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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, we 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, we 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. We also identify 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, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the 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 us to better predict tropical forest-climate feedbacks. Published version The U.S. Department of Energy Office of Biological and Environmental Research (DOE-BER), Terrestrial Ecosystem Science program supported this research under Early Career Award Number DESC0015898 to DFC, and DE-SC0008317 and DE-SC0016188 to JF and the NGEE-Tropics program support for EA, CW, and DY. JF contributed in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, California Institute of Technology. NG-R was supported by the Dorothea Schlözer Postdoctoral Programme of the Georg-August-Universität Goettingen. The Royal Society Leverhulme Africa Postdoctoral Fellowships. Grant No. LAF\R1\180025 provided funding to SA-D. Amazon FACE/CAPES grant 88887.154643/2017-00 provided support for LL. The US National Science Foundation (DEB-2016678) funded MU. European Union Horizon 2020 under the Mari Skłodovska-Curie grant agreement (No. 847693, REWIRE) provided funding to LF. Data storage and some synthesis activities were supported as part of the Next Generation Ecosystem Experiments – Tropics, funded by DOE-BER. National Science Foundation Research Coordination Grant INCyTE: DEB-1754126 to investigate nutrient cycling in terrestrial ecosystems. United Kingdom Natural Environment Research Council (NE/M019497/1, NE/S009663/1) and The Leverhulme Trust supported SAB.

Biological nitrogen fixation (BNF) isa fundamental part of nitrogen cycling in tropical forests, yet little is known about the contribution made by free-living nitrogen fixers inhabiting the often-extensive forest canopy. We used the acetylene reduction assay, calibrated with N-15(2), to measure free-living BNF on forest canopy leaves, vascular epiphytes, bryophytes and canopy soil, as well as on the forest floor in leaf litter and soil. We used a combination of calculated and published component densities to upscale free-living BNF rates to the forest level. We found that bryophytes and leaves situated in the canopy in particular displayed high mass-based rates of freeliving BNF. Additionally, we calculated that nearly 2 kg of nitrogen enters the forest ecosystem through free-living BNF every year, 40% of which was fixed by the various canopy components. Our results reveal that in the studied tropical lowland forest a large part of the nitrogen input through free-living BNF stems from the canopy, but also that the total nitrogen inputs by free-living BNF are lower than previously thought and comparable to the inputs of reactive nitrogen by atmospheric deposition. (C) 2020 Elsevier B.V. All rights reserved.