|Good example of where drought led to replacement of tallgrass species with more drought-tolerant mixed grass species. From Weaver and Albertson, 1936.|
If you read Weaver's work, there are some hidden lessons about how plant communities respond to drought. Weaver talked about how during the Great Drought, the shortgrass spread hundreds of miles to the east into the mixed grass region and the mixed grass hundreds of miles into the tallgrass. Yet, reading his observations, most of the expansion of xeric grasslands did not occur from migration of individual species, but expansion of local populations. Eventually, when the drought broke the humid grasslands marched back westward, but again through expansion of local populations or dormant propagules. In many cases, big bluestem (Andropogon gerardii) recovered from crowns that remained viable for almost a decade.
These findings from Weaver raises an interesting set of questions about the functional diversity of grasslands and how different grasslands would respond to drought. Essentially, when droughts hit or mean precipitation levels change, how much does ecosystem function depend on expansion of local populations vs. immigration of species?
We tackled this question with some of the data we had on drought tolerance for a global set of grasses. In short, we asked how the diversity of drought tolerance varied bioclimatically. For example, as mean precipitation declines along a gradient, is there a greater relative abundance of drought-tolerant species? Are there fewer drought-intolerant species?
Turns out that across the full range of precipitation that generates grasslands, the diversity of drought tolerance among grasses is high. In wet grasslands, there are still many drought-tolerant grasses. In dry grasslands, there are still many drought-intolerant grasses.
Given a number of assumptions, extrapolating out, almost all grasslands should on average have a broad range of drought tolerance. If precipitation declines, drought-tolerant species on average should be able to expand locally and maintain ecosystem function.
There are still a number of details to work out, but Weaver's description of grasslands seems to hold at the global scale. Functional diversity in grasslands represents the typical high spatial variability in resource availability and the climatic variability typical of grasslands.