Nutrient acquisition strategies of coexisting tree species in old-growth and secondary forests of Singapore

Background/Question/Methods

Tropical forests are globally important for biodiversity, carbon and water, yet are frequently found on highly weathered soils with low phosphorus and cation availability. Phosphorus limitation is hypothesized to affect plant and ecosystem processes in both natural and human-modified ecosystems. Old-growth tropical forests in Southeast (SE) Asian are dominated by Dipterocarpaceae tree species associated with ectomycorrhizal fungi (ECM), which have a greater capacity to use organic nutrients. In contrast, secondary forests in SE Asia and most Neotropical forests are dominated by tree species associated with arbuscular mycorrhiza (AM), which explore soil volumes for inorganic nutrients. We examined fine root traits of diverse tree species in a South East (SE) Asian lowland tropical forest remnant with patches of old-growth and secondary forest with low soil phosphorus availability. The old-growth forest has higher aboveground carbon stocks and the secondary forest has higher belowground carbon stocks (soil + roots) and flux rates (roots + microbes). We hypothesized that old-growth forest species would have root trait strategies with high phosphatase enzyme activity associated with root symbionts to exploit organic nutrients, whereas secondary forest species would have root trait strategies exploiting inorganic nutrients released by high microbial activity.

Results/Conclusions

Fine root trait strategies differed between old-growth and secondary forests and between root symbiont types among 25 tree species. However, different sets of traits distinguished between forest type and symbiont type. Old-growth forest species had higher root phosphomonoesterase activity (PME; F1,24 = 10.49**) and lower specific root surface area (SRA; F1,30 = 5.14*) than secondary forest species. In contrast, legumes had greater fine root diameter (F2,15 = 4.22*) and lower specific root length (SRL) (F2,21 = 7.34**) than species with AM or ECM associations. In a principal components analysis, these traits form three orthogonal axes of nutrient acquisition strategies, representing a trade-off between root diameter and SRL and SRA (41% variation), a shift in root tissue density and tips per unit length (32% variation) and lastly, investments in PME activity (17% variation). Together, our results support the shift from organic nutrient uptake strategies in ECM dominated old-growth forests to inorganic nutrient uptake strategies in AM dominated secondary forests, potentially impacting nutrient and carbon cycles. Future research will compare our results to the Tropical Root Trait Initiative (TropiRoot; https://tropiroottrait.github.io/TropiRootTrait/) database to gain further insights on trade-offs in nutrient acquisition strategies across tropical forests globally.