UC Irvine

The association between climate and vegetation distribution has long been acknowledged, but quantifying the limits of climate on vegetation growth, biomass, and mortality remains an unsolved problem. Accurate prediction of the effects of climate change requires an understanding of the physiological limitations on vegetation due to climate. Recent increases in forest mortality and wildfire in Western North America has been attributed to warming and drought, but the causal mechanisms have not been identified. This dissertation uses observations of weather and vegetation growth, biomass, and water use to compare diverse ecosystems' responses to temperature and water availability and identify physiological thresholds that could promote ecosystem resilience or vulnerability to climate change.

The second chapter constructs a diagnostic framework of climatic control on biomass. The study system was the western slope of the Sierra Nevada Mountains of California. Climatic limitations on growth rates and growing seasons were compared across the gradient, along with ecosystem growth, death, and biomass. A broad "sweet spot" of climate conditions was found, in which winter cold and summer drought were minimal enough to allow a year-round growing season. Outside of this favorable zone, the combination of growing season lengths and mortality rates produced a low-biomass, fast-growing savannah at the lowest elevation and a high-biomass, slow-growing lodgepole forest at the highest elevation.

The third chapter examines the mixed conifer forest within the "sweet spot". Two adaptations were identified to allow this forest to maintain year-round growth. First, photosynthesis rates were near maximum even as air temperatures dropped to freezing; this was a lower optimal temperature range than almost any other known forest. Second, this forest largely avoided moisture stress by accessing soil water throughout the summer drought period.

The fourth chapter explores relationships between annual precipitation and water use efficiency across ten diverse California ecosystems. The driest ecosystems exhibited low water use efficiency that varied with annual precipitation. Ecosystem water use efficiency at the dry sites responded to variable annual precipitation through increased surface evaporation, high vapor pressure deficit, and high internal CO2 concentrations. The wetter montane conifer sites showed little to no response of water use efficiency to dry years.