Understanding the drivers of plant community stability is crucial for predicting ecosystem responses to extreme drought events. In grasslands, drought resistance supports the maintenance of key functions such as above-ground primary productivity, making the identification of resistance drivers essential to guide management under climate change. Proposed factors contributing to grassland stability include multiple diversity facets, functional traits and long-term climate, but most assessments focus on temporal invariability under historical disturbance regimes, leaving mechanisms of extreme drought resistance and their variation across climatic contexts relatively underexplored. Here, we analysed data from 54 grassland sites of the International Drought Experiment to examine the resistance of above-ground net primary productivity to a short-term (i.e. 1 year) extreme drought. We investigated the relative importance and joint influence of functional composition (i.e. community-weighted means of leaf and root traits), plant diversity facets (taxonomic, functional and phylogenetic) and climate (aridity and rainfall variability) on drought resistance. We used structural equation models to disentangle direct, indirect, and moderating pathways linking these drivers to drought resistance. Long-term aridity appeared as one of the most important drivers of grassland resistance to drought, with more arid sites showing lower resistance. Moreover, aridity impacted resistance through indirect effects by shaping functional composition and plant diversity, and by moderating the influence of plant diversity and functional composition. Functional composition related to dehydration avoidance and dehydration tolerance was also positively associated with resistance, while diversity had a weaker relationship with resistance, mostly through functional and phylogenetic facets. Interannual rainfall variability also influenced resistance, with different effects in more arid versus humid and less arid sites. Synthesis. Widely studied stability drivers such as plant diversity and functional composition have only partial explanatory power for short-term drought resistance of above-ground productivity in grasslands at a global scale. The abiotic context, particularly long-term aridity, is crucial for understanding ecosystem responses to rainfall variation and can improve predictive models for advancing the study of ecosystem resistance to drought. Along with management practices that target high species diversity or specific traits, restoration and conservation practices should support vulnerable sites experiencing high aridity Compreender os fatores que determinam a estabilidade de comunidades vegetais & eacute; fundamental para prever as respostas dos ecossistemas a eventos de seca extrema. Em ecossistemas dominados por gram & iacute;neas, a resist & ecirc;ncia & agrave; seca & eacute; importante para a manuten & ccedil;& atilde;o de fun & ccedil;& otilde;es como a produtividade prim & aacute;ria a & eacute;rea. Assim, identificar os fatores que promovem essa resist & ecirc;ncia & eacute; crucial para orientar o manejo em um cen & aacute;rio de mudan & ccedil;as clim & aacute;ticas. Entre os principais determinantes da estabilidade em sistemas dominados por gram & iacute;neas est & atilde;o diferentes facetas da diversidade, atributos funcionais das esp & eacute;cies e o clima. No entanto, a maioria dos estudo foca na invariabilidade temporal sob regimes hist & oacute;ricos de dist & uacute;rbio, enquanto os mecanismos de resist & ecirc;ncia & agrave; seca extrema e a sua varia & ccedil;& atilde;o ao longo de gradientes clim & aacute;ticos permanecem relativamente pouco explorados. Neste estudo, analisamos dados de 54 s & iacute;tios do Experimento Internacional de Seca para avaliar a resist & ecirc;ncia da produtividade prim & aacute;ria l & iacute;quida a & eacute;rea a uma seca extrema de curta dura & ccedil;& atilde;o (um ano). Investigamos a import & acirc;ncia relativa e os efeitos conjuntos da composi & ccedil;& atilde;o funcional (isto & eacute;, m & eacute;dias ponderadas da comunidade de atributos funcionais de folhas e ra & iacute;zes), de diferentes facetas da diversidade de plantas (taxon & ocirc;mica, funcional e filogen & eacute;tica) e do clima (aridez e variabilidade interanual da precipita & ccedil;& atilde;o) sobre a resist & ecirc;ncia & agrave; seca. Usamos modelos de equa & ccedil;& otilde;es estruturais para discriminar os efeitos diretos, indiretos e de modera & ccedil;& atilde;o que conectam esses fatores & agrave; resist & ecirc;ncia & agrave; seca. A aridez destacou-se como um dos principais fatores geradores de resist & ecirc;ncia & agrave; seca, com locais mais & aacute;ridos apresentando menor resist & ecirc;ncia. Al & eacute;m disso, a aridez tamb & eacute;m influenciou a resist & ecirc;ncia de forma indireta, ao afetar a composi & ccedil;& atilde;o funcional e a diversidade de plantas, al & eacute;m de moderar a efeito da diversidade de plantas e da composi & ccedil;& atilde;o funcional na resist & ecirc;ncia. A composi & ccedil;& atilde;o funcional associada a estrat & eacute;gias de evita & ccedil;& atilde;o e toler & acirc;ncia & agrave; desidrata & ccedil;& atilde;o tamb & eacute;m apresentou rela & ccedil;& atilde;o positiva com a resist & ecirc;ncia, enquanto a diversidade de plantas mostrou associa & ccedil;& atilde;o mais fraca, contribuindo principalmente por meio das facetas funcionais e filogen & eacute;ticas. A variabilidade interanual da precipita & ccedil;& atilde;o tamb & eacute;m influenciou a resist & ecirc;ncia, com efeitos distintos entre locais mais & aacute;ridos e locais h & uacute;midos ou menos & aacute;ridos S & iacute;ntese. Fatores amplamente estudados como determinantes da estabilidade, como a diversidade das plantas e a composi & ccedil;& atilde;o funcional, explicam apenas parcialmente a resist & ecirc;ncia da produtividade a & eacute;rea a secas de curta dura & ccedil;& atilde;o em ecossistemas dominados por gram & iacute;n Em conjunto com pr & aacute;ticas de manejo voltados ao aumento da diversidade de esp & eacute;cies ou & agrave; promo & ccedil;& atilde;o de determinados atributos funcionais, a & ccedil;& otilde;es de restaura & ccedil;& atilde;o e conserva & ccedil;& atilde;o devem focar em & aacute;reas mais vulner & aacute;veis sob maior aridez.

Tropical ecosystems contain the world's largest biodiversity of vascular plants. Yet, our understanding of tropical functional diversity and its contribution to global diversity patterns is constrained by data availability. This discrepancy underscores an urgent need to bridge data gaps by incorporating comprehensive tropical root data into global datasets. Here, we provide a database of tropical root characteristics. This new database, TropiRoot 1.0, will be instrumental in evaluating an array of hypotheses pertaining to root functional ecology and plant biogeography, both within the tropics and relative to other global biomes. The data compilation was conducted by the TropiRoot Initiative, in partnership with the Fine-Root Ecology Database (FRED) and the Global Root Trait (GRooT) database, Colorado State University (CSU) and the Smithsonian Tropical Research Institute (STRI). Literature search and data extraction were conducted between 2020 and 2024. Literature was identified using Web of Science, Scopus, and complemented using the expert knowledge of members of TropiRoot. To provide broad environmental and geographical distributions, literature searches included root characteristics (traits) across global change drivers, natural gradients, and from different continents. We adopted FRED standardized data columns and streamlined the format to enhance accessibility for data extraction across various user groups. This optimized framework resulted in a smaller, yet comprehensive datasheet. To make the database compatible with other global root trait initiatives, column identification was standardized following the codes provided by FRED. These efforts culminated in data extracted from 104 new sources, resulting in more than 8000 rows of data (either species or community data). Most of the data in TropiRoot 1.0 include root characteristics such as root biomass, morphology, root dynamics, mass fraction, architecture, anatomy, physiology, and root chemistry. This initiative represents a 30% increase in the currently available data for tropical roots in FRED. TropiRoot 1.0 contains root characteristics from 25 different countries, where seven are located in Asia, six in South America, five in Central America and the Caribbean, four in Africa, two in North America, and 1 in Oceania. Due to the volume of data, when ancillary data were available, including soil data, these data were either extracted and included in the database or its availability was recorded in an additional column. Multiple contributors checked the entries for outliers during the collation process to ensure data quality. For text-based observations, we examined all cells to ensure that their content relates to their specific categories. For numerical observations, we ordered each numerical value from least to greatest and plotted the values, checking apparent outliers against the data in their respective sources and correcting or removing incorrect or impossible values. Some data (soil and aboveground) have different columns for the same variable presented in different units, including originally published units, but root characteristics data had units converted to match those reported in FRED. By filling a gap from global databases, TropiRoot 1.0 expands our knowledge of otherwise so far underrepresented regions and our ability to assess global trends. This advancement can be used to improve tropical forest representation in vegetation models. The data are freely available and should be cited when used.

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e., ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme.

Soil drying challenges microbial viability and survival, with bacteria employing various mechanisms to respond to shifts in osmolarity, including dormancy or metabolic upregulation of osmoprotectants. However, the extent to which these responses are shaped by an organism's phylogeny, or the climate history of a given environment is poorly understood. This study examines the responses of phylogenetically similar bacteria from semi-arid and humid tropical forest soils to osmotic and matric stress using synchrotron radiation-based Fourier Transform Infrared spectromicroscopy. This non-destructive approach depicts the biochemical phenotype for whole cells under control and stress conditions. We observed that, under osmotic stress, bacteria upregulated cell-signaling pathways, rapidly turned over lipid-storage compounds, and increased osmolyte production. In contrast, matric stress induced a more muted response, typically elevating the production of carbohydrate stress compounds, such as glycine betaine and trehalose. Whereas phylogenetically similar bacteria showed comparable biochemistry under control conditions, climate history played an important role in regulating responses to stress, whereby a stronger metabolic response was observed from semi-arid relative to tropical forest isolates. We conclude that bacterial stress response to drought can be more diverse than previously observed and regulated by both phylogeny and climate history.

Tropical peatlands play an important role in global carbon (C) cycling, but little is known about factors driving carbon dioxide (CO2) and methane (CH4) emissions from these ecosystems, especially production in deeper soils. This study aimed to identify source material and processes regulating C emissions originating deep in three sites in a peatland on the Caribbean coast of Panama. We hypothesized that (1) surface-derived organic matter transported down the soil profile is the primary C source for respiration products at depth and that (2) high lignin content results in hydrogenotrophic methanogenesis as the dominant CH4 production pathway throughout the profile. We used radiocarbon isotopic values to determine whether CO2 and CH4 at depth are produced from modern substrates or ancient deep peat, and we used stable C isotopes to identify the dominant CH4 production pathway. Peat organic chemistry was characterized using 13C solid-state nuclear magnetic resonance spectroscopy (13C-NMR). We found that deep peat respiration products had radiocarbon signatures that were more similar to surface dissolved organic C (DOC) than deep solid peat. These results indicate that surface-derived organic matter was the dominant source for gas production at depth in this peatland, likely because of vertical transport of DOC from the surface to depth. Lignin, which was the most abundant compound (55 %–70 % of C), increased with depth across these sites, whereas other C compounds like carbohydrates did not vary with depth. These results suggest that there is no preferential decomposition of carbohydrates but instead preferential retention of lignin. Stable isotope signatures of respiration products indicated that hydrogenotrophic rather than acetoclastic methanogenesis was the dominant production pathway of CH4 throughout the peat profile. These results show that deep C in tropical peatlands does not contribute greatly to surface fluxes of carbon dioxide, with compounds like lignin preferentially retained. This protection of deep C helps explain how peatland C is retained over thousands of years and points to the vulnerability of this C should anaerobic conditions in these wet ecosystems change.

Drying and drought in tropical forests, which have some of the highest net primary productivity on Earth, are likely to alter root dynamics, ecosystem function, and carbon (C) storage. We used a chronic drying experiment in four lowland Panamanian forests to investigate whether soil drying shifts tropical forest root production from surface to deeper soils, where moisture remains more abundant. Furthermore, we explored whether soil drying promotes resource acquisition strategies in roots, such as outsourcing to arbuscular mycorrhizal fungi (AMF) symbionts or increased specific root length (SRL). We found that chronic drying significantly reduced surface root biomass stocks, production, and turnover rates (0-20 cm soil depth), and increased AMF colonization without changes in SRL. Meanwhile, deep fine root productivity (> 60 cm depth) increased in the dry vs wet season, and in the drying experiment, except in the wettest, most infertile forest. Changes in root characteristics in these tropical forests with drying would likely alter forest-climate feedbacks and long-term soil C storage. At the same time, these results suggest that tropical forests may have an ability to adapt resource acquisition strategies under drying climates.

Changes in rainfall are predicted across tropical regions, with effects on nutrient, water, and carbon cycling. This chapter summarizes results from the first two years of a throughfall exclusion experiment in four lowland Panamanian forests that span a 1,000-mm change in rainfall and variation in soil fertility. Soil respiration (i.e., soil carbon dioxide [CO2 ] flux) declined with throughfall exclusion, with a site*season interaction, and the radiocarbon age of respired carbon was older in exclusion versus control plots. The decline in soil CO2 flux could be related to reduced fine root production and soil microbial biomass. Microbial community composition also changed with throughfall exclusion in infertile soils, and soil nutrients accumulated more in exclusion versus control plots during the dry season. The net effects on soil carbon storage will depend on the relative strengths of these effects over time. Continued research could improve predictions of tropical forest-climate feedbacks with changes in precipitation.

Root trait variation may reflect the ecological and evolutionary processes shaping biodiversity, but remains poorly quantified in the (sub)tropics. Here, we aim to further complete our knowledge of belowground functional strategies by assessing the contributions of subtropical and tropical species to global root trait diversity. We gathered root data for 1618 temperate, 341 subtropical, and 775 tropical species. We compared functional diversity among biomes and calculated the unique contribution of each biome to the global root economics space. Further, we determined if the within-variation of subtropical and tropical biomes is shaped by species niches and/or differences in evolutionary history. Root trait expressions differed among biomes, but root functional diversity did not. Furthermore, subtropical and tropical biomes accounted for 40% of the unique root functional space within the global traits space. Species climate niches and phylogenetic turnover explained variation in root traits (e.g., denser root tissue was associated with drier sites) among subtropical but not tropical species. Through their unique root traits, sub(tropical) species strongly expand the current global root trait space. This work underwrites their importance in conceptual models for more complete insights into how various belowground strategies drive plant functional biogeography and biodiversity globally. ### Competing Interest Statement The authors have declared no competing interest.

The increasing prevalence of drought events in grasslands and shrublands worldwide potentially has impacts on soil organic carbon (SOC). We leveraged the International Drought Experiment to study how SOC, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) concentrations, responds to extreme drought treatments (1-in-100-year) for 1 to 5 years at 19 sites worldwide. In more mesic areas (aridity index > 0.65), SOC and POC concentrations decreased by 7.9% (±3.9) and 15.9% (±6.2) with drought, respectively, but there were no impacts on MAOC concentrations. However, drought had no impact on SOC, POC, or MAOC concentrations in drylands (aridity index < 0.65). The response of SOC to drought varied along an aridity gradient, concomitant with interannual precipitation variability and standing SOC concentration gradients. These findings highlight the differing response magnitudes of POC and MAOC concentrations to drought and the key regulating role of aridity.

Tropical forests play a critical role in the global carbon (C) cycle. These ecosystems maintain the highest rates of net primary production (NPP) on Earth (Hengl et al., 2017), contain c. 30% of terrestrial C stocks (Jobbagy & Jackson, 2000), and have some of the largest stores of fine-root biomass globally (Jackson et al., 1996), as well as higher fine-root production and turnover rates compared with other biomes (Cusack et al., 2021). Tropical forest responses to projected warming, altered rainfall regimes, and elevated C dioxide (CO2) concentrations (IPCC, 2021) are likely to be different from other ecosystems because of their unique characteristics (Box 1), making targeted research and model development important for understanding tropical forest–climate feedbacks. There is now a critical mass of long-term global change field experiments and modeling efforts in tropical forests, yet thus far there has been little synthesis, cross-site comparison, or multi-site standardized experimentation among tropical forests to help us understand how these biomes are changing. An organized INSPIRE session at the 108th Annual Meeting of the Ecological Society of America set out to tackle just this. Speakers covered large-scale tropical forest field experiments and modeling efforts, with an emphasis on changes in ecosystem biogeochemistry under warming, drying, elevated atmospheric CO2, and changing nutrient status. In this meeting report, we provide an overview of the large-scale global change experiments presented and highlight the main objectives and opportunities for tropical forest research that emerged, including cross-site comparisons and integration with ecosystem-scale models (Fig. 1). The range and extent of large-scale tropical forest experiments and modeling efforts presented by the speakers highlighted the recent accumulation of new data and papers. Across the presentations, the importance of spatial and temporal variation in tropical forest responses to global change and variation in responses by different components of ecosystems (e.g. above- vs belowground), was apparent. Tropical forests have some unique ecosystem attributes, such that these ecosystems merit focused study and modeling efforts to understand their responses and feedbacks to global change. For example: At the same time, there are exceptions to this broader context, such as monodominant Dipterocarpaceae forests in SE Asia, and relative N scarcity in early successional and montane tropical forests. Still, tropical forests are most often characterized by moisture seasonality, scarcity of rock-derived nutrients, and high biodiversity. The extent of these characteristics varies among tropical forests. Despite the global importance of tropical forests in the global C cycle and their distinctive ecosystem characteristics, these ecosystems remain poorly characterized and underrepresented in dynamic vegetation and Earth System Models relative to other ecosystems (Bonan & Doney, 2018). Talks on warming experiments included the Tropical Responses to Altered Climate Experiment (TRACE) on canopy and soils in Puerto Rico, and the Soil Warming Experiment in Lowland Tropical Rainforest (SWELTR) in Panama. The Panama Rainforest Changes with Experimental Drying (PARCHED) experiment (Tana Wood, Puerto Rico; Andrew Nottingham, Panama; Lee Dietterich, Panama, respectively, Fig. 1) provided an example of a drying experiment. Results illustrated that there are rapid and often large changes in tropical forest C and nutrient cycling in response to temperature and moisture shifts (Nottingham et al., 2020; Reed et al., 2020; Dietterich et al., 2022; Cusack et al., 2023), including shifts in organism growth, activity, and diversity. Emerging results presented by Nottingham indicated that alterations to biogeochemical cycling rates are related to shifts in organism activity and biodiversity across trophic levels. There was substantial spatial and temporal variation in responses within and among forest sites, with shifts in C cycling, in particular, varying over time and space among individual forests included in PARCHED (Cusack et al., 2023), and interacting effects with other disturbances (e.g. hurricane and drought) in TRACE (Reed et al., 2020). This group of talks emphasized the importance of understanding tropical forest biogeochemical responses to both warming and drying and highlighted the need to assess these effects in combination, across more sites, and over decadal timescales. Presentations on nutrient fertilization experiments in tropical forests (Kelly Andersen, Brazil; Michelle Wong, Panama; Rebecca Ostertag, Hawai'i, USA) also highlighted the complexity of plant, soil, and microbial responses across time and space, and the importance of baseline site conditions such as forest successional stage, litter chemistry, and soil nutrient status. These presentations added to a recent meta-analysis of 36 large-scale fertilization experiments in lowland tropical forests, which indicated that multi-nutrient (N and P) limitation to NPP is most common and that earlier successional forests are more nutrient-limited than mature tropical forests (Wright, 2019). Ostertag's talk on the Hawai'i Long Substrate Age Gradient (LSAG) showed data from multi-nutrient (N, P) fertilization across stages of soil development, where NPP is limited by N on young soils, and by P on older, more strongly weathered soils (Vitousek, 2004). Ostertag's talk focused on nutrient effects on plant litter decomposition across plant species, showing that both litter quality and site characteristics influenced decomposition rates, and effects were strongest in the P-scarce older soil. These results followed earlier results showing that fertilization with the limiting nutrient had a larger effect on fine root production in the P-scarce site compared with the N-scarce site (Ostertag, 2001). Wong discussed results from a multi-nutrient fertilization across a forest successional gradient in Panama (Fig. 1), where biological N fixation is active in N-scarce, early successional sites, and declines in older, more P-scare forests, suggesting N limitation to NPP in early successional tropical forests (Batterman et al., 2013). Research presented by Andersen on multi-nutrient fertilization in the Brazilian Amazon Fertilisation Experiment (AFEX, Fig. 1) demonstrated that P addition alone increased NPP on P-scarce soils by increasing leaf and fine root production and turnover rates (Cunha et al., 2022). Together, these presentations illustrated that different nutrients or combinations of nutrients limit different ecosystem processes across tropical forests, which will likely lead to the emergence of complex sets of nutrient limitations to biological activity under elevated atmospheric CO2 (i.e., CO2 fertilization). A need emerged from these talks for more synthetic efforts to identify plant and ecosystem traits that will be important for overcoming or tolerating nutrient scarcity in the context of global change. Presentations on improving the representation of tropical forests in vegetation models (Matthew Craig, Oak Ridge National Lab; Jennifer Holm, Lawrence Berkeley National Lab) highlighted the importance of targeted data model integration for better tropical forest representation of biogeochemical feedbacks. For example, Holm discussed a mismatch between observed declines in the tropical C sink (Rammig, 2020), compared with the continual increase in tropical C sinks predicted by climate-driven vegetation models (Arora et al., 2020). Specifically, most of the global models used for climate projections and in the coupled model intercomparison project (CMIP6) predict a growing tropical C sink, contrary to what plot-scale empirical data currently suggest, which may be due to missing processes such as plant demography, nutrient competition, and disturbances. Holm argued for applying global models at the site scale to look at model-observation agreement to better predict at larger scales. With the emerging inclusion of finer-scale plant demography and competition into biogeochemical Earth System Models (ESMs), such as inclusion of the Functionally Assembled Terrestrial Ecosystem Simulator (FATES, Fig. 1) model (Holm et al., 2020), we can now apply lessons learned at the site level back to the global scale. Craig highlighted ongoing efforts to address cross-scale integration in dynamic vegetation demographic models (VDMs) including: improving representation of nutrient cycling and C costs for nutrient acquisition in tropical forests, and expanded representation of dynamic root responses to changing resources over soil depths. Models at the ecosystem scale can also be useful for predicting and understanding outcomes of forest management and restoration strategies. Planning the next broad-scale, coordinated experiments across tropical forests is needed to address key questions. New experiments should focus on data gaps and broader geographical representation within the tropics. Several important research needs emerged from the discussion: We would like to thank the > 75 audience participants who attended this INSPIRE session and contributed to constructive and productive discussion, as well as the Ecological Society of America for hosting the meeting. The Panama PARCHED experiment was supported by the US DOE Office of Science BER Early Career Award DE-SC0015898 and NSF Geography & Spatial Studies Grant BCS-1437591 to D.F. Cusack. The Panama SWELTR experiment was supported by grants to A.T. Nottingham including a UK NERC grant NE/T012226, a European Union Marie-Curie Fellowship FP7-2012-329360, and a STRI Tupper Fellowship. The Puerto Rico TRACE experiment was supported by US NSF Grant DEB-2140580 and DOE ESS DE-SC-0018942, DE-SC-0022095 to T.E. Wood and S. Reed. The Hawai'i LSAG work was supported in part NSF CAREER 0546868 to R. Ostertag and was facilitated through NSF EPSCoR 0237065 and 0554657. The US DOE Office of Science BER Next Generation Ecosystem Experiments (NGEE)-Tropics supported J.A. Holm, M. Craig, and the work with FATES model. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-1008 00OR22725. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This manuscript has been authored by UT-Battelle LLC under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Fine roots are key to ecosystem-scale nutrient, carbon (C), and water cycling, yet our understanding of fine root trait variation within and among tropical forests, one of Earth’s most C-rich ecosystems, is limited. We characterized root biomass, morphology, nutrient content, and arbuscular mycorrhizal fungal (AMF) colonization to 1.2 m depths across four distinct lowland Panamanian forests, and related root characteristics to soil C stocks. We hypothesized that: (H1) Fine root characteristics vary consistently with depth across seasonal tropical forests, with deeper roots exhibiting more exploratory traits, such as for deep water acquisition; (H2) fine root characteristics vary among tropical forests mainly in surface soils, where resource availability also varies. We found consistent variation with depth across the four forests, including decreased root biomass, root tissue density, and AMF, and increased specific root length. Among the forests, there was variation in some fine root characteristics, including greater surface root biomass and lower SRL in the wettest forest, and smaller fine root diameter in the driest forest. We also found that root characteristics were related to total soil C stocks, which were positively related to root biomass and negatively related to specific root length. These results indicate emergent properties of root variation with depth across tropical forests, and show site-scale variation in surface root characteristics. Future work could explore the flexibility in root characteristics under changing conditions such as drought.

This chapter presents soil carbon dynamics in the Barro Colorado Nature Monument (BCNM) and surrounding lowland Panamanian forests. I first review principles in soil carbon science for tropical forests, including soil carbon stocks, fractions, turnover times, inputs, and losses. I then review research on patterns and mechanisms of spatial and temporal variability in soil carbon dynamics, including among-site, seasonal, and depth-related variation as well as findings of experimental studies. Soil carbon stocks vary strongly among sites with soil weathering status, declining with fertility and increasing with clay content within the BCNM and in central Panama more generally. Rainfall seasonality drives seasonal variation in soil carbon inputs and losses. Experimental irrigation, nutrient addition, and litter manipulation studies have provided insight into the role of moisture, nutrient availability, and plant inputs in controlling soil carbon dynamics. Finally, I suggest future research directions, including constraining turnover times for different soil carbon fractions across depths.

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.

Soil drying challenges microbial viability and survival, with bacteria employing various mechanisms to respond to shifts in osmolarity, including dormancy or metabolic upregulation of osmoprotectants. However, the extent to which these responses are shaped by an organisms phylogeny or the climate history of a given environment is poorly understood. This study examines the responses of phylogenetically similar bacteria from semi-arid and humid tropical forest soils to osmotic and matric stress using synchrotron radiation-based Fourier Transform Infrared spectromicroscopy. This non-destructive approach depicts the biochemical phenotype for whole cells under control and stress conditions. We observed that, under osmotic stress, bacteria upregulated cell-signaling pathways, rapidly turned over lipid-storage compounds, and increased osmolyte production. In contrast, matric stress induced a more muted response, typically elevating the production of carbohydrate stress compounds, such as glycine betaine and trehalose. While phylogenetically similar bacteria showed comparable biochemistry under control conditions, climate history played an important role in regulating responses to stress, whereby a stronger metabolic response was observed from semi-arid relative to tropical forest isolates. We conclude that bacterial stress response to drought can be more diverse than previously observed, and regulated by both phylogeny and climate history. ### Competing Interest Statement The authors have declared no competing interest.

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.

AbstractTropical forests account for over 50% of the global terrestrial carbon sink, but climate change threatens to alter the carbon balance of these ecosystems. We show that warming and drying of tropical forest soils may increase soil carbon vulnerability, by increasing degradation of older carbon. In situ whole-profile heating by 4 °C and 50% throughfall exclusion each increased the average radiocarbon age of soil CO2 efflux by ~2–3 years, but the mechanisms underlying this shift differed. Warming accelerated decomposition of older carbon as increased CO2 emissions depleted newer carbon. Drying suppressed decomposition of newer carbon inputs and decreased soil CO2 emissions, thereby increasing contributions of older carbon to CO2 efflux. These findings imply that both warming and drying, by accelerating the loss of older soil carbon or reducing the incorporation of fresh carbon inputs, will exacerbate soil carbon losses and negatively impact carbon storage in tropical forests under climate change.

Tropical ecosystems face escalating global change. These shifts can disrupt tropical forests' carbon (C) balance and impact root dynamics. Since roots perform essential functions such as resource acquisition and tissue protection, root responses can inform about the strategies and vulnerabilities of ecosystems facing present and future global changes. However, root trait dynamics are poorly understood, especially in tropical ecosystems. We analyzed existing research on tropical root responses to key global change drivers: warming, drought, flooding, cyclones, nitrogen (N) deposition, elevated (e) CO2, and fires. Based on tree species- and community-level literature, we obtained 266 root trait observations from 93 studies across 24 tropical countries. We found differences in the proportion of root responsiveness to global change among different global change drivers but not among root categories. In particular, we observed that tropical root systems responded to warming and eCO(2) by increasing root biomass in species-scale studies. Drought increased the root: shoot ratio with no change in root biomass, indicating a decline in aboveground biomass. Despite N deposition being the most studied global change driver, it had some of the most variable effects on root characteristics, with few predictable responses. Episodic disturbances such as cyclones, fires, and flooding consistently resulted in a change in root trait expressions, with cyclones and fires increasing root production, potentially due to shifts in plant community and nutrient inputs, while flooding changed plant regulatory metabolisms due to low oxygen conditions. The data available to date clearly show that tropical forest root characteristics and dynamics are responding to global change, although in ways that are not always predictable. This synthesis indicates the need for replicated studies across root characteristics at species and community scales under different global change factors.