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Tropical forests are the largest contributors to global emissions of carbon dioxide (CO2) to the atmosphere via soil respiration (R s). As such, identifying the main controls on R s in tropical forests is essential for accurately projecting the consequences of ongoing and future global environmental changes to the global C cycle. We measured hourly R s in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R s. Across 3 years of measurements, mean R s was 7.16 ± 0.02 μmol CO2 m‐2 s‐1 (or 2,710 g C m‐2 year‐1) and showed significant seasonal variation. Despite small month‐to‐month variation in temperature (~4°C), we found significant positive relationships between daily and monthly R s with both air and soil temperature, highlighting the importance of temperature as a driver of R s even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily R s, with an optimal moisture value of 0.34 m3 m‐3. Given the relatively consistent climate at this site, the large range in mean monthly R s (~7 μmol CO2 m‐2 s‐1) was surprising and suggests that even small changes in climate can have large implications for ecosystem respiration. The strong positive relationship of R s with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of R s in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of R s in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO2) globally., We measured hourly soil respiration in a tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R s. Across 3 years of measurements, mean R s was 7.08 ± 0.02 μmol CO2 m‐2 s‐1 (2,728 g C m‐2 year‐1) and showed significant seasonal variation at daily and monthly timescales. Despite small month‐to‐month variation in temperature (~4 C), seasonal patterns of daily and monthly R s were strongly correlated with air temperature (R 2 ranged from 0.49 to 0.70 depending on the timescale), highlighting the importance of temperature as a driver of R s even in warm ecosystems, such as tropical forests.

The effects of climate change on tropical forests may have global consequences due to the forests’ high biodiversity and major role in the global carbon cycle. In this study, we document the effects of experimental warming on the abundance and composition of a tropical forest floor herbaceous plant community in the Luquillo Experimental Forest, Puerto Rico. This study was conducted within Tropical Responses to Altered Climate Experiment (TRACE) plots, which use infrared heaters under free‐air, open‐field conditions, to warm understory vegetation and soils + 4°C above nearby control plots. Hurricanes Irma and María damaged the heating infrastructure in the second year of warming, therefore, the study included one pretreatment year, one year of warming, and one year of hurricane response with no warming. We measured percent leaf cover of individual herbaceous species, fern population dynamics, and species richness and diversity within three warmed and three control plots. Results showed that one year of experimental warming did not significantly affect the cover of individual herbaceous species, fern population dynamics, species richness, or species diversity. In contrast, herbaceous cover increased from 20% to 70%, bare ground decreased from 70% to 6%, and species composition shifted pre to posthurricane. The negligible effects of warming may have been due to the short duration of the warming treatment or an understory that is somewhat resistant to higher temperatures. Our results suggest that climate extremes that are predicted to increase with climate change, such as hurricanes and droughts, may cause more abrupt changes in tropical forest understories than longer‐term sustained warming., We documented the effects of +4°C experimental warming on the abundance and composition of a tropical forest floor herbaceous plant community in Puerto Rico. Results showed that one year of warming did not significantly affect the cover of individual herbaceous species, fern population dynamics, species richness, or species diversity. However, hurricanes Irma and Maria, which hit Puerto Rico after one year of experimental warming, did have significant effects on this understory community.

The response of tropical forests to global warming is one of the largest uncertainties in predicting the future carbon balance of Earth. To determine the likely effects of elevated temperatures on tropical forest understory plants and soils, as well as other ecosystems, an infrared (IR) heater system was developed to provide in situ warming for the Tropical Responses to Altered Climate Experiment (TRACE) in the Luquillo Experimental Forest in Puerto Rico. Three replicate heated 4‐m‐diameter plots were warmed to maintain a 4°C increase in understory vegetation compared to three unheated control plots, as sensed by IR thermometers. The equipment was larger than any used previously and was subjected to challenges different from those of many temperate ecosystem warming systems, including frequent power surges and outages, high humidity, heavy rains, hurricanes, saturated clayey soils, and steep slopes. The system was able to maintain the target 4.0°C increase in hourly average vegetation temperatures to within ± 0.1°C. The vegetation was heterogeneous and on a 21° slope, which decreased uniformity of the warming treatment on the plots; yet, the green leaves were fairly uniformly warmed, and there was little difference among 0–10 cm depth soil temperatures at the plot centers, edges, and midway between. Soil temperatures at the 40–50 cm depth increased about 3°C compared to the controls after a month of warming. As expected, the soil in the heated plots dried faster than that of the control plots, but the average soil moisture remained adequate for the plants. The TRACE heating system produced an adequately uniform warming precisely controlled down to at least 50‐cm soil depth, thereby creating a treatment that allows for assessing mechanistic responses of tropical plants and soil to warming, with applicability to other ecosystems. No physical obstacles to scaling the approach to taller vegetation (i.e., trees) and larger plots were observed.

International audience; Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

The Luquillo Experimental Forest (LEF) has a long history of research on tropical forestry, ecology, and conservation, dating as far back as the early 19th Century. Scientific surveys conducted by early explorers of Puerto Rico, followed by United States institutions contributed early understanding of biogeography, species endemism, and tropical soil characteristics. Research in the second half of the 1900s established the LEF as an exemplar of forest management and restoration research in the tropics. Research conducted as part of a radiation experiment funded by the Atomic Energy Commission in the 1960s on forest metabolism established the field of ecosystem ecology in the tropics. Subsequent research has built on these early advances to develop new theories on ecosystem response to disturbance regimes and the role of the biota in ecosystem resilience. Recent and current research in the LEF has advanced understanding of resilience to hurricane disturbances, human land use, gamma irradiation, landslides, drought, and warming, showing that even following the most severe disturbances (e.g., landslides, agriculture) forests reestablish within 60 years. Work in the LEF has reversed the paradigm that tropical ecosystems are fragile, but instead exhibit remarkable resilience to many forms of disturbance present at multiple spatial and temporal scales. Current research is already advancing understanding of how climate change and attendant effects on the disturbance regime might affect the composition, structure, and function of tropical forest ecosystems.

For more accurate projections of both the global carbon (C) cycle and the changing climate, a critical current need is to improve the representation of tropical forests in Earth system models. Tropical forests exchange more C, energy, and water with the atmosphere than any other class of land ecosystems. Further, tropical-forest C cycling is likely responding to the rapid global warming, intensifying water stress, and increasing atmospheric CO2 levels. Projections of the future C balance of the tropics vary widely among global models. A current effort of the modeling community, the ILAMB (International Land Model Benchmarking) project, is to compile robust observations that can be used to improve the accuracy and realism of the land models for all major biomes. Our goal with this paper is to identify field observations of tropical-forest ecosystem C stocks and fluxes, and of their long-term trends and climatic and CO2 sensitivities, that can serve this effort. We propose criteria for reference-level field data from this biome and present a set of documented examples from old-growth lowland tropical forests. We offer these as a starting point towards the goal of a regularly updated consensus set of benchmark field observations of C cycling in tropical forests.

The removal of nutrients from senescing tissues, nutrient resorption, is a key strategy for conserving nutrients in plants. However, our understanding of what drives patterns of nutrient resorption in tropical trees is limited. We examined the effects of nutrient sources (stand-level and tree-level soil fertility) and sinks (reproductive effort) on nitrogen (N) and phosphorus (P) resorption. We evaluated resorption efficiency (percentage of original nutrients removed during senescence) and resorption proficiency (indicated by senesced-leaf nutrient concentrations) in a symbiotic N-fixing tree species, Pentaclethra macroloba, common to tropical forests in Costa Rica. Although tree-level soil P alone did not drive patterns in nutrient resorption, P efficiency and proficiency declined with increasing tree-level soil P when reproductive status was also considered. Nutrient resorption declined with increasing tree-level soil P in trees that were actively fruiting or that experienced high seedfall the year prio...

Global climate models project a decrease in the magnitude of precipitation in tropical regions. Changes in rainfall patterns have important implications for the moisture content and redox status of tropical soils, yet little is known about how these changes may affect microbial community structure. Specifically, does exposure to prior stress confer increased resistance to subsequent perturbation? Here we reduced the quantity of precipitation throughfall to tropical forest soils in the Luquillo Mountains, Puerto Rico. Treatments included newly established throughfall exclusion plots (de novo excluded), plots undergoing reduction for a second time (pre-excluded) and ambient control plots. Ten months of throughfall exclusion led to a small but statistically significant decline in soil water potential and bacterial populations clearly adapted to increased osmotic stress. Although the water potential decline was small and microbial biomass did not change, phylogenetic diversity in the de novo-excluded plots decreased by ∼40% compared with the control plots, yet pre-excluded plots showed no significant change. On the other hand, the relative abundances of bacterial taxa in both the de novo-excluded and pre-excluded plots changed significantly with throughfall exclusion compared with control plots. Changes in bacterial community structure could be explained by changes in soil pore water chemistry and suggested changes in soil redox. Soluble iron declined in treatment plots and was correlated with decreased soluble phosphorus concentrations, which may have significant implications for microbial productivity in these P-limited systems.

The influence of environmental gradients on the foliar nutrient economy of forests has been well documented; however, we have little understanding of what drives variability among individuals within a single forest stand, especially tropical forests. We evaluated inter- and intra-specific variation in nutrient resorption, foliar nutrient concentrations and physical leaf traits of nine canopy tree species within a 1-ha secondary tropical rain forest in northeastern Costa Rica. Both nitrogen (N) and phosphorus (P) resorption efficiency (RE) and proficiency of the nine tree species varied significantly among species, but not within. Both N and P RE were significantly negatively related to leaf specific strength. Green leaf N and P concentrations were strongly negatively related to leaf mass per area, and senesced leaf nutrient concentrations were significantly positively related to green leaf nutrient concentrations. This study reveals a strong influence of physical leaf traits on foliar nutrient and resorption traits of co-occurring species in a secondary wet tropical forest stand.

The productivity of many tropical wet forests is generally limited by bioavailable phosphorus (P). Microbial activity is a key regulator of P availability in that it determines both the supply of P through organic matter decomposition and the depletion of bioavailable P through microbial uptake. Both microbial uptake and mineralization occur rapidly, and their net effect on P availability varies with soil moisture, temperature, and soil organic matter quantity and quality. Exploring the mechanisms driving P availability at fine temporal scales can provide insight into the coupling of carbon, water, and nutrient cycles, and ultimately, the response of tropical forests to climate change. Despite the recognized importance of P cycling to the dynamics of wet tropical forests and their potential sensitivity to short-term fluctuations in bioavailable P, the diurnal pattern of P remains poorly understood. This study quantifies diurnal fluctuations in labile soil P and evaluates the importance of biotic and abiotic factors in driving these patterns. To this end, measurements of labile P were made every other hour in a Costa Rican wet tropical forest oxisol. Spatial and temporal variation in Bray-extractable P were investigated in relation to ecosystem carbon flux, soil CO2 efflux, soil moisture, soil temperature, solar radiation, and sap-flow velocity. Spatially averaged bi-hourly (every two hours) labile P ranged from 0.88 to 2.48 microg/g across days. The amplitude in labile P throughout the day was 0.61-0.82 microg/g (41-54% of mean P concentrations) and was characterized by a bimodal pattern with a decrease at midday. Labile P increased with soil CO2 efflux and soil temperature and declined with increasing sap flow and solar radiation. Together, soil CO2 efflux, soil temperature, and sap flow explained 86% of variation in labile P.

Litter-induced pulses of nutrient availability could play an important role in the productivity and nutrient cycling of forested ecosystems, especially tropical forests. Tropical forests experience such pulses as a result of wet-dry seasonality and during major climatic events, such as strong El Ninos. We hypothesized that (1) an increase in the quantity and quality of litter inputs would stimulate leaf litter production, woody growth, and leaf litter nutrient cycling, and (2) the timing and magnitude of this response would be influenced by soil fertility and forest age. To test these hypotheses in a Costa Rican wet tropical forest, we established a large-scale litter manipulation experiment in two secondary forest sites and four old-growth forest sites of differing soil fertility. In replicated plots at each site, leaves and twigs (,2 cm diameter) were removed from a 400-m 2 area and added to an adjacent 100-m 2 area. This transfer was the equivalent of adding 5-25 kg/ha of organic P to the forest floor. We analyzed leaf litter mass, (N) and (P), and N and P inputs for addition, removal, and control plots over a two-year period. We also evaluated basal area increment of trees in removal and addition plots. There was no response of forest productivity or nutrient cycling to litter removal; however, litter addition significantly increased leaf litter production and N and P inputs 4-5 months following litter application. Litter production increased as much as 92%, and P and N inputs as much as 85% and 156%, respectively. In contrast, litter manipulation had no significant effect on woody growth. The increase in leaf litter production and N and P inputs were significantly positively related to the total P that was applied in litter form. Neither litter treatment nor forest type influenced the temporal pattern of any of the variables measured. Thus, environmental factors such as rainfall drive temporal variability in litter and nutrient inputs, while nutrient release from decomposing litter influences the magnitude. Seasonal or annual variation in leaf litter mass, such as occurs in strong El Nino events, could positively affect leaf litter nutrient cycling and forest productivity, indicating an ability of tropical trees to rapidly respond to increased nutrient availability.

The stoichiometry of resources may explain bottom-up regulation of higher trophic levels. We tested the effects of soil and litter nutrient stoichiometry on the invertebrate litter fauna of a Costa Rican tropical rain forest. Animal densities were estimated from 15 sites across a phosphorus gradient. The density of the invertebrate litter fauna varied considerably, and was strongly tied to soil and litter phosphorus concentrations. An increase in phosphorus concentrations corresponded with an equally proportionate increase in animal densities. Natural variation in nutrient levels can thus serve as a predictor of density in a highly diverse tropical animal community.