Competition for Water and Light in Closed-Canopy Forests: A Tractable Model of Carbon Allocation with Implications for Carbon Sinks

Abstract

The dependence of forest productivity and community composition on rainfall is the result of complex interactions at multiple scales, from the physiology of carbon gain and water loss to competition among individuals and species. In an effort to understand the role of these multiscale interactions in the dependence of forest structure on rainfall, we build a tractable model of individual plant competition for water and light. With game-theoretic analyses, we predict the dominant plant allocation strategy, forest productivity, and carbon storage. We find that the amount and timing of rainfall are critical to forest structure. Comparing two forests that differ only in the total time plants spend in water saturation, the model predicts that the wetter forest has fewer fine roots, more leaves, and more woody biomass than the drier forest. In contrast, if two forests differ only in the amount of water available during water limitation, the model predicts that the wetter forest has more fine roots than the drier forest and equivalent leaves and woody biomass. The difference in these responses to increases in water availability has significant implications for potential carbon sinks with rising atmospheric CO2. We predict that enhanced productivity from increased leaf-level water-use efficiency during water limitation will be allocated to fine roots if plants respond competitively, producing only a small and short-lived carbon sink.

Decreased water limitation under elevated CO 2 amplifies potential for forest carbon sinks

“…6). We know from previous work (22) and confirm in SI Appendix 4, Fig. S4.2 that competitive overinvestment in fine roots consumes a large fraction of productivity in low-q (often water limited) environments, where large increases in q are possible.…”

“…In a forest at equilibrium size structure, it follows that there are only two distinct levels of resource availability that trees experience throughout their lives: an understory level and a canopy level [see ref. 22 for a demonstration that competitive trees have only two levels of allocation: one for canopy trees (described by l c , r c ) and one for understory trees (l u and r u )].…”

“…Recent theoretical work also highlights the complexity of plant dependence on water. Incentives to individuals in competition for water as a shared resource can have significant and sometimes counter intuitive influences on plant allocation strategies (19)(20)(21)(22). If water is limiting, competition belowground drives each plant to invest in fine roots at a level that maximizes its own competitive ability, but that can decrease the growth rates of all individuals when every plant adopts the same strategy: a "competitive overinvestment."…”

“…If water is limiting, competition belowground drives each plant to invest in fine roots at a level that maximizes its own competitive ability, but that can decrease the growth rates of all individuals when every plant adopts the same strategy: a "competitive overinvestment." Farrior et al (22) found that competitive overinvestment in fine roots trades off with competitive overinvestment in structural biomass (wood) used by the plants in height-structured competition for light. A tradeoff between short-lived fine roots and long-lived woody biomass has a large effect on carbon storage (23).…”

“…Productivity during water limitation is met by a tragedy of the commons: shared access to water makes the competitive dominant strategy one where all of the productivity during water limitation is spent on fine roots. This competitive overinvestment in fine roots increases with the productivity of plants during the period of water limitation (22). In contrast, the most competitive strategy invests productivity from periods without water limitation in structures that enhance competitive ability for light: leaf layers that can capture enough sunlight to pay for their own costs (or more) and woody biomass.…”