Roger C. Bales

The mixed‐conifer forests of Sierra Nevada create a complex system of water, energy and nutrient cycling. Thus, this study aims to understand the spatial and temporal variability of soil moisture and stand‐scale transpiration across three central Sierra headwater catchments and their controlling factors. The mean daily average sap flow of Abies concolor varies from 0.69 ± 0.48 to 1.27 ± 0.72 L/h. Higher spatial variability in soil moisture and stand‐scale transpiration was found in the snowmelt period, followed by the soil moisture recession period when vegetation growth is not water limited. The daily average sap flow was higher on south aspect than on north aspect, but this pattern was not consistent across all seasons. The diel pattern of the sap flow revealed a larger contribution of nocturnal sap flow during the dry period (22.3%–37.8%) and a smaller contribution during the recession period (16.3%–23.9%) across all catchments. Multiple linear regression analysis suggests that sap flow during summer (recession period) was governed by the ecosystem's energy, indicated in part by vapour pressure deficit and temperature. Soil moisture regulated the subsequent dry‐period transpiration. The energy‐limited condition in winter restricted the transpiration even when sufficient soil moisture was available. Topography, explained by topographic positioning index ( tpi ), played a major role in explaining stand‐scale transpiration, especially during higher atmospheric demand periods, whereas tree size (i.e., DBH) explained more than 10% sap flow variance in all seasons. The findings of this study will provide the basis for a better understanding of the eco‐hydrological processes of the Sierra Nevada under natural and anthropogenic‐induced changes.

After 4.5 billion years as an evolving and dynamic planet, the Earth continues to evolve but with human‐altered dynamics. Earth scientists have special opportunities and responsibilities to accelerate our understanding of Earth's changes that are transforming our most remarkable home.

Extreme droughts are a major determinant of ecosystem disturbance that impacts plant communities and feeds back into climate change through changes in plant functioning. However, the complex relationships between aboveground and belowground plant hydraulic traits and their role in governing plant responses to drought are not fully understood. In this study, we use a model, the Functionally Assembled Terrestrial Ecosystem Simulator in a configuration that includes plant hydraulics (FATES-Hydro), to investigate ecosystem responses to the 2012–2015 California drought in comparison with observations at a site in the southern Sierra Nevada that experienced widespread tree mortality during this drought. We conduct a sensitivity analysis to explore how different plant water sourcing and hydraulic strategies lead to differential responses during normal and drought conditions. The analysis shows the following. Deep roots that sustain productivity through the dry season are needed for the model to capture observed seasonal cycles of evapotranspiration (ET) and gross primary productivity (GPP) in normal years, and deep-rooted strategies are nonetheless subject to large reductions in ET and GPP when the deep soil reservoir is depleted during extreme droughts, in agreement with observations. Risky stomatal strategies lead to greater productivity during normal years as compared to safer stomatal control, but they also lead to a high risk of xylem embolism during the 2012–2015 drought. For a given stand density, stomatal and xylem traits have a stronger impact on plant water status than on ecosystem-level fluxes. Our study highlights the significance of resolving plant water sourcing strategies to represent drought impacts on plants and consequent feedbacks in models.

Increasing drought frequency and severity in a warming climate threaten forest ecosystems with widespread tree deaths. Canopy structure is important in regulating tree mortality during drought, but how it functions remains controversial. Here, we show that the interplay between tree size and forest structure explains drought-induced tree mortality during the 2012-2016 California drought. Through an analysis of over one million trees, we find that tree mortality rate follows a “negative-positive-negative” piecewise relationship with tree height, and maintains a consistent negative relationship with neighborhood canopy structure (a measure of tree competition). Trees overshadowed by tall neighboring trees experienced lower mortality, likely due to reduced exposure to solar radiation load and lower water demand from evapotranspiration. Our findings demonstrate the significance of neighborhood canopy structure in influencing tree mortality and suggest that re-establishing heterogeneity in canopy structure could improve drought resiliency. Our study also indicates the potential of advances in remote-sensing technologies for silvicultural design, supporting the transition to multi-benefit forest management.

Extreme droughts are a major determinant of ecosystem disturbance, which impact plant communities and feed back to climate change through changes in plant functioning. However, the complex relationships between above- and belowground plant hydraulic traits, and their role in governing plant responses to drought, are not fully understood. In this study, we use a plant hydraulics model, FATES-Hydro, to investigate ecosystem responses to the 2012–2015 California drought, in comparison with observations, for a site in the southern Sierra Nevada that experienced widespread tree mortality during this drought. We conduct a sensitivity analysis to explore how different plant water sourcing and hydraulic strategies lead to differential responses during normal and drought conditions. The analysis shows that: 1) deep roots that sustain productivity through the dry season are needed for the model to capture observed seasonal cycles of ET and GPP in normal years, and that deep-rooted strategies are nonetheless subject to large reductions in ET and GPP when the deep soil reservoir is depleted during extreme droughts, in agreement with observations. 2) risky stomatal strategies lead to greater productivity during normal years as compared to safer stomatal control, but lead to high risk of xylem embolism during the 2012–2015 drought. 3) for a given stand density, the stomatal and xylem traits have a stronger impact on plant water status than on ecosystem level fluxes. Our study reveals the importance of resolving plant water sourcing strategies in order to represent drought impacts on plants, and consequent feedbacks, in models.