Publications

Leaf-cutter ant nests are biogeochemical hot spots where ants live and import vegetation to grow fungus. Metabolic activity and (in wet tropical forests) soil gas flux to the nest may result in high nest CO2 concentrations if not adequately ventilated. Wind-driven ventilation mitigates high CO2 concentrations in grasslands, but little is known about exchange for forest species faced with prolonged windless conditions. We studied Atta cephalotes nests located under dense canopy (leaf area index > 5) in a wet tropical rainforest in Costa Rica, where wind events are infrequent. We instrumented nests with thermocouples and flow-through CO2 sensing chambers. The results showed that CO2 concentrations exiting leaf-cutter ant nests follow a diel pattern with higher values at night. We developed an efflux model based on pressure differences that evaluated the observed CO2 diel pattern in terms of ventilation by (1) free convection (warm, less dense air rises out the nest more prominently at night) and (2) episodic wind-forced convection events providing occasional supplemental ventilation during daytime. Average greenhouse gas emissions were estimated through nest vents at about 78 kg CO2eq nest-1 year-1. At the ecosystem level, leaf-cutter ant nest vents accounted for 0.2% to 1% of total rainforest soil emissions. In wet, clayey tropical soils, leaf-cutter ant nests act as free convection-driven conduits for exporting CO2 and other greenhouse gases produced within the nest (fungus and ant respiration, refuse decay), and by roots and soil microbes surrounding the nest. This allows A. cephalotes nests to be ventilated without reliable wind conditions.

Soil CO2 concentrations and emissions from tropical forests are modulated seasonally by precipitation. However, subseasonal responses to meteorological events (e.g., storms, drought) are less well known. Here, we present the effects of meteorological variability on short-term (hours to months) dynamics of soil CO2 concentrations and emissions in a Neotropical wet forest. We continuously monitored soil temperature, moisture, and CO2 for a three-year period (2015–2017), encompassing normal conditions, floods, a dry El Niño period, and a hurricane. We used a coupled model (Hydrus-1D) for soil water propagation, heat transfer, and diffusive gas transport to explain observed soil moisture, soil temperature, and soil CO2 concentration responses to meteorology, and we estimated soil CO2 efflux with a gradient-flux model. Then, we predicted changes in soil CO2 concentrations and emissions under different warming climate change scenarios. Observed short-term (hourly to daily) soil CO2 concentration responded more to precipitation than to other meteorological variables (including lower pressure during the hurricane). Observed soil CO2 failed to exhibit diel patterns (associated with diel temperature fluctuations in drier climates), except during the drier El Niño period. Climate change scenarios showed enhanced soil CO2 due to warmer conditions, while precipitation played a critical role in moderating the balance between concentrations and emissions. The scenario with increased precipitation (based on a regional model projection) led to increases of +11% in soil CO2 concentrations and +4% in soil CO2 emissions. The scenario with decreased precipitation (based on global circulation model projections) resulted in increases of +4% in soil CO2 concentrations and +18% in soil CO2 emissions, and presented more prominent hot moments in soil CO2 outgassing. These findings suggest that soil CO2 will increase under warmer climate in tropical wet forests, and precipitation patterns will define the intensity of CO2 outgassing hot moments.

Research on carbon gas flux to and from inland waters has increased over the past two decades, driven mainly by the need to understand (1) the global carbon budget in regard to stabilizing earth’s climate, and (2) how aquatic ecosystems and the services they provide change in response to anthropogenic pressures like climate and land use change. This paper reviews carbon flux research in support of a proposed global carbon monitoring network to inform public policy. It begins with an overview of the physical–chemical processes and quantification tools for carbon gas flux, and then highlights their application to streams, rivers, lakes, reservoirs and estuaries. Research outcomes to date point to spatiotemporal coverage gaps owing to the complexity of the aquatic ecosystems and land–water interactions, suggesting that long-term monitoring is needed to better understand their signals in response to changes in climate and land management. While better monitoring of gas flux is an important piece of the global carbon budget resolution problem, new information will need to be developed and integrated to adequately inform carbon policymaking. This information can stem from developments in large-scale carbon status and flux assessment tools, such as via remote sensing platforms, and from improved integrated watershed-to-water body modeling efforts.

Leaf-cutter ants are a prominent feature in Neotropical ecosystems, but a comprehensive assessment of their effects on ecosystem functions is lacking. We reviewed the literature and used our own recent findings to identify knowledge gaps and develop a framework to quantify the effects of leaf-cutter ants on ecosystem processes. Leaf-cutter ants disturb the soil structure during nest excavation changing soil aeration and temperature. They mix relatively nutrient-poor soil from deeper layers with the upper organic-rich layers increasing the heterogeneity of carbon and nutrients within nest soils. Leaf-cutter ants account for about 25% of all herbivory in Neotropical forest ecosystems, moving 10%–15% of leaves in their foraging range to their nests. Fungal symbionts transform the fresh, nutrient-rich vegetative material to produce hyphal nodules to feed the ants. Organic material from roots and arbuscular mycorrhizal fungi enhances carbon and nutrient turnover in nest soils and creates biogeochemical hot spots. Breakdown of organic matter, microbial and ant respiration, and nest waste material decomposition result in increased CO2, CH4, and N2O production, but the build-up of gases and heat within the nest is mitigated by the tunnel network ventilation system. Nest ventilation dynamics are challenging to measure without bias, and improved sensor systems would likely solve this problem. Canopy gaps above leaf-cutter ant nests change the light, wind and temperature regimes, which affects ecosystem processes. Nests differ in density and size depending on colony age, forest type and disturbance level and change over time resulting in spatial and temporal changes of ecosystem processes. These characteristics remain a challenge to evaluate rapidly and non-destructively. Addressing the knowledge gaps identified in this synthesis will bring insights into physical and biological processes driving biogeochemical cycles at the nest and ecosystem scale and will improve our understanding of ecosystem biogeochemical heterogeneity and larger scale ecological phenomena. A plain language summary is available for this article.

Global atmospheric methane growth rates have wildly fluctuated over the past three decades, which may be driven by the proportion of tropical land surface saturated by water. The El Niño/Southern Oscillation Event (ENSO) cycle drives large-scale climatic trends globally, with El Niño events typically bringing drier weather than La Niña. In a lowland tropical wet forest in Costa Rica, we measured methane flux bimonthly from March 2016 to June 2017 and using an automated chamber system. We observed a strong drying trend for several weeks during the El Niño in 2016, reducing soil moisture below normal levels. In contrast, soil conditions had high water content prior to the drought and during the moderate La Niña that followed. Soil moisture varied across the period studied and significantly impacted methane flux. Methane consumption was greater during the driest part of the El Niño period, while during La Niña and other time periods, soils had lower methane consumption. The mean methane flux observed was -0.022 mg CH4-C m-2 hr-1, and methane was consumed at all timepoints, with lower consumption in saturated soils. Our data show that month studied, and the correlation between soil type and month significantly drove methane flux trends. Our data indicate that ENSO cycles may impact biogenic methane fluxes, mediated by soil moisture conditions. Climate projections for Central America show dryer conditions and increased El Niño frequency, further exacerbating predicted drought. These trends may lead to negative climate feedbacks, with drier conditions increasing soil methane consumption from the atmosphere.

Catchment hydro-physical controls on the interannual variability of dissolved organic carbon (DOC) in terrestrial watershed runoff, important for water quality, ecosystem structure, and foodweb dynamics, are not well understood. To address this, we simulated water residence time (“age”) and flow path of terrestrial runoff and analyzed their mediating effect on relationships between annual runoff volume, DOC concentration, and DOC age. We applied this analysis to a snow-influenced watershed in California’s Sierra Nevada (USA) across a range of soil types, elevations (90–4210 m), and years (1950–1999). Simulated increases in annual runoff volume were accompanied by younger ages (r2 = 0.53–0.63) of DOC in quickflow, comprised of surface runoff and lateral flow through soil. Increases in annual runoff volume were also accompanied by gentler relationships between intra-annual (weekly) values of DOC concentration and runoff volume, regression-slopes of which followed a power-law relationship to annual runoff (r2 = 0.12–0.92) for approximately 70% of the watershed. Simulations including dynamics of water age and soil temperature produced annual ages of quickflow DOC ranging from 1 to 70 days over all soil types and water years. Similarity of this range to an observed, 1–69 day range in half-lives of relatively labile DOC in previous studies suggests substantial interannual and spatial variability in the biodegradability of DOC in terrestrial runoff. Simulations excluding dynamics of water age and soil temperature predicted order-of-magnitude less interannual variability in age of quickflow DOC, demonstrating the important effect of interannual variability in soil-water interaction times. These findings suggest that the distribution of DOC bioprocessing along transitions between terrestrial and aquatic systems may be strongly influenced by year-to-year variability in age of water.

Leaf-cutter ants are dominant herbivores that disturb the soil and create biogeochemical hot spots. We studied how leaf-cutter ant Atta cephalotes impacts soil CO2 dynamics in a wet Neotropical forest. We measured soil CO2 concentration monthly over 2.5 years at multiple depths in nonnest and nest soils (some of which were abandoned during the study) and assessed CO2 production. We also measured nest and nonnest soil efflux, nest vent efflux, and vent concentration. Nest soils exhibited lower CO2 accumulation than nonnest soils for the same precipitation amounts. During wet periods, soil CO2 concentrations increased across all depths, but were significantly less in nest than in nonnest soils. Differences were nonsignificant during drier periods. Surface efflux was equal across nest and nonnest plots (5 μmol CO2 m-2 s-1), while vent efflux was substantially (103 to 105 times) greater, a finding attributed to free convection and sporadic forced convection. Vent CO2 concentrations were less than in soil, suggesting CO2 efflux from the soil matrix into the nest. Legacy effects in abandoned nests were still observable after more than two years. These findings indicate that leaf-cutter ant nests provide alternative transport pathways to soil CO2 that increase total emissions and decrease soil CO2 concentrations, and have a lasting impact. Estimated total nest-soil CO2 emissions were 15 to 60% more than in nonnest soils, contributing 0.2 to 0.7% to ecosystem-scale soil emissions. The observed CO2 dynamics illuminate the significant carbon footprint of ecosystem engineer Atta cephalotes and have biogeochemical implications for rainforest ecosystems.

In this work, we develop and test proxy-based diagnostic tools for comparing freshwater ecosystem services (FWES) risks across an international array of freshwater ecosystems. FWES threats are increasing rapidly under pressure from population, climate change, pollution, land use change, and other factors. We identified spatially explicit FWES threats estimates (referred to as threat benchmarks) and extracted watershed-specific values for an array of aquatic ecosystems in the Western Hemisphere (Ramsar sites). We compared these benchmark values to values extracted for sites associated with an international FWES threat investigation. The resulting benchmark threats appeared to provide a meaningful context for the diagnostic assessment of study site selection by revealing gaps in coverage of the underlying socio-environmental problem. In an effort to simplify the method, we tested regularly updated environmental and socioeconomic metrics as potential proxies for the benchmark threats using regression analysis. Three category proxies, aggregated from (i) external (global to regional, climate-related), (ii) internal (watershed management-related), and (iii) socioeconomic and governance related proxies produced strong relationships with water supply threat benchmarks, but only weak relationships with biodiversity-related and nutrient regulation benchmark threats. Our results demonstrate the utility of advancing global FWES status and threat benchmarks for organizing coordinated research efforts and prioritizing decisions with regard to international socio-environmental problems.

The US and Mexico share a common history in many areas, including language and culture. They face ecological changes due to the increased frequency and severity of droughts and rising energy demands; trends that entail economic costs for both nations and major implications for human wellbeing. We describe an ongoing effort by the Environment Working Group (EWG), created by The University of California’s UC-Mexico initiative in 2015, to promote binational research, teaching, and outreach collaborations on the implications of climate change for Mexico and California. We synthesize current knowledge about the most pressing issues related to climate change in the US-Mexico border region and provide examples of cross-border discoveries and research initiatives, highlighting the need to move forward in six broad rubrics. This and similar binational cooperation efforts can lead to improved living standards, generate a collaborative mindset among participating universities, and create an international network to address urgent sustainability challenges affecting both countries.

Paleoenvironmental reconstructions are increasingly being used in conservation biology, ecosystem management, and evaluations of ecosystem services (ES), but their potential to contribute to the ES risk assessment process has not been explored. We propose that the long-term history of the ecosystem provides valuable information that augments and strengthens an ES risk assessment and that it should be considered routinely when undertaking risk assessments. We adjusted a standard ecosystem-based risk management (EBRM) protocol to include paleoenvironmental data, and tested the modified approach on two coastal lagoons in South America. Paleolimnological reconstructions in both lagoons indicate that salinity and nutrients (in Laguna de Rocha), and salinity (in Ci\ enaga Grande de Santa Marta), as controlled by hydrologic connectivity with the ocean and freshwater tributaries, have been the key variables behind ecosystem’s function. This understanding, applied to inform various components and steps in the EBRM protocol, suggests that the maintenance of hydrological connections should be a management priority to minimize risk to ES. This work illustrates the utility of including paleoenvironmental data in an EBRM context and highlights the need for a more holistic approach to risk management by incorporating the long-term history of ecosystem function.