Quantifying environmental controls on forest structure

structure_gbm Ecologists have described the general contours of relationships between forest structure and underlying environmental variables like topography, soil, lithology, and climate. But there’s relatively little information about the relative importance of these “state” variables, their potential interactive effects, or the functional forms of the relationships, particularly in subalpine forests. My research in this area quantifies the covariability of key forest structure metrics and abiotic environmental factors. Recent work has included using full-waveform LiDAR data to develop wall-to-wall estimates of conifer forest structure and and applying generalized inferential modeling approaches to estimate the relationships between structural metrics and variables like site topography, geology, soil properties, and snow depth. One novel finding was that snow accumulation and snow disappearance rate exerted dominant nonlinear influences across a range of structural features. This work aims to clarify topoclimatic constraints on stand development, improve understanding of water- and energy-limitation thresholds, and strengthen the quantitative basis for predicting future forest change.

Disentangling topoclimatic and species effects on tree growth responses to climate extremes

dendro-wavelet Environmental and physiological factors interact to govern a forest stand’s susceptibility to drought, and they do so in ways that are both nonstationary and nonlinear. This complexity leaves gaps in the understanding of how and why drought’s effects on forest dynamics and geochemical cycling vary across a landscape. I am interested in disentangling some of these relationships, and I use dendroecology methods to do so. I’m especially interested in evaluating the structural overshoot hypothesis, which suggests that denser stands should experience lower soil moisture, lower individual transpiration rates, and lower growth during extended low water periods than similarly sited but sparser stands.

Hydraulic redistribution

soil I am investigating the extent to which hydraulic redistribution (HR) contributes to the water physiologically available to trees in subalpine forests. I’m particularly interested in how HR’s contribution to the water budget covaries with a site’s geophysical and compositional characteristics, an underdescribed area in ecohydrology. I measure soil moisture (θ) and soil water potential (ψ) to quantify physiologically available water storage, characterize hydraulic redistribution and its effects on θ and ψ, and describe the vertical variability in the soil water characteristic (SWC) curve. I use these observations to study relationships between these subsurface hydrologic dynamics, sap flux, and other measurements of stand structure and function.

Agent attribution of forest disturbance

agent Forest disturbance patterns in the Sierra Nevada of California are shifting as a result of changing climate and land use. The new dynamics have induced high rates of tree mortality and may also yield species replacement and biodiversity loss. To advance a monitoring system that can both detect disturbances and attribute them to specific agents, we proposed and evaluated a method for disturbance agent attribution. I developed a Random Forest–based model to identify the location, timing, and agent of disturbance occurring in Stanislaus National Forest, California, U.S.A., between 1999 and 2015. Quantifying and mapping agent-explicit disturbance patterns in this way offers actionable data for ecological and hydrological modelers, as well as forest managers, who can use the information to adapt resource management strategies to new forest dynamics.