Alternate Trait‐Based Leaf Respiration Schemes Evaluated at Ecosystem‐Scale Through Carbon Optimization Modeling and Canopy Property Data.

Leaf maintenance respiration (Rleaf,m) is a major but poorly understood component of the terrestrial carbon cycle (C). Earth systems models (ESMs) use simple sub‐models relating Rleaf,m to leaf traits, applied at canopy scale. Rleaf,m models vary depending on which leaf N traits they incorporate (e.g., mass or area based) and the form of relationship (linear or nonlinear). To simulate vegetation responses to global change, some ESMs include ecological optimization to identify canopy structures that maximize net C accumulation. However, the implications for optimization of using alternate leaf‐scale empirical Rleaf,m models are undetermined. Here we combine alternate well‐known empirical models of Rleaf,m with a process model of canopy photosynthesis. We quantify how net canopy exports of C vary with leaf area index (LAI) and total canopy N (TCN). Using data from tropical and arctic canopies, we show that estimates of canopy Rleaf,m vary widely among the three models. Using an optimization framework, we show that the LAI and TCN values maximizing C export depends strongly on the Rleaf,m model used. No single model could match observed arctic and tropical LAI‐TCN patterns with predictions of optimal LAI‐TCN. We recommend caution in using leaf‐scale empirical models for components of ESMs at canopy‐scale. Rleaf,m models may produce reasonable results for a specified LAI, but, due to their varied representations of Rleaf,mfoliar N sensitivity, are associated with different and potentially unrealistic optimization dynamics at canopy scale. We recommend ESMs to be evaluated using response surfaces of canopy C export in LAI‐TCN space to understand and mitigate these risks. Plain Language Summary: While we have good understanding of plant photosynthesis, its links to climate and leaf nitrogen and process models to estimate photosynthesis; the same is not true for respiration. Measurements of leaf respiration are used to calibrate simple respiration models, which are applied at canopy‐scale. Here we investigate the risks associated with using various alternate simple respiration models in the context of viewing plant canopies as economic structures that must produce more carbon through photosynthesis than they use in respiration and tissue construction. We model the carbon economy of canopies with measured properties (leaf coverage and nitrogen) in arctic and in the tropical ecosystems, comparing the results using three different respiration models. First, we note the respiration estimates vary greatly among the models, so the models are not consistent. Second, we show that the optimal canopy properties (those most economically successful) also depend strongly on the respiration model used. This means that the choice of respiration model will have significant effects on the predictions of canopy response, and therefore C cycling, under global change. Our research highlights the need for more robust, process modeling of respiration. Key Points: Estimates of annual canopy maintenance respiration, a globally important C flux, vary widely among models calibrated from leaf trait dataUsing an optimization framework, we show that the canopy properties maximizing C export depend strongly on the respiration model usedLeaf‐scale empirical models should be applied cautiously at the canopy‐scale, particularly in Earth system models with canopy optimization. [ABSTRACT FROM AUTHOR]