Increasing atmospheric CO 2 concentrations and changing rainfall regimes are creating novel environments for plant communities around the world. The resulting changes in plant productivity and allocation among tissues will have significant impacts on forest carbon storage and the global carbon cycle, yet these effects may depend on mechanisms not included in global models. Here we focus on the role of individual-level competition for water and light in forest carbon allocation and storage across rainfall regimes. We find that the complexity of plant responses to rainfall regimes in experiments can be explained by individual-based competition for water and light within a continuously varying soil moisture environment. Further, we find that elevated CO 2 leads to large amplifications of carbon storage when it alleviates competition for water by incentivizing competitive plants to divert carbon from short-lived fine roots to long-lived woody biomass. Overall, we find that plant dependence on rainfall regimes and plant responses to added CO 2 are complex, but understandable. The insights developed here will serve as an important foundation as we work to predict the responses of plants to the full, multidimensional reality of climate change, which involves not only changes in rainfall and CO 2 but also changes in temperature, nutrient availability, and disturbance rates, among others.rainfall | forest dynamics | plant allocation | carbon storage | evolutionarily stable strategy T he fate of the terrestrial carbon sink hinges on the role of limitation by other resources (1, 2). If additional atmospheric CO 2 causes forests to run up against limitation by other resources, it is possible that a forest carbon sink caused by CO 2 fertilization could diminish or reverse. The fate of this service by plants, currently estimated to mitigate 30% of anthropogenic emissions per year (3), is one of the most uncertain components of global climate predictions (4). Despite this importance, however, the role of resource limitation in carbon sinks is poorly understood and poorly incorporated into global models (1, 2, 5, 6).Here we investigate the effect of water limitation of photosynthesis on forest carbon storage and sinks. With additional CO 2 in the atmosphere, more CO 2 diffuses into leaves, whereas approximately the same amount of water escapes. This increase in water use efficiency at the leaf level has been well documented in experiments (7, 8) and observed in biomes around the world (9, 10). However, fossil CO 2 is not the only factor altering water relations in plant communities. Rising temperatures (11), changing rainfall regimes (12), and nitrogen deposition (13) can also have effects on plant water balance. A complete understanding of forest carbon storage and carbon sinks thus requires understanding a truly complex system.To build the mechanistic models we need to predict forest carbon storage in novel circumstances, we favor bringing together model components whose behavior we can understand and test with controlled exp...