Ecosystem Controls and the Impacts of Climate on Vegetation Production and Patterns in California's Mountains
Climate change is anticipated to have widespread impacts on the biosphere, including redistribution of vegetation and increases in tree mortality. In California, climate change is predicted to lead to warmer and possibly drier conditions. The response of vegetation to these changes remains uncertain due to our limited understanding of the sensitivity of vegetation to weather and the range of potential responses. This dissertation addresses these uncertainties by examining the effects of climate-mediated tree mortality and weather controls on vegetation in California's mountains.
Climate-mediated tree mortality occurred in 2002–04 in the semi-arid San Jacinto Mountains, CA. Conifer tree mortality was widespread, rapid, and focused at low elevations. This pattern of tree mortality was consistent with reduced precipitation associated with climate variability. Increased mortality at low elevation rapidly drove mid-montane vegetation distributions upslope.
Low elevation forests are thought to be vulnerable to climate change, but a limited understanding of their function constrains predictions of possible responses to changes in climate. We found that low elevation mixed conifer forests in Southern California maintain a year-round growing season by continuing carbon uptake in the cool winters, and extracting water stored from deep soils in the dry summers. Low elevation forests may be sensitive to certain changes in climate including increased atmospheric vapor pressure deficit and reductions in precipitation.
We hypothesized that reduced temperatures at high elevations and increased temperatures and reduced water availability at low elevations shape elevation patterns of canopy level photosynthesis in the San Jacinto Mountains. Short-term meteorological controls on canopy photosynthesis were insufficient to predict the elevational pattern of production. Additional controls may also be important, including controls on leaf-area, feedbacks and thresholds to growth, fire disturbances, and edaphic properties.
Ecosystem level processes may also be affected by fire suppression. Increased forest stem density due to fire suppression in Western US forests is thought to account for a portion of the North American carbon sink. Stem density increased in California's mountains from 1930s–1990s, but this did not appear to increase carbon stored in aboveground biomass due to a concomitant loss of large trees.