Perhaps one of the oldest questions asked by ecologists is why do species occur in the environments they do? This question gets at the core of ecology and requires an understanding of the various interactions every species has with the biotic and abiotic environment. To add further complexity to this question, it must also be considered that environments are dynamic - some more dynamic than others. The changes in environments can occur on a daily basis (e.g. temperature fluctuations in a desert), seasonally (e.g. wet and dry season in the Mediterranean), or on a geological time scale. Most species are adapted to these sorts of changes in the environment. However, when rapid changes occur which are out of the norm for an environment - for example, global climate change - a species is faced with 3 options: move, adapt, or die. For some species, rapid adaptation or migration isn't out of the question. However, for sessile species with longer generation times, rapid adaptation and migration are much more of an issue. Trees are a primary example of such a group of organisms expected to struggle with rapid global climate change. In their paper 'Failure to migrate: lack of tree range expansion in response to climate change' Zhu et al. (2012) assess how tree species ranges are changing as a result of rapid climate change.
The literature prior to this paper suggested that as temperatures increase species will migrate northward in latitude in response. This assumption was evidenced by data showing that tree species have been migrating upwards in elevation as temperatures increased. The key flaw in this assumption comes from the attempt to generalize elevational tree responses to latitudinal responses. The primary difference between the two responses being that the distance of migration required to reach cooler temperatures is vastly smaller when moving up an elevation gradient than a latitudinal gradient.
The authors here instead focus on latitudinal shifts of tree species in response to climate change. A depressing 58% of the 92 tree species studied were found to be undergoing an overall contraction in range at both latitudinal extents. Of the 92 tree species ranges, 20% were following the previously expected pattern of expanding northward and contracting at southern latitudes while 16% of species were actually expanding southward whilst contracting in the north. The remaining 4% of tree species ranges were undergoing an overall expansion at both latitudinal extents.
This study suggests that most US tree species simply are not migrating poleward fast enough to keep up with temperature increases. This makes the situation for tree species (and the multitude of other species which rely on trees) all the more dire as climate change continues. While conservation efforts such as assisted migrations may be able to somewhat ameliorate the effects of climate change on tree species distributions, there will undoubtedly be major ecological shifts for tree species over the course of the 21st century.
Paper reference:
Zhu K., Woodall C.W., Clark J.S. (2012). Failure to migrate: lack of tree range expansion in response to climate change. Global Change Biology. 18(3): 1042 - 1052.
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