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Abstract Inland waters emit large amounts of carbon and are key players in the global carbon budget. Particularly high rates of carbon emissions have been reported in streams draining mountains, tropical regions, and peatlands. However, few studies have examined the spatial variability of CO₂concentrations and fluxes occurring within these systems, particularly as a function of catchment morphology. Here we evaluated spatial patterns of CO₂in three tropical, headwater catchments in relation to the river network and stream geomorphology. We measured dissolved carbon dioxide (pCO₂), aquatic CO₂emissions, discharge, and stream depth and width at high spatial resolutions along multiple stream reaches. Confirming previous studies, we found that tropical headwater streams are an important source of CO₂to the atmosphere. More notably, we found marked, predictable spatial organization in aquatic carbon fluxes as a function of landscape position. For example,pCO₂was consistently high (>10,000 ppm) at locations close to groundwater sources and just downstream of hydrologically connected wetlands, but consistently low (<1,000 ppm) in high gradient locations or river segments with larger drainage areas. Taken together, our findings suggest that catchment area and stream slope are important drivers ofpCO₂and gas transfer velocity (k) in mountainous streams, and as such they should be considered in catchment‐scale assessments of CO₂emissions. Furthermore, our work suggests that accurate estimation of CO₂emissions requires understanding of dynamics across the entire stream network, from the smallest seeps to larger streams. 

Abstract Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO₂) to the atmosphere by transforming terrestrial carbon to gaseous CO₂. Studies of these environments are scarce, and estimates of CO₂fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO₂evasion and other biogeochemical processes. The first method, kinematick₆₀₀(k_600‐K), is derived from an empirical relationship between temperature‐adjustedk(k₆₀₀) and the physical characteristics of the stream. The second method, measuredk₆₀₀(k_600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO₂(pCO₂) and CO₂evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability ink₆₀₀at small spatial scales. Values ofk_600‐Kranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values ofk_600‐Mranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability ink₆₀₀was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates ofk₆₀₀values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams. 

Species interactions have long been predicted to increase in intensity toward the tropics and low elevations because of gradients in climate, productivity, or biodiversity. Despite their importance for understanding global ecological and evolutionary processes, plant-animal interaction gradients are particularly difficult to test systematically across large geographic gradients, and evidence from smaller, disparate studies is inconclusive. By systematically measuring postdispersal seed predation using 6995 standardized seed depots along 18 mountains in the Pacific cordillera, we found that seed predation increases by 17% from the Arctic to the Equator and by 17% from 4000 meters above sea level to sea level. Clines in total predation, likely driven by invertebrates, were consistent across treeline ecotones and within continuous forest and were better explained by climate seasonality than by productivity, biodiversity, or latitude. These results suggest that species interactions play predictably greater ecological and evolutionary roles in tropical, lowland, and other less seasonal ecosystems. 

Abstract High‐altitude tropical grasslands, known as “páramos,” are characterized by high solar radiation, high precipitation, and low temperature. They also exhibit some of the highest ecosystem carbon stocks per unit area on Earth. Recent observations have shown that páramos may be a net source of CO₂to the atmosphere as a result of climate change; however, little is known about the source of this excess CO₂in these mountainous environments or which landscape components contribute the most CO₂. We evaluated the spatial and temporal variability of surface CO₂fluxes to the atmosphere from adjacent terrestrial and aquatic environments in a high‐altitude catchment of Ecuador, based on a suite of field measurements performed during the wet season. Our findings revealed the importance of hydrologic dynamics in regulating the magnitude and likely fate of dissolved carbon in the stream. While headwater catchments are known to contribute disproportionately larger amounts of carbon to the atmosphere than their downstream counterparts, our study highlights the spatial heterogeneity of CO₂fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Our findings revealed that CO₂evasion from stream surfaces was up to an order of magnitude greater than soil CO₂efflux from the adjacent terrestrial environment. Stream carbon flux to the atmosphere appeared to be transport limited (i.e., controlled by flow characteristics, turbulent flow, and water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by CO₂and carbon availability) in the lower reaches of the stream. A 4‐m waterfall along the channel accounted for up to 35% of the total evasion observed along a 250‐m stream reach. These findings represent a first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high‐altitude, tropical, headwater catchments.