Hydraulic controls on the productivity of vegetation
Creators: name: Potkay, Aaron J.
Publisher: name: No Publisher Supplied
Year: 2021

description: Water is a fundamental requirement for all life. Its distribution on the land surface controls the distribution and success of terrestrial plants. Consequently, the distribution of vegetation, their physiology, and function reflect their environment, particularly land hydrology. Our understanding of the function and role of vegetation depends on studying local water availability, the physiological efficiency at using that water, and the resulting feedbacks. In this dissertation, I focus on the links between water and vegetation across multiple spatial scales to study the interactions between the land surface, its water availability, and how vegetation accesses and uses that water. I derive theory and insights of plant water use and its controls from land surface hydrology in multiple studies focused on varying biological and spatial scales.First, I focus on lichen, which are algal‐fungal symbionts and traditionally considered some of the earliest forms of terrestrial vegetation. Because lichen activity is controlled by moisture dynamics, understanding water transport is prerequisite to understanding their fundamental biology. I propose a model of water distributions within foliose lichens governed by laws of fluid motion. My model differentiates between water stored in symbionts, on extracellular surfaces, and in distinct morphological layers. I parameterize the model with hydraulic properties inverted from laboratory measurements of Flavoparmelia caperata and validate for wetting and drying. I ask: (1) Where is the bottleneck to water transport? (2) How do hydration and dehydration dynamics differ? and (3) What causes these differences? I found that resistance to vapor flow is concentrated at thallus surfaces and acts as the bottleneck for equilibrium, while internal resistances are small. The model captures hysteresis in hydration and desiccation, which are shown to be controlled by nonlinearities in hydraulic capacitance. Muting existing nonlinearities slowed drying and accelerated wetting, while exaggerating nonlinearities accelerated drying and slowed wetting. The hydraulic nonlinearity of F. caperata is considerable, which may reflect its preference for humid and stable environments. The model establishes the physical foundation for future investigations of transport of water, gas, and sugar between symbionts.Second, I switch focus from small lichen to comparatively larger trees. Lichen thalli are small enough that water can efficiently flow between symbionts through simple cell-to-cell pathways (Potkay et al., 2020). However, trees’ great size challenges this simple pathway, and they require a highly efficient vasculature as an additional means to transport water across their lengthy structures. This vasculature spans virtually the entirety of the tree from root, through stem, and to leaves. Each of these organs (e.g. root, stem, leaf) must be individually conductive enough to not limit total transport, since if any one organ underperforms, then the entire trees’ transport would suffer. Each organs’ conductance depends on the individual organs’ vascular anatomy and structure as well as the tree’s “choice” to invest biomass among different organs. If one organ were not performing well, then the tree could compensate by partitioning more of their total biomass into the limiting organ. Hence, my second chapter focus on how these vascular, internal hydraulics control trees’ biomass partitioning or allocation to different organs, hereon referred to as allometry and how allometry changes with environment, particularly soil water availability. I present the Tree Hydraulics and Optimal Resource Partitioning (THORP) model, which predicts the optimal tree allometries for different environments from their internal vascular transport. Here, optimal refers biologically to the evolutionary drive to develop strategies that improve efficiency, survival, and ability to compete and refers mathematically to the maximization of an objective function, often represented by canopy photosynthesis. Following this optimality principle, THORP predicts the allocation fractions to organs as proportional to their ratio of marginal gain to marginal cost, where gain is net canopy photosynthesis rate, and costs are senescence rates. Root total biomass and profile shape are predicted simultaneously by a unified optimization principle. Optimal partitioning is solved by a numerically efficient analytical solution. THORP’s predictions agree with reported tree biomass partitioning in response to size, water limitations, elevated CO2 and pruning, suggesting that tree allometry is optimal, resulting from natural selection. Roots were sensitive to soil moisture profiles and grew down to the groundwater table when present. Groundwater buffered against water stress regardless of meteorology, stabilizing allometry and root profiles as deep as c. 30 m. Much of plant allometry can be explained by hydraulic considerations. However, nutrient limitations cannot be fully ignored. Rooting mass and profiles were synchronized with hydrological conditions and groundwater even at considerable depths, illustrating that the below ground shapes whole‐tree allometry.Third, I maintain focus on tree hydraulics; however, I interpret their growth from the perspective of the underlying mechanisms rather than through the lens of optimality (i.e. how should a tree grow). The mechanisms underlying the environmental and internal limitations to tree growth are not fully understood and thus not represented in large-scale vegetation models. The prevailing theory has been that tree growth rate is limited by photosynthesis (source-limitation), while growing evidence suggests that growth (at least for mature trees) is more likely limited by cell turgor at the growth sites leaf, wood or root), hereon referred to as sink-limitations. I develop a simple, analytically-solved, mechanistic, turgor-driven growth model (TDGM) and a phloem transport model (PTM) to explore the mechanics of phloem transport, evaluate the hypothesis that turgor constrains growth, and explore the envelope of the exponents in metabolic scaling (the relationship between growth rate and tree size). My results show that mechanistic, sink-limited growth schemes based on plant turgor limitations are feasible for large-scale model implementations with minimal computational demands. The PTM predicts nearly uniform sugar concentrations regardless of phloem conductances, water potential gradients, and the strength of sink-demands, supporting the osmoregulatory flow and high pressure manifold hypotheses, and enabled TDGM implementation without explicit coupling to the PTM, further simplifying computation. I test the TDGM by comparing online and offline predictions (i.e. those coupled to and calculated independently from THORP, respectively) of whole-tree growth rate to well-established observations and allometric theory. The simple TDGM predicts realistic tree heights, growth rates, and metabolic scaling over decadal to centurial timescales, suggesting that tree growth is generally sink- and turgor-limited. Like observed trees, the TDGM captures tree-size- and resource- based deviations from the classical ¾ power-law metabolic scaling for which turgor is responsible.Fourth, I discuss a parsimonious method of capturing basic land surface hydrologic processes in existing Earth system models (ESMs). ESMs calculate the atmospheric and oceanic fluxes of matter and energy at the resolutions too course to simulate hydrologic process of the land surface. Land hydrology, in turn, regulates the same physical and biochemical processes that ESMs aim to simulate. While increasing the spatial resolution of ESMs could capture hydrology, such a solution would be computationally infeasible. Thus another, minimally computationally expensive solution is needed. Here, I explain one such solution. The land surface of the Community Land Model (CLM) is parameterized according to Fan et al.’s (2019) “stadium solution” based on a global terrain analysis so hilltop-to-valley lateral groundwater flow can be simulated within the CLM’s existing resolution. This new land surface hydrologic parameterization influences simulated solar insolation and surface temperatures, according to the topographic partitioning of aspect and solar exposure, as well as the transpiration and photosynthetic carbon assimilation of vegetation. Groundwater convergence steadily supplied water to valleys, allowing valley vegetation to transpire and assimilate carbon at greater rates and for longer periods than hilltop vegetation.Predictions of Earth’s future climate depend on vegetation models, which do not fully capture either vegetation’s sensitivity to water availability and stress (Fatichi et al., 2014) or the hydrology that governs the water locally available to vegetation (Clark et al., 2015; Fan et al., 2019). Thus, current predictions cannot adequately capture important feedbacks between climate and vegetation. The four studies presented here emphasize local soil hydrology and the water’s role on vegetation. Several models are presented in this dissertation, which are grounded in physiological mechanisms and the physics of water’s motion within the ground and plants, are computationally simple, and could be integrated into existing climate models to improve predictions of future climate, particularly the terrestrial C cycle under global change. These studies either advance our understanding of physiology (Chapter 1 &amp; 3), enabling simplifying assumptions to future model development and climate prediction, or directly propose means of capturing vegetation’s sensitivity to water for global models, including how water shapes their portioning of biomass between organs (Chapter 2) and growth rates (Chapter 3) or is locally available based on hydrologic processes (Chapter 4).