Abstract
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.
Highlights
TROPICAL ROOT ACQUISITION OF NUTRIENTS AND WATERTropical forests play a dominant role in regulating global water, nutrient, and carbon (C) cycles (Field et al, 1998; Jobbagy and Jackson, 2000; Cleveland et al, 2011), and fine root biomass, production, and uptake activity mediate all of these biogeochemical cycles
Tropical forests play a dominant role in regulating global water, nutrient, and carbon (C) cycles (Field et al, 1998; Jobbagy and Jackson, 2000; Cleveland et al, 2011), and fine root biomass, production, and uptake activity mediate all of these biogeochemical cycles
Within this section we review commonly measured fine root traits and dynamics that are linked to water and nutrient acquisition (Figure 1), including: (1) fine root biomass stocks and depth distributions; (2) fine root dynamics, including patterns in production, mortality, turnover, and phenology; (3) fine root morphological traits; (4) fine root physiological traits; (5) fine root symbiotic associations (i.e., “symbiotic traits”), including mycorrhizas and N-fixing microbes
Summary
Tropical forests play a dominant role in regulating global water, nutrient, and carbon (C) cycles (Field et al, 1998; Jobbagy and Jackson, 2000; Cleveland et al, 2011), and fine root biomass, production, and uptake activity mediate all of these biogeochemical cycles. A better understanding of how commonly measured tropical root traits relate to nutrient and water uptake and plant function in tropical forests could help us improve predictive models of forest-climate feedbacks for tropical biomes. When water or nutrients are relatively more limiting to growth than aboveground resources (e.g., light), plants can increase the relative allocation of C to root biomass, increasing root:shoot ratios (Brouwer, 1963; Shipley and Meziane, 2002; Roa-Fuentes et al, 2021) Under these conditions, plants tend to express fine root morphological, physiological, and phenological traits that maximize resource acquisition (Violle et al, 2007). For fine root traits that are associated with the acquisition of multiple resources, the responses to water versus nutrient scarcity can differ, which might lead to tradeoffs among traits expressed, and spatial and temporal offsets in resource availability versus plant growth. We suggest section “Frontiers in Tropical Root Acquisition Research” for research and data-model integration to improve our understanding of how tropical forest fine roots mediate and respond to changes in resource availability
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