Competition for light and water increases tree carbon allocation to fine roots and leaves in a next-generation dynamic vegetation model

Abstract:

The response of the terrestrial carbon (C) cycle to climate change is a key uncertainty in Earth system models (ESMs). An important component of this uncertainty concerns plant functional diversity, which is typically represented in ESMs by ~10 plant functional types (PFTs) with fixed traits. The PFT framework is widely viewed to be inadequate, and several alternatives have been proposed. However, few global vegetation models, particularly those designed as ESM components, include the individual-level competitive mechanisms that largely determine how plant functional traits are distributed in time and space in real ecosystems. We have developed a new land model, designed as an ESM component, that represents forest dynamics and height-structured competition for light according to a simple canopy space-filling algorithm, the perfect plasticity approximation (PPA). The new land model, LM3-PPA, allows for an arbitrary number of PFTs (or species) whose spatial-temporal distributions are determined by the outcome of individual-level competition for light and water (and, in future model versions, belowground nutrients). We performed experiments with LM3-PPA to determine how competition for light and water affects tree C allocation to leaves, fine roots, and wood across climate gradients and in response to episodic drought. Under chronically moist conditions, competitively-optimal (fitness-maximizing), NPP-maximizing, and biomass-maximizing trees all had similar C allocation. However, under chronically dry conditions, competitively-optimal trees allocated more C to both fine roots and leaves, and less C to wood, compared to NPP- or biomass-maximizing strategies. When subject to episodic drought, the most drought-tolerant allocational strategies had relatively low allocation to leaves (and thus low leaf area and low water demand). Thus, the “over-investment” in leaves that results from resource competition increases the vulnerability of forests to drought in the current version of LM3-PPA, which allows for individual-level plasticity in tree crown shapes but not in plant C allocation. Our results imply that ESM projections may depend strongly not only on trait variation across species or PFTs, but also on individual plasticity in traits.