Introduction. How tropical forests respond to increasing water stress over the coming decades remains a great unknown. Resolving this requires an understanding of the functional strategies that trees adopt with respect to water use and how these vary across biomes. Strategies are expressed by different combinations of functional traits, which relate to a particular aspect of plant function. Characterising strategies is, however, more challenging than characterising plants by individual functional traits. In principle plants with the same values of one trait may have completely different values of another trait, leading to a very different overall behaviour. As a result, we know relatively little about how plant strategies with respect to water stress vary across space.

Objectives. We combined trait data and process-based modelling to explore the viability of different tree functional strategies across tropical forests. We aimed to identify the most successful strategies to dealing with water deficit in each location as well as the diversity of strategies that can survive.

Methods. We compiled a global dataset of tree functional traits related to hydraulic strategy. We analysed these for trade-offs, noting consistency in patterns between tropical and temperate species. We sampled from this trade-off space to create individual tree functional strategy options, which we then applied in a dynamic vegetation model with a novel and detailed representation of plant hydraulics. We ran simulations pan-tropically to assess variation in functional diversity and in successful strategies with respect to productivity and biomass.

Results. We found a strong relationship between water deficit and functional diversity across the tropics, consistent with that previously reported for species diversity in the Amazon. The western Amazon and South-East Asia stand out as particular hotspots of hydraulic functional diversity. The most successful strategies in these regions in terms of biomass and net primary productivity both tended towards higher risk taking than in drier regions, but with differences in isohydricity.

Conclusions. Our results provide a set of hypotheses of the spatial variation in successful hydraulic functional strategy which requires testing against field observations.