Climate Change

In California, the forests of the Sierra Nevada, Southern Cascades, Klamath-Siskiyou ecoregions and most western coniferous forests have undergone a number of dramatic changes in both composition and structure, and hence function, during the 20th Century. Although there are a number of reasons for these observations, changing climate is likely a strong force driving forests toward more drought stress in recent decades. Over the past century, the Sierra Nevada has warmed about 1°C in nighttime low temperatures and 0.5°C in mean temperature, and has experienced little systematic change in precipitation.

Scientists project that temperatures will increase between 3-6°C in the Sierra Nevada over the next 50 to 80 years. Som computer models predict an increase in net precipitation while others predict a decrease. However, models consistently suggest that by the late 21st Century, the Sierra Nevada will experience:

1. Decreased annual precipitation in the form of snow, and loss of snowpack.
2. Increasing temperatures that drive increasing dry season soil moisture stress.
3. A higher fraction of annual precipitation in fewer storm events (i.e. more intense storms and flooding).
4. An increased frequency of drought.

The consequences of increasing temperatures and moisture stress are projected to lead to changes in forest composition. Areas that appear most vulnerable to changing climate are likely to be along the margins of current forest distributions, e.g. the southern extent of subalpine conifer forests may retreat northwards over time. While these predictions are the best available science, climate projections and the implications for ecological change must be considered with caution because of uncertainty in these models.

In addition to drought stress in forests, these climate trends also have a profound influence on fire and other interacting disturbances, such as insect infestations and disease. Currently, most changes in forest structure are principally a result of larger, more intense fires in more fuel laden environments, leading to large patches of high tree mortality. Forest managers should anticipate that climate change and subsequent drought will exacerbate conditions for severe fires. Managing forests to create more resilient conditions will improve chances of maintaining forests in the future.

In 2012, the Forest Service sought to develop a largescale vulnerability assessment and associated climate adaptation strategies for focal resources of the Sierra Nevada. This project was a collaborative, multi-stakeholder effort led by the Forest Service to provide information and tools for Forest Planning and management (e.g., NEPA analyses, forest plan revisions, climate scorecard) and other natural resource management and conservation efforts to prepare for climate change impacts in the Sierra Nevada.