Despite numerous dire predictions by climate models and observational studies, there remains a great need for empirical data from more local studies to test model predictions and provide more specific information to land managers about species responses to warming. One way to determine current trajectories of climate-induced change is by re-sampling historic data sets and comparing modern and historic conditions. The Vegetation Type Mapping (VTM) Project sampled 18,000 vegetation plots throughout California from 1929-1934. I re-sampled 139 of their plots in high elevations (2500-3400 m) of the central Sierra Nevada from 2007-2009 to look for differences in stand structure and composition. Modern plots were sampled using methods that extracted equivalent data as that of VTM crews, focusing on the structure and composition of woody vegetation. Stem measurements were lumped into the four diameter size classes used by VTM crews and analysis was carried out on stem counts in each size class for modern versus historic plots.
Results/Conclusions
Modern stem abundance of the smallest size class was higher for all eight conifer species that occur in the study range. These differences represent increases of 16-147% over historic levels, including significant increases of 43 and 62%, respectively, for the two highest-elevation species, mountain hemlock (Tsuga mertensiana) and whitebark pine (Pinus albicualis). This trend was observed across all microsite types and elevations sampled in the study but was stronger in the northern half of the area sampled. In addition, stem abundance in the two largest size classes was lower than historic levels for the three largest species. Taken together, these results indicate major shifts in stand structure during the past 75 years and do not represent anomalous, single-species or specific microsite changes. Also, since re-sampling was focused on high elevations in national parks and wilderness areas, these changes cannot be attributed to human disturbances such as logging and fire suppression. The marked increase of small trees observed may be a result of increased recruitment and survival afforded by lengthened growing seasons and warmer temperatures in the summer months. This result is surprising, given the prediction that high-elevation vegetation in California will be significantly reduced over the next 100 years.