/m2 at the interface between tundra and boreal forest). However, most of the carbon in the tundra occurs in the soil (about 94%), whereas about half (46%) of the carbon in the boreal forest occurs in the vegetation. The carbon stored in the tundra (about 102 Pg) is about 40% of that stored in the boreal forests (excluding the boreal woodlands). The tundra, boreal forest, and boreal woodlands together store 461 Pg of carbon; this is equivalent to about 71 years of annual global carbon emissions (based on emission data for the 1960s) of CO2 from fossil fuels (about 6.5 Pg of carbon per year). In contrast to the boreal forest, tundra has a high albedo and reflects about 80% of incoming radiation and this can lead to local cooling. Displacement of tundra vegetation by shrubs increases winter soil temperatures by 2 ºC.
Feedbacks that change the rate of climate change (although probably not the direction) will affect the rates of changes in biodiversity. For example, the effect of shrubs on soil temperatures is expected to increase decomposition rates and nutrient cycling, and so further shrub expansion. Also, it is possible that glacial dynamics (as well as more generally the dynamics of frozen ground) will have an effect. Glaciers have expanded and contracted in response to climatic variations. For example, in Iceland the maximum extent of the glaciers in historical times occurred in 1890. The majority of the glaciers contracted during the first half of the 20th century, particularly during the warm 1930s. Then from about 1940 the climate cooled, slowing the retreat of the glaciers, and some even started to advance again. This dynamic behavior of glaciers can have a marked effect on the biodiversity of nunataks (hills or mountains completely surrounded by glacial ice), which often contain a large proportion of the regional biodiversity. For example, there are over 100 species of vascular plants growing on Esjufjöll, a 9 km long nunatak within the glacier Vatnajökull, which is more than 20% of Iceland’s total vascular plant flora.
Glacial dynamics are not entirely related to temperature. In Norway, there is some evidence that inland glaciers are currently retreating while coastal glaciers are advancing in response to greater quantities of snowfall. This indicates the difficulties of predicting the effects of climate change on glaciers. The different rates of warming at different seasons of the year, as well as changes in seasonal precipitation patterns, especially for snow, will all determine the future dynamics of glaciers. These in turn influence the nunataks, the extent of areas of new ground available for primary ecological succession after glacial retreat, and the loss of ecosystems covered by advancing glaciers.
Chapter 10: Principles of Conserving the Arctic’s Biodiversity
10.2 Conservation of arctic ecosystems and species
10.2.1 Marine environments
10.2.2 Arctic freshwater environments
10.2.3 Environments north of the treeline
10.2.4 Arctic boreal forest environments
10.2.5 Human-modified habitats
10.2.6 Conservation of arctic species
10.2.7 Incorporating traditional knowledge
10.2.8 Implications for biodiversity conservation
10.3 Human impacts on the biodiversity of the Arctic
10.4 Effects of climate change on the biodiversity of the Arctic
10.5 Managing biodiversity conservation in a changing environment
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