The hidden life of tropical roots: functional root traits and their response to climatic disturbances

Roots play a critical role in plant nutrition, and terrestrial carbon cycling. However, they are often understudied compared to their aboveground counterparts; especially in the tropics, where more carbon is cycled than in any other ecosystem. Some tropical forests, like in Puerto Rico, are more represented in scientific studies than others. However, this information is sparse, complicating the interpretation of root trait patterns. Trees in Puerto Rico have adapted mechanisms for withstanding hurricane disturbances, including in their roots. Additionally, as many tropical forests, some in Puerto Rico have low available phosphorus (P); thus, trees rely on root traits to enhance P acquisition. For example, higher root mycorrhizal colonization, branching, and exudation of phosphatase enzymes can each optimize P uptake. Yet, the high cost invested on these traits is a trade-off, which results in a diversity of traits among species. The aim of this dissertation was to synthesize root studies from Puerto Rico for the past 50 years, measure root response to warming and hurricanes disturbances, and test root trait strategies related to P acquisition from common species of the island. I found from previous studies that rooting depth in Puerto Rico is shallow ( \u3c 20cm), root nutrient concentrations do not vary much across forests, and there is under-representation of forests outside the Luquillo Experimental Forest. I measured a negative effect of warming on root production and biomass after 7 months of experimental warming. I captured an overall increase of root biomass after 10 months of the hurricanes (Irma and María) possibly due to the understory plant composition change. Yet, the increase in root biomass was less in previously warmed plots than in control plots, suggesting a legacy effect of the warming treatment on the recovery of roots. When testing root traits related to P acquisition, I found that species with high mycorrhizal colonization also had high root phosphatase activity, but low branching ratio. This suggests a combined contribution of phosphatase enzymes from the plant and their fungal partners to obtain soil P, and that plants will either invest in more branched roots, or more mycorrhizal colonization and more phosphatase exudation

Toward a Coordinated Understanding of Hydro-Biogeochemical Root Functions in Tropical Forests for Application in Vegetation Models

Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest–climate feedbacks for these carbon-rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine-root strategies for soil resource exploration, (2) coupling and trade-offs in fine-root water vs nutrient acquisition, and (3) aboveground–belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground–belowground hydro-nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate

Leaf-cutting ant (Atta cephalotes) nests may be hotspots of methane and carbon dioxide emissions in tropical forests

Leaf-cutting ants of the genus Atta are widely distributed throughout the American tropics and subtropics and rival other herbivores in the consumption of surrounding foliage. Although numerous studies have been conducted on the role these insects play in herbivory and organic matter dynamics, only a handful of studies have examined their impacts on soil greenhouse gas emissions. Our study investigated fluxes of carbon dioxide (CO2) and methane (CH4) from three nests of Atta cephalotes using a portable greenhouse gas analyzer, and measured CO2 and CH4 emissions from soils containing nest holes that ranged 5.2–152.1 g CO2-C and −1.1 to 15,264.7 mg CH4-C m-2 day-1, respectively. Fluxes of CO2 and CH4 were positively correlated above nest holes, but not in patches of soil away from leaf-cutting ant nests. Nearby non-nest soil emissions were significantly lower, ranging from 0.6 to 6.0 g CO2-C and −1.3 to 0.77 mg CH4-C m-2 day-1. Fluxes of both gases among nests and among holes within a single nest were highly variable. This preliminary dataset is small in scale both temporarily and geographically, but the discovery of substantial greenhouse gas fluxes from Atta cephalotes nests may have important implications for carbon budgets of tropical and subtropical American forests. Further work will be necessary to determine the mechanisms behind enhanced greenhouse gas emissions from leaf-cutting ant nests, and how this may alter ecosystem-scale CO2 emissions and CH4 sink strength in tropical forest soils

Experimental warming and its legacy effects on root dynamics following two hurricane disturbances in a wet tropical forest

Tropical forests are expected to experience unprecedented warming and increases in hurricane disturbances in the coming decades; yet, our understanding of how these productive systems, especially their belowground component, will respond to the combined effects of varied environmental changes remains empirically limited. Here we evaluated the responses of root dynamics (production, mortality, and biomass) to soil and understory warming (+4°C) and after two consecutive tropical hurricanes in our in situ warming experiment in a tropical forest of Puerto Rico: Tropical Responses to Altered Climate Experiment (TRACE). We collected minirhizotron images from three warmed plots and three control plots of 12 m . Following Hurricanes Irma and María in September 2017, the infrared heater warming treatment was suspended for repairs, which allowed us to explore potential legacy effects of prior warming on forest recovery. We found that warming significantly reduced root production and root biomass over time. Following hurricane disturbance, both root biomass and production increased substantially across all plots; the root biomass increased 2.8-fold in controls but only 1.6-fold in previously warmed plots. This pattern held true for both herbaceous and woody roots, suggesting that the consistent antecedent warming conditions reduced root capacity to recover following hurricane disturbance. Root production and mortality were both related to soil ammonium nitrogen and microbial biomass nitrogen before and after the hurricanes. This experiment has provided an unprecedented look at the complex interactive effects of disturbance and climate change on the root component of a tropical forested ecosystem. A decrease in root production in a warmer world and slower root recovery after a major hurricane disturbance, as observed here, are likely to have longer-term consequences for tropical forest responses to future global change