Response of forest trees to global environmental changes

Characteristics of tree species may uniquely situate them to withstand environmental changes. Paleoecological evidence indicates that the geographic ranges of tree species have expanded and contracted several times since the last glacial epoch in response to directional environmental changes. For most tree species, these range fluctuations have been accomplished without any apparent loss of genetic diversity. A possible explanation that distinguishes most trees from many herbaceous plants is that much of the genetic variation within tree species is found within rather than among their populations. Thus, the extinction of a relatively large proportion of a tree species' populations would result in relatively little overall loss of genetic diversity. Furthermore, phylogeographic studies indicate that for some tree species, habitat heterogeneity (elevation, slope aspect, moisture, etc.) in glacial refugia may have preserved adaptive genetic variation that, when recombined and exposed to selection in newly colonized habitats, gave rise to the local adaptation currently seen. The maintenance of genetic diversity in the face of extensive habitat fragmentation is also a concern. Many forest trees, however, may be buffered from the adverse effects of habitat fragmentation. First, the longevity of individual trees may retard population extinction and allow individuals and populations to survive until habitat recovery occurs. Second, considerable evidence is available that both animal and wind-pollinated tree species in fragments experience levels of pollen flow that are sufficient to counteract the effects of genetic drift. The combination of individual longevity, high intra-population genetic diversity and the potential for high rates of pollen flow should make tree species especially resistant to extinction and the loss of genetic diversity during changing environmental conditions. (C) 2004 Elsevier B.V. All rights reserved.