A mechanistic model of tree competition and facilitation for Mediterranean forests: Scaling from leaf physiology to stand dynamics
Introduction
A central goal of plant ecology is to understand the mechanisms controlling the structure and dynamics of plant communities (e.g., Crawley, 1986, Tilman, 1988). It is generally accepted that community structure is an emergent property of species responses to resource gradients and interactions, such as competition. Gradient analyses have shown spatial and temporal regularities in community structure across sites with respect to resource variation, suggesting the existence of a few common underlying ecological mechanisms for all plant communities (Whittaker, 1975). Over the last decades, the development of mechanistic theories of plant communities have allowed us to identify critical ecological processes for explaining and predicting changes in community structure along environmental and disturbance gradients (Tilman, 1982, Smith and Huston, 1989). Chiefly, the mechanistic theory of plant competition developed by Tilman (1988) is based on an individual-based model (ALLOCATE) that describes community structure as a feedback between individual plant responses to resource availability and resource depletion by plants. In the model, species ability to gather above- and below-ground resources is governed by an inevitable tradeoff between shoot-stem and root allocations. According to ALLOCATE predictions, this tradeoff explains species relative positions along environmental gradients and a large number of successional and segregation patterns along productivity gradients. The mechanisms controlling successional dynamics in plant communities have been explored only in a few systems, mainly in temperate regions, where light and nutrients are the main limiting resource. The constrains and tradeoffs associated with water availability, however, have not been explicitly incorporated in plant mechanistic theory despite the fact that water is along light and nutrients a critical resource influencing the productivity and composition of plant communities worldwide (Woodward, 1986, Stephenson, 1990).
The development of a mechanistic model for any plant system requires understanding two separate processes (e.g., Goldberg, 1990). First, we need to describe how plants respond to resource heterogeneity and the interspecific tradeoffs implied (plant performance sub-model). Secondly, we need to understand how biotic and abiotic processes interact dynamically to control resource supply rates (resource sub-model). Both individual responses and resource variation interact dynamically to determine the nature of the interspecific mechanisms driving successional change in plant communities; e.g., competition, facilitation or neutral interactions. A large number of studies have focused on plant functional responses and tradeoffs in response to light and nutrient limitations (e.g., Shugart, 1984, Brokaw, 1985, Chazdon, 1988, Tilman, 1988, Canham, 1989, Grubb et al., 1996, Kobe et al., 1995). In contrast, whole-plant responses to combined gradients in light and water availability are poorly understood and studies on plant–water relations have typically focused on leaf-level ecophysiological processes (e.g., Tenhunen et al., 1990). Shade and drought effects do not act independently on plant performance (Holmgren et al., 1997, Sack, 2004) but rather show interactive effects, which are qualitatively different from those described for light and nutrient limitations. Similarly, as a result of the interaction between irradiance and soil moisture, spatial and temporal distribution of soil moisture can be qualitatively from temperate forests (e.g., Joffre and Rambal, 1993, Kitzberger et al., 2000). All these evidences suggest that water shortage results in qualitatively different mechanisms of community assembly than nutrient limitations. Finally, another difficulty in developing mechanistic models of forest dynamics is the existing compromise between model complexity and mathematical tractability. Mechanistic theories developed so far rely either on simple analytical or heuristic models that omit too many critical biological details (e.g., Tilman, 1982, Smith and Huston, 1989) or on size-structured numerical models (e.g., Shugart, 1984, Tilman, 1988, Mouillot et al., 2001) that are difficult to analyze and interpret from a biological point of view. Asymmetric resource competition in forest size-structured populations, however, can be described through partial differential equation systems (Kohyama, 1991), which have been shown to emerge from approximate aggregations of individual-based models (e.g., Pacala and Deutschman, 1995, Lischke et al., 1998). Thus, the development of analytical models of stand dynamics seems a feasible goal.
In this study, we develop a simulator of stand dynamics that incorporates the main constrains and tradeoffs experienced by plants in water-limited forests. The model is specifically suited to describe stand structure and dynamics of Mediterranean evergreen forests, which represent a transition stage between northern more humid temperate forests and semi-arid vegetation (Archibold, 1995). The model is a size-structured individual-based simulator of forest stand dynamics coupled to a stochastic soil water balance. Individual tree performance is described by a physiologically-based big-leaf model which describes carbon gain and transpiration as a function of climatic and edaphic conditions. Differences among species are based on morphological (size and allocation patterns) rather than on physiological differences. All plants sharing the same morphology and germinating at the same time define a cohort of individuals. Trees compete for light and water, with shorter trees being shaded by taller trees and soil water is depleted through cumulative transpiration. Competition for water is asymmetrical with trees with a higher root density experiencing a higher water supply. We use the simulator to explore general patterns that may emerge at different levels of biological organization. First we explore how the leaf-level tradeoff between carbon gain and water loss expresses at the whole-plant-level and the influence of allocation strategy on plant performance along a water availability gradient. Secondly, we ask whether plant competitive mechanisms may change along annual rainfall and rainfall frequency gradients and, in particular whether shifts from competitive to positive interactions are observed. Third, we investigate how stand structure and composition are modified by changes in the rainfall regime and disturbances. Specifically, we investigate how annual rainfall and rainfall stochasticity influence standing crop and coexistence of species differing in their allocation strategy. Finally, we propose a general analytical framework to describe stand dynamics in light and water-limited size-structured populations.
Section snippets
The forest stand model
The simulator is a size-structured individual-based model that describes the temporal dynamics of overlapping tree cohorts structured by light and water competition (Fig. 1). It results from coupling of two different models: a model of tree growth and mortality, and a resource dynamics model (light and soil water availability). Both models are interconnected, as tree performance is a function of resource availability, which depends in turn on tree transpiration and light interception by canopy
Resource allocation and whole-plant performance
Leaf area index (LAI) had a strong effect on both mortality and annual carbon gain. A larger LAI implies a larger photosynthetic apparatus and larger light interception but also higher water loss and reductions in stomatal conductance (see reduction in gF in Fig. 3). Diminishing payoffs in terms of carbon gain result in hyperbolic increments of annual net carbon assimilation per unit of ground with respect to LAI and in a decrease of net assimilation per unit of leaf (Fig. 3). This effect was
Resources and biomass invested by plants
According to the classical tradeoff between above- and below-ground resource uptake (Tilman, 1988) a larger allocation to photosynthetic apparatus results in higher rates of light interception while a larger investment in roots increases below-ground resources uptake rates. Our results imply however that tradeoffs associated with light and water competition express across functional levels in a qualitatively different manner than tradeoffs associated with nutrient competition. Water use in
Acknowledgements
MAZ was supported by grant REN2002-04041-C02-02/ GLO and REN2000-745, CICYT, Ministerio de Ciencia y Tecnología (M.C.y T.), Spain. We also acknowledge research networks GLOBIMED (CICYT, M.C. y T.) and REDBOME (Junta de Andalucía) for promoting stimulating discussions on Mediterranean forest ecology. Gianluca Biondi and Noemi Gizzi from Politecnico di Milano assisted with program implementation and figure elaboration at several stages of this work. Comments from Jose M. Rey Benayas, Pedro Villar
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