From tropics to treeline: assessing and extending metabolic theory for plant mortality using globally-distributed forest data

Abstract:

Understanding the drivers of global variation in forest structure and dynamics is a longstanding goal of ecology. Metabolic scaling theory (MST) predicts such patterns from first principles of how plants use resources, grow, fill space, and die. It makes several predictions for mortality rates in plant communities under demographic steady state. Specifically, MST predicts that background mortality rates will scale with the -2/3 power of stem diameter, and that the normalization constant of this relationship will be governed by plant growth rates. However, empirical evaluations of these predictions are needed. Further, to predict global variation in plant mortality rates, MST can be extended to include the effects of climate variables such as temperature. Here we test and extend MST predictions for mortality rates in plant communities using long-term data collected from nine Forest MacroSystems (FMS) network sites that span broad latitudinal climate gradients. We observed a negative correlation between stem diameter and mortality rates as predicted by MST, although scaling exponents and normalization constants varied across sites. We then show that variation among sites can be reconciled by unpacking the mortality normalization constant to reveal the influence of temperature on plant mortality rates. Importantly, we find that the temperature-dependence of plant mortality rates agrees with current globally-averaged estimates for the temperature-dependence of photosynthesis, as hypothesized by MST. These results shed light on the ‘zeroth-order’ mechanisms driving rates of mortality across the globe and can help inform predictions of climate effects in global change biology.