Partly this reluctance of the vegetation to change may relate to the extreme conditions at lops 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!
liven where the treclinc has not moved noticeably, a shift in composition of the existing forest further downslope may reveal the cffccts 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 plants endemic to Tasmania—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 /one 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.
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