15.4.2.2 Projected changes in biodiversity, vegetation zones and productivity
Where soils are adequate for forest expansion, species richness will increase as relatively species-rich forest displaces tundra (see Figure 15.3; Callaghan et al., 2005). Some species in isolated favourable microenvironments far north of their main distribution are very likely to spread rapidly during warming. Except for the northernmost and highest-Arctic species, species will generally extend their ranges northwards and higher in altitude, while the dominance and abundance of many will decrease. Likely rates of advance are uncertain; although tree-line advance of up to 25 km/yr during the early Holocene have been recorded, rates of 2 km/yr and less are more probable (Payette et al., 2002; Callaghan et al., 2005). Trophic structure is relatively simple in the Arctic, and decreases in the abundance of keystone species are expected to lead to ecological cascades, i.e., knock-on effects for predators, food sources etc. Local changes in distribution and abundance of genetically different populations will be the initial response of genetically diverse species to warming (Crawford, 2004). Arctic animals are likely to be most vulnerable to warming-induced drying (invertebrates); changes in snow cover and freeze-thaw cycles that affect access to food and protection from predators; changes that affect the timing of behaviour (e.g., migration and reproduction); and influx of new competitors, predators, parasites and diseases. Southern species constantly reach the Arctic but few become established (Chernov and Matveyeva, 1997). As a result of projected climate change, establishment will increase and some species, such as the North American mink, will become invasive, while existing populations of weedy southern plant species that have already colonised some Arctic areas are likely to expand. The timing of bird migrations and migration routes are likely to change as appropriate Arctic habitats become less available (Callaghan et al., 2005; Usher et al., 2005).
Warming experiments that adequately reproduced natural summer warming impacts on ecosystems across the Arctic showed that plant communities responded rapidly to 1-3°C warming after two growing seasons, that shrub growth increased as observed under natural climate warming, and that species diversity decreased initially (Walker et al., 2006). Experimental warming and nutrient addition showed that mosses and lichens became less abundant when vascular plants increased their growth (Cornelissen et al., 2001; Van Wijk et al., 2003). CO2-enrichment produced transient plant responses, but microbial communities changed in structure and function (Johnson et al., 2002) and frost hardiness of some plants decreased (Beerling et al., 2001) making them more susceptible to early frosts. Supplemental ultraviolet-B caused few plant responses but did reduce nutrient cycling processes (Callaghan et al., 2005; Rinnan et al., 2005). Such reductions could potentially reduce plant growth.
A ‘moderate’ projection for 2100 for the replacement of tundra areas by forest is about 10% (Sitch et al., 2003; see Figure 15.3), but estimates of up to 50% have also been published (White et al., 2000). However, impacts of changing hydrology, active layer depth and land use are excluded from these models. These impacts can be large: for example, Vlassova (2002) suggests that 475,000 km2 of tree-line forest has been destroyed in Russia, thereby creating tundra-like ecosystems. Narrow coastal strips of tundra (e.g., in parts of the Russian European Arctic) will be completely displaced as forest reaches the Arctic Ocean. During 1960 to 2080, tundra is projected to replace about 15 to 25% of the polar desert, and net primary production (NPP) will increase by about 70% (2.8 to 4.9 Gt of carbon) (Sitch et al., 2003). Geographical constraints on vegetation relocation result in large sub-regional variations in projected increases of NPP, from about 45% in fragmented landmasses to about 145% in extensive tundra areas (Callaghan et al., 2005).
Climate warming is likely to increase the incidence of pests, parasites and diseases such as musk ox lung-worm (Kutz et al., 2002) and abomasal nematodes of reindeer (Albon et al., 2002). Large-scale forest fires and outbreaks of tree-killing insects that are triggered by warm weather are characteristic of the boreal forest and are likely to increase in extent and frequency (Juday et al., 2005), creating new areas of forest tundra. During the 1990s, the Kenai Peninsula of south-central Alaska experienced a massive outbreak of spruce bark beetle over 16,000 km2 with 10-20% tree mortality (Juday et al., 2005). Also following recent climate warming, spruce budworm has reproduced further north, reaching problematic numbers in Alaska (Juday et al., 2005), while autumn moth defoliation of mountain birch trees, associated with warm winters in northern Fennoscandia, has occurred over wide areas and is projected to increase (Callaghan et al., 2005).