Twentieth century shifts in abundance and composition of vegetation types of the Sierra Nevada, CA, US

Questions: Has tree density changed consistently across vegetation types? Do changes in component species correspond with changes across vegetation types? Do patterns of changes suggest potential drivers of change? Location: Northern two-thirds of the Sierra Nevada, CA, USA, ca. 45 000 km². Methods: Using two data sets that cover the span of elevations and land jurisdictions in the study area, we classified 4321 historical plots and 1000 modern plots into nine broad groups of vegetation types that are widely used by land managers and researchers in the region. We compared tree density and composition between historical and modern plots across and within these nine types. Results: In the modern data set, tree density was significantly higher in eight of nine vegetation types. Total density was significantly higher in modern plots for all west slope types, especially for montane hardwood, where modern forests are 128% denser, and mixed conifer forests, which are 69% denser. Relative density of component species was also very different between data sets in these forests, and suggests a shift in dominance toward shade-tolerant conifers and evergreen oaks. Fire suppression is likely a driving factor in these types but density was also significantly higher in high-elevation types such as sub-alpine forest (+20%), where neither fire suppression nor logging have had major impacts on structure. East slope forest types (eastside P. jeffreyi forest and piñon-juniper woodland) were very similar in both modern and historical data sets, with no significant differences in density or composition. Conclusion: West slope forest types, especially montane hardwood and mixed conifer forest, appear the most altered types of the mountain range. These types are more productive but have also been subject to greater disturbance than high-elevation and east slope forest types. Climate change may also be driving changes across the study area. Species such as Quercus chrysolepis and Calocedrus decurrens, which have each increased markedly in abundance, appear well positioned to dominate in the near future, especially under continued fire suppression and a warmer climate.