According to one FAO study, tree species tend to shift both to higher latitudes and to higher altitudes in response to global warming. Under this scenario, tree ranges in the northern hemisphere have the potential to expand 100 km northward, while their southernmost boundaries could retreat by the same magnitude for each degree of warming beyond current regional temperatures.
Similarly, scientists predict that loblolly pine (Pinus taeda), an important industrial forest species in the south-eastern region of the United States, could shift up to 350 km northward in response to a three degree increase in global temperatures, according to the FAO study.
Such shifts in forest distributions have already been observed. In Sweden during the first half of the twentieth century, the range of birch (Betula pubescens) expanded northward into the tundra in response to warming.
Shift to higher altitudes increases vulnerability
In addition to moving north, tree species could move to higher altitudes as a result of global warming.
FAO's study cited work by scientists in the Austrian Alps who found that alpine plant species have "migrated" toward higher altitudes at rates ranging from less than one metre to nearly four metres per year over the last century. The central Alps warmed by 0.7 degrees Celsius during that same period.
However, such a trend would make many species more vulnerable to genetic and environmental pressures, since mountain habitats are typical limited in size, which in turn would limit species populations and therefore the diversity of their gene pool.
For some species, little impact - for others, extinction
Not all tree species will respond by shifting their ranges, notes FAO. Some may have a greater capacity to adapt to new climatic conditions and could more or less continue to occupy their current distribution ranges.
For other species, however, climate change might outpace their ability to adapt, leading to extinction. In fact, warns FAO, in the coming century or so the Earth's climate is expected to change more rapidly than the rate at which many forest ecosystems would be able to adapt or re-establish themselves in more propitious climates, raising the spectre of large-scale species and forest die-offs.
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