Showing papers by "John D. Aber published in 1999"

Hemlock woolly adelgid impacts on community structure and N cycling rates in eastern hemlock forests

Abstract: Mortality of dominant tree species caused by introduced pests and pathogens have been among the most pervasive and visible impacts of humans on eastern U.S. forests in the 20th century, yet little is known about the ecosystem-level consequences of these invasions. In this study we quantified the impacts of the introduced hemlock woolly adelgid (Adelges tsugae Annand) on community structure and ecosystem processes in eastern hemlock (Tsuga canadensis (L.) Carr.) forests in southern New England. Data were collected at six hemlock-dominated sites spanning a continuum from 0 to 99% mortality. Light availability to the understory and seedling regeneration both increased in stands affected by the adelgid. Differences in soil organic matter, total C, and total N pools between infested and noninfested sites were not associated with hemlock decline. Net N mineralization, nitrification, and N turnover increased at sites experiencing hemlock mortality. Inorganic N availability and nitrification rates increased dramatically with adelgid infestation and hemlock mortality, suggesting that nitrate leaching is likely in regions experiencing hemlock mortality. In the longer term, ecosystem processes at infested stands are likely to be driven by the successional dynamics that follow hemlock mortality. Resume : La mortalite causee chez les principales especes d'arbres par des ravageurs et des pathogenes introduits compte parmi les impacts les plus etendus et les plus visibles de l'homme sur les forets de l'est des Etats-Unis au cours du vingtieme siecle. Pourtant, on connait peu de choses relativement aux consequences de ces invasions sur les ecosystemes. Dans le cadre de cette etude, nous avons quantifie les effets du puceron lanigere de la pruche ( Adelges tsugae Annand) sur la structure des communautes et sur les processus ecosystemiques dans des forets de pruche du

Controls on N Retention and Exports in a Forested Watershed

Abstract: We conducted a 15N-tracer study in a fertilized, forested catchment at the Bear Brook Watersheds in Maine (BBWM), USA, in order to characterize N cycling processes, identify sinks for ammonium-N additions, and determine the contribution of the experimental ammonium additions to nitrate exports from the treated catchment. Distributions of 15N in plant tissues, soils, precipitation and streamwater collected before adding tracers showed that nitrate-N (the dominant form of inorganic N deposition at the site) inputs under ambient conditions were depleted in 15N relative to plants and that soil was enriched in 15N relative to plants. The 15N content of streamwater nitrate was within the range of 15N contents in natural plant tissues, suggesting that nitrate deposited from the atmosphere is reduced and assimilated into soil and plant N pools before being leached as nitrate from the catchment. Variations in 15N natural abundances also suggested that most N uptake by trees is from the forest floor and that nitrification occurs in soils at this catchment under ambient conditions. Changes in 15N contents of plant tissues, soils and streamwater after adding a 15N tracer to the ammonium sulfate fertilizer applied to the treated catchment showed that soils were the dominant sink for the labeled ammonium. Surface soils (Oea horizon plus any underlying mineral soil to 5cm depth) assimilated 19 to 31 percent of the 42 kg ha−1 of 15N-labelled ammonium-N during the tracer study. Aboveground biomass assimilated 8 to 17 percent of the labeled ammonium-N additions. Of the three forest types on the catchment, the soil:biomass assimilation ratio of labeled-N was highest in the spruce forest, intermediate in the beech-dominated hardwood forest and lowest in the mixed hardwood-spruce forest. Although ammonium sulfate additions led to increases in streamwater nitrate, only 2 of the 13 kg ha−1 of nitrate-N exported from the catchment during the 2 years of tracer additions was derived from the 42 kg ha−1 of labeled ammonium-N additions.

Soil detrital processes controlling the movement of 15n tracers to forest vegetation

Abstract: Controlled field experiments to study the effects of heightened atmospheric inputs of nitrogen (N) to forests typically demonstrate that most N enters nonextractable pools in soil, while some N is taken up by vegetation, and varying amounts are exported. In a few experimental manipulations of N inputs to forests, 15N has been added as a tracer to more closely study the fates and redistributions of NH4+ and NO3− at the ecosystem level. We developed TRACE, a biogeochemical process model based on previous models, to interpret ecosystem-level 15N field data following applications of 15N-enriched NO3− or NH4+ at the Harvard Forest, Massachusetts, USA. We simulated the forms, masses, atom%, and timing of 15N applications in ambient and chronically fertilized plots over two growing seasons in coniferous and deciduous forest stands. Incorporating principles of stable-isotope redistributions, such as mass balance and pool dilution, into the process model provided a strong means of comparing alternative model formulations against field data. TRACE explicitly illustrated the manner in which rates of gross N turnover in soils could be high enough to provide strong sinks for 15N in ambient plots, while limited enough to allow much greater uptake of 15N by vegetation in fertilized plots. Ectorganic horizons, including litter and humified matter, were key in retaining 15N inputs. We found that fine root uptake and turnover could not account for the rapid movement of 15N into soil pools; direct assimilation into soil pools was required for both NH4+ and NO3− in both deciduous and coniferous forests. Such high rates of N assimilation could not be accounted for by microbial biomass production using detrital C as the substrate. These findings have far-ranging implications for understanding the reciprocal effects of N deposition on forest C budgets, and forest C cycling on ecosystem N retention.

Leaching of nutrient cations from the forest floor: effects of nitrogen saturation in two long-term manipulations

Abstract: Nitrogen saturation results in greater mobility of nitrate, which in turn is often correlated with concentrations of nutrient cations in soil solution and streamwater. At the Harvard Forest, U.S.A., under long-term NH4NO3 inputs, a Pinus resinosa Ait. forest has exhibited signs of N saturation more rapidly than a mixed-Quercus forest. We test the hypothesis that increased nitrate leaching causes increased concentrations of nutrient cations in soil solution. Over 2 years (years 6 and 7 of treatment) we measured SO42-, NO3-, Cl-, Ca2+, K+, Mg2+, Na+, H+, and NH4+ in throughfall solution and in forest-floor (Oa) leachate. Concentrations of NO3- in forest-floor leachate increased with rates of N amendment and correlated positively with cation concentrations, with stronger overall correlations in the pine forest: r2 values were 0.51 (pine forest) and 0.39 (oak forest) for Ca2+, 0.45 (pine) and 0.16 (oak) for K+, and 0.62 (pine) and 0.50 (oak) for Mg2+. In summer and fall, the oak forest showed some negative re...

Sources of Variability in Net Primary Production Predictions at a Regional Scale: A Comparison Using PnET-II and TEM 4.0 in Northeastern US Forests

Abstract: Because model predictions at continental and global scales are necessarily based on broad characterizations of vegetation, soils, and climate, estimates of carbon stocks and fluxes made by global terrestrial biosphere models may not be accurate for every region. At the regional scale, we suggest that attention can be focused more clearly on understanding the relative strengths of predicted net primary productivity (NPP) limitation by energy, water, and nutrients. We evaluate the sources of variability among model predictions of NPP with a regional-scale comparison between estimates made by PnET-II (a forest ecosystem process model previously applied to the northeastern region) and TEM 4.0 (a terrestrial biosphere model typically applied to the globe) for the northeastern US. When the same climate, vegetation, and soil data sets were used to drive both models, regional average NPP predictions made by PnET-II and TEM were remarkably similar, and at the biome level, model predictions agreed fairly well with NPP estimates developed from field measurements. However, TEM 4.0 predictions were more sensitive to regional variations in temperature as a result of feedbacks between temperature and belowground N availability. In PnET-II, the direct link between transpiration and photosynthesis caused substantial water stress in hardwood and pine forest types with increases in solar radiation; predicted water stress was relieved substantially when soil water holding capacity (WHC) was increased. Increasing soil WHC had little effect on TEM 4.0 predictions because soil water storage was already sufficient to meet plant demand with baseline WHC values, and because predicted N availability under baseline conditions in this region was not limited by water. Because NPP predictions were closely keyed to forest cover type, the relative coverage of low- versus high-productivity forests at both fine and coarse resolutions was an important determinant of regional NPP predictions. Therefore, changes in grid cell size and differences in the methods used to aggregate from fine to coarse resolution were important to NPP predictions insofar as they changed the relative proportions of forest cover. We suggest that because the small patches of high-elevation spruce-fir forest in this region are substantially less productive than forests in the remainder of the region, more accurate NPP predictions will result if models applied to this region use land cover input data sets that retain as much fine-resolution forest type variability as possible. The differences among model responses to variations in climate and soil WHC data sets suggest that the models will respond quite differently to scenarios of future climate. A better understanding of the dynamic interactions between water stress, N availability, and forest productivity in this region will enable models to make more accurate predictions of future carbon stocks and fluxes.

Applying AVIRIS at the sub-regional scale: forest productivity and nitrogen and cation cycling

Abstract: Human activities have greatly altered the carbon and nitrogen dynamics of temperate forest ecosystems. These changes stem not only from elevated CO2 and nitrogen deposition, but also from land use practices that date back decades to many centuries. In the northeastern U.S., forests have a several hundred year history of humaninduced disturbance, the effects of which are only beginning to be understood. Almost all forested lands in the region were at one time logged or cleared for agriculture. In the mountainous portions of northern New England, logging was severe and was often followed by slash fires and substantial soil erosion. Although most of the region has since returned to forest, these disturbances have left an imprint that can be seen in present day species distribution, forest productivity, and nutirent cycling.