Heat-waves with higher intensity and frequency and longer durations are expected in the future due to global warming, which could have dramatic impacts in agriculture, economy and ecology. This field study examined how plant responded to heat-stress (HS) treatment at different timing in naturally occurring vegetation. HS treatment (5 days at 40.5 • C) were applied to 12 1 m 2 plots in restored prairie vegetation dominated by a warm-season C 4 grass, Andropogon gerardii, and a warm-season C 3 forb, Solidago canadensis, at different growing stages. During and after each heat stress (HS) treatment, temperature were monitored for air, canopy, and soil; net CO 2 assimilation (A net ), quantum yield of photosystem II ( PSII ), stomatal conductance (g s ), and internal CO 2 level (C i ), specific leaf area (SLA), and chlorophyll content of the dominant species were measured. One week after the last HS treatment, all plots were harvested and the biomass of above-ground tissue and flower weight of the two dominant species were determined. HS decreased physiological performance and growth for both species, with S. canadensis being affected more than A. gerardii, indicated by negative HS effect on both physiological and growth responses for S. canadensis. There were significant timing effect of HS on the two species, with greater reductions in the net photosynthetic rate and productivity occurred when HS was applied at later-growing season. The reduction in aboveground productivity in S. canadensis but not A. gerardii could have important implications for plant community structure by increasing the competitive advantage of A. gerardii in this grassland. The present experiment showed that HS, though ephemeral, may promote long-term effects on plant community structure, vegetation dynamics, biodiversity, and ecosystem functioning of terrestrial biomes when more frequent and severe HS occur in the future.Keywords: global climate change, photosynthesis, aboveground productivity, Solidago canadensis, Andropogon gerardii Abbreviations: A net , net photosynthetic rate (µmol m −2 s −1 ); ANPP, Aboveground net primary production (g); g s , stomatal conductance to water vapor (mol m −2 s −1 ); HS, heat stress; LAI, leaf area index (m 2 m −2 ); LWC, (leaf water content, %); SLA, specific leaf area (m 2 kg −1 ); W a , aboveground biomass (g); W f , biomass of flowers (g); i WUE, intrinsic water use efficiency (the ratio of A net to g s) ; PSII , quantum yield of electron transport of photoystem II.
[1] Estuarine and coastal wetlands exhibit high rates of carbon burial and storage in anaerobic sediments, but the extent to which carbon sequestration is offset by methane (CH 4 ) emissions from these ecosystems remains unclear. In this study we combine measurements of sediment-air CH 4 fluxes with monitoring of belowground CH 4 pools in a New Jersey tidal marsh in order to clarify mechanistic links between environmental drivers, subsurface dynamics, and atmospheric emissions. Measurements were conducted in an unvegetated mud flat and adjacent low marsh vegetated with Spartina alterniflora and Phragmites australis. Pore water measurements throughout the year revealed long-term CH 4 storage in mud flat sediments, leading to a seasonal lag in emissions that extended into winter months. CH 4 reservoirs and fluxes in vegetated sediments were well described by an empirical temperature-response model, while poor model agreement in unvegetated sediments was attributed to decouplings between production and flux due to storage processes. This study highlights the need to incorporate sediment gas exchange rates and pathways into biogeochemical process models.Citation: Reid, M. C., R. Tripathee, K. V. R. Schäfer, and P. R. Jaffé (2013), Tidal marsh methane dynamics: Difference in seasonal lags in emissions driven by storage in vegetated versus unvegetated sediments,
Net ecosystem exchange (NEE) of tidal brackish wetlands in urban areas is largely unknown, albeit it is an important ecosystem service. High carbon dioxide (CO 2 ) uptake of estuaries can potentially be achieved by creating conditions that foster CO 2 uptake and sequestration. Thus, this study sought to assess NEE in a mesohaline tidal urban wetland that has been restored and determine the biophysical drivers of NEE in order to investigate uptake strength and drivers thereof. Beginning in 2009, NEE was measured using the eddy covariance technique in a restored urban estuarine wetland. Maximum NEE rates observed were À30 μmol m À2 s À1 under high light conditions in the summer. Monthly mean NEE showed this ecosystem to be a CO 2 source in the winter, but a CO 2 sink in summer. Conditional Granger causality showed the influence of net radiation on half daily to biweekly timescales on NEE and the influence of water level at half daily time scales. The overall productivity of this wetland is within the expected range of tidal brackish marshes and it was a sink for atmospheric CO 2 in two out of the 3 years of this study and had a continued increase over the study period.
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