While elevational patterns in tree communities are often examined, shrubs are typically ignored despite their abundance in many systems. The Northern Sierras represent a contact zone between plants of North Temperate biogeographic origin and plants with a southern biogeographic origin, providing an opportunity to compare elevational responses of shrubs from these two groups. We examined elevation patterns in leaf morphology along a 625 m elevational gradient on the western side of the Sierra Mountains of California for two sclerophyllous evergreen shrubs (Arctostaphylos nevadensis and Quercus vaccinifolia, southern origin) and two deciduous shrubs (Amelanchier alnifolia and Prunus emarginata, North Temperate origin). At seven sites along this elevational gradient, we collected leaves from up to five individuals of each species. Leaf area was measured for four leaves per individual, per site. Epidermal peels were collected from two leaves per plant and used to assess stomatal density and stomatal length. For each individual, 10 leaf images were taken at 400x from the epidermal peels and their measures averaged to produce a single value for that individual.
Results/Conclusions
As would be expected, there were large differences among species in leaf morphology that were fairly consistent along the elevational gradient. The two north temperate species were similar in stomatal density and leaf area, though Amelanchier had longer stomata than Prunus. Despite having nearly identical stomatal lengths, Quercus had nearly 4x higher stomatal density than Arctostaphylos. This large divergence placed the North Temperate plants as morphological intermediates between the southern taxa. Though there was significant variation among collection sites in leaf morphology, no species showed either clinal or unimodal trait responses to the elevational gradient. Even in multivariate analyses species traits varied idiosyncratically with elevation. The lack of clear elevational responses in leaf morphology suggests that either local environmental conditions strongly select only species well-suited to local conditions, or that local environmental heterogeneity is sufficient to mask elevational patterns in these species. Understanding the drivers of elevational responses will be crucial for predicting impacts of projected climatic changes on Sierra vegetation.