In recent decades, we have accumulated a considerable amount of data on the hydraulic traits of tropical forest tree species. These data are used to understand the physiology of water transport and its variation across species, ecosystems and precipitation gradients. It has been postulated that in areas with seasonal precipitation, episodic droughts, or precipitation gradients, fundamental trade-offs among hydraulic traits must exist in order for coexistence of a diversity of plant functional types (PFTs) to occur. However, strong evidence from direct observations for such a trade-offs has been elusive. Therefore, we use a process-based terrestrial ecosystem model with mechanistic representation of water transport, the Ecosystem Demography model (ED2-hydro), to perform a controlled set of simulations that explore how trade-offs emerge among xylem hydraulic conductivity (Kx), xylem vulnerability (xylem-P50), stem capacitance (Cs), stomatal control (stomatal-P50) and rooting depth, thus enabling coexistence of multiple PFTs. ED2-hydro was forced with local meteorological drivers and soil conditions from Barro Colorado Island, Panama. ED2-hydro was parameterized with various combinations of Kx, xylem-P50, Cs, stomatal-P50, and rooting depth between four PFTs and run in a large ensemble of simulations to identify robust trade-off relationships.
Results/Conclusions
When all trait values that result in coexistence across all four PFTs are considered, none of the ED2-hydro hydraulic traits appear to be strongly correlated, which is similar to observations of species-averaged traits compared within and across tropical ecosystems. However, ED2-hydro predicts that strong trade-offs between hydraulic traits do exist, but they only emerge when the traits of individuals that are in competition with each other are directly compared. For example, in ecosystems with variable precipitation, coexistence between hydraulically-safe and hydraulically-efficient PFTs occurs along an axis where the difference in xylem-P50 and the ratio of Kx for safe versus efficient PFTs remains inversely proportional. The model also predicts that coexistence occurs under variable precipitation when only the rooting depth differs, while holding the other hydraulic traits equivalent across PFTs. Based on our results from ED2-hydro, we hypothesize that hydraulic traits are strongly coordinated among individuals growing in direct competition at the forest-gap scale. We also recommend a series of field measurements to test if the relationships predicted by this modeling framework promote coexistence between PFTs with alternative hydraulic strategies.