The Science

Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, scientists know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, researchers identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils.

The Impact

Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between emphasis in models characterizing nutrient and water uptake rates and carbon costs versus emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing better prediction of tropical forest-climate feedbacks.

Summary

In this review, researchers identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. The review also identifies interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions increased colonization rates under water scarcity scenarios in some forests rather than reducing mycorrhizal colonization of fine roots as might be expected.

Principal Investigator

Daniela Cusack
Colorado State University & STRI
daniela.cusack@colostate.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

A New Phytologist Workshop supported the collaboration of the TropiRoot group that produced this review. Support was received from a National Science Foundation Research Coordination Grant INCyTE: DEB-1754126 to investigate nutrient cycling in terrestrial ecosystems. The U.S. Department of Energy’s (DOE) Office of Science Biological and Environmental Research (BER) Program supported this research under Early Career Award Numbers DE-SC0015898, DE-SC0008317, and DE-SC0016188. Research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Support was also received from the Dorothea Schlözer Postdoctoral Programme of the Georg-August-Universität Göttingen. Data storage and some synthesis activities were supported as part of BER’s Next Generation Ecosystem Experiments–Tropics.

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References