Seven decades of forest change in the Sierra Nevada, CA, USA

Background/Question/Methods

Sustainable management of temperate montane forests requires balanced consideration of ecological disturbances, land-use practices, and the effects of global change. In the Sierra Nevada, California, USA, various studies have assessed forest change over time and its causes but most of these have been at local or watershed scales. Using extensive historical and modern forest inventories, we compared montane forest structure in the early 1930s with modern (2001-2010) conditions across the northern two-thirds of the Sierra Nevada (~45,000 km²), along an elevational gradient from near sea level to ~4000 m. We examined changes in density of all tree species combined in 500-m elevation bands, as well as species-level changes in density and frequency for all of the most common tree species. We compared density changes in unburned areas with areas that had burned between 1940 and 2000.

Results/Conclusions

We observed a general trend of increased density of small trees (10.2 - 30.4 cm) and decreased density of large trees (> 61.0 cm) for most elevations and individual species, resulting in net increased tree densities at all elevations. The increase in small trees was greatest at middle elevations (500 - 2000 m), where modern densities were 103 to 140% greater than historical levels, and at the highest elevations (> 3000 m), where densities were 124% greater. Decreases in large tree densities were significant for nearly all elevations, ranging from 12 to 82% reductions from historical levels; decline in large trees was remarkably consistent across species. Changes in individual species frequencies and densities from low and middle elevations appear to favor shade-tolerant species over shade-intolerant pines. Modern tree densities in plots that had burned since 1940 resembled unburned densities in the historical dataset. Long-term fire suppression and logging have probably contributed to these changes in low to middle elevation conifer forests. Change in macroclimate may be driving changes at high elevations as well as the decline of large trees at all elevations. As climates continue to warm, it is likely to play a greater and greater role in forest change, and the magnitude of change is probably dependent on the future roles of stressors like wildfire, fire suppression, air pollution, and insect and disease outbreaks.