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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2014)

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.

    Cover page: Climate controls on ecosystem production, biomass, and water cycling
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2016)
    The mid-elevation forest of California's Sierra Nevada poses a bioclimatic paradox. Mid-elevation trees experience a montane Mediterranean climate, with near-freezing winter days and rain-free summers. The asynchrony between warmth and water input suggests low primary production, limited by photosynthetic dormancy in winter cold, and again in summer and early autumn with drought, yet this forest is characterized by tall trees and high biomass. We used eddy covariance in a mid-elevation Sierra stand to understand how winter cold and summer drought limit canopy photosynthesis and production. The trees exhibited canopy photosynthesis year-round. Trees avoided winter dormancy, and daytime CO2uptake continued despite a deep snowpack and near-freezing temperatures. Photosynthesis on sunny days continued at half of maximum rates when air temperature was 0 °C. Likewise, the vegetation avoided summer drought dormancy, and high rates of daytime CO2uptake and transpiration continued despite a 5-month period with only negligible water input. We attribute this drought avoidance to deep rooting and availability of deep soil water. Year-round photosynthesis helps explain the large biomass observed in the Sierra Nevada, and implies adaptive strategies that may contribute to the resiliency or vulnerability of Sierran vegetation to climate change.
      Cover page: A montane Mediterranean climate supports year-round photosynthesis and high forest biomassCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Merced Previously Published Works (2012)
      Rising temperatures and declining water availability have influenced the ecological function of mountain forests over the past half-century. For instance, warming in spring and summer and shifts towards earlier snowmelt are associated with an increase in wildfire activity and tree mortality in mountain forests in the western United States. Temperature increases are expected to continue during the twenty-first century in mountain ecosystems across the globe, with uncertain consequences. Here, we examine the influence of interannual variations in snowpack accumulation on forest greenness in the Sierra Nevada Mountains, California, between 1982 and 2006. Using observational records of snow accumulation and satellite data on vegetation greenness we show that vegetation greenness increases with snow accumulation. Indeed, we show that variations in maximum snow accumulation explain over 50% of the interannual variability in peak forest greenness across the Sierra Nevada region. The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited mid-elevations, between 2,000 and 2,600a €‰m. Ina situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.
        Cover page: Elevation-dependent influence of snow accumulation on forest greeningCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Merced Previously Published Works (2022)
        Understanding potential response of forest carbon (C) and nutrient storage to warming is important for climate mitigation policies. Unfortunately, those responses are difficult to predict in seasonally dry forests, in part, because ecosystem processes are highly sensitive to both changes in temperature and precipitation. We investigated how warming might alter stocks of C, nitrogen (N), and phosphorus (P) in vegetation and the entire regolith (soil + weathered bedrock or “saprock”) using a space-for-time substitution along a bioclimatic gradient in the Sierra Nevada, California. The pine-oak and mixed-conifer forests between 1,160–2,015 m elevation have more optimal climates (not too dry or hot) for ecosystem productivity, soil weathering, and cycling of essential elements than the oak savannah (405 m) and subalpine forest (2,700 m). We found decreases in overstory vegetation nutrient stocks with decreasing elevation because of enhanced water limitation and greater occurrence of disturbances. Stocks of C, N, and P in the entire regolith peaked at the pine-oak and mixed-conifer forests across the bioclimatic gradient, driven by thicker regolith profiles and greater nutrient input rates. These observations suggest long-term warming will decrease ecosystem nutrient storage at the warmer, transitional pine-oak zone, but will increase nutrient storage at the colder, subalpine zone. Assuming steady-state conditions, we found the mean residence time of ecosystem C decreased with projected rising air temperatures and increased following a major drought event across the bioclimatic gradient. Our study emphasizes potentially elevation-dependent changes in nutrient storage and C persistence with warming in seasonally dry forests.
          Cover page: Climate Warming Alters Nutrient Storage in Seasonally Dry Forests: Insights From a 2,300 m Elevation GradientCreative Commons 'BY-NC' version 4.0 license
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