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Eastern hemlock [Tsuga canadensis (L.) Carrière serve an important ecological role in riparian ecosystems in the southern Appalachians. Significant hemlock mortality is occurring due to infestation by the hemlock woolly adelgid (HWA) (Adelges tsugae Annand), a non-native invasive pest. Our objective was to quantify the impacts of HWA and hemlock mortality on nutrient cycling pools and processes. In 2004, we established eight research plots in riparian areas with > 50% basal area in hemlock and four reference plots in riparian areas without hemlock (hardwood). All hemlock plots were infested with HWA. In four of the hemlock plots, all hemlock trees were girdled to induce defoliation and rapid mortality. By fall 2006, there was 90 and 10% mortality in the girdled and nongirdled hemlock plots, respectively. Measurements included soil temperature and moisture, nutrient pools, N transformations, litterfall and forest floor amount and chemistry, and throughfall and soil solution chemistry. From 2004 to 2008, litterfall composition changed, with an initial increase in the hemlock needle percentage followed by a decline. Hemlock plots had cooler spring soil temperatures than hardwood plots. Hemlock plots had greater surface soil and forest floor total C than hardwood plots; soil C content did not change during the 4 yr of measurement. There were no differences in N mineralization rates or soil solution N concentrations among treatments. Differences between litterfall and forest floor nutrient contents in hemlock and hardwood plots suggest that as hemlocks are replaced by hardwood species, nutrient cycling rates and processes will be similar to hardwood reference plots. [ABSTRACT FROM AUTHOR]

We characterized structural and functional attributes along hillslope gradients in headwater catchments. We endeavored to identify parameters that described significant transitions along the hillslope. On each of four catchments, we installed eight 50 m transects perpendicular to the stream. Structural attributes included woody and herbaceous vegetation; woody debris and forest floor mass, nitrogen (N) and carbon (C); total soil C and N; litterfall amount and quality by species; and microclimatic conditions. Functional attributes included litter decomposition, soil microarthropods, soil CO2 evolution, soil solution chemistry, and soil extractable N. Forest floor mass, N and C, and soil depth increased with distance from the stream and transitioned between 10 and 20 m. In contrast, litterfall N rate (kilograms of nitrogen per hectare per day), downed woody debris, soil A-horizon C and N, and soil solution NO3 concentration all decreased with distance, and exhibited significant transitions. Certain overstory species were more abundant in the uplands than near the stream. Herbaceous diversity and richness were similar across the hillslope, but species distributions varied in response to hillslope moisture content. Taken together, these results suggest that at 10–20 m from the stream, transitions occur that separate riparian from upland conditions and may provide valuable insight into riparian zone definition. [ABSTRACT FROM AUTHOR]

The article presents simple mathematical and graphical models derived from data quantifying the distribution of water flux within the stems and the seasonal variability of daily water use of eastern hemlock trees growing in New England and the southern Appalachians in the U.S. The models can be used to estimate tree water use based on tree size and climate conditions. They can also be used to ensure that insecticide is applied during periods of rapid water uptake and movement through the tree stem.

Summary: Long-term measurements of soil nitrogen (N) transformations along an environmental gradient within the Coweeta Hydrologic Laboratory basin in western North Carolina showed a strong seasonal pattern and suggested that vegetation community type—through its influence on soil properties—was an important regulating factor. Our objective was to determine the relative effects of biotic vs. abiotic factors on soil N transformations. During the 1999 and 2000 growing seasons we transplanted soil cores from each of the five gradient plots to all other gradient plots for their 28-day in situ incubation. N mineralization and nitrification rates in soils from the northern hardwood (NH) site were significantly increased when soils were transplanted to warmer sites. N mineralization rates also increased in transplanted soil from the dry mixed-oak/pine site to a wetter site. Multiple regression analysis of N mineralization from all five sites found that biotic (total soil N and C:N ratios) and climatic factors (moisture and temperature) regulate N mineralization. Regression analyses of individual sites showed that N mineralization rates responded to variation in temperature and moisture at only the high elevation northern hardwood site and moisture alone on the dry warm mixed-oak/pine site. N mineralization was unrelated to temperature or moisture at any of the other sites. Results indicate that soil properties plus climatic conditions affect soil N transformations along the environmental gradient at Coweeta. Environmental controls were significant only at the extreme sites; i.e., at the wettest and warmest sites and soils with highest and lowest C and N contents. The high degree of temperature sensitivity for the northern hardwood soils indicates potentially large responses to climatic change at these sites. [Copyright &y& Elsevier]

Summary: We measured forest floor CO2 flux in three age classes of forest in the southern Appalachians: 20-year-old, 85-year-old, and old-growth. Our objectives were to quantify differences in forest floor CO2 flux among age classes, and determine the relative importance of abiotic and biotic driving variables. Forest floor CO2 flux was measured using an openflow infrared gas analyzer measurement system for 24h periods and samples were taken every 2 months over a 2-year period. Litter/soil interface, soil temperature (5cm depth), soil moisture (%), forest floor moisture (%), forest floor mass, fine root (⩽2mm) mass, coarse root mass (>2mm), forest floor C and N (%), fine root C and N, coarse root C and N, and soil N and C were co-measured during each sample period. Results showed significant nonlinear relationships (r 2=0.68 to 0.81) between litter/soil interface temperature and forest floor CO2 flux for all three forest age classes, but no differences in temperature response parameters. These results indicated no differences in forest floor CO2 flux among age classes. Considerable temporal variation in abiotic and biotic variables was observed within and among forests. Biotic variables correlated with forest floor CO2 flux included indices of litter and root quality. Differences in biotic variables correlated with forest floor CO2 flux among forests may have been related to shifts in the relative importance of heterotrophic and autotrophic respiration components to overall forest floor CO2 flux. [Copyright &y& Elsevier]

Microbial biomass, nitrifiers and denitrifiers in surface soil (0–10 cm) were quantified in a riparian zone restoration project at Coweeta, North Carolina, USA. Four treatments are included in this study: (1) a degraded (+N) riparian zone with continued compaction, vegetation removal, and nutrient addition (mow, roll, and nutrient addition); (2) a degraded (−N) riparian zone with continued compaction and vegetation removal, but without nutrient addition (mow and roll only); (3) a restored riparian area (no grazing, cessation of manure and N fertilizer application and re-vegetation with natural regrowth for 2 years); and (4) reference riparian zone (no riparian zone degradation has occurred within the last 10 years). Mean microbial biomass C increased from winter (316 mg kg−1) to summer (593 mg kg−1), and decreased from summer to fall (265 mg kg−1). There were no significant changes in microbial biomass C with the cessation of manure and chemical fertilizer application in the degraded−N plot and with re-vegetation in the restored plot as compared to the degraded+N plot. Microbial biomass C level in the restored plot was comparable to that in the reference plot for most seasons. Restored plots had significant greater populations of denitrifiers than reference plots in the spring. Nitrifier numbers were lower in the degraded−N and restored plots than the degraded+N. Ammonium oxidizers in summer were more abundant (25,000 g−1 soil) in the degraded+N plot compared to (1000 g−1 soil) in the degraded−N and restored plots, and NO2− oxidizers in the same period were more abundant (130,000 g−1 soil) in the degraded+N plot than that in the degraded−N and restored plots about (40,000 g−1 soil). The soil NO3− concentrations were considerably lower in the degraded−N and restored plots than the degraded+N plot. Our results imply either cessation of manure and N fertilizer application or cessation of manure and N fertilizer and re-vegetation could contribute to restoration of degraded riparian zone through reducing numbers of nitrifiers. [Copyright &y& Elsevier]

Concentrated animal feeding operations (CAFOs) pose wide ranging environmental risks to many parts of the US and across the globe, but datasets for CAFO risk assessments are not readily available. Within the United States, some of the greatest concentrations of CAFOs occur in North Carolina. It is also one of the only states with publicly accessible location data for classes of CAFOs that are required to obtain water quality permits from the U.S. Environmental Protection Agency (EPA); however, there are no public data sources for the large number of CAFOs that do not require EPA water quality permits. We combined public records of CAFO locations with data collected in North Carolina by the Waterkeeper and Riverkeeper Alliances to examine the distribution of both permitted and non-permitted CAFOs across the state. Over half (55%) of the state's 6646 CAFOs are located in the Coastal Plain, a low-lying region vulnerable to flooding associated with regular cyclonic and convective storms. We identified 19% of CAFOs ≤ 100 m of the nearest stream, and some as close as 15 m to the nearest stream, a common riparian buffer width for water quality management. Future climate scenarios suggest large storm events are expected to become increasingly extreme, and dry interstorm periods could lengthen. Such extremes could exacerbate the environmental impacts of CAFOs. Understanding the potential impacts of CAFO agroecosystems will require remote sensing to identify CAFOs, fieldwork to determine the extent of environmental footprints, and modeling to identify thresholds that determine environmental risk under changing conditions. [ABSTRACT FROM AUTHOR]

The article discusses the importance of research on forest drought due to climate change that causes warm temperature and precipitation severity in the U.S. It cites that the U.S. Department of Agriculture initiated a study "Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis" which evaluate that changes and summarizes the scientific foundation to manage drought resilience and adaptation.

As human activity and climate variability alter the movement of water through the environment the need to better understand hydrologic cycle responses to these changes has grown. A reasonable starting point for gaining such insight is studying changes in streamflow given the importance of streamflow as a source of renewable freshwater. Using a wavelet assisted method we analyzed trends in the magnitude of annual scale streamflow variability from 967 watersheds in the continental U.S. (CONUS) over a 70 year period (1940–2009). Decreased annual variability was the dominant pattern at the CONUS scale. Ecoregion scale results agreed with the CONUS pattern with the exception of two ecoregions closely divided between increases and decreases and one where increases dominated. A comparison of trends in reference and non-reference watersheds indicated that trend magnitudes in non-reference watersheds were significantly larger than those in reference watersheds. Boosted regression tree (BRT) models were used to study the relationship between watershed characteristics and the magnitude of trends in streamflow. At the CONUS scale, the balance between precipitation and evaporative demand, and measures of geographic location were of high relative importance. Relationships between the magnitude of trends and watershed characteristics at the ecoregion scale exhibited differences from the CONUS results and substantial variability was observed among ecoregions. Additionally, the methodology used here has the potential to serve as a robust framework for top-down, data driven analyses of the relationships between changes in the hydrologic cycle and the spatial context within which those changes occur. [ABSTRACT FROM AUTHOR]

Abstract Autotrophic respiration may regulate how ecosystem productivity responds to changes in temperature, atmospheric [CO2 ] and N deposition. Estimates of autotrophic respiration are difficult for forest ecosystems, because of the large amount of biomass, different metabolic rates among tissues, and seasonal variation in respiration rates. We examined spatial and seasonal patterns in autotrophic respiration in a Pinus strobus ecosystem, and hypothesized that seasonal patterns in respiration rates at a common temperature would vary with [N] for fully expanded foliage and fine roots, with photosynthesis for foliage, and with growth for woody tissues (stems, branches, and coarse roots). We also hypothesized that differences in [N] would largely explain differences in maintenance or dormant-season respiration among tissues. For April–November, mean respiration at 15 °C varied from 1.5 to 2.8 μmol kg-1 s-1 for fully expanded foliage, 1.7–3.0 for growing foliage, 0.8–1.6 for fine roots, 0.6–1.1 (sapwood) for stems, 0.5–1.8 (sapwood) for branches, and 0.2–1.5 (sapwood) for coarse roots. Growing season variation in respiration for foliage produced the prior year was strongly related to [N] (r 2 = 0.94), but fine root respiration was not related to [N]. For current-year needles, respiration did not covary with [N]. Night-time foliar respiration did not vary in concert with previous-day photosynthesis for either growing or fully expanded needles. Stem growth explained about one-third of the seasonal variation in stem respiration (r 2 = 0.38), and also variation among trees (r 2 = 0.43). We did not determine the cause of seasonal variation in branch and coarse root respiration, but it is unlikely to be directly related to growth, as the pattern of respiration in coarse roots and branches was not synchronized with stem growth. Seasonal variations in... [ABSTRACT FROM AUTHOR]

We merge concepts from stomatal optimization theory and cohesion-tension theory to examine the dynamics of three mechanisms that are potentially limiting to leaf-level gas exchange in trees during drought: (1) a 'demand limitation' driven by an assumption of optimal stomatal functioning; (2) 'hydraulic limitation' of water movement from the roots to the leaves; and (3) 'non-stomatal' limitations imposed by declining leaf water status within the leaf. Model results suggest that species-specific 'economics' of stomatal behaviour may play an important role in differentiating species along the continuum of isohydric to anisohydric behaviour; specifically, we show that non-stomatal and demand limitations may reduce stomatal conductance and increase leaf water potential, promoting wide safety margins characteristic of isohydric species. We used model results to develop a diagnostic framework to identify the most likely limiting mechanism to stomatal functioning during drought and showed that many of those features were commonly observed in field observations of tree water use dynamics. Direct comparisons of modelled and measured stomatal conductance further indicated that non-stomatal and demand limitations reproduced observed patterns of tree water use well for an isohydric species but that a hydraulic limitation likely applies in the case of an anisohydric species. [ABSTRACT FROM AUTHOR]

Coweeta Hydrologic Laboratory, located in western North Carolina, USA, is a 2,185 ha basin wherein forest climate monitoring and watershed experimentation began in the early 1930s. An extensive climate and hydrologic network has facilitated research for over 75 years. Our objectives in this paper were to describe the monitoring network, present long-term air temperature and precipitation data, and analyze the temporal variation in the long-term temperature and precipitation record. We found that over the period of record: (1) air temperatures have been increasing significantly since the late 1970s, (2) drought severity and frequency have increased with time, and (3) the precipitation distribution has become more extreme over time. We discuss the implications of these trends within the context of regional and global climate change and forest health. [ABSTRACT FROM AUTHOR]

Flow variability is a potential indicator of land use impacts on aquatic ecosystems and a dominating factor for lotic habitats. Vegetation management effects on the stream habitat conditions must be better understood to propose forest management activities that are compatible with general ecosystem management objectives (integrity, diversity, sustainability, etc.). In our study, we used long term flow data (1936-2004) from four gauged experimental watersheds (W1, W2, W17, W18) of Coweeta Hydrologic Laboratory in US to assess the impacts of pine conversion on flow characteristics by using paired watershed experimentation. In W1, all trees and shrubs were cut and burned in 1956-57 and white pine (Pinus strobus) was planted in 1957. In W17, white pine was planted in 1956. W2 and W18 have been kept untreated as reference watersheds for W1 and W17, respectively. After analyzing long-term daily flow series with flow duration curves and frequency analyzes, we found that the timing and magnitude of 7Q flows were changed significantly due to conversion but flow variability was not affected. Overall findings revealed that pine conversion has significantly influenced some flow characteristics but stream habitat conditions were not affected potentially. [ABSTRACT FROM AUTHOR]

Summary: Hierarchical Bayesian (HB) modeling allows for multiple sources of uncertainty by factoring complex relationships into conditional distributions that can be used to draw inference and make predictions. We applied an HB model to estimate the parameters and state variables of a parsimonious hydrological model – GR4J – by coherently assimilating the uncertainties from the model, observations, and parameters at Coweeta Basin in western North Carolina. A state-space model was within the Bayesian hierarchical framework to estimate the daily soil moisture levels and their uncertainties. Results show that the posteriors of the parameters were updated from and relatively insensitive to priors, an indication that they were dominated by the data. The uncertainties of the simulated streamflow increased with streamflow increase. By assimilating soil moisture data, the model could estimate the maximum capacity of soil moisture accounting storage and predict storm events with higher precision compared to not assimilating soil moisture data. This study has shown that hierarchical Bayesian model is a useful tool in water resource planning and management by acknowledging stochasticity. [Copyright &y& Elsevier]

Presents a study which examined nitrogen trace gas emissions from a riparian ecosystem in southern Appalachia. Methods; Results; Discussion.

Shifts in streamflow, due to future climate and land use change, may pose risks to nearby human communities. Projecting the spatial distribution and impacts of these risks requires consideration of biophysical and socioeconomic factors. Models like the Soil and Water Assessment Tool (SWAT) can project spatial distributions of hydrologic risk due to shifting biophysical factors like climate and land use, but cannot account for socioeconomic factors influencing a community's capacity to adapt to future streamflow changes. To address this limitation, we used a risk matrix to classify subbasins in a large river basin in the southeastern USA based on (1) percent increase in SWAT simulated 10-year and extreme high flows due to climate and land use change between baseline (1982–2002) and projected (2050–2070) periods and (2) degree of community vulnerability according to a Social Vulnerability Index (SVI). We compared spatial distributions of high-risk subbasins based on SWAT results, SVI results, and the integration of SWAT and SVI results using a risk matrix. Large increases in simulated 10-year and extreme high flows occurred in middle and lower parts of the river basin, and socially vulnerable communities were distributed throughout. We identified 16, 7, and 14 unique high-risk subbasins using SWAT results, SVI results, and SWAT and SVI results, respectively. By using a risk matrix, we identified subbasins with vulnerable communities that are projected to experience future increases in streamflow due to climate and land use change. These results serve as a starting point for subsequent climate change adaptation planning. [ABSTRACT FROM AUTHOR]

The interactions of terrestrial C pools and fluxes with spatial and temporal variation in climate are not well understood. We conducted this study in the southern Appalachian Mountains where complex topography provides variability in temperature, precipitation, and forest communities. In 1990, we established five large plots across an elevation gradient allowing us to study the regulation of C and N pools and cycling by temperature and water, in reference watersheds in Coweeta Hydrologic Laboratory, a USDA Forest Service Experimental Forest, in western NC, USA. Communities included mixed-oak pine, mixed-oak, cove hardwood, and northern hardwood. We examined 20-year changes in overstory productivity and biomass, leaf litterfall C and N fluxes, and total C and N pools in organic and surface mineral soil horizons, and coarse wood, and relationships with growing season soil temperature and precipitation. Productivity increased over time and with precipitation. Litterfall C and N flux increased over time and with increasing temperature and precipitation, respectively. Organic horizon C and N did not change over time and were not correlated to litterfall inputs. Mineral soil C and N did not change over time, and the negative effect of temperature on soil pools was evident across the gradient. Our data show that increasing temperature and variability in precipitation will result in altered aboveground productivity. Variation in surface soil C and N is related to topographic variation in temperature which is confounded with vegetation community. Data suggest that climatic changes will result in altered aboveground and soil C and N sequestration and fluxes. [ABSTRACT FROM AUTHOR]

Increasing air temperature is expected to extend growing season length in temperate, broadleaf forests, leading to potential increases in evapotranspiration and net carbon uptake. However, other key processes affecting water and carbon cycles are also highly temperature-dependent. Warmer temperatures may result in higher ecosystem carbon loss through respiration and higher potential evapotranspiration through increased atmospheric demand for water. Thus, the net effects of a warming planet are uncertain and highly dependent on local climate and vegetation. We analyzed five years of data from the Coweeta eddy covariance tower in the southern Appalachian Mountains of western North Carolina, USA, a highly productive region that has historically been underrepresented in flux observation networks. We examined how leaf phenology and climate affect water and carbon cycling in a mature forest in one of the wettest biomes in North America. Warm temperatures in early 2012 caused leaf-out to occur two weeks earlier than in cooler years and led to higher seasonal carbon uptake. However, these warmer temperatures also drove higher winter ecosystem respiration, offsetting much of the springtime carbon gain. Interannual variability in net carbon uptake was high (147 to 364 g C m −2  y −1 ), but unrelated to growing season length. Instead, years with warmer growing seasons had 10% higher respiration and sequestered ∼40% less carbon than cooler years. In contrast, annual evapotranspiration was relatively consistent among years (coefficient of variation = 4%) despite large differences in precipitation (17%, range = 800 mm). Transpiration by the evergreen understory likely helped to compensate for phenologically-driven differences in canopy transpiration. The increasing frequency of high summer temperatures is expected to have a greater effect on respiration than growing season length, reducing forest carbon storage. [ABSTRACT FROM AUTHOR]

Trees alter their use and allocation of nutrients in response to drought, and changes in soil nutrient cycling and trace gas flux (N2O and CH4) are observed when experimental drought is imposed on forests. In extreme droughts, trees are increasingly susceptible to attack by pests and pathogens, which can lead to major changes in nutrient flux to the soil. Extreme droughts often lead to more common and more intense forest fires, causing dramatic changes in the nutrient storage and loss from forest ecosystems. Changes in the future manifestation of drought will affect carbon uptake and storage in forests, leading to feedbacks to the Earth's climate system. We must improve the recognition of drought in nature, our ability to manage our forests in the face of drought, and the parameterization of drought in earth system models for improved predictions of carbon uptake and storage in the world's forests. [ABSTRACT FROM AUTHOR]

Over the past century forest regrowth in Europe and North America expanded forest carbon (C) sinks and offset C emissions but future C accumulation is uncertain. Policy makers need insights into forest C dynamics as they anticipate emissions futures and goals. We used land use and forest inventory data to estimate how forest C dynamics have changed in the southeastern United States and attribute changes to land use, management, and disturbance causes. From 2007-2012, forests yielded a net sink of C because of net land use change (+6.48 Tg C yr−1) and net biomass accumulation (+75.4 Tg C yr−1). Forests disturbed by weather, insect/disease, and fire show dampened yet positive forest C changes (+1.56, +1.4, +5.48 Tg C yr−1, respectively). Forest cutting caused net decreases in C (−76.7 Tg C yr−1) but was offset by forest growth (+143.77 Tg C yr−1). Forest growth rates depend on age or stage of development and projected C stock changes indicate a gradual slowing of carbon accumulation with anticipated forest aging (a reduction of 9.5% over the next five years). Additionally, small shifts in land use transitions consistent with economic futures resulted in a 40.6% decrease in C accumulation. [ABSTRACT FROM AUTHOR]

Mountain watersheds are primary sources of freshwater, carbon sequestration, and other ecosystem services. There is significant interest in the effects of climate change and variability on these processes over short to long time scales. Much of the impact of hydroclimate variability in forest ecosystems is manifested in vegetation dynamics in space and time. In steep terrain, leaf phenology responds to topoclimate in complex ways, and can produce specific and measurable shifts in landscape forest patterns. The onset of spring is usually delayed at a specific rate with increasing elevation (often called Hopkins' Law; Hopkins, 1918), reflecting the dominant controls of temperature on greenup timing. Contrary with greenup, leaf senescence shows inconsistent trends along elevation gradients. Here, we present mechanisms and an explanation for this variability and its significance for ecosystem patterns and services in response to climate. We use moderate-resolution imaging spectro-radiometer ( MODIS) Normalized Difference Vegetation Index ( NDVI) data to derive landscape-induced phenological patterns over topoclimate gradients in a humid temperate broadleaf forest in southern Appalachians. These phenological patterns are validated with different sets of field observations. Our data demonstrate that divergent behavior of leaf senescence with elevation is closely related to late growing season hydroclimate variability in temperature and water balance patterns. Specifically, a drier late growing season is associated with earlier leaf senescence at low elevation than at middle elevation. The effect of drought stress on vegetation senescence timing also leads to tighter coupling between growing season length and ecosystem water use estimated from observed precipitation and runoff generation. This study indicates increased late growing season drought may be leading to divergent ecosystem response between high and low elevation forests. Landscape-induced phenological patterns are easily observed over wide areas and may be used as a unique diagnostic for sources of ecosystem vulnerability and sensitivity to hydroclimate change. [ABSTRACT FROM AUTHOR]

Legacy effects of past land use and disturbance are increasingly recognized, yet consistent definitions of and criteria for defining them do not exist. To address this gap in biological- and ecosystem-assessment frameworks, we propose a general metric for evaluating potential legacy effects, which are computed by normalizing altered system function persistence with duration of disturbance. We also propose two distinct legacy-effect categories: signal effects from lags in transport and structural effects from physical landscape changes. Using flux records for water, sediment, nitrogen, and carbon from long-term study sites in the eastern United States from 1500 to 2000, we identify gaps in our understanding of legacy effects and reveal that changes in basin sediment dynamics precede instrumented records. These sediment dynamics are not generally incorporated into interpretations of contemporary records, although their potential legacy effects are substantial. The identification of legacy effects may prove to be a fundamental component of landscape management and effective conservation and restoration practice. [ABSTRACT FROM AUTHOR]

Understanding changes in community composition caused by invasive species is critical for predicting effects on ecosystem function, particularly when the invasive threatens a foundation species. Here we focus on dynamics of forest structure, composition and microclimate, and how these interact in southern Appalachian riparian forests following invasion by hemlock woolly adelgid, HWA, Adelges tsugae. We measured and quantified changes in microclimate; canopy mortality; canopy and shrub growth; understory species composition; and the cover and diversity in riparian forests dominated by eastern hemlock Tsuga canadensis over a period of seven years. Treatments manipulated hemlock mortality either through invasion (HWA infested stands) or girdling (GDL) hemlock trees. Mortality was rapid, with 50% hemlock tree mortality occurring after six years of invasion, in contrast to more than 50% mortality in two years following girdling. Although 50% of hemlock trees were still alive five years after infestation, leaf area lost was similar to that of girdled trees. As such, overall responses over time (changes in light transmittance, growth, soil moisture) were identical to girdled stands with 100% mortality. Our results showed different growth responses of the canopy species, shrubs and ground layer, with the latter being substantially influenced by presence of the evergreen shrub, rhododendron Rhododendron maximum. Although ground layer richness in the infested and girdled stands increased by threefold, they did not approach levels recorded in hardwood forests without rhododendron. Increased growth of co-occurring canopy trees occurred in the first few years following hemlock decline, with similar responses in both treatments. In contrast, growth of rhododendron continued to increase over time. By the end of the study it had a 2.6-fold higher growth rate than expected, likely taking advantage of increased light available during leaf-off periods of the deciduous species. Increased growth and dominance of rhododendron may be a major determinant of future responses in southern Appalachian ecosystems; however, our results suggest hemlock will be replaced by a mix of Acer, Betula, Fagus and Quercus canopy genera where establishment is not limited by rhododendron. [ABSTRACT FROM AUTHOR]

The microbial conversion of organic nitrogen (N) to plant available forms is a critical determinant of plant growth and carbon sequestration in forests worldwide. In temperate zones, microbial activity is coupled to variations in temperature, yet at the ecosystem level, microbial N mineralization seems to play a minor role in determining patterns of N loss. Rather, N losses often appear to vary with seasonality in hydrology and plant demand, while exports over longer periods are thought to be associated with increasing rates of anthropogenic N deposition. We analyzed long-term (21-32 years) time series of climate and stream and atmospheric chemistry from two temperate deciduous forest watersheds in the southeastern USA to understand the sensitivity of internal forest N cycles to climate variation and atmospheric deposition. We evaluated the time series with a simple analytical model that incorporates key biotic constraints and mechanisms of N limitation and cycling in plant-soil systems. Through maximum likelihood analysis, we derive biologically realistic estimates of N mineralization and its temperature sensitivity ( Q). We find that seasonality and long-term trends in stream nitrate (NO) concentrations can in large part be explained by the dynamics of internal biological cycling responding to climate rather than external forcing from atmospheric chemistry. In particular, our model analysis suggests that much of the variation in N cycling in these forests results from the response of microbial activity to temperature, causing NO losses to peak in the growing season and to accelerate with recent warming. Extrapolation of current trends in temperature and N deposition suggests that the upturn in temperature may increase future N export by greater than threefold more than from increasing deposition, revealing a potential direct effect of anthropogenic warming on terrestrial N cycles. [ABSTRACT FROM AUTHOR]

The recent infestation of southern Appalachian eastern hemlock stands by hemlock woolly adelgid (HWA) is expected to have dramatic and lasting effects on forest structure and function. We studied the short-term changes to the carbon cycle in a mixed stand of hemlock and hardwoods, where hemlock was declining due to either girdling or HWA infestation. We expected that hemlock would decline more rapidly from girdling than from HWA infestation. Unexpectedly, in response to both girdling and HWA infestation, hemlock basal area increment (BAI) reduced substantially compared to reference hardwoods in 3 years. This decline was concurrent with moderate increases in the BAI of co-occurring hardwoods. Although the girdling treatment resulted in an initial pulse of hemlock needle inputs, cumulative litter inputs and O horizon mass did not differ between treatments over the study period. Following girdling and HWA infestation, very fine root biomass declined by 20–40% in 2 years, which suggests hemlock root mortality in the girdling treatment, and a reduction in hemlock root production in the HWA treatment. Soil CO2 efflux ( Esoil) declined by approximately 20% in 1 year after both girdling and HWA infestation, even after accounting for the intra-annual variability of soil temperature and moisture. The reduction in Esoil and the concurrent declines in BAI and standing very fine root biomass suggest rapid declines in hemlock productivity from HWA infestation. The accelerated inputs of detritus resulting from hemlock mortality are likely to influence carbon and nutrient fluxes, and dictate future patterns of species regeneration in these forest ecosystems. [ABSTRACT FROM AUTHOR]

Natural forests in southern China have been severely logged due to high human demand for timber, food, and fuels during the past century, but are recovering in the past decade. The objective of this study was to investigate how vegetation cover changes in composition and structure affected the water budgets of a 9.6-km2 Dakeng watershed located in a humid subtropical mountainous region in southern China. We analyzed 27 years (i.e., 1967-1993) of streamflow and climate data and associated vegetation cover change in the watershed. Land use/land cover census and Normalized Difference of Vegetation Index (NDVI) data derived from remote sensing were used to construct historic land cover change patterns. We found that over the period of record, annual streamflow ( Q) and runoff/precipitation ratio did not change significantly, nor did the climatic variables, including air temperature, Hamon’s potential evapotranspiration (ET), pan evaporation, sunshine hours, and radiation. However, annual ET estimated as the differences between P and Q showed a statistically significant increasing trend. Overall, the NDVI of the watershed had a significant increasing trend in the peak spring growing season. This study concluded that watershed ecosystem ET increased as the vegetation cover shifted from low stock forests to shrub and grasslands that had higher ET rates. A conceptual model was developed for the study watershed to describe the vegetation cover-streamflow relationships during a 50-year time frame. This paper highlighted the importance of eco-physiologically based studies in understanding transitory, nonstationary effects of deforestation or forestation on watershed water balances. [ABSTRACT FROM AUTHOR]

It is critical that evapotranspiration (ET) be quantified accurately so that scientists can evaluate the effects of land management and global change on water availability, streamflow, nutrient and sediment loading, and ecosystem productivity in watersheds. The objective of this study was to derive a new semi-empirical ET modeled using a dimension analysis method that could be used to estimate forest ET effectively at multiple temporal scales. The model developed describes ET as a function of water availability for evaporation and transpiration, potential ET demand, air humidity, and land surface characteristics. The model was tested with long-term hydrometeorological data from five research sites with distinct forest hydrology in the United States and China. Averaged simulation error for daily ET was within 0.5 mm/day. The annual ET at each of the five study sites were within 7% of measured values. Results suggest that the model can accurately capture the temporal dynamics of ET in forest ecosystems at daily, monthly, and annual scales. The model is climate-driven and is sensitive to topography and vegetation characteristics and thus has potential to be used to examine the compounding hydrologic responses to land cover and climate changes at multiple temporal scales. [ABSTRACT FROM AUTHOR]

Abstract: Many researchers are using sap flux to estimate tree-level transpiration, and to scale to stand- and catchment-level transpiration; yet studies evaluating the comparability of sap flux-based estimates of transpiration (E t) with alternative methods for estimating E t at this spatial scale are rare. Our ability to accurately scale from the probe to the tree to the watershed has not yet been demonstrated, nor do we know the relative impact of the main sources of variability on our scaled estimates. Accounting for the variability in the radial distribution of sap flux within the sapwood, the variability of transpiration among trees and between plots within the catchment, and the variability in stand density, sapwood area, and leaf area are critical for making landscape inferences about transpiration. During 2004 and 2005, we continuously monitored 40 trees in three plots within a 13.5-ha gauged watershed comprising a 50-year-old eastern white pine plantation within the Coweeta Basin in western North Carolina, USA. We scaled sap flux-based estimates of stand transpiration (E t) and surface area-based estimates of stand interception (E i) to the catchment and compared these with water balance estimates of evapotranspiration (E, precipitation minus runoff, P − R o). For both years, the sum of sap flux scaled E t and E i were 14 and 7% lower than evapotranspiration estimated from P − R o. Our results show that a considerable amount of variation exists at each scaling step encountered; however, a simple scaling exercise revealed that omitting among plot variation affected the sap flux scaled E t estimate by 48%. Thus, the largest source of variability in scaling to the landscape was landscape variation in stand density and sapwood area. [Copyright &y& Elsevier]

Summary: It is widely recognized that vegetation restoration will have positive effects on watershed health by reducing soil erosion and non-point source pollution, enhancing terrestrial and aquatic habitat, and increasing ecosystem carbon sequestration. However, the hydrologic consequences of forestation on degraded lands are not well studied in the forest hydrology community as a whole. China has the largest area of forest plantations in the world now, and the hydrologic consequences of massive forestation are unknown. We applied a simplified hydrological model across the diverse physiographic region to estimate the potential magnitude of annual water yield response to forestation. Our study suggests that the average water yield reduction may vary from about 50mm/yr (50%) in the semi-arid Loess Plateau region in northern China to about 300mm/yr (30%) in the tropical southern region. We conclude that forestation in China that often involves a combination of tree planting and engineering (e.g., terracing) may have even a higher potential to greatly reduce annual water yield in headwater watersheds, especially in the semi-arid Loess Plateau region. However, the forestation area is relatively small for most large basins with mixed landuses in China, thus the regional effects of forestation on water resource management may not be of major concern. Comprehensive science-based evaluation of roles of forests on regulating regional water resources is critical to the current forestation endeavors in China. [Copyright &y& Elsevier]

• A computationally efficient method for large scale GPP estimation is proposed • Vegetation structural change in forest contributed most to the GPP increase in China • Afforestation enhanced GPP significantly in China • Solar radiation decline dominated the negative effect of climate variation on China's GPP • CO 2 fertilization offset the negative effect of climate variation on China's GPP during 2001-2016 Terrestrial gross primary production (GPP) is the largest carbon flux from the atmosphere to the terrestrial ecosystems. However, how GPP dynamics respond to vegetation structural change (VSC), climate change, and rising CO 2 concentration are still unclear. In this study, we developed a process-based model (i.e., Farquhar GPP Model) to characterize GPP dynamics in China from 2001 to 2016, and further identify the dominant drivers for GPP variation related to VSC, climate change, and rising atmospheric CO 2 through model scenario design. During the study period, VSC showed an overwhelmingly positive impact on annual total GPP (i.e., 39.2 Tg C yr−1, P<0.001), followed by the fertilization effect of rising CO 2 (i.e., 11.9 Tg C yr−1, P<0.001). However, climate variation would be expected to have an overall negative effect (i.e., -9.9 Tg C yr−1, P<0.001), which is primarily related to the decline of the incoming solar radiation over this period. Consequently, China's GPP showed a remarkable upward trend (i.e., 40.5 Tg C yr−1, P<0.001) during the study period. Our study highlighted the strong positive GPP contribution from VSC, which is mainly related to forest increase under the background of forest cover changes primarily from widespread forest conservation and restoration programs implemented in China since the late 1990s. Therefore, our study suggests that forest conservation and restoration could effectively increase terrestrial ecosystem GPP, despite the negative effects of climate change. [ABSTRACT FROM AUTHOR]

Land surface phenology (LSP) has been widely used as the "footprint" of urbanization and global climate change. Shifts of LSP have cascading effects on food production, carbon sequestration, water consumption, biodiversity, and public health. Previous studies mainly focused on investigating the effects of urbanization on the spatial patterns of LSP by comparing phenological metrics, e.g. start of season (SOS) and end of season (EOS), between urban center and the surrounding rural regions. However, it remains unclear how urbanization-induced land cover conversions and climate change jointly influence the temporal variations of SOS and EOS within the urban ecosystem. To fill this knowledge gap, we utilized daily two-band enhanced vegetation index, daily meteorological record, and annual land cover dataset to investigate the respective impacts of urbanization and climate change on temporal shifts of LSP between the post- and the pre-urbanization periods over 196 large cities in the northern mid-latitudes. We found 51% of the cities experienced an advanced SOS with an average of −6.39 ± 5.82 days after urbanization has occurred, while the remaining 49% of the cities had a delayed SOS with an average of 7.56 ± 5.63 days. We also found a later EOS at 53% of the cities and an earlier EOS at 47% of the cities with an average of 8.43 ± 7.59 and −5.57 ± 4.99 days between the post- and pre-urbanization periods, respectively. Multiple linear regression analysis indicates that climate variables (i.e. temperature, precipitation, and insolation) play dominant roles in regulating the temporal shifts of LSP. Furthermore, the earlier SOS and later EOS were significantly correlated with the amplitude of urbanization (i.e. increase of impervious surface area) in cities after controlling for effects of climate factors. These patterns were generally consistent across eight climate zones. Our findings provide critical information in modeling natural and anthropogenic effects on urban ecosystem, with important benefits for urban sustainability and biodiversity conservation. • Post - and pre-urbanization periods were identified for 196 large cities. • Half of the cities experienced earlier SOS and delayed EOS after urbanization. • Climate change played a dominant role in regulating temporal shifts of phenology. • Urbanization can either amplified or off-set climate effects on phenology. • Influences of urbanization and climate change vary by climate background. [ABSTRACT FROM AUTHOR]