Response Of Vegetation To The 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 ice around the Arctic has decreased rapidly, glaciers everywhere are melting back fast, and permafrost is thawing in areas where it has been stable for centuries or millennia. Already there are 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 3.9*). The general pattern of the warming appears to correspond to a natural climate fluctuation that has always

Figure 3.9. The greening trend around the Arctic from satellite data. Source: data from Stowe et al. (2004), figure by Zhou and Myeni (2004). (Note: NDVI is a measure of the "green-ness" of the image. The higher the NDVI the more vegetation.)

Figure 3.9. The greening trend around the Arctic from satellite data. Source: data from Stowe et al. (2004), figure by Zhou and Myeni (2004). (Note: NDVI is a measure of the "green-ness" of the image. The higher the NDVI the more vegetation.)

Figure 3.10. Instrumental temperature record of the last 120 years (from CDIAC).

Figure 3.10. Instrumental temperature record of the last 120 years (from CDIAC).

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 continuous covering of tundra, comparison of aerial photographs taken in the 1940s 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 3.11a, 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 native to Antarctica: a grass (Deschampsia antarctica) and a tiny member of the cabbage family (Colobanthus quitensis). 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 glaciers are melting back almost everywhere, a strong sign that there is warming going on. Change in vegetation on mountains is harder to find and

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Climate Northern Canada
Figure 3.11. (a, b) Arctic shrub cover change in northern Canada. The numbered areas in the foreground show the change most clearly. Source: Stow et al. (2004).

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 meltback of glaciers is occurring nearby. Partly this reluctance of the vegetation to change may relate to the extreme conditions at tops of mountains: soils are thin and poorly developed, and plants cannot establish themselves and grow very easily even if the climate is now warm enough. So, a response to warming will often take time, perhaps the time taken for microbes to get to work breaking down minerals that seedlings will eventually be able to use. Also, plants grow and develop slowly in the cold climate of high mountains, even if some warming has occurred. It can take them a long time to respond to the warmth by becoming larger or setting more seed. Such is the slowness of ecosystems in the high mountains that some ecologists believe that treelines are now rising in response to a warming event that occurred 150 years ago, not the current burst of warming!

Even where the treeline has not moved noticeably, a shift in composition of the existing forest further downslope may reveal the effects of increasing temperatures. For example, in northeastern China, Changbai Mountain has not shown much change in the treeline, but my co-worker Yangjian Zhang has shown that the ancient forest on its upper slopes has thickened and the species composition has shifted towards more warmth-demanding species—certainly the trend that would be expected for a climate warming.

Near the top of mountains in Tasmania (the big island off southeastern Australia), the high alpine tundra zone—which has several beautiful types of cushion plant—is disappearing as trees are able to seed themselves higher and higher up the mountains, due to less severe winters and warmer summers. The mountains in Tasmania are only just tall enough to have a tundra zone at the top. At the rate things are going, in a few more decades these mountains will probably be forested right up to the top; there will be no tundra zone left in Tasmania and these alpine plants will only survive in cultivation.

In ecology, there always seem to be some exceptions to a trend. In parts of the timberline in the lowlands of northern Siberia, trees are retreating south as the tundra expands. The change in tree cover seems to have gone totally in the opposite direction to what would be expected from the warming trend observed across the region. However, the retreat is apparently due to a reduction in rain and snowfall seen in the climate records, rather than any trend towards coldness.

Although the most striking shifts are evident at the coldest limits of the world's vegetation, changes may also be occurring almost unnoticed in other parts of the world that are not being so closely watched, or where there is not such a striking boundary in vegetation structures as on the edge of a biome. In many parts of the world, trees now seem to be growing faster on average than at any time in the last few centuries, perhaps as a result of the warming. In some areas, the trees in native forests are growing faster and dying younger than they used to several decades ago. This increased "turnover", as it is called, may be a result of warmer temperatures allowing each tree to grow and complete its life cycle more rapidly. Such speeding-up of forest growth has been detected across much of the western USA in a recent study by Mark Harmon of the University of Oregon. A similar trend has been found by Oliver Phillips and colleagues across the Amazon Basin, and also in a widespread study in the rainforests of central Africa. However, the pattern is not present everywhere: for example, in at least some tropical forest localities in Malaysia and Central America, in a certain percentage of the forest plots within South America and Africa, and in parts of the eastern USA, forest growth and turnover seem to be slowing down. Exactly what is at work behind the general trend towards faster forest growth and turnover is not clear, although a change in climate is widely suspected. Other factors might be important too: for example, the direct C02 fertilization effect (see Chapter 8). In Europe, faster forest growth might be partly due to the beneficial effects of nitrogen and sulfur pollution on the trees, making up for nutrient-deficient soils.

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