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|Publications||Journal Articles||Controls of Methylmercury Bioaccumulation in Forest Floor Food Webs
Abstract - Compared to the extensive research on aquatic ecosystems, very little is known about the sources and trophic transfer of methylmercury (MeHg) in terrestrial ecosystems. In this study, we examine energy flow and trophic structure using stable carbon (d13C) and nitrogen (d15N) isotope ratios, respectively, and MeHg levels in basal resources and terrestrial invertebrates from four temperate forest ecosystems. We show that MeHg
levels in biota increased significantly (p < 0.01) with d13C and d15N at all sites, implying the importance of both microbially processed diets (with increased d13C) and trophic level (with increased d15N) at which organisms feed, on MeHg levels in forest floor biota. The trophic magnification slopes of MeHg (defined as the slope of log10MeHg vs d15N) for these forest floor food webs (0.20-0.28) were not significantly different (p > 0.05) from those observed for diverse temperate freshwater systems (0.24 ± 0.07; n = 78), demonstrating for the first time the nearly equivalent efficiencies with which MeHg moves up the food chain in these contrasting ecosystem types. Our results suggest that in situ production of MeHg within the forest floor and efficient biomagnification both elevate MeHg levels in carnivorous invertebrates in temperate forests, which can contribute to significant bioaccumulation of this neurotoxin in terrestrial apex predators.
|Experimental N and P additions alter stream macroinvertebrate community composition via taxon-level responses to shifts in detrital resource stoichiometry
Abstract - 1. Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities,
partly by altering stoichiometric imbalances between consumers and
their food. Testing relationships between resource stoichiometry and consumer
assemblage structure requires ecosystem-level manipulations that have been
lacking to date.
2. We analysed patterns of macroinvertebrate community composition in five detritus-
based headwater streams subject to experimental whole-stream N and P additions
that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1,
128:1) over 2 years, following a 1-year pre-treatment period.
3. We predicted that shifts in leaf litter stoichiometry would drive overall patterns of
community composition via greater responses of shredders to enrichment than
other taxa, as shredders dominate primary consumer biomass and experience
larger consumer–resource elemental imbalances than other functional groups in
stream ecosystems. Specifically, we expected litter C:P to be a significant predictor
of shredder biomass given the greater relative imbalances between shredder
and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment
were related to other taxon-level traits, including body size and stoichiometry,
larval life span and growth rate.
4. Whole-community composition shifted similarly across the five streams after enrichment,
largely driven by increased shredder and predator biomass. These shifts
were limited to the autumn/winter seasons and related to decreased leaf litter C:P,
highlighting important links between the quality of seasonal litter subsidies and
5. Among 10 taxa that drove structural shifts, two declined while other taxa from
the same functional/taxonomic groups responded positively, suggesting that specific
life-history traits may determine sensitivity to enrichment.
6. Increases in total shredder biomass, and in biomass of several common shredders,
were associated with lower litter C:P. Body C:P did not predict shredder response
to enrichment. However, weak negative relationships between shredder response
and body size, and larval life span, suggest that small-bodied and short-lived taxa
may be more responsive to shifting resource stoichiometry.
|Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians
Abstract - The loss of Tsuga canadensis from invasion by hemlock woolly adelgid (Adelges tsugae, HWA) has altered ecosystem
structure and function in forests across the eastern United States. In southern Appalachian forests, T.
canadensis co-occurred with hardwood species and an evergreen Rhododendron maximum shrub layer in riparian
and cove positions. In this region, HWA infestation was detected in 2003, with mortality reaching 97% by 2014.
In this study we examined responses of light, soil moisture, tree seedling density, and overstory and understory
vegetation growth from 2004 to 2014 following HWA infestation and mortality of T. canadensis. We hypothesized
that seedling recruitment and vegetation growth would continue to increase over time as observed with
initial trends reported through 2009, and that species that associate with ectomycorrhizal (ECM) fungi may
benefit more than those that do not due to the pulse of organic material in these stands from dead T. canadensis
trees. Light transmission measured at 1m above the ground increased from 2006 to 2009, but gradually decreased
from 2015 to 2017. Basal area of overstory non-Tsuga trees increased only marginally, and there was no
recruitment of tree seedlings to the overstory size class, even though seedling density of deciduous species
increased initially. Increased basal area and stem density of R. maximum may explain the light and seedling
responses, as this species can inhibit tree seedling recruitment by limiting light and nutrients. Overstory species
with the highest basal area increment (BAI) in the post-T. canadensis stands were Pinus rigida, Betula lenta and
Quercus coccinea, which all associate with ECM fungi. However, not all ECM tree species grew significantly more
following T. canadensis mortality compared to pre-mortality growth rates—only those ECM species that had high
growth rates prior to mortality did. After a decade, growth of both overstory trees and R. maximum has not
compensated for the loss of T. canadensis. Active management of R. maximum, which may involve the removal of
the evergreen shrub and soil organic layer, may be required to allow for diverse tree seedling recruitment; and
subsequently, restore riparian forest structure, diversity, and function.
|The Temporal Distribution and Duration of Mississippian Polities in Alabama, Georgia, Mississippi, and Tennessee
Abstract - To aid our understanding of prehispanic social change in a subcontinental context, this article presents data and analysis relating to the occupational histories of 351 Mississippian platform mound sites in Alabama, Georgia, Mississippi, and Tennessee. Based on the premise that sites with platform mounds served as the administrative and ritual centers for Mississippian polities, our study demonstrates that polities in the study area rose and fell with some regularity, and in many cases, new polities succeeded old ones in the same locations. Our work expands on a previous analysis of 47 northern Georgia area sites. Through a theoretical framework tailored for macroregional processes and a rule-based approach in collecting and standardizing data from previous work, this study serves as an example for incorporating different processes and regions to provide a more coherent and complete picture of the Mississippian macroregion. Our results show that polity cycling was typical in our study area, and we argue that the rise and fall of polities is best described within a theoretical framework emphasizing collapse and resilience. By treating collapse as a normal feature of Mississippian polities, we can better understand the interconnectedness of Mississippian polities across regions.
|Integrating Ecophysiological and Agent-Based Models to Simulate How Behavior Moderates Salamander Sensitivity to Climate
Abstract - Developing rigorous ecological models is a fundamental goal of conservation biologists seeking to forecast biotic responses to climate change. A limitation of many models is they are amechanistic and lack integration of behavior, which is fundamental to animal biology. We integrated biophysical and agent-based models (ABM) to examine how behavior could affect the sensitivity of Plethodontid salamander activity time to climate. Specifically, our model used a temperature differential to stimulate plant climbing, a widely observed behavior among salamanders, which would allow salamanders to reduce body temperatures and associated dehydration rates. Consistent with expectations, predicted activity time was positively correlated with precipitation. The model showed that climbing plants increased activity time in drier conditions, particularly for smaller salamanders. The predicted importance of climbing behavior, a form of behavioral plasticity, was highly sensitive to assumptions about the threshold of water loss an individual was willing to tolerate. Because activity time is associated with fitness, increased activity time as a consequence of climbing behavior could moderate salamander sensitivity to shifts in weather patterns. Our results demonstrate the potential and importance of integrating behaviors into ecophysiological models when evaluating a species' potential sensitivity to climate.
|Rainfall partitioning varies across a forest age chronosequence in the southern Appalachian Mountains
Abstract - Evaporation of precipitation from plant surfaces, or interception, is a major component of the global water budget. Interception has been measured and/or modelled across a wide variety of forest types; however, most studies have focused on mature, second-growth forests, and few studies have examined interception processes across forest age classes. We present data on two components of interception, total canopy interception (Ei) and litter interception—that is, Oi + Oe horizon layers—(Eff), across a forest age chronosequence, from 2 years since harvest to old growth. We used precipitation, throughfall, and stemflow collectors to measure total rainfall (P) and estimate Ei; and collected litter biomass and modelled litter wetting and drying to estimate evaporative loss from litter. Canopy Ei, P minus throughfall, increased rapidly with forest age and then levelled off to a maximum of 21% of P in an old-growth site. Stemflow also varied across stands, with the highest stemflow (~8% of P) observed in a 12-year-old stand with high stem density. Modelled Eff was 4–6% of P and did not vary across sites. Total stand-level interception losses (Ei + Eff) were best predicted by stand age (R2 = 0.77) rather than structural parameters such as basal area (R2 = 0.49) or leaf area (R2 < 0.01). Forest age appears to be an important driver of interception losses from forested mountain watersheds even when stand-level structural variables are similar. These results will contribute to our understanding of water budgets across the broader matrix of forest ages that characterize the modern forest landscape.
|Foodwebs based on unreliable foundations: spatiotemporal masting merged with consumer movement, storage, and diet
Abstract - Mast-fruiting trees represent a pulsed resource that both supports and destabilizes consumer populations. Whereas a reliable resource is abundant on average and with limited variation in time and space, masting is volatile and localized, and that variability ramifies throughout food-webs. Theory is developed to evaluate how the space–time structure of masting interacts with consumers who exploit alternative hosts, forage widely in space, and store reserves in time. We derive the space–time–species covariance in resource supply and combine it with the space–time–diet breadth of consumers, or ambit. Direct connection to data is made possible with Mast Inference and Forecasting (MASTIF), a state-space autoregressive model that fits seed-trap and canopy observations and predicts resource availability within the canopy and on the forest floor with full uncertainty. A resource score can be assigned to each consumer–habitat combination that integrates the benefits of a high mean supply weighed against the variance cost. As the consumer ambit increases, the volatility of an unreliable resource shifts from a variance cost to a mean benefit. Consumers foraging in the canopy (arboreal arthropods and rodents, song birds) experience space-time covariance between host trees. Consumers on the forest floor (seed and damping-off fungi, arthropods, rodents, ground-nesting birds, mammals) experience instead a redistribution of that covariance by dispersal. For consumers lacking mobility, demographic storage in the form of episodic birth cohorts following mast years is important for population persistence. Consumers additionally compensate volatility with diet breadth. Depending on the dominant masting strategies of host tree species in the diet, habitats differentially limit consumers depending on the misalignment between consumer ambit and spatiotemporal covariance of hosts. The impact of adding or subtracting a diet item can be gauged with the standard error (SE) rule or the benefit of an added diet item balanced against the variance cost, both of which depend on the existing diet, the abundance of the new host, and the consumer's foraging ambit. Results rank habitats by their capacities to support wildlife and other consumers from a resource perspective. Results are connected directly to data, with full uncertainty, by MASTIF.
|Drought sensitivity of an N2-fixing tree may slow temperate deciduous forest recovery from disturbance
Abstract - Increased drought intensity and frequency due to climate change may reduce the abundance and activity of nitrogen (N2)-fixing plants, which supply new N to terrestrial ecosystems. As a result, drought may indirectly reduce ecosystem productivity through its effect on the N cycle. Here, we manipulated growing season net rainfall across a series of plots in an early successional mesic deciduous forest to understand how drought affects the aboveground productivity of the N2-fixing tree Robinia pseudoacacia and three co-occurring nonfixing tree species. We found that lower soil moisture was associated with reduced productivity of R. pseudoacacia but not of nonfixing trees. As a result, the relative biomass and density of R. pseudoacacia declined in drier soils over time. Greater aboveground biomass of R. pseudoacacia was also associated with greater total soil N, extractable inorganic N, N mineralization rates, and productivity of nonfixing trees. These soil N effects may reflect current N2 fixation by R. pseudoacacia saplings, or the legacy effect of former trees in the same location. Our results suggest that R. pseudoacacia promotes the growth of nonfixing trees in early succession through its effect on the N cycle. However, the sensitivity of R. pseudoacacia to dry soils may reduce N2 fixation under scenarios of increasing drought intensity and frequency, demonstrating a mechanism by which drought may indirectly diminish potential forest productivity and recovery rate from disturbance.
|Understanding PES from the ground up: a combined choice experiment and interview approach to understanding PES in Costa Rica
Abstract - Payments for environmental services (PES) are part of a suite of market-based conservation mechanisms that have gained international attention for their potential to produce ecosystem services across private landholdings. The Costa Rican government, a PES pioneer, provides landowners with remuneration for conserving forest cover on their properties. The efficiency and effectiveness of PES programs have been critiqued, because little is understood about the ability of payments to alter landowner behavior. Unique to the existent literature, we combine data from semi-structured interviews with an economic nonmarket valuation technique, the stated choice experiment, to explore preferences for PES and the impact of PES on decision making in the mixed-use Bellbird Biological Corridor of Costa Rica. The choice experiment reveals farmer preferences for PES, while interviews provide context for the choice experiment results and illuminate the implications for policy. Results show that farmer objections to PES design and program implementation influence willingness to accept payments for conservation efforts. Results further indicate that a significant share of farmers in the study area are resistant to the PES program irrespective of competitive payments offered. We suggest that this overall resistance to the program is a key driver of program inefficiency. We further suggest that potential program improvements require better engagement with multifaceted landowner values, and that PES may not constitute an effective mechanism for engaging small-scale farmers in conservation.
|Transpiration and subsurface controls of streamflow recession characteristics
Abstract - In headwater catchments, streamflow recedes between periods of rainfall at a predictable rate generally defined by a power–law relationship relating streamflow decay to streamflow. Research over the last four decades has applied this relationship to predictions of water resource availability as well as estimations of basin-wide physiographic characteristics and ecohydrologic conditions. However, the interaction of biophysical processes giving rise to the form of these power–law relationships remains poorly understood, and recent investigations into the variability of streamflow recession characteristics between discrete events have alternatively suggested evapotranspiration, water table elevation, and stream network contraction as dominant factors, without consensus. To assess potential temporal variability and
interactions in the mechanism(s) driving streamflow recession, we combine long-term observational data from a headwater stream in the southern Appalachian Mountains with state and flux conditions from a process-based ecohydrologic model. Streamflow recession characteristics are nonunique and vary systematically with seasonal
fluctuations in both rates of transpiration and watershed wetness conditions, such that transpiration dominates recession signals in the early growing season and diminishes in effect as the water table elevation progressively drops below and decouples with the root zone with topographic position. As a result of this decoupling, there exists a seasonal hysteretic relationship between streamflow decay and both evapotranspiration and watershed wetness conditions. Results indicate that for portions of the year, forest transpiration may actively compete with subsurface
drainage for the same water resource that supplies streamflow, though for extended time periods, these processes exploit distinct water stores. Our analysis raises concerns about the efficacy of assessing humid headwater systems using traditional recession analysis, with recession curve parameters treated as static features of the
watershed, and we provide novel alternatives for evaluating interacting biological and geophysical drivers of streamflow recession.
|Spatiotemporal sensitivity of thermal stress for monitoring canopy hydrological stress in near real-time
Abstract - Monitoring drought in real-time using minimal field data is a challenge for ecosystem management and conservation. Most methods require extensive data collection and in-situ calibration and accuracy is difficult to evaluate. Here, we demonstrated how the space-borne canopy “thermal stress”, defined as surface-air temperature difference, provides a reliable surrogate for drought-induced water stress in vegetation. Using physics-based relationships that accommodate uncertainties, we showed how changes in canopy water flux from ground-based measurements relate to both the surface energy balance and remotely-sensed thermal stress. Field measurements of evapotranspiration in the southeastern and northwestern US verify this approach based on sensitivity of evapotranspiration to thermal stress in a large range of atmospheric and climate conditions. We found that a 1?°C change in the thermal stress is comparable to 1–1.2?mm day-1 of evapotranspiration, depending on site and climate conditions. We quantified temporal and spatial sensitivity of evapotranspiration to the thermal stress and showed that it has the strongest relationship with evapotranspiration during warm and dry seasons, when monitoring drought is essential. Using only air and surface temperatures, we predicted the inter-annual anomaly in thermal stress across the contiguous United States over the course of 15 years and compared it with conventional drought indices. Among drought metrics that were considered in this study, the thermal stress had the highest correlation values. Our sensitivity results demonstrated that the thermal stress is a particularly strong indicator of water-use in warm seasons and regions. This simple metric can be used at varying time-scales to monitor surface evapotranspiration and drought in large spatial extents in near real-time.
|Ecosystem processes at the watershed scale: Influence offlowpath patterns of canopy ecophysiology on emergentcatchment water and carbon cycling
Abstract - Forest canopy water use and carbon cycling traits (WCT) can vary substantially and in spatially organized patterns, with significant impacts on watershed ecohydrology. In many watersheds, WCT may vary systematically along and between hydrologic flowpaths as an adaptation to available soil water, nutrients, and microclimate-mediated atmospheric water demand. We hypothesize that the emerging patterns of WCT at the hillslope to catchment scale provide a more resistant ecohydrological system, particularly with respect to drought stress, and the maintenance of high levels of productivity. Rather than attempting to address this hypothesis with species-specific patterns, we outline broader functional WCT groups and explore the sensitivity of water and carbon balances to the representation of canopy WCT functional organization through a modelling approach. We use a well-studied experimental watershed in North Carolina where detailed mapping of forest community patterns are sufficient to describe WCT functional organization. Ecohydrological models typically use broad-scale characterizations of forest canopy composition based on remotely sensed information (e.g., evergreen vs. deciduous), which may not adequately represent the range or spatial pattern of functional group WCT at hillslope to watershed scales. We use three different representations of WCT functional organizations: (1) restricting WCT to deciduous/conifer differentiation, (2) utilizing more detailed, but aspatial, information on local forest community composition, and (3) spatially distributed representation of local forest WCT. Accounting for WCT functional organization information improves model performance not only in terms of capturing observed flow regimes (especially watershed-scale seasonal flow dynamics) but also in terms of representing more detailed canopy ecohydrologic behaviour (e.g., root zone soil moisture, evapotranspiration, and net canopy photosynthesis), especially under dry condition. Results suggest that the well-known zonation of forest communities over hydrologic gradients is not just a local adaptation but also provides a property that regulates hillslope to catchment-scale behaviour of water use and drought resistance.
|Revisiting the Hewlett and Hibbert (1963) hillslope drainage experiment and modeling effects of decadal pedogenic processes and leaky soil boundary conditions
Abstract - Subsurface flow dominates water movement from hillslopes to streams in most forested headwater catchments. Hewlett and Hibbert (1963) constructed an idealized hillslope model (0.91 × 0.91 × 15.0 m; 21.8°) using reconstituted C horizon soil to investigate importance of interflow, a type of subsurface flow. They saturated the model, covered it to prevent evaporation, and allowed free drainage for 145 d. The resulting recession drainage curve suggested two phases: fast drainage of saturated soil in the first 1.5 d, then slow drainage of unsaturated soil. Hydrologists interpreted the latter as evidence interflow could sustain baseflow, even during extended drought. Since that experiment, typical forest vegetation grew in the model, providing root and litter inputs for 55 years. We removed all above-ground live biomass and repeated the experiment physically and numerically (HYDRUS-2D), hypothesizing that pedogenesis would change the drainage curve and further elucidate the role of unsaturated flow from hillslopes. Contrary to this hypothesis, drainage curves in our twice-repeated physical experiments and numerical simulation were unchanged for the first ~10 days, indicating pedogenesis and biological processes had not largely altered bulk hydraulic conductivities or soil moisture release characteristics. However, drainage unexpectedly ceased after about two weeks (14.3 ± 2.52 d), an order of magnitude sooner than in the original experiment, due to an apparent leak in the hillslope analogous to commonly observed bedrock fractures in natural systems. Thus, our results are a more natural recession behavior that highlights how incorporation of alternative hydrologic outputs can reduce drainage duration and volume from soils to baseflow.
|Soil Bacterial and Fungal Communities Exhibit Distinct Long-Term Responses to Disturbance in Temperate Forests
Abstract - In Appalachian ecosystems, forest disturbance has long-term effects on microbially driven biogeochemical processes such as nitrogen (N) cycling. However, little is known regarding long-term responses of forest soil microbial communities to disturbance in the region. We used 16S and ITS sequencing to characterize soil bacterial (16S) and fungal (ITS) communities across forested watersheds with a range of past disturbance regimes and adjacent reference forests at the Coweeta Hydrologic Laboratory in the Appalachian mountains of North Carolina. Bacterial communities in previously disturbed forests exhibited consistent responses, including increased alpha diversity and increased abundance of copiotrophic (e.g., Proteobacteria) and N-cycling (e.g., Nitrospirae) bacterial phyla. Fungal community composition also showed disturbance effects, particularly in mycorrhizal taxa. However, disturbance did not affect fungal alpha diversity, and disturbance effects were not consistent at the fungal class level. Co-occurrence networks constructed for bacteria and fungi showed that disturbed communities were characterized by more connected and tightly clustered network topologies, indicating that disturbance alters not only community composition but also potential ecological interactions among taxa. Although bacteria and fungi displayed different long-term responses to forest disturbance, our results demonstrate clear responses of important bacterial and fungal functional groups (e.g., nitrifying bacteria and mycorrhizal fungi), and suggest that both microbial groups play key roles in the long-term alterations to biogeochemical processes observed following forest disturbance in the region.
|An Expanded Investigation of Atmospheric Rivers in the Southern Appalachian Mountains and Their Connection to Landslides
Abstract - Previous examination of rain gauge observations over a five-year period at high elevations within a river basin of the southern Appalachian Mountains showed that half of the extreme (upper 2.5%) rainfall events were associated with an atmospheric river (AR). Of these extreme events having an AR association, over 73% were linked to a societal hazard at downstream locations in eastern Tennessee and western North Carolina. Our analysis in this study was expanded to investigate AR effects in the southern Appalachian Mountains on two river basins, located 60 km apart, and examine their influence on extreme rainfall, periods of elevated precipitation and landslide events over two time periods, the ‘recent’ and ‘distant’ past. Results showed that slightly more than half of the extreme rainfall events were directly attributable to an AR in both river basins. However, there was disagreement on individual ARs influencing extreme rainfall events in each basin, seemingly a reflection of its proximity to the Blue Ridge Escarpment and the localized terrain lining the river basin boundary. Days having at least one landslide occurring in western North Carolina were found
to be correlated with long periods of elevated precipitation, which often also corresponded to the influence of ARs and extreme rainfall events.