Search for: All records

Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments. 

Summary Coarse roots represent a globally important belowground carbon pool, but the factors controlling coarse root decomposition rates remain poorly understood relative to other plant biomass components. We compiled the most comprehensive dataset of coarse root decomposition data including 148 observations from 60 woody species, and linked coarse root decomposition rates to plant traits, phylogeny and climate to address questions of the dominant controls on coarse root decomposition.We found that decomposition rates increased with mean annual temperature, root nitrogen and phosphorus concentrations. Coarse root decomposition was slower for ectomycorrhizal than arbuscular mycorrhizal associated species, and angiosperm species decomposed faster than gymnosperms. Coarse root decomposition rates and calcium concentrations showed a strong phylogenetic signal.Our findings suggest that categorical traits like mycorrhizal association and phylogenetic group, in conjunction with root quality and climate, collectively serve as the optimal predictors of coarse root decomposition rates.Our findings propose a paradigm of the dominant controls on coarse decomposition, with mycorrhizal association and phylogeny acting as critical roles on coarse root decomposition, necessitating their explicit consideration in Earth‐system models and ultimately improving confidence in projected carbon cycle–climate feedbacks. 

Abstract Anthropogenic disturbances alter trajectories of ecological succession, introduce spatiotemporal variability in the composition of communities, and potentially create communities that differ substantially from those prior to disturbance. Invasive species are introduced or spread by human activities, with considerable effect on native ecosystems throughout the world. We evaluate the temporal stability of woody plant metacommunity structures and the mechanisms that give rise to them in a tropical disturbance‐mediated environment. We used data collected over 20 years to (1) evaluate elements of metacommunity structure, (2) identify the gradients along which metacommunities are structured, and (3) quantify the relative contributions of environmental and spatial factors on variation in species composition. Analyses were conducted separately for combinations of life zone (areas defined by edaphic features and climate) and species origin (native versus non‐native). Native species exhibited compartmentalized structures (i.e., groups of species with similar distributions that are replaced by other such groups along a gradient), whereas non‐natives exhibited random structures. Metacommunities based on all species were consistently compartmentalized, except in dry forest, which exhibited random structure. Compartmentalized structures occurred along gradients defined by life zone and soil type, whereas no environmental factors were consistently associated with random structures. Metacommunity structure was stable through time despite a complex disturbance regime. Dry forests, which have experienced the most extensive and intensive history of anthropogenic disturbances of any life zone on Puerto Rico are characterized by degraded and fragmented landscapes, with species that do not respond to a common environmental gradient. 

Abstract Elevational gradients represent platforms for exploring the effects of environmental variation on biodiversity. The environmental correlates of these spatial gradients are likely to be modified during the Anthropocene, as species respond to global change drivers including warming and increased frequency of extreme events. We quantified variation in the abundance of four functional groups of canopy arthropods (i.e., folivores, sap‐suckers, detritivores, and predators), as well as in aspects of biodiversity on each of six host‐plant species along two elevational transects in the Luquillo Mountains of Puerto Rico: a mixed forest transect, traversing tabonuco, palo colorado, and elfin forests, and a palm forest transect, comprising only patches dominated by sierra palm (Prestoea acuminata). We expected gradients in arthropod abundance and biodiversity to be host‐tree specific, and for gradients on palm to differ between transects due to a combination of mechanisms associated with host selection, rescue effects, habitat structure, and source pool dynamics. In general, abundance and biodiversity declined with elevation. The ways in which abundance declined with increasing elevation was contingent on host tree identity and on arthropod functional group, whereas all aspects of biodiversity declined with elevation in consistent manners regardless of host tree identity or transect. Similarly, turnover (beta components of biodiversity between sequential elevational strata) did not differ between transects. Decreases in productivity with increasing elevation may be responsible for gradients in abundance or biodiversity. However, host‐specific and functional group‐specific gradients suggest that elevational effects manifest differently depending on tree species identity and resource bases that are consumer specific. 

Abstract The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termedDSi regime, mediates the timing of DSi delivery to downstream waters and thus governs river biogeochemical function and aquatic community condition. Previous work identified five distinct DSi regimes across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple DSi regimes over time. Several potential drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, and yet the large‐scale spatiotemporal controls on DSi regimes have not been identified. We evaluate the role of environmental variables on the behavior of DSi regimes in nearly 200 rivers across the Northern Hemisphere using random forest models. Our models aim to elucidate the controls that give rise to (a) average DSi regime behavior, (b) interannual variability in DSi regime behavior (i.e., Annual DSi regime), and (c) controls on DSi regime shape (i.e., minimum and maximum DSi concentrations). Average DSi regime behavior across the period of record was classified accurately 59% of the time, whereas Annual DSi regime behavior was classified accurately 80% of the time. Climate and primary productivity variables were important in predicting Average DSi regime behavior, whereas climate and hydrologic variables were important in predicting Annual DSi regime behavior. Median nitrogen and phosphorus concentrations were important drivers of minimum and maximum DSi concentrations, indicating that these macronutrients may be important for seasonal DSi drawdown and rebound. Our findings demonstrate that fluctuations in climate, hydrology, and nutrient availability of rivers shape the temporal availability of fluvial DSi. 

Abstract Models project that climate change is increasing the frequency of severe storm events such as hurricanes. Hurricanes are an important driver of ecosystem structure and function in tropical coastal and island regions and thus impact tropical forest carbon (C) cycling. We used the DayCent model to explore the effects of increased hurricane frequency on humid tropical forest C stocks and fluxes at decadal and centennial timescales. The model was parameterized with empirical data from the Luquillo Experimental Forest (LEF), Puerto Rico. The DayCent model replicated the well-documented cyclical pattern of forest biomass fluctuations in hurricane-impacted forests such as the LEF. At the historical hurricane frequency (60 years), the dynamic steady state mean forest biomass was 80.9 ± 0.8 Mg C/ha during the 500-year study period. Increasing hurricane frequency to 30 and 10 years did not significantly affect net primary productivity but resulted in a significant decrease in mean forest biomass to 61.1 ± 0.6 and 33.2 ± 0.2 Mg C/ha, respectively (p < 0.001). Hurricane events at all intervals had a positive effect on soil C stocks, although the magnitude and rate of change of soil C varied with hurricane frequency. However, the gain in soil C stocks was insufficient to offset the larger losses from aboveground biomass C over the time period. Heterotrophic respiration increased with hurricane frequency by 1.6 to 4.8%. Overall, we found that an increasing frequency of tropical hurricanes led to a decrease in net ecosystem production by − 0.2 ± 0.08 Mg C/ha/y to − 0.4 ± 0.04 Mg C/ha/y for 30–10-year hurricane intervals, respectively, significantly increasing the C source strength of this forest. These results demonstrate how changes in hurricane frequency can have major implications for the tropical forest C cycle and limit the potential for this ecosystem to serve as a net C sink. 

Abstract Background and AimsUnderstanding shifts in the demographic and functional composition of forests after major natural disturbances has become increasingly relevant given the accelerating rates of climate change and elevated frequency of natural disturbances. Although plant demographic strategies are often described across a slow–fast continuum, severe and frequent disturbance events influencing demographic processes may alter the demographic trade-offs and the functional composition of forests. We examined demographic trade-offs and the shifts in functional traits in a hurricane-disturbed forest using long-term data from the Luquillo Forest Dynamics Plot (LFPD) in Puerto Rico. MethodsWe analysed information on growth, survival, seed rain and seedling recruitment for 30 woody species in the LFDP. In addition, we compiled data on leaf, seed and wood functional traits that capture the main ecological strategies for plants. We used this information to identify the main axes of demographic variation for this forest community and evaluate shifts in community-weighted means for traits from 2000 to 2016. Key ResultsThe previously identified growth–survival trade-off was not observed. Instead, we identified a fecundity–growth trade-off and an axis representing seedling-to-adult survival. Both axes formed dimensions independent of resprouting ability. Also, changes in tree species composition during the post-hurricane period reflected a directional shift from seedling and tree communities dominated by acquisitive towards conservative leaf economics traits and large seed mass. Wood specific gravity, however, did not show significant directional changes over time. ConclusionsOur study demonstrates that tree demographic strategies coping with frequent storms and hurricane disturbances deviate from strategies typically observed in undisturbed forests, yet the shifts in functional composition still conform to the expected changes from acquisitive to conservative resource-uptake strategies expected over succession. In the face of increased rates of natural and anthropogenic disturbance in tropical regions, our results anticipate shifts in species demographic trade-offs and different functional dimensions. 

Abstract Hurricanes are major disturbances with important consequences to stream ecosystems as they create major floods and remove riparian vegetation. Understanding their impacts is a priority, as hurricane intensity is expected to increase due to global climate change.Mayfly assemblages in streams fill a diversity of ecological roles and functions. They are important consumers of algae by scraping benthic biofilms and detritivores associated with fine particles and leaf litter. Other taxa are filterers and even predators. Mayflies are also important prey items in aquatic and terrestrial food webs.Here, we assessed the effects of two consecutive hurricanes that impacted Puerto Rico in 2017 to understand how hurricane‐induced changes in the environment alter mayfly composition, secondary production and emergence.The study was conducted in the Luquillo Experimental Forest, Puerto Rico. Mayflies were sampled as nymphs and emerging adults for 6 months before and 17 months after hurricanes Irma and María hit the island in September 2017. Leaf litter inputs, canopy cover and chlorophyllaconcentrations were monitored along with mayflies.Mayfly assemblages were dominated by two genera of Leptophlebiidae before the hurricane,Neohagenulus (two species: N. julioTraver, 1938,N. luteolusTraver, 1938) andBorinquena (one species: B. carmencitaTraver, 1938). Both genera decreased in density after the hurricanes and were replaced with the BaetidaeCloeodes maculipesTraver, 1938 as the dominant taxon. This pattern was observed in both nymph and emerging adult densities.The secondary production of Leptophlebiidae species was highest before hurricane disturbance, with the BaetidaeC. maculipesshowing the opposite pattern.Neohagenulushad an annual production of 445 mg m⁻² year⁻¹,C. maculipesof 153 mg m⁻² year⁻¹andB. carmencitaof 68 mg m⁻² year⁻¹.Overall, the mayfly assemblages in our studied stream are vulnerable to hurricane disturbances. Expected increases in hurricane impacts might result in assemblage shifts that could change assemblage composition and alter energy flows within the ecosystem. 

Abstract Large scale disturbances are known to significantly alter aspects of both species diversity and ecosystem function. In the Caribbean, hurricane events are a significant form of disturbance, the effects of which have been shown to alter food web function, especially in the terrestrial environment. Although hurricanes have been studied from a variety of their effects on ecosystems, there is little research on how these storms affect species along elevational gradients. Within terrestrial habitats, ants form the basis of many food webs, being both numerically dominant and functioning in a variety of roles within the food web. On September 20th, 2017 Hurricane Maria, a category 4 storm, crossed over the island of Puerto Rico, causing significant damage to both human and natural systems. We collected data on ant abundance and composition from 150 samples of leaf litter along a 700 m elevational gradient during June the year of and after the storm event. Ant abundance increased by 400% after the storm with many common ant species seeming to benefit, especially at lower elevations. There were subtle changes in ant richness, with declines generally after the storm, but yet again this response was dependent on elevation. This is one of the first studies to consider how terrestrial insect communities are affected by large hurricane events across elevations, and our results are in contrast to past work showing declines in ant abundance after such storms. Abstract in Spanish is available with online material. 

Abstract Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30‐year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes (“soilR”) and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0–10 cm and 35–60 cm depths relative to a modeled control soil with constant inputs over the 30‐year period. Between 1988 and 2018, the occluded C stock increased and δ¹³C in all pools decreased, while changes in particulate and mineral‐associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris‐deposited C, and possible microbial scavenging of old and young C in the particulate and mineral‐associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.