Climate and plant trait strategies determine tree carbon allocation to leaves and mediate future forest productivity

Abstract

Forest leaf area has enormous leverage on the carbon cycle because it mediates both forest productivity and resilience to climate extremes. Despite widespread evidence that trees are capable of adjusting to changes in environment across both space and time through modifying carbon allocation to leaves, many vegetation models use fixed carbon allocation schemes independent of environment, which introduces large uncertainties into predictions of future forest responses to atmospheric CO 2 fertilization and anthropogenic climate change. Here, we develop an optimization-based model, whereby tree carbon allocation to leaves is an emergent property of environment and plant hydraulic traits. Using a combination of meta-analysis, observational datasets, and model predictions, we find strong evidence that optimal hydrauliccarbon coupling explains observed patterns in leaf allocation across large environmental and CO 2 concentration gradients. Furthermore, testing the sensitivity of leaf allocation strategy to a diversity in hydraulic and economic spectrum physiological traits, we show that plant hydraulic traits in particular have an enormous impact on the global change response of forest leaf area. Our results provide a rigorous theoretical underpinning for improving carbon cycle predictions through advancing model predictions of leaf area, and underscore that tree-level carbon allocation to leaves should be derived from first principles using mechanistic plant hydraulic processes in the next generation of vegetation models. K E Y W O R D Saridity gradient, carbon allocation, climate change, CO 2 fertilization, leaf area, plant hydraulic traits, sapwood area, vegetation model

Widespread drought‐induced tree mortality at dry range edges indicates that climate stress exceeds species' compensating mechanisms

“…The importance of climate variability in mediating mortality patterns, where more variable sites experienced higher dry edge mortality, further underscores this conclusion. High interannual variability might lead plants to shift structural allocation in order to remain competitive in wet years, such as increased allocation to leaf area, which would increase stress and mortality during extreme drought, an extension of the concept of “structural overshoot” (Jump et al, ; Trugman et al, ). These results emphasize that understanding the timescales over which trees “optimize” and respond to environmental variability, for example in adjusting allocation patterns or hydraulic/anatomical traits, may be quite important for modeling the vulnerability of forests to climate‐induced mortality events.…”

“…, and interannual coefficient of variation (CV) of annual temperature ("Temp," red), annual precipitation ("Pcp") from two different datasets (blue), potential evapotranspiration (PET; dark red), precipitation minus PET (P-PET; darkblue), mean soil moisture (SM; brown), and mean climatic water deficit (CWD; brown). Error bars show the 95% confidence intervals competitive in wet years, such as increased allocation to leaf area, which would increase stress and mortality during extreme drought, an extension of the concept of "structural overshoot" Trugman et al, 2019). These results emphasize that understanding the timescales over which trees "optimize" and respond to environmental variability, for example in adjusting allocation patterns or hydraulic/anatomical traits, may be quite important for modeling the vulnerability of forests to climate-induced mortality events.…”

“…While our results indicate that compensating mechanisms were not sufficient to buffer drier populations from higher mortality rates during drought, a number of potential compensating mechanisms could in theory operate at the tree or ecosystem level to reduce drought vulnerability of drier populations. Drier populations within a species may exhibit more drought-tolerant traits, such as more embolism-resistant xylem (Anderegg, 2015;López et al, 2013), rooting distribution differences (Williams & Ehleringer, 2000), and lower leaf area per sapwood or root area (Martínez-Vilalta et al, 2009;Rosas et al, 2019;Trugman et al, 2019). At a community/ecosystem level, lower tree densities and lower leaf area found in drier populations can reduce water loss and local water stress during drought (Martínez-Vilalta et al, 2009;Rosas et al, 2019;Trugman et al, 2019) and thus could buffer those populations from experiencing mortality.…”

Leveraging plant hydraulics to yield predictive and dynamic plant leaf allocation in vegetation models with climate change

“…Optimality approaches show significant promise for predicting the interaction between plant biophysics and environment and have been implemented in the context of plant hydraulics, as illustrated by the HOTTER model example (Trugman et al, ), and for multiple resource limitations (i.e., Dybzinski, Farrior, & Pacala, ; Farrior et al, ; Rastetter & Shaver, ; Thomas & Williams, ) in simple models. However, the calculation of the marginal costs and benefits associated with allocation trade‐offs can be extremely computationally expensive.…”

Functional traits and diversity of intertidal seaweeds: from individuals to communities

“…Yet under accelerating anthropogenic forcing, macroalgal beds are experiencing major changes in community composition as, for instance, kelp retreat towards the poles while warmer-water and non-native species expand . Such community restructuringand its ecosystem ramifications -is mediated by species' functional traits (Lavorel and Garnier, 2002;Suding et al 2008;Trugman et al 2019).…”