Mary Beth Adams

Long-term impacts of forest treatments on water yield: a summary for northeastern USA

Abstract Long-term changes in annual water yield are summarized and compared for 11 catchment studies in the northeastern USA. Substantial increases in water yield of up to 350 mm year−1 were obtained in the first year by clearing forest vegetation and controlling regrowth with herbicides. Commercial clearcutting with natural regrowth resulted in initial increases in water yield of 110–250 mm year−1. This range in response was due to differences in precipitation and configuration of cuttings. Unless regrowth was controlled with herbicides, yield increases declined quickly after cutting, seldom persisting for more than 10 years. However, yield increases were readily extended over 20 years or more with intermediate cuttings and/or repeated control of regrowth with herbicides. Nearly all increases in water yield occur during the growing season as augmentation of baseflow. Changes in species composition after forest cutting on several study catchments eventually resulted in decreased water yields compared with those from uncut, control catchments. Results are discussed in terms of implications for surface water supplies, global climate change, nutrient cycling, hydrological modeling, and long-term research.

Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems

By synthesizing more than twenty years of research at the Fernow Experimental Forest, we have documented 7 symptoms of nitrogen saturation in two adjacent watersheds. The symptoms include: 1) high relative rates of net nitrification, 2) long-term increases in stream-water concentrations of nitrate and base cations, 3) relatively high nitrate concentrations in solution losses, 4) little seasonal variability in stream-water nitrate concentrations, 5) a high discharge of nitrate from a young aggrading forest, 6) a rapid increase in nitrate loss following fertilization of a young aggrading forest, and 7) low retention of inorganic nitrogen when compared with other forested sites. These data support current conceptual models of nitrogen saturation and provide a strong, and perhaps the best, example of nitrogen saturation in the United States.

Ecology and decline of red spruce in the eastern United States.

This book focuses on the recent decline of red spruce and the role of acid rain and associated air pollutants in this decline. The purpose of the book is to summarize a large body of recent research on this important environmental issue. The book is divided into three sections: section one summarizes the features of spruce-fir forests in the eastern U.S. and examines the ecology of the forests, their soils, and the atmospheric conditions experienced by these forests. Section two looks at experimental results from many air pollution studies and evaluates mechanisms of air pollution effects on red spruce trees. Section three synthesizes the current state of knowledge regarding the widespread red spruce decline in forests of the eastern United States.

Pervasive interactions between ungulate browsers and disturbance regimes promote temperate forest herbaceous diversity.

Disruptions to historic disturbance and herbivory regimes have altered plant assemblages in forests worldwide. An emerging consensus suggests that these disruptions often result in impoverished forest biotas. This is particularly true for eastern U.S. deciduous forests where large gaps and understory fires were once relatively common and browsers were far less abundant. Although much research has focused on how disturbance and browsers affect tree diversity, far less attention has been devoted to forest understories where the vast majority (>75%) of the vascular species reside. Here we test the hypothesis that the reintroduction of disturbances resembling historic disturbance regimes and moderate levels of ungulate browsing enhance plant diversity. We explore whether once-common disturbances and their interaction with the top-down influence of browsers can create conditions favorable for the maintenance of a rich herbaceous layer in a region recognized as a temperate biodiversity hotspot in West Virginia, U.S.A. We tested this hypothesis via a factorial experiment whereby we manipulated canopy gaps (presence/absence) of a size typically found in old-growth stands, low-intensity understory fire (burned/unburned), and deer browsing (fenced/unfenced). We tracked the abundance and diversity of more than 140 herb species for six years. Interactions among our treatments were pervasive. The combination of canopy gaps and understory fire increased herbaceous layer richness, cover, and diversity well beyond either disturbance alone. Furthermore, we documented evidence that deer at moderate levels of abundance promote herbaceous richness and abundance by preferentially browsing fast-growing pioneer species that thrive following co-occurring disturbances (i.e., fire and gaps). This finding sharply contrasts with the negative impact browsers have when their populations reach levels well beyond those that occurred for centuries. Although speculative, our results suggest that interactions among fire, canopy gaps, and browsing provided a variable set of habitats and conditions across the landscape that was potentially capable of maintaining much of the plant diversity found in temperate forests.

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24- year-old, treated with (NH4)2SO4 since 1989 at the rate of 35 kg N·ha–1·year–1). Net nitrification was 141, 114, and 115 kg N·ha–1·year–1, for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO3– of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO3– in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.

Ecosystem Processes and Human Influences Regulate Streamflow Response to Climate Change at Long-Term Ecological Research Sites

Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Niño—Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes.

Impact of harvesting and atmospheric pollution on nutrient depletion of eastern US hardwood forests

The eastern hardwood forests of the US may be threatened by the changing atmospheric chemistry and by changes in harvesting levels. Many studies have documented accelerated base cation losses with intensive forest harvesting. Acidic deposition can also alter nutrient cycling in these forests. The combination of increased harvesting, shorter rotations, and more intensive harvesting, along with the potential for N and S saturation due to changing atmospheric chemistry in the eastern US, raises concerns about the long-term productivity of these commercially important eastern hardwood forests. We review the literature describing the effects of intensive harvesting and acidic atmospheric deposition on budgets of base nutrients which presents evidence that the ambient levels of N and S deposition are leading to N and S saturation and elevated base leaching from the soil in some eastern forests, and we discuss potential concerns for long-term productivity. We also discuss criteria and indicators for monitoring sustainability of the soils of these forests.

Historic disturbance regimes promote tree diversity only under low browsing regimes in eastern deciduous forest

Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large-scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade-tolerant, fire-intolerant species that replace and suppress many fire-tolerant species); (2) the gap hypothesis (small gaps typical of todays forests promote dominance of a few shade-tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few unpalatable species). We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size ;255 m 2 ), and browsing by deer, and we followed the fates of .28 000 seedlings and saplings for five years. Understory tree communities in control plots were dominated (up to 90% )b yFagus grandifolia, averaging little more than two species, whereas overstories were diverse, with 10-15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity, because browsers reduced the benefits of gaps and created depauperate understories following fire. Consequently, two major disturbances that once promoted tree diversity no longer do so because of browsing. Our findings appear to reconcile equivocal views on the role of fire and gaps. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances promote diversity (three- to fivefold). Our results apply to large portions of eastern North America where deer are overabundant, and we provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre-European settlement forests could maintain high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.

Stream water and soil solution responses to 5 years of nitrogen and sulfur additions at the Fernow Experimental Forest, West Virginia

To examine the effects of elevated N and S inputs on a central hardwood forest, a whole-watershed acidification experiment was initiated in 1989 on the Fernow Experimental Forest, West Virginia. Annual experimental additions of 40 kg S ha−1 year−1 and 35 kg N ha−1 year−1 as ammonium sulfate fertilizer were applied to a 34 ha watershed with a 25-year-old stand of central Appalachian hardwoods. An adjacent watershed served as the control. After 5 years of treatment (total additions of 275 kg S ha−1 and 220 kg N ha−1), stream water NO3−, Ca2+, Mg2+ concentrations and export increased. Soil solution concentrations provide evidence that the treatment watershed is nitrogen-saturated, which was unexpected for such a young stand. No statistically significant changes in annual SO42− export were observed, but peak stream water concentrations of SO42− did increase during the treatment period. Changes in soil solution chemistry suggest that the treated watershed also may be approaching SO42− saturation.

Nitrification potentials and landscape, soil and vegetation characteristics in two Central Appalachian watersheds differing in NO3- export

Two watersheds within 1 km of each other in the Central Appalachian mountains of West Virginia have similar management histories and receive 13 kg of N in atmospheric deposition, but NO3 � export from one watershed (W4) has increased over the last 30 years, and is now approximately five times greater than NO3 � export from the other (W10). We measured net nitrification potentials (NNP) and other landscape, soil, and plant community variables (1) to determine whether differences in leaching could be attributed to differences in NNP, (2) to identify other significant differences between the watersheds, (3) to identify variables that could account for both between- and within-watershed variability in NNP, and (4) to identify readily measured variables that distinguish plots with relatively high or low NNP. NNPs in W4 were 0.84 kg N ha � 1 per day, approximately three times higher than those on W10. Watershed 4 lay at a slightly higher elevation, had gentler slopes, a thinner forest floor, lower C:N in the 0‐10 cm soil layer, lower tree density, greater basal area in Acer saccharum, less basal area in Quercus prinus and Amelanchier arborea, more frequent occurrences of A. saccharum seedlings, Laportea canadensis, Polystichum acrostichoides, Trillium sp., Uvularia sessilifolia and Viola spp., and fewer occurrences of Gaultheria procumbens, and Viburnum acerifolum. NNPs were correlated with many soil characteristics related to base cation supply, C:N and water holding capacity (WHC). Several two- and three-variable regression models, which were mostly based on soil characteristics, accounted for a large proportion of the variability in NNP (adjusted R 2 > 0:60), as well as for the difference between watershed means (t-test of residuals indicate no significant difference). A regression model based on basal area of A. saccharum and A. rubrum and the presence or absence of Trillium accounted for 50% of the variability in NNP. At this highdeposition site, plots with soils that had higher pHs, greater base cation supply and WHC, and lower C:N were more susceptible to NO3 � leaching and N saturation. # 2002 Elsevier Science B.V. All rights reserved.