Theory and tests for coordination among hydraulic and photosynthetic traits in co-occurring woody species

Co-occurring plants show wide variation in their hydraulic and photosynthetic traits. Here, we extended ‘least-cost’ optimality theory to derive predictions for how variation in key hydraulic traits potentially affects the cost of acquiring and using water in photosynthesis and how this, in turn, should drive variation in photosynthetic traits. We tested these ideas across 18 woody species at a temperate woodland in eastern Australia, focusing on hydraulic traits representing different aspects of plant water balance, that is storage (sapwood capacitance, C_S), demand vs supply (branch leaf : sapwood area ratio, A_L : A_S and leaf : sapwood mass ratio and M_L : M_S), access to soil water (proxied by predawn leaf water potential, Ψ_PD) and physical strength (sapwood density, WD). Species with higher A_L : A_S had higher ratio of leaf-internal to ambient CO₂ concentration during photosynthesis (c_i : c_a), a trait central to the least-cost theory framework. C_S and the daily operating range of tissue water potential (∆Ψ) had an interactive effect on c_i : c_a. C_S, WD and Ψ_PD were significantly correlated with each other. These results, along with those from multivariate analyses, underscored the pivotal role leaf : sapwood allocation (A_L : A_S), and water storage (C_S) play in coordination between plant hydraulic and photosynthetic systems. This study uniquely explored the role of hydraulic traits in predicting species-specific photosynthetic variation based on optimality theory and highlights important mechanistic links within the plant carbon–water balance.