The carbon uptake of terrestrial ecosystems is tightly linked to water losses by transpiration, due to the exchange of both water and CO2 through leaf stomata. Water can thus be interpreted as a resource that is consumed to acquire carbon needed for plant growth. This inherent coupling requires plants to manage available water in such a way as to avoid the occurrence of water stress, while preserving carbon uptake. Accordingly, it has been hypothesized that plants optimally regulate transpiration and water transport through their tissues to maximize photosynthesis and growth. To test this hypothesis, we develop optimization models and explore emerging patterns in vegetation gas exchanges and growth.