No abstract

To estimate the robustness of hydrologic models under projected future climate change, researchers test transferability between climatically contrasting observed periods. This approach can only assess the performance changes induced by altered precipitation and related environmental dynamics (e.g., greening under wet conditions), since the instrumental record does not contain temperatures or carbon dioxide levels that are similar to future climate change projections. Additionally, there is an inherent assumption that long‐term persistence of changes in precipitation will not further impact catchment response. In this study, we undertake a series of virtual catchment experiments using an ecohydrologic model that simulates dynamic vegetation growth, nutrient cycling, and subsurface hydrology. These experiments explore a number of climate change scenarios. We compare simulations based on persistent altered climate states against simulations designed to represent historical periods with the same precipitation but limited time for ecohydrologic adaptation. We find that persistence of precipitation changes as well as increased temperature and elevated carbon dioxide levels can all substantially impact streamflow under drier future conditions. For wetter future scenarios, simulated differences in the flow regime were smaller, but there was still notable divergence in modeled low flows and other hydrologic variables. The results suggest that historical periods with equivalent precipitation statistics cannot necessarily be used as proxies for future climate change when examining catchment runoff response and/or model performance. The current literature likely underestimates the potential for nonstationarity in hydrologic assessments, especially for drier future scenarios.

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.

The importance of terrestrial and aquatic ecosystems in controlling nitrogen dynamics in streams is a key interest of ecologists studying dissolved inorganic nitrogen (DIN) export from watersheds. In this study, we coupled a stream model with a terrestrial ecohydrological model and conducted a global sensitivity analysis to evaluate the relative importance of both ecosystems to nitrogen export. We constructed two scenarios (''normal'' and high nitrate loads) to explore conditions under which terrestrial (lateral nitrate flux) or aquatic ecosystems (instream nutrient processes) may be more important in controlling DIN export. In a forest catchment, although the forest ecosystem controls the nitrogen load to streams, sensitivity results suggested that most nitrogen output from the terrestrial ecosystem was taken up by instream microbial immobilization associated with benthic detritus and retained in detritus. Later the immobilized nitrogen was remineralized as DIN. Therefore, the intra-annual pattern of DIN concentration in the stream was low in fall and became high in spring. Not only was instream microbial immobilization saturated with the high nitrogen load scenario, but also the net effect of immobilization and mineralization on DIN export was minimized because nitrogen cycling between organic and inorganic forms was accelerated. Overall, our linked terrestrial-aquatic model simulations demonstrated that stream process could significantly affect the amount and timing of watershed nitrogen export when nitrogen export from the terrestrial system is low. However, when nitrogen export from the terrestrial system is high, the effect of stream processes is minimal.

The potential for increased loads of dissolved organic carbon (DOC) in streams and rivers is a concern for regulating the water quality in water supply watersheds. With increasing hydroclimatic variability related to global warming and shifts in forest ecosystem community and structure, understanding and predicting the magnitude and variability of watershed supply and transport of DOC over multiple time scales have become important research and management goals. In this study, we use a distributed process‐based ecohydrological model (Regional Hydro‐Ecological Simulation System [RHESSys]) to explore controls and predict streamflow DOC loads in Biscuit Brook. Biscuit Brook is a forested headwater catchment of the Neversink Reservoir, part of the New York City water supply system in the Catskill Mountains. Three different model structures of RHESSys were proposed to explore and evaluate hypotheses addressing how vegetation phenology and hydrologic connectivity between deep groundwater and riparian zones influence streamflow and DOC loads. Model results showed that incorporating dynamic phenology improved model agreement with measured streamflow in spring, summer, and fall and fall DOC concentration, compared with a static phenology. Additionally, the connectivity of deep groundwater flux through riparian zones with dynamic phenology improved streamflow and DOC flux in low flow conditions. Therefore, this study suggests the importance of inter‐annual vegetation phenology and the connectivity of deep groundwater drainage through riparian zones in the hydrology and stream DOC loading in this forested watershed and the ability of process‐based ecohydrological models to simulate these dynamics. The advantage of a process‐based modelling approach is specifically seen in the sensitivity to forest ecosystem dynamics and the interactions of hydroclimate variability with ecosystem processes controlling the supply and distribution of DOC. These models will be useful to evaluate different forest management approaches toward mitigating water quality concerns.

Activation of the innate immune system promotes polyclonal antibody secretion to eliminate invading pathogens. Inherent in this process is the potential to activate autoreactive B cells and induce autoimmunity. We showed previously that TLRstimulated dendritic cells and macrophages regulate B cell tolerance to Smith antigen, in part through the secretion of interleukin-6 (IL-6). In this manuscript, we show that neutralization of IL-6 fails to abrogate macrophage-mediated repression and identify soluble CD40 ligand IntroductionRecognition of microorganisms by Toll-like receptors (TLRs) promotes inflammation and stimulates the innate immune system to produce antibody, responses that are beneficial in clearing infections. However, TLR ligation of autoreactive B cells can lead to transient or persistent autoimmunity. 1 Studies of rheumatoid factorspecific B cells show that immune complexes containing TLR and B-cell receptor (BCR) ligands induce proliferation of autoreactive B cells. [2][3][4] Likewise, anti-double-stranded DNA (dsDNA)-specific B cells proliferate in response to BCRmediated internalization of chromatin. 5 Because most nuclear self-antigens contain BCR and TLR ligands, these findings suggest that stimulation of autoreactive B cells through the BCR and/or TLR activates some autoreactive B cells.During T-dependent immune responses, CD40 stimulation induces B-cell proliferation, increases the expression of costimulatory molecules, and promotes germinal center formation leading to high affinity, class-switched antibodies. Continuous exposure to CD40 ligand (CD40L) promotes the formation of memory cells by blocking B lymphocyte-induced maturation protein-1 (Blimp-1) expression and arresting plasma cell differentiation. 6 CD40/CD40L interactions also regulate autoreactive B cells that encounter activated CD4 ϩ T H cells. 7 Hen egg lysozyme (HEL)-specific B cells that have been continuously exposed to self-antigen up-regulate Fas in response to CD40L stimulation. Subsequent encounter with an activated HEL-specific T cell induces Fasdependent B-cell apoptosis, thereby preventing autoimmunity. Thus, persistent exposure to self-antigen modulates Fas and CD40 to induce apoptosis or prevent terminal differentiation.We recently identified a mechanism of tolerance that regulates autoreactive B cells during innate immune responses. In response to lipopolysaccharide (LPS), dendritic cells (DCs) and macrophages (M⌽s) regulate HEL-, p-azophenylarsonate (Ars)-and low-affinity (2-12H/V8), Smith antigen (Sm)-specific B cells through the secretion of interleukin-6 (IL-6). 8 Regulation of immunoglobulin (Ig) secretion is selective in that chronically antigen-experienced (anergic) B cells are repressed, whereas transiently stimulated naive B cells are not. This indicates that tolerance within the B-cell compartment extends beyond antigeninduced receptor desensitization and that persistent BCR ligation affects other receptors. Herein, we report that in addition to IL-6, M⌽s secrete soluble CD40L (sCD40L), which selectively r...

No abstract

No abstract