Description: [1]  Northern inland waters emit CO 2 and CH 4 to the atmosphere but the importance of these emissions is poorly understood due to a lack of integrated catchment scale estimates of carbon (C) emissions from lakes and streams. In this study we quantified the annual emission of CO 2 and CH 4 from 27 lakes and 23 stream segments in a 15 km 2 subarctic catchment in northern Sweden. All lakes and streams were net sources of C to the atmosphere on an annual basis. Streams dominated (96%) the aquatic CO 2 emission while lakes (61%) dominated the aquatic CH 4 emission. Total aquatic C emission from the catchment was estimated to be 9.1 g C m -2  yr -1 (98% as CO 2 ). Although streams only accounted for 4% of the aquatic area in the catchment, they accounted for 95% of the total emission. The C emissions from lakes and streams were considerably larger than previously reported downstream waterborne export of C from the catchment indicating that that the atmospheric losses of C in the aquatic systems is an important component in the catchment C balance.

Description: Vegetation cover in dry regions is a key variable determining desertification. Soils exposed to rainfall by desertification can form physical crusts that reduce infiltration, exacerbating water stress on the remaining vegetation. Paradoxically, field studies show that crust removal is associated with plant mortality in desert systems, while artificial biological crusts can improve plant regeneration. Here, it is shown how physical crusts can act as either drivers of, or buffers against desertification depending on their environmental context. The behavior of crusts is first explored using a simplified theory for water movement on a uniform, partly vegetated slope subject to stationary hydrologic conditions. Numerical model runs supplemented with field data from a semiarid Long-Term Ecological Research (LTER) site are then applied to represent more realistic environmental conditions. When vegetation cover is significant, crusts can drive desertification, but this process is potentially self-limiting. For low vegetation cover, crusts mitigate against desertification by providing water subsidy to plant communities through a runoff-runon mechanism.

Description: [1]  The Little River (LR) in southern Georgia, USA has experienced lengthening droughts since monitoring began in 1972. We evaluated the impacts of drought on riverine carbon cycling using a nine-year dataset of dissolved organic carbon (DOC) coupled with laboratory experiments in the LR, as well as long-term datasets in three additional rivers within the Suwannee River basin. Longer drought periods reduced downstream DOC export, but also led to higher DOC concentrations in the following hydroperiod. Within a hydroperiod, DOC concentration was positively correlated with temperature and negatively correlated with river discharge, but also negatively correlated with total discharge during the previous hydroperiod. Among hydroperiods, DOC concentration was more strongly correlated with antecedent hydrological conditions than with current hydrological conditions across broad spatial scales: in three additional rivers within the Suwannee River basin (Alapaha, Withlacoochee, Okapilco), and in headwater and downstream reaches of the LR. Microbial DOC consumption and CO 2 production were elevated when DOC concentration was high. Despite dramatic hydrologic changes, DOC composition appeared stable, with optical analyses suggesting that LR DOC is primarily composed of three terrestrial humic-like fluorescence groups. If the current climatic trend of intensifying droughts, elevated temperatures, and decreased discharge continue, our results suggest the net effect may be for a more localized riverine carbon cycle with reduced downstream transport of DOC, but higher local mineralization rates due to elevated DOC concentrations.

Description: Climate warming is expected to lengthen growing seasons of temperate forest ecosystems and increase gross primary productivity. Simultaneously, warming is expected to increase summer ecosystem respiration, which could offset gains accrued from longer growing seasons. These responses have been observed during anomalously warm years, but the role of future climate change on phenological tradeoffs and how they affect net primary productivity (NPP) at regional scales in temperate forests remains unexplored. We simulated scenarios of climate change on monthly forest NPP throughout 18 million hectares of temperate forests in New England, USA through year 2100. Using an ecophysiological model coupled to a forest landscape model, we simulated scenarios of climate change on monthly NPP. A high emission scenario (RCP 8.5), resulted in longer growing seasons that offset mid-summer ecosystem respiration costs and produced greater annual NPP throughout the study landscape compared to simulations using the current climate. In spring and autumn months, temperature was positively associated with greater NPP; in summer months, the relationship was negative. Spatially, the greatest increase in NPP occurred in the warmer southern region under a warm climate scenario with increased precipitation. Under a warm scenario with drier conditions, the greatest increase in NPP occurred in the cooler northern region. Phenological tradeoffs will affect NPP of future forests and their potential to serve as a negative feedback to climate change. Barring other limitations, longer growing seasons will offset greater respiratory demands and contribute to increases in NPP throughout the temperate forests of New England in the future.

Description: Wildfire represents the single largest disturbance to the ecohydrological function of northern peatlands. Alterations to peatland thermal behavior as a result of wildfire will modify the carbon balance of these important long-term global carbon stores and regulate post-fire ecosystem recovery. We simulate the 3-D thermal behavior of a peatland that has been disturbed by wildfire to identify how changes in peat temperatures emerge from changes to the surface energy balance and peat thermal properties. Peat temperatures are simulated within two adjacent peatlands, one area having burned 4 years previously, the second which has been wildfire-free for 75 years. We demonstrate that there is only a small alteration to the thermal response in Sphagnum fuscum hummocks that are not severely burnt within the wildfire. In contrast, wildfire produces important changes to the energy balance of Sphagnum hollows. A large reduction in the latent heat flux post-fire increases surface temperatures by up to 30°C during daytime summer conditions. However, temperatures through the peat profile are insensitive to these increases in surface temperature. The low surface moisture content of near-surface peat insulates the profile from these higher temperatures and, at depths below 0.015 m, only small differences are identifiable between burned and unburned hollow temperatures. Nevertheless, we argue that these alterations to near-surface temperatures and evaporation rates likely substantially influence the thermal and hydrological conditions post-wildfire, impacting the peatland recovery.

Description: [1]  Forested peatlands represent an important global carbon pool, storing 48.0 Pg of carbon within continental western Canada alone. Peatland hydrology regulates the carbon dynamics and future stability of this carbon store and provides a critical control on regional water dynamics. Drying associated with land-use change and climate change has the potential to increase tree growth, modifying the density, size and spatial arrangement of trees. This can reduce peatland evaporation and offset the associated increase in transpiration. To determine the magnitude of this negative ecohydrological feedback, we simulated spatial variations in radiation, turbulent energy fluxes and temperatures in peatlands with real and idealized tree densities and distributions. For a random tree distribution, an increase in tree density from 0 to 4 trees per m 2 reduced available energy at the peat surface, decreasing average evaporation by 25%. At higher tree densities, feather moss species covered a larger fraction of the ground because of lower light availability. In combination with the lower energy availability, this change in moss composition reduced evaporation by ~70%. The reduction in evaporation was greater (83%) when the effects of increased canopy cover on peatland aerodynamic properties were incorporated. Additionally, we found that evaporation was dependent on the spatial arrangement of trees, with evaporation being higher when trees were clustered. Overall, our model showed that the trade-off between reduced evaporation and increased transpiration with increasing tree densities reduced landscape variation in evapotranspiration, with simulated evapotranspiration remaining approximately constant across a broad range of peatland ecosystems despite varying canopy densities.