Tradeoffs and Synergies in Tropical Forest Root Traits and Dynamics for Nutrient and Water Acquisition: Field and Modeling Advances
Authors: Cusack Daniela Francis , Addo-Danso Shalom D. , Agee Elizabeth A. , Andersen Kelly M. , Arnaud Marie , Batterman Sarah A. , Brearley Francis Q. , Ciochina Mark I. , Cordeiro Amanda L. , Dallstream Caroline , Diaz-Toribio Milton H. , Dietterich Lee H. , Fisher Joshua B. , Fleischer Katrin , Fortunel Claire , Fuchslueger Lucia , Guerrero-Ramírez Nathaly R. , Kotowska Martyna M. , Lugli Laynara Figueiredo , Marín César , McCulloch Lindsay A. , Maeght Jean-Luc , Metcalfe Dan , Norby Richard J. , Oliveira Rafael S. , Powers Jennifer S. , Reichert Tatiana , Smith Stuart W. , Smith-Martin Chris M. , Soper Fiona M. , Toro Laura , Umaña Maria N. , Valverde-Barrantes Oscar , Weemstra Monique , Werden Leland K. , Wong Michelle , Wright Cynthia L. , Wright Stuart Joseph , Yaffar Daniela 
Published: December 2021

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.