Timothy H. Morin

Species‐specific transpiration responses to intermediate disturbance in a northern hardwood forest

Intermediate disturbances shape forest structure and composition, which may in turn alter carbon, nitrogen, and water cycling. We used a large-scale experiment in a forest in northern lower Michigan where we prescribed an intermediate disturbance by stem girdling all canopy-dominant early successional trees to simulate an accelerated age-related senescence associated with natural succession. Using 3 years of eddy covariance and sap flux measurements in the disturbed area and an adjacent control plot, we analyzed disturbance-induced changes to plot level and species-specific transpiration and stomatal conductance. We found transpiration to be ~15% lower in disturbed plots than in unmanipulated control plots. However, species-specific responses to changes in microclimate varied. While red oak and white pine showed increases in stomatal conductance during postdisturbance (62.5 and 132.2%, respectively), red maple reduced stomatal conductance by 36.8%. We used the hysteresis between sap flux and vapor pressure deficit to quantify diurnal hydraulic stress incurred by each species in both plots. Red oak, a ring porous anisohydric species, demonstrated the largest mean relative hysteresis, while red maple, bigtooth aspen, and paper birch, all diffuse porous species, had the lowest relative hysteresis. We employed the Penman-Monteith model for LE to demonstrate that these species-specific responses to disturbance are not well captured using current modeling strategies and that accounting for changes to leaf area index and plot microclimate are insufficient to fully describe the effects of disturbance on transpiration.

Environmental drivers of methane fluxes from an urban temperate wetland park

Methane (CH4) emissions were measured at the Wilma H. Schiermeier Olentangy River Wetland Research Park (ORWRP) over three summers and two winters using an eddy covariance system. We used an empirical model to determine the main environmental drivers of methane emissions. Methane emissions covary strongly with water vapor fluxes, CO2 fluxes, and soil temperature. We adjust our models to account for the heterogeneous environment of the wetland by including the flux footprint distribution among different microsites as a predictive variable in the methane model. We used a forward linear stepwise model in combination with an Akaike information criteria-based model selection process and neural network modeling to determine which environmental variables are most effective in modeling methane emissions in our site. Different models and environmental variables best represented methane fluxes in the winter and summer and also during the day or night within each season. We parameterized an optimal empirical model for methane emissions from the ORWRP that is used for gap filling of site-level methane fluxes over 2 years. Some of the most effective variables for modeling methane were carbon, water vapor, and heat fluxes, all of which typically have the same data gaps as the time series of methane flux. In order to determine if these variables were useful for modeling methane despite the additional gap-filling error, we determined through an error propagation experiment that eddy covariance gap-filling models for methane may be best developed by including other gap-filled fluxes as predictors, despite the high level of shared gaps and subsequent gap-fill error propagation.

Observations of stem water storage in trees of opposing hydraulic strategies

Hydraulic capacitance and water storage form a critical buffer against cavitation and loss of conductivity within the xylem system. Withdrawal from water storage in leaves, branches, stems, and roots significantly impacts sap flow, stomatal conductance, and transpiration. Storage quantities differ based on soil water availability, tree size, wood anatomy and density, drought tolerance, and hydraulic strategy (anisohydric or isohydric). However, the majority of studies focus on the measurement of storage in conifers or tropical tree species. We demonstrate a novel methodology using frequency domain reflectometry (FDR) to make continuous, direct measurements of wood water content in two hardwood species in a forest in Michigan. We present results of a two month study comparing the water storage dynamics between a mature red oak and red maple, two species with differing wood densities, hydraulic architecture, and hydraulic strategy. We also include results pertaining to the use of different probe lengths to ...

Carbon dioxide fluxes of an urban tidal marsh in the Hudson‐Raritan estuary

Net ecosystem exchange (NEE) of tidal brackish wetlands in urban areas is largely unknown, albeit it is an important ecosystem service. High carbon dioxide (CO2) uptake of estuaries can potentially be achieved by creating conditions that foster CO2 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 −30 µmol m−2 s−1 under high light conditions in the summer. Monthly mean NEE showed this ecosystem to be a CO2 source in the winter, but a CO2 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 CO2 in two out of the 3 years of this study and had a continued increase over the study period.

Contrasting strategies of hydraulic control in two codominant temperate tree species

Biophysical controls on plant water status exist at the leaf, stem, and root levels. Therefore, we pose that hydraulic strategy is a combination of traits governing water use at each of these three levels. We studied sap flux, stem water storage, stomatal conductance, photosynthesis, and growth of red oaks (Q. rubra) and red maples (A. rubrum). These species differ in stomatal hydraulic strategy, xylem architecture, and may root at different depths. Stable isotope analysis of xylem water was used to identify root-water uptake depth. Oaks were shown to access a deeper water source than maples. During non-limiting soil moisture conditions, transpiration was greater in maples than oaks. However, during a soil dry down, transpiration and stem water storage decreased by more than 80% and 28% in maples, but only by 31% and 1% in oaks. We suggest that the preferential use of deep water by red oaks allows the species to continue transpiration and growth during soil water limitations. In this case, deeper roots may provide a buffer against drought-induced mortality. Using 14 years of growth data, we show that maple growth correlates with mean annual soil moisture at 30 cm, but oak growth does not. The observed responses of oak and maple to drought were not able to be explained by leaf and xylem physiology alone. We employed the FETCH2 plant-hydrodynamics model to demonstrate the influence of root, stem, and leaf controls on tree-level transpiration. We conclude that all three levels of hydraulic traits are required to define hydraulic strategy.

Methanogenesis in oxygenated soils is a substantial fraction of wetland methane emissions

The current paradigm, widely incorporated in soil biogeochemical models, is that microbial methanogenesis can only occur in anoxic habitats. In contrast, here we show clear geochemical and biological evidence for methane production in well-oxygenated soils of a freshwater wetland. A comparison of oxic to anoxic soils reveal up to ten times greater methane production and nine times more methanogenesis activity in oxygenated soils. Metagenomic and metatranscriptomic sequencing recover the first near-complete genomes for a novel methanogen species, and show acetoclastic production from this organism was the dominant methanogenesis pathway in oxygenated soils. This organism, Candidatus Methanothrix paradoxum, is prevalent across methane emitting ecosystems, suggesting a global significance. Moreover, in this wetland, we estimate that up to 80% of methane fluxes could be attributed to methanogenesis in oxygenated soils. Together, our findings challenge a widely held assumption about methanogenesis, with significant ramifications for global methane estimates and Earth system modeling.Methane production is traditionally not found in oxygenated soils, a paradigm incorporated in global greenhouse gas modelling efforts. Here the authors show geochemical and biological evidence of active methanogenesis in bulk-oxic wetland soils, attributing up to 80% of the total methane budget for the site.

A Numerical Case Study of the Implications of Secondary Circulations to the Interpretation of Eddy-Covariance Measurements Over Small Lakes

We use a large-eddy simulation (LES) to study the airflow patterns associated with a small inland lake surrounded by a forest of height one-tenth the radius of the lake. We combine LES results with scalar dispersion simulations to model potential biases in eddy-covariance measurements due to the heterogeneity of surface fluxes and vertical advection. The lake-to-forest transition can induce a non-zero vertical velocity component, affecting the interpretation of flux measurements. Significant horizontal gradients of mean

The seasonal and diurnal dynamics of methane flux at a created urban wetland

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