Tree functional traits organize hydrological and biogeochemical dynamics in tropical rainforest

Background/Question/Methods

Tropical forests harbor staggering biological diversity and are critical to global water and biogeochemical cycles.  Tree diversity can be exceptionally high in tropical forests, but while the functional traits of some tree species are known, it is unclear how these properties influence nutrient dynamics at the ecosystem scale and how trait-driven processes respond to climate variability.  Natural monodominant tropical forests (one species >60% of canopy) are often thought to result from positive plant-soil feedbacks, but evidence for a biogeochemical signature of the phenomena is mixed.  Here, we examine stable water isotopes and chemistry in old-growth watershed forests characterized by monodominance of the legume Mora excelsa or diverse tree communities on the island of Trinidad.  We compared soil availability and ecosystem losses of water and nutrients between forest types, examined how dominant tree functional traits influence ecosystem nutrient constraints and dynamics, and assessed the sensitivity of these patterns to inter-annual variation in rainfall.    

Results/Conclusions

We show that across 25 forests and three years of high rainfall variability, losses of inorganic nitrogen and phosphorus in streams and levels of nitrate in soils were consistently 2-3 times lower in Mora than in diverse forest.  These differences persisted during a historic drought and a prolonged wet season.  Nutrient and water isotope responses to rainfall variability were synchronous across forests and tracked landscape-level shifts in flowpaths, with strong positive coupling of discharge and nitrogen and phosphorus but negative discharge-concentration relationships for cations.  Our analysis indicates that the canopy and rooting architecture of Mora routes water through shallow nutrient-poor soils compared with deeper nutrient-rich soils in diverse forests.  However, flowpath variation could not explain the magnitude difference between forest types in nitrogen accumulation and loss.  Mora does not associate with nitrogen fixing bacteria or ectomycorrhizal fungi implying that low nutrient levels result from an alternate functional strategy to that of diverse forests. We suggest that Mora “engineers” ecosystem nutrient cycles through physical and chemical alteration of surface soils.  Our results provide evidence for ecosystem-scale expression of plant species traits and suggest potential plant-soil feedbacks that structure the biogeochemistry of tropical forests.