Coupled whole‐tree optimality and xylem hydraulics explain dynamic biomass partitioning

Abstract

Summary Trees partition biomass in response to resource limitation and physiological activity. It is presumed that these strategies evolved to optimize some measure of fitness. If the optimization criterion can be specified, then allometry can be modeled from first principles without prescribed parameterization. We present the Tree Hydraulics and Optimal Resource Partitioning (THORP) model, which optimizes allometry by estimating 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. 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. 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.

One Stomatal Model to Rule Them All? Toward Improved Representation of Carbon and Water Exchange in Global Models

“…Only one of the better performing models required calibration of a parameter explicating stomatal sensitivity to the soil moisture conditions (Medlyn), confirming the viability of hydraulics‐based stomatal models for predicting vegetation responses in novel climate spaces. Notably, the better constrained optimization approaches (e.g., ProfitMax, SOX opt ), both in terms of parameterization and functional representations, embed the potential for acclimation and plasticity in the plant hydraulic and photosynthetic responses (Sabot et al., 2020; Sperry et al., 2019), with a link to plant carbon allocation patterns (Potkay et al., 2021; Trugman et al., 2019) of major significance for the future of global coupled climate‐vegetation modeling.…”

Do stomata optimize turgor‐driven growth? A new framework for integrating stomata response with whole‐plant hydraulics and carbon balance

“…(2004) for Scots pine allometric relationships for Scots pine's carbon pools given by Potkay et al . (2021b)…”

“…We parameterized our GOSM with values reported or estimated from the existing literature to represent an evergreen conifer, many of which were synthesized by Potkay et al . (2021b) (Table 3). The majority of the parameters originate from studies of Scots pine ( Pinus sylvestris ); however, several parameters represent loblolly pine ( Pinus taeda ).…”

How woody plants adjust above‐ and below‐ground traits in response to sustained drought

“…Furthermore, during a drought, or drought recovery (Hikino et al ., 2022), the proportion of carbon allocated belowground also tends to increase (Zhang et al ., 2019; Brunn et al ., 2022) together with rooting depth in some cases; in a long‐term drought experiment in Queensland, rainforest trees increased average rooting depth (Pivovaroff et al ., 2021). Overall, greater root dry matter per unit leaf area (Potkay et al ., 2021) may help maintain water supply, but greater belowground allocation may also limit the potential for aboveground traits to respond to drought (Zhou et al ., 2020; Agee et al ., 2021; Pagay et al ., 2022), while increases in rooting depth could be maladaptive under nondrought conditions if nutrient uptake is reduced (Berkelhammer et al ., 2022).…”