Biological Sciences

Ecosystems are open systems that exchange matter and energy with their environment. They differ in their efficiency in doing so as a result of their location on Earth, structure and disturbance, including anthropogenic legacy. Entropy has been proposed to be an effective metric to describe these differences as it relates energy use efficien-cies of ecosystems to their thermodynamic environment (i.e., temperature) but has rarely been studied to understand how ecosystems with different disturbance legacies respond when confronted with environmental variability. We studied three sites in a longleaf pine ecosystem with varying levels of anthropogenic legacy and plant functional diversity, all of which were exposed to extreme drought. We quantified ra-diative (eff rad), metabolic and overall entropy changes-as well as changes in exported to imported entropy (eff flux) in response to drought disturbance and environmental variability using 24 total years of eddy covariance data (8 years per site). We show that structural and functional characteristics contribute to differences in energy use efficiencies at the three study sites. Our results demonstrate that ecosystem function during drought is modulated by decreased absorbed solar energy and variation in the partitioning of energy and entropy exports owing to differences in site enhanced vegetation index and/or soil water content. Low eff rad and metabolic entropy as well as slow adjustment of eff flux at the anthropogenically altered site prolonged its recovery from drought by approximately 1 year. In contrast, stands with greater plant functional diversity (i.e., the ones that included both C 3 and C 4 species) adjusted their entropy exports when faced with drought, which accelerated their recovery. Our study provides a path forward for using entropy to determine ecosystem function across different global ecosystems.

Dairy farms are predominantly carbon sources, due to high livestock emissions from enteric fermentation and manure. Integrated crop–livestock systems (ICLSs) have the potential to offset these greenhouse gas (GHG) emissions, as recycling products within the farm boundaries is prioritized. Here, we quantify seasonal and annual greenhouse gas budgets of an ICLS dairy farm in Wisconsin USA using satellite remote sensing to estimate vegetation net primary productivity (NPP) and Intergovernmental Panel on Climate Change (IPCC) guidelines to calculate farm emissions. Remotely sensed annual vegetation NPP correlated well with farm harvest NPP (R2 = 0.9). As a whole, the farm was a large carbon sink, owing to natural vegetation carbon sinks and harvest products staying within the farm boundaries. Dairy cows accounted for 80% of all emissions as their feed intake dominated farm feed supply. Manure emissions (15%) were low because manure spreading was frequent throughout the year. In combinat...

ABSTRACT Background/Question/Methods The frequency and magnitude of fire determines structure and regulates function of savanna ecosystems worldwide, yet our understanding of prescribed fire impacts on carbon dynamics in these systems is rudimentary and becomes more uncertain with changing soil water availability. While we understand that fires occur cyclically and return at frequencies based on fire regime, the impact that fire has on carbon dynamics has often been viewed as a single event. We combined prescribed fire, eddy covariance (EC) techniques and fuel consumption plots over an ongoing study which currently includes 6 years of EC measurements and 3 dormant season burn cycles to examine the cyclic response of fire on longleaf pine forest carbon dynamics along an edaphic gradient (mesic, intermediate and xeric sites), in southwestern Georgia, USA. We estimated linear and nonlinear models, as well as used time series methods new to the EC community, to quantify drivers of carbon dynamics in these systems while addressing the questions: 1) How does fire influence net ecosystem exchange (NEE), gross ecosystem exchange (GEE), and ecosystem respiration (Reco), and how long do these variables take to recover post-fire? 2) Do environmental conditions or prescribed fire have a greater influence on these ecosystems’ carbon dynamics? Results/Conclusions We determined from this study that our mesic site was a moderate sink of carbon, while the intermediate and xeric sites were carbon neutral during the course of this study. When loss associated with fire was combined with NEE rates, all sites became moderate carbon sources in the years that included a burn (2009, 2011 and 2013). Analyses of assimilation and respiration parameters (e.g., maximum photosynthesis, quantum efficiency, and daytime ecosystem respiration) showed a positive trend pre-fire and a negative trend post-fire for maximum ecosystem CO2 uptake rates, and the opposite relationship for daytime ecosystem respiration rates. Within 30 to 60 days following fire, ecosystem physiological activity was statistically similar to pre-fire and appeared to be driven by the pine canopy. Our results suggest that prescribed fire (low intensity, high frequency) does not re-set this ecosystem through its removal of litter and understory, but rather maintains the existing structure and function (in this case, carbon flux rates) because longleaf pine ecosystems have evolved with fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), environmental variation over the study was more important in determining the carbon dynamics of the longleaf pine ecosystem.

ABSTRACT Background/Question/Methods Subtropical longleaf pine (Pinus palustris) savannas are important ecosystems, in terms of their biodiversity (> 40 species per m2) as well as ecosystem services, such as carbon sequestration. The structure and function of these ecosystems are maintained with frequent low intensity fires. However, the role that cyclic fire plays in soil respiration is relatively unknown. Soil respiration is a major component in the global carbon cycle and greatly dependent on air temperature and soil moisture. Approximately half of the soil respiration is attributed to metabolic activity in plants, supporting root growth. We collected soil respiration measurements in longleaf pine savannas in southwestern Georgia, USA, over a four-year period, to increase our understanding of carbon dynamics in these ecosystems. The study included monthly observations of soil respiration (Fsoil), air temperature (Tair) and soil moisture (VWC), over three fire cycles, at three sites with differing water holding capacities from 2009 to 2013. We used mixed model approaches to answer the following research questions: 1) How does Fsoil contribute to the net ecosystem exchange (NEE) of longleaf pine savannas? 2) How does VWC affect Fsoil in longleaf pine ecosystems? 3) Does seasonal fire alter Fsoil rates at the three sites? Results/Conclusions Our results show that Fsoil accounts for approximately 60% of NEE at all sites. All sites vary significantly in terms of VWC, ranging on average from 7% at the xeric site to 11% at the mesic site, with higher variability at the mesic and lower variation at the xeric site. Our mixed model indicated that Fsoil was significantly different by site and year, and varied with VWC and Tair (α = 0.05). We found that lower VWC resulted in higher Fsoil, and that Fsoil was higher following drought years (2012). However, lower VWC at the xeric site did not cause higher Fsoil rates, in contrast to the mesic site. Whereas Tair was positively correlated with Fsoil, VWC showed negative correlation with Fsoil. We did not find significant effects of seasonal burn on Fsoil. We conclude that Fsoil largely depends on the prevailing weather conditions and soil texture. Hence, drier soil conditions as one consequence of global climate change may lead to increased Fsoil in these ecosystems and therefore alter NEE.

The southeastern US has experienced an increase in the number of extreme heat events since the 1970s, due in part to global change. Despite rising temperatures, greater variability in weather has also led to more freeze events across parts of the Southeast, particularly during El Nino winters. Structural variation in forest stands and plant functional diversity can lead to localized micrometeorological differences that may alter the recovery of different forests from these temperature-induced disturbances. This variation can lead to differences in energy and entropy dynamics, which drive the metabolic response of these systems. Using thermodynamic metrics, we quantified metabolic energy and entropy production in response to extreme heat and freeze events at three longleaf pine savanna sites spanning an edaphic moisture gradient (i.e., xeric, intermediate, mesic). The sites also differed in anthropogenic legacy, with soil tillage occurring at the intermediate site, resulting in great...

Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenologica...

Biogenic volatile organic compounds (BVOCs) play critical roles in ecological and earth-system processes. Ecosystem BVOC models rarely include soil and litter fluxes and their accuracy is often challenged by BVOC dynamics during periods of rapid ecosystem change like spring leaf out. We measured BVOC concentrations within the air space of a mixed deciduous forest and used a hybrid Lagrangian/Eulerian canopy transport model to estimate BVOC flux from the forest floor, canopy, and whole ecosystem during spring. Canopy flux measurements were dominated by a large methanol source and small isoprene source during the leaf-out period, consistent with past measurements of leaf ontogeny and theory, and indicative of a BVOC flux situation rarely used in emissions model testing. The contribution of the forest floor to whole-ecosystem BVOC flux is conditional on the compound of interest and is often non-trivial. We created linear models of forest floor, canopy, and whole-ecosystem flux for each study compound and used information criteria-based model selection to find the simplest model with the best fit. Most published BVOC flux models do not include vapor pressure deficit (VPD), but it entered the best canopy, forest floor, and whole-ecosystem BVOC flux model more than any other study variable in the present study. Since VPD is predicted to increase in the future, future studies should investigate how it contributes to BVOC flux through biophysical mechanisms like evaporative demand, leaf temperature and stomatal function.

Globally, soil CO2 efflux rates (Fs) have been linked to changes in soil water content (SWC), rainfall and temperature and/or productivity. However, within an ecosystem, Fs can vary based on site structure and function, which can be affected by a combination of abiotic and biotic factors. This becomes particularly important when an ecosystem is faced with disturbances, such as drought or fire. Site-specific compensatory responses to disturbances may therefore alter C mineralization, as well as root respiration. Hence, single location Fs estimates may not be a representative for ecosystems across their distributional ranges. We conducted a 6-year study along an edaphic moisture gradient of longleaf pine ecosystems that were maintained with prescribed fire, using eddy covariance and soil respiration measurements to address how Fs varies with changes in ecosystem structure and function, as well as disturbances. Lower air temperatures (Tair) decreased Fs at all sites, but that response was also affected by productivity and SWC. Productivity significantly altered Fs rates at all sites, especially when we accounted for changes in temperature and SWC. Plant regrowth post-fire temporarily increased Fs (10–40%), whereas drought reduced Fs at all sites. Our results show that site productivity, Fs and the degree to which ecosystems adapt to climate variations and disturbance can be site specific. Hence, model forecasting of carbon dynamics would strongly benefit from multi-location measurements of Fs across the distributional range of an ecosystem.