Response Of Vegetation To Tiie Present Warming Of Climate

There are of course many aspects of plant ecology that seem tightly controlled by temperature: the broad-scale distribution of biomes across the continents is one example (Chapter 2). Temperature also determines exactly how high up a mountain trees can grow, and the precise time of year that trees start to leaf out, or when spring flowers appear. It also determines how fast a tree can grow, with variations in climate showing up in the width of the annual rings.

Because of the amplifying factors that operate near the poles (Chapter 5). GCMs predict that climate should be changing most dramatically close to the poles; and indeed climate station data show that these areas are warming particularly rapidly.

Land and ocean ice data seem to corroborate the view that this warming is both intense and sustained; sea icc around the Arctic has dccrcascd rapidly, glacicrs everywhere arc melting back fast, and permafrost is thawing in areas where it has been stable for centuries or millennia. Already there arc signs that the rapid warming that has occurred over the past few decades has had some effects on biological processes, in at least some parts of the world.

In the mid and high latitudes of the northern hemisphere, most of the temperature-sensitive aspects of plant behavior are showing at least some signs of shifting in response to the recent warming trend. However, the trend cannot be found everywhere, partly because the warming itself is somewhat patchy, and perhaps also because other environmental factors can intrude and complicate the picture.

As one would expect from the temperature trends, vegetation around the Arctic has begun to change. On the broadest scale, satellite data show a greening of the Arctic since the 1980s, especially in the northernmost parts of Canada and Alaska, and northwestern Siberia and Scandinavia (Figure 3.7*). The general pattern of the warming appears to correspond to a natural climate fluctuation that has always occurred across these regions: the Arctic Oscillation. What is unusual now is how-intense and sustained this phase is. From all the climate indicators that we have available, there has been no other period in the past thousand years where the Arctic experienced such warm temperatures for so long. This suggests that something beyond the natural background of climate fluctuation may be at work. On the ground, this warming translates into noticeable changes in the structure and species composition of tundra vegetation in northern Alaska and Canada. Many of the small ponds that dot the landscape have drained, as a result of the layer of icy soil (permafrost) that held them in place melting away, so terrestrial vegetation is taking over from the aquatic communities that lived there before. Shrubby vegetation of dwarf willows and alders is pushing into the grassy tundra on Alaska's north slope. On the far northern islands of Canada, where climate has always been too cold for a

Www Climate And Vegetation Drawing

NDVI TREND (19A2-I909)

Figure 3.7. The greening trend around the Arctic from satellite data. Source: data from Stowc et ai (2004), figure by Zhou and Myeni (2004). (Note: NDVI is a measure of the "grecn-ness" of the image. The higher the NDVI the more vegetation.)

NDVI TREND (19A2-I909)

Figure 3.7. The greening trend around the Arctic from satellite data. Source: data from Stowc et ai (2004), figure by Zhou and Myeni (2004). (Note: NDVI is a measure of the "grecn-ness" of the image. The higher the NDVI the more vegetation.)

continuous covering of tundra, comparison of aerial photographs taken in the 1930s and today shows that there has been an expansion of shrubby vegetation out from the most sheltered spots, which were the only places it was able to grow before (Figure 3.8a. b). It seems then that the landscape in the far north is changing, because of the warming that has occurred during that period.

At the other end of the world, on the rocky edges of the Antarctic Peninsula, a noticeable warming has occurred over recent decades. On the west side of the Peninsula, temperatures have gone up by 2.6 C since the 1940s. This warming has resulted in a veritable population explosion of the only two types of vascular plants known from Antarctica: a grass (Deschampsia an l arc tic a) and a tiny member of the cabbage family (Colohanthus qui/ensis). At sites where these two species have been monitored over more than 30 years, they have expanded from scattered plants and clumps to form the first "lawns" on Antarctica.

Mountain tops around the world also seem to be experiencing warmer temperatures. Mountain glacicrs arc melting back almost everywhere, a strong sign that there is warming going on. Change in vegetation on mountains is harder to find and interpret than melting of glaciers, but it is certainly widespread. Some of the most striking changes are in the Ural Mountains of western Russia, where the treeline over a very broad area has migrated 60 80 m upslope. Similar upwards migration of the treeline has occurred in the mountains of Scandinavia, in the western USA, the Alps and the mountains of Tasmania. However, in some areas of the world, mountain vegetation has not responded, even where mcltback of glacicrs is occurring nearby.

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