Declining water yield from forested mountain watersheds in response to climate change and forest mesophication.

Climate change and forest disturbances are threatening the ability of forested mountain watersheds to provide the clean, reliable, and abundant fresh water necessary to support aquatic ecosystems and a growing human population. Here, we used 76 years of water yield, climate, and field plot vegetation measurements in six unmanaged, reference watersheds in the southern Appalachian Mountains of North Carolina, USA to determine whether water yield has changed over time, and to examine and attribute the causal mechanisms of change. We found that annual water yield increased in some watersheds from 1938 to the mid-1970s by as much as 55%, but this was followed by decreases up to 22% by 2013. Changes in forest evapotranspiration were consistent with, but opposite in direction to the changes in water yield, with decreases in evapotranspiration up to 31% by the mid-1970s followed by increases up to 29% until 2013. Vegetation survey data showed commensurate reductions in forest basal area until the mid-1970s and increases since that time accompanied by a shift in dominance from xerophytic oak and hickory species to several mesophytic species (i.e., mesophication) that use relatively more water. These changes in forest structure and species composition may have decreased water yield by as much as 18% in a given year since the mid-1970s after accounting for climate. Our results suggest that changes in climate and forest structure and species composition in unmanaged forests brought about by disturbance and natural community dynamics over time can result in large changes in water supply.

Can forest management be used to sustain water-based ecosystem services in the face of climate change?

Forested watersheds, an important provider of ecosystems services related to water supply, can have their structure, function, and resulting streamflow substantially altered by land use and land cover. Using a retrospective analysis and synthesis of long-term climate and streamfiow data (75 years) from six watersheds differing in management histories we explored whether streamflow responded differently to variation in annual temperature and extreme precipitation than unmanaged watersheds. We show significant increases in temperature and the frequency of extreme wet and dry years since the 1980s. Response models explained almost all streamflow variability (adjusted R2 > 0.99). In all cases, changing land use altered streamflow. Observed watershed responses differed significantly in wet and dry extreme years in all but a stand managed as a coppice forest. Converting deciduous stands to pine altered the streamflow response to extreme annual precipitation the most; the apparent frequency of observed extreme wet years decreased on average by sevenfold. This increased soil water storage may reduce flood risk in wet years, but create conditions that could exacerbate drought. Forest management can potentially mitigate extreme annual precipitation associated with climate change; however, offsetting effects suggest the need for spatially explicit analyses of risk and vulnerability.

Simulated effects of sulfur deposition on nutrient cycling in class I wilderness areas.

We predicted the effects of sulfate (SO(4)) deposition on wilderness areas designated as Class I air quality areas in western North Carolina using a nutrient cycling model (NuCM). We used three S deposition simulations: current, 50% decrease, and 100% increase. We measured vegetation, forest floor, and root biomass and collected soil, soil solution, and stream water samples for chemical analyses. We used the closest climate stations and atmospheric deposition stations to parameterize NuCM. The areas were: Joyce Kilmer (JK), Shining Rock (SR), and Linville Gorge (LG). They differ in soil acidity and nutrients, and soil solution and stream chemistry. Shining Rock and LG have lower soil solution base cation and higher acidic ion concentrations than JK. For SR and LG, the soil solution Ca/Al molar ratios are currently 0.3 in the rooting zone (A horizon), indicating Al toxicity. At SR, the simulated Ca/Al ratio increased to slightly above 1.5 after the 30-yr simulation regardless of S deposition reduction. At LG, Ca/Al ratios ranged from 1.6 to 2.4 toward the end of the simulation period, the 100% increase scenario had the lower value. Low Ca/Al ratios suggest that forests at SR and LG are significantly stressed under current conditions. Our results also suggest that SO(4) retention is low, perhaps contributing to their high degree of acidification. Their soils are acidic, low in weatherable minerals, and even with large reductions in SO(4) and associated acid deposition, it may take decades before these systems recover from depletion of exchangeable Ca, Mg, and K.

EMERGY-based environmental systems assessment of a multi-purpose temperate mixed-forest watershed of the southern Appalachian Mountains, USA.

Emergy (with an 'm') synthesis was used to assess the balance between nature and humanity and the equity among forest outcomes of a US Forest Service ecosystem management demonstration project on the Wine Spring Creek watershed, a high-elevation (1600 m), temperate forest located in the southern Appalachian mountains of North Carolina, USA. EM embraces a holistic perspective, accounting for the multiple temporal and spatial scales of forest processes and public interactions, to balance the ecological, economic, and social demands placed on land resources. Emergy synthesis is a modeling tool that allows the structure and function of forest ecosystems to be quantified in common units (solar emergy-joules, sej) for easy and meaningful comparison, determining 'system-value' for forcing factors, components, and processes based on the amount of resources required to develop and sustain them, whether they are money, material, energy, or information. The Environmental Loading Ratio (ELR), the units of solar emergy imported into the watershed via human control per unit of indigenous, natural solar emergy, was determined to be 0.42, indicating that the load on the natural environment was not ecologically damaging and that excess ecological capacity existed for increasing non-ecological activities (e.g. timbering, recreation) to achieve an ELR of 1.0 (perfect ecological-economic balance). Three forest outcomes selected to represent the three categories of desired sustainability (ecological, economic, and social) were evaluated in terms of their solar emergy flow to measure outcome equity. Direct economic contribution was an order of magnitude less (224 x 10(12)solar emergy-joules (sej) ha(-1)) than the ecological and social contributions, which were provided at annual rates of 3083 and 2102 x 10(12)sejha(-1), respectively. Emergy synthesis was demonstrated to holistically integrate and quantify the interconnections of a coupled nature-human system allowing the goals of ecological balance and outcome equity to be measured quantitatively.

Nitrogen trace gas emissions from a riparian ecosystem in southern Appalachia.

In this paper, we present two years of seasonal nitric oxide (NO), ammonia (NH3), and nitrous oxide (N2O) trace gas fluxes measured in a recovering riparian zone with cattle excluded and adjacent riparian zone grazed by cattle. In the recovering riparian zone, average NO, NH3, and N2O fluxes were 5.8, 2.0, and 76.7 ng N m(-2) S(-1) (1.83, 0.63, and 24.19 kg N ha(-1) y(-1)), respectively. Fluxes in the grazed riparian zone were larger, especially for NO and NH3, measuring 9.1, 4.3, and 77.6 ng N m(-2) S(-1) (2.87, 1.35, and 24.50 kg N ha(-1) y(-1)) for NO, NH3, and N2O, respectively. On average, N2O accounted for greater than 85% of total trace gas flux in both the recovering and grazed riparian zones, though N2O fluxes were highly variable temporally. In the recovering riparian zone, variability in seasonal average fluxes was explained by variability in soil nitrogen (N) concentrations. Nitric oxide flux was positively correlated with soil ammonium (NH4+) concentration, while N2O flux was positively correlated with soil nitrate (NO3-) concentration. Ammonia flux was positively correlated with the ratio of NH4+ to NO3-. In the grazed riparian zone, average NH3 and N2O fluxes were not correlated with soil temperature, N concentrations, or moisture. This was likely due to high variability in soil microsite conditions related to cattle effects such as compaction and N input. Nitric oxide flux in the grazed riparian zone was positively correlated with soil temperature and NO3- concentration. Restoration appeared to significantly affect NO flux, which increased approximately 600% during the first year following restoration and decreased during the second year to levels encountered at the onset of restoration. By comparing the ratio of total trace gas flux to soil N concentration, we show that the restored riparian zone is likely more efficient than the grazed riparian zone at diverting upper-soil N from the receiving stream to the atmosphere. This is likely due to the recovery of microbiological communities following changes in soil physical characteristics.

Watershed Ecosystem Analysis as a Basis for Multiple-Use Management of Eastern Forests.

There is ever-increasing competition for the many uses and natural resources of forests in the eastern United States. Multiple-use management has long been a stated goal for these forests, but application has been problematic and seldom satisfactory to all users. There is a need to incorporate more science into management decisions for Eastern forests, and thereby convincingly demonstrate to forest managers and the public why certain combinations of uses may or may not be compatible. One proven approach for doing this is to use watershed ecosystem analysis. Small watersheds, usually <100 ha in area, serve as a convenient ecosystem for studying how forests function in terms of cycling energy, nutrients, and water. Results of these studies allow assessments of forest health and productivity, and evaluations of impacts of both natural and human-related disturbances. This paper provides illustrations of how watershed ecosystem analysis can be used to study the effects of current harvesting practices, acidic deposition, and past land use. The paper also shows how recommendations for land use are derived from watershed ecosystem analysis, and how they are put into practice.

Assessing seasonal leaf area dynamics and vertical leaf area distribution in eastern white pine (Pinus strobus L.) with a portable light meter.

We evaluated the ability of a portable light meter (Sunfleck Ceptometer, Decagon Devices, Pullman, WA, USA) to quantify seasonal photosynthetically active radiation (PAR, 400-700 nm) interception, projected stand leaf area index (LAI), and vertical LAI distribution in a 32-year-old eastern white pine (Pinus strobus L.) plantation. Canopy PAR transmittance measured with the ceptometer was converted to LAI with the Beer-Lambert Equation. The ceptometer was sensitive to changes in PAR transmittance resulting from foliage growth. Predicted stand LAI ranged from 3.5 in the dormant season to a maximum of 5.3 in late July. Predicted LAI values were within 9% of values determined from destructive sampling. Published canopy extinction coefficients (k) were inadequate for converting PAR transmittance data to stand LAI because a significant amount of PAR was intercepted by dead branches and stems below the forest canopy. Because of interception by dead branches and stems, we estimated k = 0.84, which is substantially higher than previously reported values. The ceptometer was also sensitive to seasonal changes in PAR transmittance within the canopy. However, in contrast to predictions based on the Beer-Lambert Law, the relationship between proportional PAR transmittance (Q(i)/Q(o)) and cumulative LAI within the canopy was linear. Thus, vertical LAI distribution was best estimated with a linear model, as opposed to the non-linear model assumed in the Beer-Lambert Equation. We hypothesize that the linear relationship was a result of a gap in the canopy which was not represented by the cumulative leaf area distribution estimation procedure.