Background/Question/Methods Understanding vegetation change requires an understanding of the different allocation patterns and strategies of plants. There are conspicuous differences among the allocation patterns of the dominant vegetation in the world's biomes, ranging from relatively greater investment in height-generating stem to relatively greater investments in resource gathering leaves and roots. We present a tractable model of competition for water and light that allows us to understand and accurately predict plant allocation patterns.

Models predicting changes in vegetation distribution due to climate change often lack the rigorous individual-based resource competition that has been so successful in predicting smaller scale forest size and structure. Using such detail in global climate models is impractical with current computing standard. Such a model would be difficult to constrain and understand its results. We present a model of height-structured competition for water and light that predicts vegetation changes based on the long-term outcome of that competition. The tractability of the model rests on the advances made in the Perfect Plasticity Approximation model (Purves et al. 2007 PLoS One, Purves et al. 2008 PNAS, Strigul et al. 2008 Ecol. Monogr.).  

Results/Conclusions We focus predictions of carbon allocation to basic structures in trees: leaves, fine roots, and wood (branches, stem, and coarse roots). With this tractable model of competition we are able to make reasonable predictions in comparison with global data. We also find that competitive pressure through ecological and evolutionary time (not growth optimization) is crucial in driving patterns of root allocation and its relation to water availability. The model predicts that timing of rainfall is critically important for the performance and dominance of different root allocation strategies. This result emphasizes the need for future models that are mechanistic and tractable enough at the level of plant competition to accurately handle greater detail of environmental variation.

Variations in plant allocation patterns across the globe, and thus carbon storage, is a direct result of competitive interactions among individual plants over ecological and evolutionary time. The model we present is a first step towards implementing a more mechanistic representation of resource competition in global models.