Soil Base Saturation Combines with Beech Bark Disease to Influence Composition and Structure of Sugar Maple-Beech Forests in an Acid Rain-Impacted Region

Sugar maple, an abundant and highly valued tree species in eastern North America, has experienced decline from soil calcium (Ca) depletion by acidic deposition, while beech, which often coexists with sugar maple, has been afflicted with beech bark disease (BBD) over the same period. To investigate how variations in soil base saturation combine with effects of BBD in influencing stand composition and structure, measurements of soils, canopy, subcanopy, and seedlings were taken in 21 watersheds in the Adirondack region of NY (USA), where sugar maple and beech were the predominant canopy species and base saturation of the upper B horizon ranged from 4.4 to 67%. The base saturation value corresponding to the threshold for Al mobilization (16.8%) helped to define the species composition of canopy trees and seedlings. Canopy vigor and diameter at breast height (DBH) were positively correlated (P < 0.05) with base saturation for sugar maple, but unrelated for beech. However, beech occupied lower canopy positions than sugar maple, and as base saturation increased, the average canopy position of beech decreased relative to sugar maple (P < 0.10). In low-base saturation soils, soil-Ca depletion and BBD may have created opportunities for gap-exploiting species such as red maple and black cherry, whereas in high-base saturation soils, sugar maple dominated the canopy. Where soils were beginning to recover from acidic deposition effects, sugar maple DBH and basal area increased progressively from 2000 to 2015, whereas for beech, average DBH did not change and basal area did not increase after 2010.     

Nitrogen cycling in a northern hardwood forest: Do species matter?

To investigate the influence of individual tree species on nitrogen (N) cycling in forests, we measured key characteristics of the N cycle in small single-species plots of five dominant tree species in the Catskill Mountains of New York State. The species studied were sugar maple (Acer saccharum), American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), eastern hemlock (Tsuga canadensis), and red oak (Quercus rubra). The five species varied markedly in N cycling characteristics. For example, hemlock plots consistently showed characteristics associated with "slow" N cycling, including low foliar and litter N, high soil C:N, low extractable N pools, low rates of potential net N mineralization and nitrification and low NO ₃ ^– amounts trapped in ion-exchange resin bags buried in the mineral soil. Sugar maple plots had the lowest soil C:N, and the highest levels of soil characteristics associated with NO ₃ ^– production and loss (nitrification, extractable NO ₃ ^– , and resin bag NO ₃ ^– ). In contrast, red oak plots had near-average net mineralization rates and soil C:N ratios, but very low values of the variables associated with NO ₃ ^– production and loss. Correlations between soil N transformations and litter concentrations of N, lignin, lignin:N ratio, or phenolic constituents were generally weak. The inverse correlation between net nitrification rate and soil C:N that has been reported in the literature was present in this data set only if red oak plots were excluded from the analysis. This study indicates that tree species can exert a strong control on N cycling in forest ecosystems that appears to be mediated through the quality of soil organic matter, but that standard measures of litter quality cannot explain the mechanism of control.     

Plant and soil natural abundance <Emphasis Type="Italic">δ</Emphasis><Superscript>15</Superscript>N: indicators of relative rates of nitrogen cycling in temperate forest ecosystems

Watersheds within the Catskill Mountains, New York, receive among the highest rates of nitrogen (N) deposition in the northeastern United States and are beginning to show signs of N saturation. Despite similar amounts of N deposition across watersheds within the Catskill Mountains, rates of soil N cycling and N retention vary significantly among stands of different tree species. We examined the potential use of δ ¹⁵N of plants and soils as an indicator of relative forest soil N cycling rates. We analyzed the δ ¹⁵N of foliage, litterfall, bole wood, surface litter layer, fine roots and organic soil from single-species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine root and organic soil δ ¹⁵N values were highest within sugar maple stands, which correlated significantly with higher rates of net mineralization and nitrification. Results from this study suggest that fine root and organic soil δ ¹⁵N can be used as an indicator of relative rates of soil N cycling. Although not statistically significant, δ ¹⁵N was highest within foliage, wood and litterfall of beech stands, a tree species associated with intermediate levels of soil N cycling rates and forest N retention. Our results show that belowground δ ¹⁵N values are a better indicator of relative rates of soil N cycling than are aboveground δ ¹⁵N values.     

Impacts of moso bamboo (Phyllostachys pubescens) invasion on dry matter and carbon and nitrogen stocks in a broad‐leaved secondary forest located in Kyoto,western Japan

In western and central Japan, the expansion of exotic moso bamboo (Phyllostachys pubescens Mazel ex J. Houz.) populations into neighboring vegetation has become a serious problem. Although the effects of bamboo invasion on biodiversity have been well studied, shifts in nutrient stocks and cycling, which are fundamental for ecosystem functioning, are not fully understood. To explore the effects of P. pubescens invasion on ecosystem functions we examined above‐ and below‐ground dry matter and carbon (C) and nitrogen (N) stocks in a pure broad‐leaved tree stand, a pure bamboo stand, and two tree–bamboo mixed stands with different vegetation mix ratios in the secondary forest of Kyoto, western Japan. In the process of invasion, bamboo shoots offset broad‐leaved tree deaths; thus, no clear trend was apparent in total above‐ or below‐ground biomass or in plant C and N stocks during invasion. However, the ratio of above‐ground to below‐ground biomass (T/R ratio at the stand level) decreased with increasing bamboo dominance, especially in the early stages of invasion. This shift indicates that rapid bamboo rhizomatous growth is a main driver of substantial changes in stand structure. We also detected rises in the C/N ratio of forest‐floor organic matter during bamboo invasion. Thus major impacts of P. pubescens invasion into broad‐leaved forests include not only early shifts in biomass allocation, but also changes in the distribution pattern of C and N stored in plants and soil.     

Competition for nitrogen sources between European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus) seedlings

To investigate the short‐term consequences of direct competition between beech and sycamore maple on root N uptake and N composition, mycorrhizal seedlings of both tree species were incubated for 4 days (i.e. beech only, sycamore maple only or both together) in an artificial nutrient solution with low N availability. On the fourth day, N uptake experiments were conducted to study the effects of competition on inorganic and organic N uptake. For this purpose, multiple N sources were applied with a single label. Furthermore, fine roots were sampled and analysed for total amino acids, soluble protein, total nitrogen, nitrate and ammonium content. Our results clearly show that both tree species were able to use inorganic and organic N sources. Uptake of inorganic and organic N by beech roots was negatively affected in the presence of the competing tree species. In contrast, the presence of beech stimulated inorganic N uptake by sycamore maple roots. Both the negative effect of sycamore maple on N uptake of beech and the positive effect of beech on N uptake of sycamore maple led to an increase in root soluble protein in beech, despite an overall decrease in total N concentration. Thus, beech compensated for the negative effects of the tree competitor on N uptake by incorporating less N into structural N components, but otherwise exhibited the same strategy as the competitor, namely, enhancing soluble protein levels in roots when grown under competition. It is speculated that enhanced enzyme activities of so far unknown nature are required in beech as a defence response to inter‐specific competition.     

Functional Group Identity Does not Predict Invader Impacts: Differential Effects of Nitrogen-fixing Exotic Plants on Ecosystem Function

The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits rather than broadly defined functional groups alone when predicting invader impacts.     

The phosphorus status of northern hardwoods differs by species but is unaffected by nitrogen fertilization

Northern hardwood forests in the eastern US exhibit species-specific influences on nitrogen (N) cycling, suggesting that their phosphorus (P) cycling characteristics may also vary by species. These characteristics are increasingly important to understand in light of evidence suggesting that atmospheric N deposition has increased N availability in the region, potentially leading to phosphorus limitation. We examined how P characteristics differ among tree species and whether these characteristics respond to simulated N deposition (fertilization). We added NH₄NO₃ fertilizer (50 kg ha^?1 year^?1) to single-species plots of red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.), eastern hemlock (Tsuga canadensis (L.) Carr.), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britt.), in the Catskill Mountains, New York from 1997 to 2007. Species differences were observed in foliar, litter and root P concentrations, but all were unaffected by a cumulative N fertilization of 550 kg/ha. Similarly, measures of soil P availability and biotic P sufficiency differed by species but were unaffected by N fertilization. Results suggest species exhibit unique relationships to P as well as N cycles. We found little evidence that N fertilization leads to increased P limitation in these northern hardwood forests. However, species such as sugar maple and red oak may be sufficient in P, whereas beech and hemlock may be less sufficient and therefore potentially more sensitive to future N-stimulated P limitation.     

Partitioning of the source of leaf calcium of American beech and sugar maple using leaf Ca/Sr ratios: a predominantly surficial but variable depth of Ca uptake

Background and Aims

Reduced availability of calcium (Ca) has been linked to maple forest decline. We therefore aimed at assessing the contribution of the different soil horizons to leaf Ca of competing beech (Fagus grandifolia Ehrh.) and sugar maple (Acer saccharum Marsh.) to better understand the dynamics of Ca uptake.

Methods

Leaf Ca was partitioned using the Ca/Sr ratio approach in two mature forests of southern Quebec. A mass balance was also used at one site to validate the results obtained with the Ca/Sr approach.

Results

The L and F horizons contributed most of the leaf Ca of beech and maple with likely small contributions from the upper B and/or H/Ahe horizons. Leaf Ca/Sr ratios of beech were however more variable than those of maple. Using a mass balance, the organic horizons and upper mineral soil horizons were found to provide ca. 80 and 20 % of tree Ca uptake, respectively.

Conclusion

Beech and maple Ca uptake depth apportionment is on average similar but beech is likely more plastic in sourcing soil Ca. The low contribution of the mineral soil to leaf Ca at our sites can be linked to less favorable conditions for Ca uptake likely associated with low Ca/Al ratios.     

Foliar production and decomposition rates in urban forests invaded by the exotic invasive shrub, <Emphasis Type="Italic">Lonicera maackii</Emphasis>

Exotic invasive shrubs can form dense monocultures in forest understories, which can have cascading effects on ecosystem structure and function. Amur honeysuckle, an exotic shrub that forms dense canopies in eastern forests, has the potential to alter plant community structure and ecosystem functions, such as primary production and decomposition. The goal of this study was to examine foliar productivity and leaf litter decomposition in forests invaded by Amur honeysuckle (Lonicera maackii) and to determine the extent to which the presence of this dominant exotic species may alter ecosystem function in these forests. We found that forests invaded by Amur honeysuckle had 16 times greater honeysuckle foliar biomass and 1.5 times lower total foliar biomass than forests of equivalent tree basal area, but having few honeysuckle shrubs. This suggests that productivity of native tree and shrub species may be reduced where honeysuckle density is high. Additionally, honeysuckle litter decayed four times faster and released nitrogen more rapidly than sugar maple litter, and sugar maple litter decayed 19% faster in forests invaded by Amur honeysuckle. These findings suggest that forests invaded by Amur honeysuckle may exhibit lower rates of organic matter accrual and less nitrogen retention in the forest floor. Since honeysuckle leaves develop in early spring before those of other shrubs or trees in the area, the rapid release of nitrogen from honeysuckle litter that we measured in early spring is timed to benefit this invasive species. The temporally coincident phenologies of nitrogen release during decomposition with the foliar growth needs of this shrub indicates that a potential positive feedback loop may exist between these processes that promotes continued growth and dominance of honeysuckle shrubs in these forested systems.     

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO<Subscript>2</Subscript> and varying soil resource availability

Rising atmospheric [CO₂] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO₂] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO₂. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO₂]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO₂]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO₂]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO₂] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO₂], rather than changes in substrate chemistry.     

Variability of soil N cycling and N2O emission in a mixed deciduous forest with different abundance of beech

The mixture of other broadleaf species into beech forests in Central Europe leads to an increase of tree species diversity, which may alter soil biochemical processes. This study was aimed at 1) assessing differences in gross rates of soil N cycling among deciduous stands of different beech (Fagus sylvatica L.) abundance in a limestone area, 2) analyzing the relationships between gross rates of soil N cycling and forest stand N cycling, and 3) quantifying N₂O emission and determining its relationship with gross rates of soil N cycling. We used ¹⁵N pool dilution techniques for soil N transformation measurement and chamber method for N₂O flux measurement. Gross rates of mineral N production in the 0–5 cm mineral soil increased across stands of decreasing beech abundance and increasing soil clay content. These rates were correlated with microbial biomass which, in turn, was influenced by substrate quantity, quality and soil fertility. Leaf litter-N, C:N ratio and base saturation in the mineral soil increased with decreasing beech abundance. Soil mineral N production and assimilation by microbes were tightly coupled, resulting in low N₂O emissions. Annual N₂O emissions were largely contributed by the freeze-thaw event emissions, which were correlated with the amount of soil microbial biomass. Our results suggest that soil N availability may increase through the mixture of broadleaf species into beech forests.     

Control of Nitrogen Loss from Forested Watersheds by Soil Carbon:Nitrogen Ratio andTree Species Composition

Leaching losses of nitrate from forests can have potentially serious consequences for soils and receiving waters. In this study, based on extensive sampling of forested watersheds in the Catskill Mountains of New York State, we examine the relationships among stream chemistry, the properties of the forest floor, and the tree species composition of watersheds. We report the first evidence from North America that nitrate export from forested watersheds is strongly influenced by the carbon:nitrogen (C:N) ratio of the watershed soils. We also show that variation in soil C:N ratio is associated with variation in tree species composition. This implies that N retention and release in forested watersheds is regulated at least in part by tree species composition and that changes in species composition caused by introduced pests, climate change, or forest management could affect the capacity of a forest ecosystem to retain atmospherically deposited N. Received 4 March 2002; Accepted 4 June 2002.     

Sapling herbivory,invertebrate herbivores and predators across a natural tree diversity gradient in Germany’s largest connected deciduous forest

Tree species-rich forests are hypothesised to be less susceptible to insect herbivores, but so far herbivory–diversity relationships have rarely been tested for tree saplings, and no such study has been published for deciduous forests in Central Europe. We expected that diverse tree communities reduce the probability of detection of host plants and increase abundance of predators, thereby reducing herbivory. We examined levels of herbivory suffered by beech (Fagus sylvatica L.) and maple saplings (Acer pseudoplatanus L. and Acer platanoides L.) across a tree species diversity gradient within Germany’s largest remaining deciduous forest area, and investigated whether simple beech or mixed stands were less prone to damage caused by herbivorous insects. Leaf area loss and the frequency of galls and mines were recorded for 1,040 saplings (>13,000 leaves) in June and August 2006. In addition, relative abundance of predators was assessed to test for potential top-down control. Leaf area loss was generally higher in the two species of maple compared to beech saplings, while only beech showed a decline in damage caused by leaf-chewing herbivores across the tree diversity gradient. No significant patterns were found for galls and mines. Relative abundance of predators on beech showed a seasonal response and increased on species-rich plots in June, suggesting higher biological control. We conclude that, in temperate deciduous forests, herbivory–tree diversity relationships are significant, but are tree species-dependent with bottom-up and top-down control as possible mechanisms. In contrast to maple, beech profits from growing in a neighbourhood of higher tree richness, which implies that species identity effects may be of greater importance than tree diversity effects per se. Hence, herbivory on beech appeared to be mediated bottom-up by resource concentration in the sampled forest stands, as well as regulated top-down through biocontrol by natural enemies. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

Winter climate change implications for decomposition in northeastern forests: comparisons of sugar maple litter with herbivore fecal inputs

Forests in northeastern North America are influenced by varying climatic and biotic factors; however, there is concern that rapid changes in these factors may lead to important changes in ecosystem processes such as decomposition. Climate change (especially warming) is predicted to increase rates of decomposition in northern latitudes. Warming in winter may result in complex effects including decreased levels of snow cover and an increased incidence of soil freezing that will effect decomposition. Along with these changes in climate, moose densities have also been increasing in this region, likely affecting nutrient dynamics. We measured decomposition and N release from ¹⁵N‐labeled sugar maple leaf litter and moose feces over 20 months in reference and snow removal treatment (to induce soil freezing) plots in two separate experiments at the Hubbard Brook Experimental Forest in New Hampshire, USA. Snow removal/soil freezing decreased decomposition of maple litter, but stimulated N transfer to soil and microbial biomass. Feces decomposed more rapidly than maple litter, and feces N moved into the mineral soil more than N derived from litter, likely due to the lower C : N ratio of feces. Feces decomposition was not affected by the snow removal treatment. Total microbial biomass (measured as microbial N and C) was not significantly affected by the treatments in either the litter or feces plots. These results suggest that increases in soil freezing and/or large herbivore populations, increase the transfer rate of N from plant detritus or digested plants into the mineral soil. Such changes suggest that altering the spatial and temporal patterns of soil freezing and moose density have important implications for ecosystem N cycling.     

Base-cation Cycling by Individual Tree Species in Old-growth Forests of Upper Michigan,USA

The influence of individual tree species on base-cation (Ca, Mg, K, Na) distribution and cycling was examined in sugar maple (Acer saccharum Marsh.), basswood (Tilia americana L.), and hemlock (Tsuga canadensis L.) in old-growth northern hardwood – hemlock forests on a sandy, mixed, frigid, Typic Haplorthod over two growing seasons in northwestern Michigan. Base cations in biomass, forest floor, and mineral soil (0–15 cm and 15–40 cm) pools were estimated for five replicated trees of each species; measured fluxes included bulk precipitation, throughfall, stemflow, litterfall, forest-floor leachate, mineralization + weathering, shallow-soil leachate, and deep-soil leachate. The three species differed in where base cations had accumulated within the single-tree ecosystems. Within these three single-tree ecosystems, the greatest quantity of base cations in woody biomass was found in sugar maple, whereas hemlock and basswood displayed the greatest amount in the upper 40 cm of mineral soil. Base-cation pools were ranked: sugar maple > basswood, hemlock in woody biomass; sugar maple, basswood > hemlock in foliage; hemlock > sugar maple, basswood in the forest floor, and basswood > sugar maple, hemlock in the mineral soil. Base-cation fluxes in throughfall, stemflow, the forest-floor leachate, and the deep-soil leachate (2000 only) were ranked: basswood > sugar maple > hemlock. Our measurements suggest that species-related differences in nutrient cycling are sufficient to produce significant differences in base-cation contents of the soil over short time intervals (<65 years). Moreover, these species-mediated differences may be important controls over the spatial pattern and edaphic processes of northern hardwood-hemlock ecosystems in the upper Great Lakes region.     

Anthropogenic N Deposition Increases Soil C Storage by Decreasing the Extent of Litter Decay: Analysis of Field Observations with an Ecosystem Model

Recent meta-analyses of experimental studies simulating increased anthropogenic nitrogen (N) deposition in forests reveal greater soil carbon (C) storage under elevated levels of atmospheric N deposition. However, these effects have not yet been included in ecosystem-scale models of soil C and N cycling and it is unclear whether increased soil C storage results from slower decomposition rates or a reduced extent of decomposition (for example, an increase in the amount of litter entering slowly decaying humus pools). To test these alternatives, we conducted a meta-analysis of litter decomposition data. We then used the results from our meta-analysis to model C and N cycling in four sugar maple forests in Michigan using an ecosystem process model (TRACE). We compared model results testing our alternative hypotheses to field data on soil C storage from a 17-year N deposition experiment. Using data from published litter decomposition studies in forests, we determined that, on average, exogenous N inputs decreased lignin decomposition rates by 30% and increased cellulose decomposition by 9%. In the same set of litter decomposition studies increased exogenous N availability increased the amount of litter entering slowly decaying humus pools in a manner significantly related to the lignocellulose index of decaying litter. Incorporating changes to decomposition rates in TRACE did not accurately reproduce greater soil C storage observed in our field study with experimentally elevated N deposition. However, when changes in the extent of decomposition were incorporated in TRACE, the model produced increased soil C storage by increasing the amount of litter entering the humus pool and accurately represented C storage in plant and soil pools under experimental N deposition. Our modeling results and meta-analysis indicate that the extent of litter decay as humus is formed, rather than slower rates of litter decay, is likely responsible for the accumulation of organic matter, and hence soil C storage, under experimental N deposition. This effect should be incorporated in regional to global-scale models simulating the C balance of forest ecosystems in regions receiving elevated N deposition.     

Influence of Tree Species on Forest Nitrogen Retention in the Catskill Mountains, New York, USA

This study examines the effect of four tree species on nitrogen (N) retention within forested catchments of the Catskill Mountains, New York (NY). We conducted a 300-day ¹⁵N field tracer experiment to determine how N moves through soil, microbial, and plant pools under different tree species and fertilization regimes. Samples were collected from single-species plots of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga canadensis L.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marsh). Using paired plots we compared the effects of ambient levels of N inputs (11 kg N/ha/y) to additions of 50 kg N/ha/y that began 1.5 years prior to and continued throughout this experiment. Total plot ¹⁵N recovery (litter layer, organic and mineral soil to 12 cm, fine roots, and aboveground biomass) did not vary significantly among tree species, but the distribution of sinks for ¹⁵N within the forest ecosystem did vary. Recovery in the forest floor was significantly lower in sugar maple stands compared to the other species. ¹⁵Nitrogen recovery was 22% lower in the fertilized plots compared to the ambient plots and red oak stands had the largest drop in ¹⁵N recovery as a result of N fertilization. Aboveground biomass became a significantly greater ¹⁵N sink with fertilization, although it retained less than 1% of the tracer addition. These results indicate that different forest types vary in the amount of N retention in the forest floor, and that forest N retention may change depending upon N inputs.     

Hemlock Woolly Adelgid in New England Forests: Canopy Impacts Transforming Ecosystem Processes and Landscapes

Exotic insect pests may strongly disrupt forest ecosystems and trigger major shifts in nutrient cycling, structure, and composition. We examined the relationship between these diverse effects for the hemlock woolly adelgid (HWA, Adelges tsugae Annand) in New England forests by studying its impacts on local canopy processes in stands differing in infestation levels and linking these impacts to shifts in canopy nutrient cycling and stand and landscape effects. HWA initiated major changes in canopy biomass and distribution. Whereas uninfested trees exhibit a significant decline in canopy biomass from the center to the periphery and a positive correlation between total needle litter and estimated biomass, infested trees have significantly less total canopy biomass, produce less new foliage, shed relatively more needles, and exhibit no correlation between litter and canopy biomass. Foliar N content of infested trees was 20%–40% higher than reference trees, with the strongest increase in young foliage supporting the highest densities of HWA. Foliar %C was unaffected by HWA or foliar age. Epiphytic microorganisms on hemlock needles exhibited little variation in abundance within canopies, but colony-forming units of bacteria, yeast, and filamentous fungi were 2–3 orders of magnitude more abundant on medium and heavily infested than uninfested trees. Throughfall chemistry, quantity, and spatial pattern were strongly altered by HWA. Throughfall exhibits a strong gradient beneath uninfested trees, decreasing in volumes from the canopy periphery to the trunk by more than 45%. The amount of throughfall beneath infested trees exhibits no spatial pattern, reaches 80%–90% of the bulk precipitation, and is characterized by significantly higher concentrations of nitrogen compounds, dissolved organic carbon, and cations. Across the southern New England landscape there is a strong south-to-north gradient of decreasing hemlock tree and sapling mortality and understory compositional change that corresponds to the duration of infestation. Regionally, black birch (Betula lenta L.) is profiting most from hemlock decline by significantly increasing in density and cover. These findings suggest that it is necessary to study the connections between fast/small-scale processes such as changes in nutrient cycling in tree canopies and slow/integrative processes like shifts in biogeochemieal cycling and compositional changes at forest stands and landscapes to better understand the effects of an exotic pest species like HWA on forest ecosystem structure and function.     

Exotic plant invasion alters chaparral ecosystem resistance and resilience pre- and post-wildfire

Perturbations such as wildfire and exotic plant invasion have significant impacts on soils, and the extent to which invaded soils are resistant or resilient to these disturbances varies by ecosystem type. Replacement of shrublands by herbaceous exotics pre- and post-wildfire may drastically alter soil chemical and biological properties for an unknown duration. We assessed above and belowground resistance and resilience to exotic plant invasion both before and after a chaparral wildfire. We hypothesized that exotic plant species would change chemical characteristics of chaparral soils by altering litter and microbial inputs, and that controlling exotics and seeding native species would restore chemical characteristics to pre-invaded conditions. We additionally hypothesized that exotic plant species would slow succession above- and belowground, as well as recovery of post-wildfire chaparral structure and function. Plant species composition and soil nutrient pools and cycling rates were evaluated in mature and invaded chaparral pre- and post-wildfire. Exotic plant species were weeded and native species were seeded to assess impacts of exotic competition on native species recovery. Invasion did not impact all soil characteristics before fire, but increased soil C/N ratio, pH, and N cycling rates, and reduced NO₃-N availability. After fire, invasives slowed succession above- and belowground. Removal of exotics and seeding natives facilitated succession and resulted in plant composition similar to uninvaded, post-wildfire chaparral. The chaparral ecosystem was not resistant to impacts of invasion as indicated by altered soil chemistry and C and N cycling rates; however, short-term restoration led to recovery of extractable nitrogen availability indicating resilience of chaparral soils. This suggests that the permanence of exotic plant species, once established, represents a greater ecological challenge than exotic plant impacts on soils.     

Mechanisms for carbon and nutrient release and retention in beech forest gaps

Nutrient cycling and water balance in forest gaps has received little attention until now, although gap regeneration is important to natural dynamics of temperate forests. Gaps of 30 m diameter, cut in a mature beech forest, exhibited a distinct change in microclimatic conditions in comparison with the surrounding stand. Soil moisture in gaps remained very high throughout the observation period. Disruption of the N cycle in gaps led to substantial nitrate losses; seepage water nitrate concentrations were 10–18 mg NO₃-N L^-1. Excess nitrification was a significant cause of soil acidification and aluminium release. The pH in subsoil seepage water decreased by 0.25. Liming in gaps promoted the establishment of a herbaceous vegetation, which functioned as an important nutrient sink, and thus is recommended for tree regeneration in highly acidified forest ecosystems as it increases the resilience of the ecosystem to nutrient losses.