Fine-scale environmental controls on seedling recruitment in early life stages in a Mediterranean-type climate
Plants in Mediterranean-type climates face significant environmental challenges across life stages, and seedlings are one of the most sensitive stages to variations in climate. Environmental controls may promote or restrict seedling survival in a linear fashion, or may drive mortality at the seedling stage in a threshold-type fashion. In order to investigate linear versus threshold relationships of seedlings with microclimates, we studied the establishment of two foothill (Pinus sabiniana and Quercus douglasii) and three montane (P. jeffreyi, P. ponderosa and Q. kellogii) tree species in the Sierra Nevada and Tehachapi Mountains in California over three years. Establishment of seedlings and associated fine-scale micro-environmental variables were monitored at 24 common gardens located in microenvironments at four sites along an elevation gradient. Environmental variables were related to seedling emergence and first through third year survival using generalized linear models (GLM) and classification and regression trees (CART).
Predictions of seedling recruitment based on temperature variables were significantly improved by the use of finer-scale abiotic predictors such as surface temperature as compared to air temperature at 2 m. Survival of all species differed significantly by microenvironment within sites. We detected threshold responses to summer daily maximum temperature and mid-summer soil moisture related to seedling mortality in Q. douglasii, Q. kellogii and P. sabiniana using CART analyses. In the gradient response-type (GLM), seedling survival was best correlated with factors that affected plant water balance, namely maximum temperature and soil water content. Response curves became more significantly related to predictors in years two and three in most species. All species established better in cooler and wetter locations and if our abnormally warm and dry study years reflect future-type conditions, this may indicate decreasing climate suitability for recruitment of these species at lower elevation sites. The establishment of species beyond the current distribution of conspecific adult trees, such as the survival of foothill species in montane environments, may reflect changes in recruitment under future climates.