Microclimate controls on tree species establishment in mountainous regions

Background/Question/Methods

Global climate change is causing shifts in ranges of tree species. Many studies investigating the potential responses of tree species to climate change rely on temperature tolerances inferred from range boundaries.  Field trials assessing differential growth responses within microclimates, and additionally, using seeds of different geographic provenances, provide a more consistent basis for investigating these controls on tree species establishment.  This project investigates the following questions as a basis for further investigation into tree species response to climate change:  what is the distribution of microenvironments in mountain landscapes under the current climate, and how does tree species establishment differ across these environments? This study focuses on 5 tree species: Quercus douglasii, Q. kelloggii, Pinus ponderosa, P. jeffreyi and P. sabiniana.  Microclimate conditions and establishment were assessed at four sites: a foothill and a montane site in both the Sierra Nevada and the Tehachapi Ranges. At each site, arrays of 24 temperature sensors were established, sampling microenvironment heterogeneity across approximately 2km area. Six common gardens including all species and reciprocal-seeding from the two mountain ranges were established:  on north slopes, south slopes and valley floors, and these were surrounded by dense grids of 20 temperature loggers in 0.16 ha areas.

Results/Conclusions

As anticipated, sensor networks at each site recorded substantial differences in temperature by topographic aspect and height above the soil surface. The magnitude of these differences varied significantly across the landscape.  Differential success of tree species establishment was also seen across microenvironments.  Topographically-modulated microclimates may play a significant role in determining habitat extent and connectivity, thereby influencing extinction risk and regulating rates of range expansion under climate change.  New methods for interpolating weather station data and for downscaling coarse climate models show promise for portraying this microclimate variation.