Microclimates mediate water fluxes from vegetation

Earth is the blue planet, unique in our solar system for its ability to sustain a water cycle that spans three phases: solid (ice), liquid (water), and gas (water vapor). The global cycling of water between the earth’s land surface, subsurface, cryosphere, oceans, and atmosphere is fundamental to earth’s radiative balance and energy transport, and sustaining life in every ecosystem. The global water cycle is also a driver of feedbacks between the land surface and the atmosphere over a range of scales, from minute exchanges through stomata on leaves to eco-climate teleconnections wherein continental-scale changes in vegetation cover could alter climate and ecosystem dynamics in a different hemisphere.At fine spatial scales, microclimatic variation influences the strength and type of these feedbacks, and plays a role in determining patterns of vegetation vulnerability. Microclimates arise in hilly terrains from the midlatitudes to the poles due to differences in solar gain on opposing slopes. This leads to differences in the daily timing, duration, and intensity of sunlight exposure, and variable associations between sunlight and other climatic variables such as air temperature and humidity. These slope-aspect-induced climate differences are ecologically important, and impact vegetation-mediated water balance between the earth surface and the atmosphere. This thesis investigates climate–vegetation feedbacks arising from the impact of microclimate variation on water fluxes from forest vegetation. The primary investigation approach was a field study based at the University of California’s Angelo Coast Range Reserve in Northern California. There, I installed sensors to collect continuous high-resolution (∼5 minutes) data in a study of water flux differences in Pacific madrone (Arbutus menziesii) and Douglas fir (Pseudotsuga menziesii) across a slope-induced microclimatic gradient from spring 2018 to fall 2020. The field site has a Mediterranean climate with wet winters and dry summers. Chapters 2, 3, 4, and 5 present our instrumentation and field installation, our data on sap velocity from 14 Pacific madrone and 6 Douglas fir trees spanning adjacent north and south slopes at the Reserve, as well as our analysis of sap velocity variations in the context of high-resolution in situ observations of air temperature, relative humidity, soil moisture, and insolation. The environmental observations highlight climatic gradients on small scales, showing neighboring wet and dry zones in the soil, and seasonally evolving sub-canopy gradients in air temperature and humidity (chapter 2). A cross-species comparison of transpiration between Pacific madrone and Douglas fir trees demonstrates the impact of being tall: despite operating with slower peak sap velocities over the dry season, Douglas firs consistently transpired as much or more than their Pacific madrone neighbors, thanks to a longer exposure to sunlight granted by their height (chapter 3). A cross-slope comparison of transpiration in a single species (Pacific madrone) yielded a surprise: integrated summer transpiration is higher on the drier south slope than the north slope, which has abundant rock moisture but less sunlight (chapter 4). Analysis of the Pacific madrone sap flow data from both slopes in an environmental response model emphasizes the difference between tree populations on each slope in their aggregate physiological responses to specific aspects of their environments (chapter 5). Analysis of my field data suggests that the tree populations on each slope acclimate to their respective microclimates in functionally relevant ways. In particular, we hypothesize that south slope trees use water more sparingly under water-limited conditions, and yet still transpire more water over their longer and sunnier days. We speculate that differing proportions of sun-adapted and shade-adapted leaves, differences in stomatal regulation, and cross-slope root-zone moisture differences could explain some of the observed and modeled differences. Yet, the analysis of chapter 5 does not identify mechanisms of acclimation with the observations available. To explore these and other hypotheses, in the final chapter we turn to the rich parameter space of a model with high process resolution, CLM-FATES, a component of an Earth System Model (6). Though this aspect of the work is ongoing, we demonstrate that, in a model forest of broadleaf evergreen trees, differences in light availability alone are not sufficient to explain the cross-slope transpiration differences observed in the field. We also show that, because different stand structures have different physical properties, substantive plasticity in light- and water-use efficiency can evolve even while holding plant functional traits constant. This thesis advances our understanding of how water cycling by trees varies with local environment and climate. It contributes to improved representation of the complex Earth system in climate models by adding to current understanding of the processes that affect the cycling of water. The results open new research avenues in representing vegetation function in land surface models, especially in rough, hilly or mountainous terrains which are characterized by a mosaic of highly variable microclimates. This representation will, in turn, be a useful tool to anticipate tree mortality, species shifts, or even extinctions, and to guide climate mitigation strategies involving ecosystems.

Advisor: Fung, Inez