Landscape dynamics of seedling recruitment microsites for foothill and montane tree species in the Sierra Nevada and Tehachapi Mountains
Localized climates and soil moisture regimes in Sierra Nevada foothill and montane landscapes may allow tree species to re-distribute locally under a changing climate, thereby buffering species' exposure to regional climate change. To better understand the coupling of microenvironments to regional trends in tree establishment, we measured and modeled microclimate and tree seedling establishment at four landscapes in the southern Sierra Nevada and Tehachapi Ranges. We asked: What is the relationship between regional climate and spatiotemporal dynamics of microclimates? What microclimate factors govern plant establishment and initial growth? Can we model those factors at appropriately fine scales? Since 2011, we have measured microclimate with weather stations and surface temperature arrays. Simultaneously, seedling establishment trials have been conducted for two oak species and three pine species at 24 experimental gardens located across a range of topographic positions at the four study landscapes. Climate water deficit (CWD) and surface air temperatures are being modeled at 30 m resolution using the Basin Characterization Model, downscaled PRISM data and downscaled global climate model outputs for the period 1901-2100. We are applying the modeled relationship between measured seedling establishment and microclimate to project interannual variation in seedling establishment across our study landscapes.
Pine recruitment has been low overall. Results are presented for foothill and montane oak species, Quercus douglasii and Quercus kelloggii, respectively. Both species emerge in foothill and montane sites but survivorship for both species is higher at cooler, wetter montane locations. Year 1 survival is strongly associated with accumulated winter-through-summer CWD and maximum summer surface temperatures. Modeled extent and location of species-specific establishment sites manifest high inter-annual variation as well as long term historical and projected future trends. Mapping the frequency of good establishment years within multi-decadal windows helps visualize landscape dynamics in tree establishment sites. Results are being incorporated into spatially explicit population models to understand the emergence of tree species range shifts from local processes.