Climate change and the detection of possible elevation shifts of forest species in the Sierra Nevada, California

Background/Question/Methods Climate change threatens to shift vegetation latitudinally towards the poles or the equator and elevationally up mountain slopes. These vegetation shifts can alter ecological functioning, including fire regimes, seedling recruitment, and tree mortality, and the provision of ecosystem services, including carbon sequestration, watershed protection, and biodiversity conservation. Field observations of vegetation shifts will provide important information to natural resource managers who need to adapt management practices to climate change. The objectives of our research were: (1) to detect any past shift of tree species along an elevation gradient in the Sierra Nevada range, California, (2) to assess the potential causes of any observed shifts, and (3) to establish a transect of plots to monitor any future shifts. We established 32 Whittaker plots at 8 sites along a 19 km transect that covered an elevation change of 1400 m. The transect crossed three vegetation types, starting at oak/Douglas-fir forest at low elevation, proceeding through mid-elevation mixed conifer, and ending in red fir forest at upper elevations. We tagged, counted, and measured a sample of trees and extracted tree cores to estimate tree ages.

Results/Conclusions Analysis of 1145 measured trees characterized the three vegetation types. Oak/Douglas-fir forest generally occurs up to 1300 m elevation, dominated by California black oak (Quercus kellogii), canyon live oak (Quercus chrysolepis), and Douglas-fir (Pseudotsuga menziesii). Mixed conifer forest occurs at an elevation range of 1300-1800 m, dominated by sugar pine (Pinus lambertiana) and white fir (Abies concolor). Red fir (Abies magnifica) forest occurs above 1800 m. Estimation of past and current species composition based on tree age estimates from 197 tree cores provided indications of a possible vegetation shift: (1) statistically significant increase of Douglas-fir at mid-elevation and significant decrease at lower elevations, (2) significant decrease of red fir at one high elevation site, and (3) significant increase of canyon live oak at one mid-elevation site. Meteorological observations 7 km from the transect show a statistically significant increase of mean annual temperature in the period 1948-2004, a sign of global climate change. Because logging, burning, grazing, or other local human disturbance has not occurred along most of the transect since European settlement of the region, climate change may have caused the observed vegetation changes, although fire exclusion may have also affected species composition.