UC Berkeley

Unprecedented anthropogenic global changes challenge the ability of societies to sustain desirable features of the environment. Some argue that we have entered a new global epoch where human activity is the major driver of environmental change. This is resoundingly true for American western forests, which have seen dramatic changes in disturbance regimes, species composition, and hydrologic and nutrient cycles due to fire suppression, air pollution, land use change, and climate change. These novel stressors have resulted in unprecedented conditions that may require new adaptive approaches to management focused on building resilience. The following research examines novel approaches to revitalizing a disturbance-dependent foundation tree species in the Sierra Nevada and reconstructs temporal and spatial components of historical fire regimes in the Sierra Nevada. These research threads help us understand how Sierran ecosystems functioned before Euro-American management, how these ecosystems are behaving today, and give insight into how we can manage for ecological resilience in the century to come.

Aspen (Populus tremuloides) comprises only a small fraction (1%) of the Sierra Nevada landscape, yet contributes significant biological diversity to this range. There is currently a high level of concern in the Western United States about declining vigor in mature aspen stands that often lack sufficient regeneration to ensure their long-term persistence. It is also highly uncertain if aspen will be able to accommodate the rapid climate changes predicted for the next century via migration through seedling establishment. I the first two studies following, I report on the efficacy of aspen revitalization management strategies, post-wildfire regeneration dynamics, experimental human assisted migration, and recent aspen seedling establishment in the Lake Tahoe Basin and eastern Sierra Nevada. I find substantial evidence that greater disturbance severity yields increased aspen sprout density and growth rates. I also find compelling evidence that post-fire aspen ramets are robust transplant material, having higher transplant survival rates than ramets from unburned stands as well as greenhouse-grown seedlings.

Fire is a key ecological process in dry mixed-conifer forests that historically burned frequently. Many of these forests on the western slope of the Sierra Nevada have been highly altered by a century of fire suppression, mining, logging, and land-use change, which have homogenized forest structure over large areas. Historical spatial and temporal patterns of fire can be used to inform current and future disturbance-based management seeking to restore ecosystem heterogeneity and resilience that had been supported by frequent low to moderate-severity fires prior to the twentieth century. Temporal patterns of historical fire are well known in these forests, but there is a high degree of uncertainty regarding the spatial dynamics of the pre-settlement fire regime. In the final study presented here, I reconstruct the spatial and temporal dynamics of wildfire from 1750-1900 in a 3000 ha mixed-conifer forest in the southern Sierra Nevada using data from 118 fire scared tree samples. Fire was once common in these forests that have not burned for 80-100 years, with mean fire return intervals from both spatially explicit and non-spatial temporal reconstructions ranging from 3-11 years. A vast majority of fires in this area (97%) occurred late in the growing season or during tree dormancy, and no significant controls on fire frequency were identified by slope aspect. Spatially explicit fire frequency reconstructions can aid in landscape-scale disturbance-based management aimed at increasing forest resilience and reducing fire risk.