Paulo C. Olivas

Increased CO2 loss from vegetated drained lake tundra ecosystems due to flooding

Tundra ecosystems are especially sensitive to climate change, which is particularly rapid in high northern latitudes resulting in significant alterations in temperature and soil moisture. Numerous studies have demonstrated that soil drying increases the respiration loss from wet Arctic tundra. And, warming and drying of tundra soils are assumed to increase CO 2 emissions from the Arctic. However, in this water table manipulation experiment (i.e., flooding experiment), we show that flooding of wet tundra can also lead to increased CO 2 loss. Standing water increased heat conduction into the soil, leading to higher soil temperature, deeper thaw and, surprisingly, to higher CO 2 loss in the most anaerobic of the experimental areas. The study site is located in a drained lake basin, and the soils are characterized by wetter conditions than upland tundra. In experimentally flooded areas, high wind speeds (greater than ~4 m s −1 ) increased CO 2 emission rates, sometimes overwhelming the photosynthetic uptake, even during daytime. This suggests that CO 2 efflux from C rich soils and surface waters can be limited by surface exchange processes. The comparison of the CO 2 and CH 4 emission in an anaerobic soil incubation experiment showed that in this ecosystem, CO 2 production is an order of magnitude higher than CH 4 production. Future increases in surface water ponding, linked to surface subsidence and thermokarst erosion, and concomitant increases in soil warming, can increase net C efflux from these arctic ecosystems.

Effects of Fine-Scale Topography on CO2 Flux Components of Alaskan Coastal Plain Tundra: Response to Contrasting Growing Seasons

Abstract Arctic regions hold considerable reservoirs of soil organic carbon. However, most of this carbon is in a potential labile state, and expected changes in temperature and water availability could strongly affect the carbon balance of tundra ecosystems. Plant community composition and soil carbon are closely tied to microtopography and position relative to the water table. We evaluated CO2 fluxes and moss contribution to ecosystem photosynthesis in response to fine-scale topography across a drained lake bed in Barrow, Alaska, during two contrasting growing seasons. CO2 exchange was assessed through static chamber measurements in three vegetation classes distinguished by plant dominance and topographic position within low-centered polygons. Gross primary production (GPP) and ecosystem respiration (ER) were the lowest under high soil moisture conditions in 2006. ER responded more strongly to wet conditions, resulting in a larger summer sink in 2006 than in 2005 (64 vs. 17g CO2 m−2, respectively). Microsites responded differently to contrasting weather conditions. Low elevation microsites presented a strong reduction in ER as a result of increased water availability. A maximum of 48% of daytime GPP and 33% of seasonal daytime GPP was contributed by moss on average across microtopographic positions. The interaction between fine-scale microtopography and variation in temperature and water availability can result in considerable differences in CO2 sink activity of the polygonal tundra.

Seasonal patterns in energy partitioning of two freshwater marsh ecosystems in the Florida Everglades

We analyzed energy partitioning in short- and long-hydroperiod freshwater marsh ecosystems in the Florida Everglades by examining energy balance components (eddy covariance derived latent energy (LE) and sensible heat (H) flux). The study period included several wet and dry seasons and variable water levels, allowing us to gain better mechanistic information about the control of and changes in marsh hydroperiods. The annual length of inundation is ~5 months at the short-hydroperiod site (25°26′16.5″N, 80°35′40.68″W), whereas the long-hydroperiod site (25°33′6.72″N, 80°46′57.36″W) is inundated for ~12 months annually due to differences in elevation and exposure to surface flow. In the Everglades, surface fluxes feed back to wet season precipitation and affect the magnitude of seasonal change in water levels through water loss as LE (evapotranspiration (ET)). At both sites, annual precipitation was higher than ET (1304 versus 1008 at the short-hydroperiod site and 1207 versus 1115 mm yr−1 at the long-hydroperiod site), though there were seasonal differences in the ratio of ET:precipitation. Results also show that energy balance closure was within the range found at other wetland sites (60 to 80%) and was lower when sites were inundated (60 to 70%). Patterns in energy partitioning covaried with hydroperiods and climate, suggesting that shifts in any of these components could disrupt current water and biogeochemical cycles throughout the Everglades region. These results suggest that the complex relationships between hydroperiods, energy exchange, and climate are important for creating conditions sufficient to maintain Everglades ecosystems.

El Niño Southern Oscillation (ENSO) Enhances CO2 Exchange Rates in Freshwater Marsh Ecosystems in the Florida Everglades

This research examines the relationships between El Niño Southern Oscillation (ENSO), water level, precipitation patterns and carbon dioxide (CO2) exchange rates in the freshwater wetland ecosystems of the Florida Everglades. Data was obtained over a 5-year study period (2009–2013) from two freshwater marsh sites located in Everglades National Park that differ in hydrology. At the short-hydroperiod site (Taylor Slough; TS) and the long-hydroperiod site (Shark River Slough; SRS) fluctuations in precipitation patterns occurred with changes in ENSO phase, suggesting that extreme ENSO phases alter Everglades hydrology which is known to have a substantial influence on ecosystem carbon dynamics. Variations in both ENSO phase and annual net CO2 exchange rates co-occurred with changes in wet and dry season length and intensity. Combined with site-specific seasonality in CO2 exchanges rates, El Niño and La Niña phases magnified season intensity and CO2 exchange rates at both sites. At TS, net CO2 uptake rates were higher in the dry season, whereas SRS had greater rates of carbon sequestration during the wet season. As La Niña phases were concurrent with drought years and extended dry seasons, TS became a greater sink for CO2 on an annual basis (−11 to −110 g CO2 m−2 yr−1) compared to El Niño and neutral years (−5 to −43.5 g CO2 m−2 yr−1). SRS was a small source for CO2 annually (1.81 to 80 g CO2 m−2 yr−1) except in one exceptionally wet year that was associated with an El Niño phase (−16 g CO2 m−2 yr−1). Considering that future climate predictions suggest a higher frequency and intensity in El Niño and La Niña phases, these results indicate that changes in extreme ENSO phases will significantly alter CO2 dynamics in the Florida Everglades.

Water uptake of Alaskan tundra evergreens during the winter–spring transition

PREMISE OF THE STUDY The cold season in the Arctic extends over 8 to 9 mo, yet little is known about vascular plant physiology during this period. Evergreen species photosynthesize under the snow, implying that they are exchanging water with the atmosphere. However, liquid water available for plant uptake may be limited at this time. The study objective was to determine whether evergreen plants are actively taking up water while under snow and/or immediately following snowmelt during spring thaw. METHODS In two in situ experiments, one at the plot level and another at the individual species level, (2)H-labeled water was used as a tracer injected beneath the snow, after which plant stems and leaves were tested for the presence of the label. In separate experiments, excised shoots of evergreen species were exposed to (2)H-labeled water for ∼5 s or 60 min and tested for foliar uptake of the label. KEY RESULTS In both the plot-level and the species-level experiments, some (2)H-labeled water was found in leaves and stems. Additionally, excised individual plant shoots exposed to labeled water for 60 min took up significantly more (2)H-label than shoots exposed ∼5 s. CONCLUSIONS Evergreen tundra plants take up water under snow cover, some via roots, but also likely by foliar uptake. The ability to take up water in the subnivean environment allows evergreen tundra plants to take advantage of mild spring conditions under the snow and replenish carbon lost by winter respiration.

Ecosystem resistance in the face of climate change: a case study from the freshwater marshes of the Florida Everglades

Shaped by the hydrology of the Kissimmee-Okeechobee-Everglades watershed, the Florida Everglades is composed of a conglomerate of wetland ecosystems that have varying capacities to sequester and store carbon. Hydrology, which is a product of the regions precipitation and temperature patterns combined with water management policy, drives community composition and productivity. As shifts in both precipitation and air temperature are expected over the next 100 years as a consequence of climate change, CO2 dynamics in the greater Everglades are expected to change. To reduce uncertainties associated with climate change and to explore how projected changes in atmospheric CO2 concentration and climate can alter current CO2 exchange rates in Everglades freshwater marsh ecosystems, we simulated fluxes of carbon among the atmosphere, vegetation, and soil using the DAYCENT model. We explored the effects of low, moderate, and high scenarios for atmospheric CO2 (550, 850, and 950 ppm), mean annual air temperature (+1...