In order to predict the effects of climate change on natural systems, it is critical that we understand how species have responded to past anthropogenic climate change. Montane plants and animals have been observed to shift their geographic distributions upslope in response to climate change, which manifests as changes in local community composition. However, there is tremendous variation in the magnitude and direction of upslope shift and compositional change, leading to heterogeneity in the degree of climate tracking. We tested the hypothesis that tropical floras and faunas have heightened sensitivity to climate change and therefore should track changes in temperature through time more closely than do temperate-zone floras and faunas. We tested this hypothesis using two approaches. First, using a meta-analysis of resurvey data, we examined how observed upslope shifts matched expected shifts based on warming for both species assemblages (150 estimates) and individual species (5,872 estimates). Second, using repeated forest inventories in the temperate montane forests of United States (more than 200 plots) and the tropical montane forests of Central and South America (more than 60 plots), we compared the rates of compositional change (i.e., thermophilization rates) to concurrent rates of warming to generate an index of climatic tracking. We then tested for differences in climate tracking across latitudes.
Results/Conclusions
Our first meta-analysis showed that tropical assemblages and species were tracking climate more closely than are temperate assemblages and species. Tropical assemblages have shifted upslope by about 79% of the amount predicted by local warming rates, compared to a tracking score of 33% observed for temperate assemblages. Our second analysis using forest inventory data suggested that the thermophilization rates of plant communities, as well as the degree of climate tracking, were greater at lower latitudes. On average, tropical sites have a thermophilization rate of 0.076±0.003 °C per decade compared to a warming rate of 0.169±0.002 °C per decade; temperate sites have an average thermophilization rate of 0.004±0.001 °C per decade compared to a warming rate of 0.250±0.000 °C per decade. Both lines of evidence suggest widespread upslope shifts of montane species but that rates of shifts are faster in the tropics. This latitudinal gradient in distributional responses is consistent with accumulating evidence that tropical montane biotas are disproportionately sensitive to ongoing and future climate change. This study showcases how distributional responses can be quantitatively analyzed using both past ecological studies and current large-scale inventory data.