Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to sto... more Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15 N tracer application), total ecosystem 15 N retention was negatively correlated with foliar natural-abundance 15 N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15 N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15 N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 1816 tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15 N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg NÁha À1 Áyr À1 above which most ecosystems showed net losses of applied 15 N tracer in response to N fertilizer addition.
We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net eco... more We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net ecosystem production (NEP) near Toolik Lake, Alaska to compare effects of temperature, moisture and nutrients on tundra C balances. We measured fluxes using closed chambers in control, warmed and fertilized plots in wet, moist and dry tundra. ER rankings of tundra types differed between years. In 1992 ER was [approximately]70 g C m[sup [minus]2]y[sup [minus]1] in wet and moist tundra and was 50% lower in dry tundra. In 1993 ER was >150 g C m[sup [minus]2]y[sup [minus]1] in moist tundra and [approximately]55 g C m[sup [minus]2]y[sup [minus]1] in wet and dry tundra. CH[sub 4] emissions ranged from 3.5 to 7 g C m[sup [minus]2]y[sup [minus]1] in wet and from 0.6 to 2.8 g C m[sup [minus]2]y[sup [minus]1] in moist tundra. Dry tundra consumed about 0.1 g CH[sub 4]-C m[sup [minus]2]y[sup [minus]1]. In wet tundra ER increased slightly with warming but dramatically with fertilization. Wet tundra NEP increased with fertilization but not with warming. CH[sub 4] emissions from wet tundra increased with warming but decreased with fertilization. Warming and fertilization increased ER but neither treatment affected NEP in moist tundra. CH[sub 4] emissions from moist tundra responded similarly but less dramatically to treatments than did wet tundra CH[sub 4] fluxes. Warming did not affect ER or NEP in dry tundra, fertilization increased both process. Consumption of CH[sub 4] in dry tundra increased with warming but decreased with fertilization.
ABSTRACT Background/Question/Methods Sandy soils in forests of the upper Great Lakes region store... more ABSTRACT Background/Question/Methods Sandy soils in forests of the upper Great Lakes region store ~80 Mg C ha-1 and function together with forest vegetation as part of the Northern Hemisphere carbon sink. Our study is focused on the effects of exotic earthworm communities on carbon cycling in sandy soils of northern temperate forests and on whether they alter the amount, distribution, and stability of carbon as has been shown for finer textured forest soils. We measured earthworm species diversity, population density, and stable isotope contents (13C and 15N) across an aspen-dominated forest at the University of Michigan Biological Station in northern lower Michigan. Surveys were conducted using an electro-shocking technique in a 33 hectare treated area of the FASET (Forest Accelerated Succession ExperimenT) manipulation, where aspen and birch trees were girdled in May 2008, and in a reference (untreated) stand. We characterized spatial and seasonal patterns of earthworm abundance and community diversity using counts of individuals in electro-shocked plots, and differences in resource use using stable isotope abundances of earthworm species. To investigate possible community-specific effects on soil carbon dynamics, a mesocosm experiment was designed using eight earthworm community treatments. Rates of soil CO2 efflux were measured and the effects of earthworm communities relative to reference mesocosms (containing no earthworms) were quantified. Results/Conclusions Earthworm communities were dominated by five species of European-origin: Dendrobaena octaedra, Apporectodea trapezoides, A. caliginosa, Lumbricus rubellus, and L. terrestris. Earthworm densities range from <20 individuals m-2 to >150 individuals m-2. Both densities and relative abundances of species and developmental classes varied seasonally. The relationship between earthworm densities and soil moisture was strongest in summer, followed by fall. No relationship was found in the spring when juvenile densities are highest and soils are moist. The isotopic signatures varied according to species ecological groups and spatially across plots. Ecological groups were separated by 15N signatures, while 13C signatures separated earthworm communities across horizontal space. While some earthworm communities increase rates of soil CO2 efflux (e.g. L. rubellus/A. caliginosa community increased CO2 efflux by up to 31%), others decreased the rate of soil CO2 efflux (e.g. A. caliginosa/L. terrestris community decreased CO2 efflux by up to 20%). These data provide insight into the structure of earthworm communities in sandy temperate forest soils. Further, preliminary data suggest that understanding relationships between community composition and resource loading may enhance our ability to predict future impacts on soil carbon dynamics in northern temperate forests.
We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net eco... more We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net ecosystem production (NEP) near Toolik Lake, Alaska to compare effects of temperature, moisture and nutrients on tundra C balances. We measured fluxes using closed chambers in control, warmed and fertilized plots in wet, moist and dry tundra. ER rankings of tundra types differed between years. In 1992 ER was [approximately]70 g C m[sup [minus]2]y[sup [minus]1] in wet and moist tundra and was 50% lower in dry tundra. In 1993 ER was >150 g C m[sup [minus]2]y[sup [minus]1] in moist tundra and [approximately]55 g C m[sup [minus]2]y[sup [minus]1] in wet and dry tundra. CH[sub 4] emissions ranged from 3.5 to 7 g C m[sup [minus]2]y[sup [minus]1] in wet and from 0.6 to 2.8 g C m[sup [minus]2]y[sup [minus]1] in moist tundra. Dry tundra consumed about 0.1 g CH[sub 4]-C m[sup [minus]2]y[sup [minus]1]. In wet tundra ER increased slightly with warming but dramatically with fertilization. Wet tundra NEP increased with fertilization but not with warming. CH[sub 4] emissions from wet tundra increased with warming but decreased with fertilization. Warming and fertilization increased ER but neither treatment affected NEP in moist tundra. CH[sub 4] emissions from moist tundra responded similarly but less dramatically to treatments than did wet tundra CH[sub 4] fluxes. Warming did not affect ER or NEP in dry tundra, fertilization increased both process. Consumption of CH[sub 4] in dry tundra increased with warming but decreased with fertilization.
ABSTRACT Urbanization alters both biotic and abiotic ecosystem properties within, surrounding and... more ABSTRACT Urbanization alters both biotic and abiotic ecosystem properties within, surrounding and even at great distances from urban areas. Ecosystem responses to these land changes are complex and interacting as a consequence of connectivity of resources, energy, and information among social, physical and biological systems. Research on urban ecology has generally focused on studies within single urban ecosystems, but the time is ripe for synthesis and comparison. For understanding of urban ecosystems and associated environmental problems to advance, research must consider urban areas in the context of physicochemical, social, geographical, and other contexts. We present testable hypotheses on the effects of urbanization at multiple scales (local, regional, continental, and global). Comparative studies across gradients, as well as spatially continuous coverage of land change, contribute to an approach to test these hypotheses. We argue that such research will permit robust forecasts of how land change and human settlement patterns alter ecosystem services and resource utilization across the North American continent, and that these forecasts can be applied globally.
... Richard Bowden rbowden@alleg.edu Department of Environmental Science, Allegheny College Meadv... more ... Richard Bowden rbowden@alleg.edu Department of Environmental Science, Allegheny College Meadville, PA 16335, USA Richard Boone ffrdb@uaf.edu Institute of Arctic Biology, University of Alaska Fairbanks Fairbanks, AK 99775-7000, USA ...
ABSTRACT Background/Question/Methods Studies of the stable isotope composition (13C and 15N) of f... more ABSTRACT Background/Question/Methods Studies of the stable isotope composition (13C and 15N) of fungal sporocarps have produced various explanations for variation in isotopic signatures of these conspicuous participants in forest C and N cycling. Mechanisms responsible for the 15N enrichment of ectomycorrhizal (ECM) sporocarps are particularly relevant from a biogeochemical perspective, and probably include variation in substrate use, mycelial development and morphology, and N transfer to phytobiont hosts. We measured soil N availability using ion exchange resin incubations, fungal sporocarp 15N and 13C abundances, and percent N and 15N contents of tree foliage in an oligotrophic temperate forest to test whether 15N signatures and trophic strategies of fungal sporocarps (saprotrophic and ECM) vary with soil N availability. We also investigated soil N availability as a driver of variation in foliar 15N signatures and ECM C and N transfers as quantified according to a published mass-balance model. Results/Conclusions Our data on saprotrophic fungi showed evidence for two divergent trophic groups with unique responses to increasing soil N availability. The first group, typified by Armillaria mellea, used isotopically depleted N to decompose relatively 13C-enriched organic C fractions. Fungal biomass from this group showed increasing 15N enrichment along a gradient of increasing soil N availability. The second group was exemplified by Leotia lubrica, which used relatively 13C-depleted, recent-C inputs to forest floors to access 15N-enriched, humic N fractions. Fungal sporocarps from this group became progressively 15N-depleted as soil N availability increased. ECM sporocarps were consistently more 15N-enriched than these co-occurring saprotrophs but nonetheless showed the same relationships between 15N enrichment and soil N availabilty. ECM sporocarps from the genera Amanita, Laccaria, Lactarius, and Russula all showed greater 15N enrichment with increasing soil N availability, while Boletus spp. and Leccinum spp. showed progressive 15N-depletion along the same gradient. Foliage from two of the dominant tree species at the site, Acer rubrum and Quercus rubra, showed 15N enrichment with increasing soil N availability, and isotopically heavier foliage also had higher N content. N transfers from ECM hyphae to Q. rubra appeared to be relatively less important as an N source as soil N availability increased, despite an increasing proportion of hyphal N being transferred at higher levels of soil N availability. C allocation to ECM hyphae declined nonlinearly with increasing soil N availability. These relationships between soil N availability and ECM C and N transfers are deserving of further study by independent methods.
The eastern U.S. receives elevated rates of N deposition compared to preindustrial times, yet rel... more The eastern U.S. receives elevated rates of N deposition compared to preindustrial times, yet relatively little of this N is exported in drainage waters. Net uptake of N into forest biomass and soils could account for a substantial portion of the difference between N deposition and solution exports. We quantified forest N sinks in biomass accumulation and harvest export for 16 large river basins in the eastern U.S. with two separate approaches: (1) using growth data from the USDA Forest Service's Forest Inventory and Analysis (FIA) program, and (2) using a model of forest nitrogen cycling (PnET-CN) linked to FIA information on forest age-class structure. The model was also used to quantify N sinks in soil and dead wood, and nitrate losses below the rooting zone. Both methods agreed that net growth rates were highest in the relatively young forests on the Schuylkill watershed, and lowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removed an average of 2.7 kg N ha −1 yr −1 (range: 1-5 kg N ha −1 yr −1 ), and standing stocks increased by 4.0 kg N ha −1 yr −1 (-3 to 8 kg N ha −1 yr −1 ). Together, these sinks for N in woody biomass amounted to a mean of 6.7 kg N ha −1 yr −1 (2-9 kg N ha −1 yr −1 ), or 73% (15-115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood and soil were small; soils were only a significant net sink for N during simulations of reforestation of degraded agricultural sites. Predicted losses of nitrate depended on the combined effects of N deposition, and both shortand long-term effects of disturbance. Linking the model with forest inventory information on age-class structure provided a useful step toward incorporating realistic patterns of forest disturbance status across the landscape.
ABSTRACT Belowground carbon (C) and nitrogen (N) cycles are tightly coupled in many northern temp... more ABSTRACT Belowground carbon (C) and nitrogen (N) cycles are tightly coupled in many northern temperate forests. However, disturbances that cause tree mortality can decouple linkages between belowground components of these biogeochemical cycles. We measured a suite of belowground processes following a treatment to accelerate the natural senescence of early-successional dominants in a northern temperate forest, hypothesizing that tree mortality would decrease belowground C allocation, increase belowground N availability and cycling rates, and trigger N leaching in this highly N-limited forest. In the Forest Accelerated Succession ExperimenT, we stem girdled >6700 aspen and birch trees (39% of basal area) on 40 ha of forestland at the University of Michigan Biological Station to test these and other hypotheses related to forest development and biogeochemistry. Here, we present the cascade of changes in belowground C and N cycling that occurred in the first two growing seasons following initiation of FASET in April 2008. Aspen and birch mortality decreased stand-level fine root nonstructural carbohydrate contents, accelerating fine root turnover and causing a net reduction in fine root biomass. These decreases in belowground C allocation and root functioning increased forest floor net NH4+ availability, which accelerated nitrification rates, increased net NO3- availability, and led to small NO3- leaching losses. These belowground perturbations were registered by the forest canopy through shifts in foliar 13C and 15N natural abundances, which indicated water stress among girdled aspens and faster, leakier cycling in a source N pool shared by foliage of all tree species. We interpret the results of this initial disturbance phase of the FASET study in the context of our conceptual model of N availability and ecosystem C storage over longer-term (successional) time scales. We predict that, following minor N leaching losses during disturbance, reorganization of belowground N cycling along a more active, faster-cycling trajectory will enable the forest to increase rates of C storage during its next successional stage, promoting resilience of C and N cycling to this experimental disturbance.
This paper describes the potential impacts of warming temperatures and changing precipitation on ... more This paper describes the potential impacts of warming temperatures and changing precipitation on plants, wildlife, invasive species, pests, and agricultural ecosystems across the multi-state region centered on Chicago, Illinois. We examine a geographic area that captures much of Lake Michigan, including a complex mosaic of urbanization and agriculture surrounding southern Lake Michigan. We consider species currently present within this broad region as well as species that are expected to move into or out of the area as climate zones shift northward through the coming century. Our analysis draws upon disparate data sources to compile projections. We conclude that a complex mixture of land use poses particular challenges to natural ecosystems in this region under climate change. Dispersal is likely to be limited for some species, and some populations of native taxa may already be reduced due to habitat loss. Other species can persist, even thrive, within a mixed landscape mosaic, provided natural areas and green spaces are available. If such spaces are somehow connected, they can provide opportunities for native species to inhabit and move through the metropolitan region (perhaps even better than the landscapes previously dominated by agriculture). Strategies for adapting regional agriculture and minimizing pest outbreaks also call for creative management intervention. With additional research, Chicago and its surrounding environs have an opportunity to provide leadership on effective management of natural resources under climate change.
Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to sto... more Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15 N tracer application), total ecosystem 15 N retention was negatively correlated with foliar natural-abundance 15 N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15 N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15 N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 1816 tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15 N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg NÁha À1 Áyr À1 above which most ecosystems showed net losses of applied 15 N tracer in response to N fertilizer addition.
We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net eco... more We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net ecosystem production (NEP) near Toolik Lake, Alaska to compare effects of temperature, moisture and nutrients on tundra C balances. We measured fluxes using closed chambers in control, warmed and fertilized plots in wet, moist and dry tundra. ER rankings of tundra types differed between years. In 1992 ER was [approximately]70 g C m[sup [minus]2]y[sup [minus]1] in wet and moist tundra and was 50% lower in dry tundra. In 1993 ER was >150 g C m[sup [minus]2]y[sup [minus]1] in moist tundra and [approximately]55 g C m[sup [minus]2]y[sup [minus]1] in wet and dry tundra. CH[sub 4] emissions ranged from 3.5 to 7 g C m[sup [minus]2]y[sup [minus]1] in wet and from 0.6 to 2.8 g C m[sup [minus]2]y[sup [minus]1] in moist tundra. Dry tundra consumed about 0.1 g CH[sub 4]-C m[sup [minus]2]y[sup [minus]1]. In wet tundra ER increased slightly with warming but dramatically with fertilization. Wet tundra NEP increased with fertilization but not with warming. CH[sub 4] emissions from wet tundra increased with warming but decreased with fertilization. Warming and fertilization increased ER but neither treatment affected NEP in moist tundra. CH[sub 4] emissions from moist tundra responded similarly but less dramatically to treatments than did wet tundra CH[sub 4] fluxes. Warming did not affect ER or NEP in dry tundra, fertilization increased both process. Consumption of CH[sub 4] in dry tundra increased with warming but decreased with fertilization.
ABSTRACT Background/Question/Methods Sandy soils in forests of the upper Great Lakes region store... more ABSTRACT Background/Question/Methods Sandy soils in forests of the upper Great Lakes region store ~80 Mg C ha-1 and function together with forest vegetation as part of the Northern Hemisphere carbon sink. Our study is focused on the effects of exotic earthworm communities on carbon cycling in sandy soils of northern temperate forests and on whether they alter the amount, distribution, and stability of carbon as has been shown for finer textured forest soils. We measured earthworm species diversity, population density, and stable isotope contents (13C and 15N) across an aspen-dominated forest at the University of Michigan Biological Station in northern lower Michigan. Surveys were conducted using an electro-shocking technique in a 33 hectare treated area of the FASET (Forest Accelerated Succession ExperimenT) manipulation, where aspen and birch trees were girdled in May 2008, and in a reference (untreated) stand. We characterized spatial and seasonal patterns of earthworm abundance and community diversity using counts of individuals in electro-shocked plots, and differences in resource use using stable isotope abundances of earthworm species. To investigate possible community-specific effects on soil carbon dynamics, a mesocosm experiment was designed using eight earthworm community treatments. Rates of soil CO2 efflux were measured and the effects of earthworm communities relative to reference mesocosms (containing no earthworms) were quantified. Results/Conclusions Earthworm communities were dominated by five species of European-origin: Dendrobaena octaedra, Apporectodea trapezoides, A. caliginosa, Lumbricus rubellus, and L. terrestris. Earthworm densities range from <20 individuals m-2 to >150 individuals m-2. Both densities and relative abundances of species and developmental classes varied seasonally. The relationship between earthworm densities and soil moisture was strongest in summer, followed by fall. No relationship was found in the spring when juvenile densities are highest and soils are moist. The isotopic signatures varied according to species ecological groups and spatially across plots. Ecological groups were separated by 15N signatures, while 13C signatures separated earthworm communities across horizontal space. While some earthworm communities increase rates of soil CO2 efflux (e.g. L. rubellus/A. caliginosa community increased CO2 efflux by up to 31%), others decreased the rate of soil CO2 efflux (e.g. A. caliginosa/L. terrestris community decreased CO2 efflux by up to 20%). These data provide insight into the structure of earthworm communities in sandy temperate forest soils. Further, preliminary data suggest that understanding relationships between community composition and resource loading may enhance our ability to predict future impacts on soil carbon dynamics in northern temperate forests.
We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net eco... more We measured ecosystem respiration (ER: or CO[sub 2] flux), methane (CH[sub 4]) fluxes and net ecosystem production (NEP) near Toolik Lake, Alaska to compare effects of temperature, moisture and nutrients on tundra C balances. We measured fluxes using closed chambers in control, warmed and fertilized plots in wet, moist and dry tundra. ER rankings of tundra types differed between years. In 1992 ER was [approximately]70 g C m[sup [minus]2]y[sup [minus]1] in wet and moist tundra and was 50% lower in dry tundra. In 1993 ER was >150 g C m[sup [minus]2]y[sup [minus]1] in moist tundra and [approximately]55 g C m[sup [minus]2]y[sup [minus]1] in wet and dry tundra. CH[sub 4] emissions ranged from 3.5 to 7 g C m[sup [minus]2]y[sup [minus]1] in wet and from 0.6 to 2.8 g C m[sup [minus]2]y[sup [minus]1] in moist tundra. Dry tundra consumed about 0.1 g CH[sub 4]-C m[sup [minus]2]y[sup [minus]1]. In wet tundra ER increased slightly with warming but dramatically with fertilization. Wet tundra NEP increased with fertilization but not with warming. CH[sub 4] emissions from wet tundra increased with warming but decreased with fertilization. Warming and fertilization increased ER but neither treatment affected NEP in moist tundra. CH[sub 4] emissions from moist tundra responded similarly but less dramatically to treatments than did wet tundra CH[sub 4] fluxes. Warming did not affect ER or NEP in dry tundra, fertilization increased both process. Consumption of CH[sub 4] in dry tundra increased with warming but decreased with fertilization.
ABSTRACT Urbanization alters both biotic and abiotic ecosystem properties within, surrounding and... more ABSTRACT Urbanization alters both biotic and abiotic ecosystem properties within, surrounding and even at great distances from urban areas. Ecosystem responses to these land changes are complex and interacting as a consequence of connectivity of resources, energy, and information among social, physical and biological systems. Research on urban ecology has generally focused on studies within single urban ecosystems, but the time is ripe for synthesis and comparison. For understanding of urban ecosystems and associated environmental problems to advance, research must consider urban areas in the context of physicochemical, social, geographical, and other contexts. We present testable hypotheses on the effects of urbanization at multiple scales (local, regional, continental, and global). Comparative studies across gradients, as well as spatially continuous coverage of land change, contribute to an approach to test these hypotheses. We argue that such research will permit robust forecasts of how land change and human settlement patterns alter ecosystem services and resource utilization across the North American continent, and that these forecasts can be applied globally.
... Richard Bowden rbowden@alleg.edu Department of Environmental Science, Allegheny College Meadv... more ... Richard Bowden rbowden@alleg.edu Department of Environmental Science, Allegheny College Meadville, PA 16335, USA Richard Boone ffrdb@uaf.edu Institute of Arctic Biology, University of Alaska Fairbanks Fairbanks, AK 99775-7000, USA ...
ABSTRACT Background/Question/Methods Studies of the stable isotope composition (13C and 15N) of f... more ABSTRACT Background/Question/Methods Studies of the stable isotope composition (13C and 15N) of fungal sporocarps have produced various explanations for variation in isotopic signatures of these conspicuous participants in forest C and N cycling. Mechanisms responsible for the 15N enrichment of ectomycorrhizal (ECM) sporocarps are particularly relevant from a biogeochemical perspective, and probably include variation in substrate use, mycelial development and morphology, and N transfer to phytobiont hosts. We measured soil N availability using ion exchange resin incubations, fungal sporocarp 15N and 13C abundances, and percent N and 15N contents of tree foliage in an oligotrophic temperate forest to test whether 15N signatures and trophic strategies of fungal sporocarps (saprotrophic and ECM) vary with soil N availability. We also investigated soil N availability as a driver of variation in foliar 15N signatures and ECM C and N transfers as quantified according to a published mass-balance model. Results/Conclusions Our data on saprotrophic fungi showed evidence for two divergent trophic groups with unique responses to increasing soil N availability. The first group, typified by Armillaria mellea, used isotopically depleted N to decompose relatively 13C-enriched organic C fractions. Fungal biomass from this group showed increasing 15N enrichment along a gradient of increasing soil N availability. The second group was exemplified by Leotia lubrica, which used relatively 13C-depleted, recent-C inputs to forest floors to access 15N-enriched, humic N fractions. Fungal sporocarps from this group became progressively 15N-depleted as soil N availability increased. ECM sporocarps were consistently more 15N-enriched than these co-occurring saprotrophs but nonetheless showed the same relationships between 15N enrichment and soil N availabilty. ECM sporocarps from the genera Amanita, Laccaria, Lactarius, and Russula all showed greater 15N enrichment with increasing soil N availability, while Boletus spp. and Leccinum spp. showed progressive 15N-depletion along the same gradient. Foliage from two of the dominant tree species at the site, Acer rubrum and Quercus rubra, showed 15N enrichment with increasing soil N availability, and isotopically heavier foliage also had higher N content. N transfers from ECM hyphae to Q. rubra appeared to be relatively less important as an N source as soil N availability increased, despite an increasing proportion of hyphal N being transferred at higher levels of soil N availability. C allocation to ECM hyphae declined nonlinearly with increasing soil N availability. These relationships between soil N availability and ECM C and N transfers are deserving of further study by independent methods.
The eastern U.S. receives elevated rates of N deposition compared to preindustrial times, yet rel... more The eastern U.S. receives elevated rates of N deposition compared to preindustrial times, yet relatively little of this N is exported in drainage waters. Net uptake of N into forest biomass and soils could account for a substantial portion of the difference between N deposition and solution exports. We quantified forest N sinks in biomass accumulation and harvest export for 16 large river basins in the eastern U.S. with two separate approaches: (1) using growth data from the USDA Forest Service's Forest Inventory and Analysis (FIA) program, and (2) using a model of forest nitrogen cycling (PnET-CN) linked to FIA information on forest age-class structure. The model was also used to quantify N sinks in soil and dead wood, and nitrate losses below the rooting zone. Both methods agreed that net growth rates were highest in the relatively young forests on the Schuylkill watershed, and lowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removed an average of 2.7 kg N ha −1 yr −1 (range: 1-5 kg N ha −1 yr −1 ), and standing stocks increased by 4.0 kg N ha −1 yr −1 (-3 to 8 kg N ha −1 yr −1 ). Together, these sinks for N in woody biomass amounted to a mean of 6.7 kg N ha −1 yr −1 (2-9 kg N ha −1 yr −1 ), or 73% (15-115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood and soil were small; soils were only a significant net sink for N during simulations of reforestation of degraded agricultural sites. Predicted losses of nitrate depended on the combined effects of N deposition, and both shortand long-term effects of disturbance. Linking the model with forest inventory information on age-class structure provided a useful step toward incorporating realistic patterns of forest disturbance status across the landscape.
ABSTRACT Belowground carbon (C) and nitrogen (N) cycles are tightly coupled in many northern temp... more ABSTRACT Belowground carbon (C) and nitrogen (N) cycles are tightly coupled in many northern temperate forests. However, disturbances that cause tree mortality can decouple linkages between belowground components of these biogeochemical cycles. We measured a suite of belowground processes following a treatment to accelerate the natural senescence of early-successional dominants in a northern temperate forest, hypothesizing that tree mortality would decrease belowground C allocation, increase belowground N availability and cycling rates, and trigger N leaching in this highly N-limited forest. In the Forest Accelerated Succession ExperimenT, we stem girdled >6700 aspen and birch trees (39% of basal area) on 40 ha of forestland at the University of Michigan Biological Station to test these and other hypotheses related to forest development and biogeochemistry. Here, we present the cascade of changes in belowground C and N cycling that occurred in the first two growing seasons following initiation of FASET in April 2008. Aspen and birch mortality decreased stand-level fine root nonstructural carbohydrate contents, accelerating fine root turnover and causing a net reduction in fine root biomass. These decreases in belowground C allocation and root functioning increased forest floor net NH4+ availability, which accelerated nitrification rates, increased net NO3- availability, and led to small NO3- leaching losses. These belowground perturbations were registered by the forest canopy through shifts in foliar 13C and 15N natural abundances, which indicated water stress among girdled aspens and faster, leakier cycling in a source N pool shared by foliage of all tree species. We interpret the results of this initial disturbance phase of the FASET study in the context of our conceptual model of N availability and ecosystem C storage over longer-term (successional) time scales. We predict that, following minor N leaching losses during disturbance, reorganization of belowground N cycling along a more active, faster-cycling trajectory will enable the forest to increase rates of C storage during its next successional stage, promoting resilience of C and N cycling to this experimental disturbance.
This paper describes the potential impacts of warming temperatures and changing precipitation on ... more This paper describes the potential impacts of warming temperatures and changing precipitation on plants, wildlife, invasive species, pests, and agricultural ecosystems across the multi-state region centered on Chicago, Illinois. We examine a geographic area that captures much of Lake Michigan, including a complex mosaic of urbanization and agriculture surrounding southern Lake Michigan. We consider species currently present within this broad region as well as species that are expected to move into or out of the area as climate zones shift northward through the coming century. Our analysis draws upon disparate data sources to compile projections. We conclude that a complex mixture of land use poses particular challenges to natural ecosystems in this region under climate change. Dispersal is likely to be limited for some species, and some populations of native taxa may already be reduced due to habitat loss. Other species can persist, even thrive, within a mixed landscape mosaic, provided natural areas and green spaces are available. If such spaces are somehow connected, they can provide opportunities for native species to inhabit and move through the metropolitan region (perhaps even better than the landscapes previously dominated by agriculture). Strategies for adapting regional agriculture and minimizing pest outbreaks also call for creative management intervention. With additional research, Chicago and its surrounding environs have an opportunity to provide leadership on effective management of natural resources under climate change.
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Papers by K. Nadelhoffer