Showing papers in "Ecosystems in 2005"

An Exploratory Framework for the Empirical Measurement of Resilience

Abstract: Deliberate progress towards the goal of long-term sustainability depends on understanding the dynamics of linked social and ecological systems. The concept of social-ecological resilience holds promise for interdisciplinary syntheses. Resilience is a multifaceted concept that as yet has not been directly operationalized, particularly in systems for which our ignorance is such that detailed, parameter-rich simulation models are difficult to develop. We present an exploratory framework as a step towards the operationalization of resilience for empirical studies. We equate resilience with the ability of a system to maintain its identity, where system identity is defined as a property of key components and relationships (networks) and their continuity through space and time. Innovation and memory are also fundamental to understanding identity and resilience. By parsing our systems into the elements that we subjectively consider essential to identity, we obtain a small set of specific focal variables that reflect changes in identity. By assessing the potential for changes in identity under specified drivers and perturbations, in combination with a scenario-based approach to considering alternative futures, we obtain a surrogate measure of the current resilience of our study system as the likelihood of a change in system identity under clearly specified conditions, assumptions, drivers and perturbations. Although the details of individual case studies differ, the concept of identity provides a level of generality that can be used to compare measure of resilience across cases. Our approach will also yield insights into the mechanisms of change and the potential consequences of different policy and management decisions, providing a level of decision support for each case study area.

Surrogates for Resilience of Social–Ecological Systems

Abstract: From its roots in ecology, resilience (Holling 1973) has more recently been applied to social-ecological systems, or SES. Theories of changing resilience explicitly address the persistence or breakdown of diverse states of complex systems (Gunderson and Holling 2002). These ideas have attracted interest from interdisciplinary research groups interested in change, conservation or restoration of SES (Berkes and others 2003; Scheffer and others 2003). From a practical standpoint, resilience theory provides a conceptual foundation for sustainable development (Folke and others 2002). The transition from the ory to practice, however, requires assessment or estimation of resilience (Carpenter and others 2001). So far, there is little experience with esti mating resilience of SES, and little understanding of the sensitivity of resilience measures to changes in SES. This shortage of practical field experience is a barrier to building understanding through empirical study of resilience in SES. Direct measurement of resilience is difficult be cause it requires measuring the thresholds or boundaries that separate alternate domains of dynamics for SES. The only sure way to detect a threshold in a complex system is to cross it (Carpenter 2003). Yet threshold-crossings do not occur very often. In the natural sciences, much understanding of thresholds has come from delib erate manipulations of ecosystems, or before/after studies of large disturbances (Turner and Dale

Prevalence of Heterotrophy and Atmospheric CO2 Emissions from Aquatic Ecosystems

Abstract: Recent, parallel developments in the study of freshwater and marine ecosystems have provided evidence that net heterotrophic systems (those in which respiratory organic matter destruction exeeds photosynthetic production) are more prevalent than hitherto believed, including most rivers, oligo- to mesotrophic lakes and some oligotrophic regions of the ocean. In parallel, these aquatic ecosystems have been shown to act as CO2 sources to the atmosphere, as expected from the heterotrophic nature of the communities they contain. The prevalence of net heterotrophic aquatic ecosystems indicates that they must receive significant inputs of organic carbon from adjacent ecosystems, assigning an important role to the lateral exchanges of carbon between land and aquatic ecosystems, between coastal and open ocean ecosystems, as well as internal redistribution within large or complex aquatic ecosystems in determining their metabolic status and the gaseous exchange with the atmosphere. The examination of the carbon budget of ecosystems requires, therefore, an integrative approach that accounts for exchanges between compartments often studied in isolation. These recent findings conform a new paradigm of the functioning of aquatic ecosystems, and the metabolic connectivity between ecosystems in the biosphere.

Nitrogen Retention, Removal, and Saturation in Lotic Ecosystems

Abstract: Increased nitrogen (N) loading to lotic ecosystems may cause fundamental changes in the ability of streams and rivers to retain or remove N due to the potential for N saturation. Lotic ecosystems will saturate with sustained increases in the N load, but it is unclear at what point saturation will occur. Rates of N transformation in lotic ecosystems will vary depending on the total N load and whether it is an acute or chronic N load. Nitrogen saturation may not occur with only pulsed or short-term increases in N. Overall, saturation of microbial uptake will occur prior to saturation of denitrification of N and denitrification will become saturated prior to nitrification, exacerbating increases in nitrate concentrations and in N export downstream. The rate of N export to downstream ecosystems will increase proportionally to the N load once saturation occurs. Long term data sets showed that smaller lotic ecosystems have a greater capacity to remove instream N loads, relative to larger systems. Thus, denitrification is likely to become less important as a N loss mechanism as the stream size increases. There is a great need for long-term studies of N additions in lotic ecosystems and clear distinctions need to be made between ecosystem responses to short-term or periodic increases in N loading and alterations in ecosystem functions due to chronic N loading.

Ecosystem Respiration in a Cool Temperate Bog Depends on Peat Temperature But Not Water Table

Abstract: Ecosystem respiration (ER) is an important but poorly understood part of the carbon (C) budget of peatlands and is controlled primarily by the thermal and hydrologic regimes. To establish the relative importance of these two controls for a large ombrotrophic bog near Ottawa, Canada, we analyzed ER from measurements of nighttime net ecosystem exchange of carbon dioxide (CO2) determined by eddy covariance technique. Measurements were made from May to October over five years, 1998 to 2002. Ecosystem respiration ranged from less than 1 μmol CO2 m−2 s−1 in spring (May) and fall (late October) to 2–4 μmol CO2 m−2 s−1 during mid-summer (July-August). As anticipated, there was a strong relationship between ER and peat temperatures (r2 = 0.62). Q10 between 5° to 15°C varied from 2.2 to 4.2 depending upon the choice of depth where temperature was measured and location within a hummock or hollow. There was only a weak relationship between ER and water-table depth (r2 = 0.11). A laboratory incubation of peat cores at different moisture contents showed that CO2 production was reduced by drying in the surface samples, but there was little decrease in production due to drying from below a depth of 30 cm. We postulate that the weak correlation between ER and water table position in this peatland is primarily a function of the bog being relatively dry, with water table varying between 30 and 75 cm below the hummock tops. The dryness gives rise to a complex ER response to water table involving i) compensations between production of CO2 in the upper and lower peat profile as the water table falls and ii) the importance of autotrophic respiration, which is relatively independent of water-table position.

Contrasting Effects of Substrate and Fertilizer Nitrogen on the Early Stages of Litter Decomposition

Abstract: Commonly observed positive correlations between litter nitrogen (N) concentrations and decomposition rates suggest that N frequently limits decomposition in its early stages. However, numerous studies have found little, if any, effect of N fertilization on decomposition. I directly compared internal substrate N and externally supplied inorganic N effects on decomposition in sites varying in soil N availability. I decomposed eight substrates (with initial %N from 0–2.5) in control and N-fertilized plots at eight grassland and forest sites in central Minnesota. N fertilization increased decomposition at only two of eight sites, even though decomposition was positively related to litter N at all sites and to soil N availability across sites. The effect of externally supplied N on decomposition was independent of litter N concentration, but was greater at sites with low N availability. The inconsistent effects of substrate and externally supplied N may have arisen because decomposers use organic N preferentially as an N source; because inorganic N availability across sites or with fertilization induced changes in microbial community attributes (for example, lower C:N or greater efficiency) that reduced the response of decomposition to increased inorganic N supply; or because the positive correlation between litter N or site N availability with decomposition was spurious, caused by tight correlations between litter or site N and some other factor that truly limited decomposition. These inconsistent effects of substrate N and external N supply on decomposition suggest that the oft-observed relationship between litter N and decomposition may not indicate N limitation of decomposition.

Aquatic Terrestrial Linkages Along a Braided-River: Riparian Arthropods Feeding on Aquatic Insects

Abstract: Rivers can provide important sources of energy for riparian biota. Stable isotope analysis (d 13 C, d 15 N) together with linear mixing models, were used to quantify the importance of aquatic insects as a food source for a riparian arthropod assemblage inhabiting the shore of the braided Tagliamento River (NE Italy). Proportional aquatic prey contributions to riparian arthropod diets differed considerable among taxa. Carabid beetles of the genus Bembidion and Nebria picicornis fed entirely on aquatic insects. Aquatic insects made up 80% of the diet of the dominant staphylinid beetle Paederidus rubrothoracicus. The diets of the dominant lycosid spiders Arctosa cinerea and Pardosa wagleri consisted of 56 and 48% aquatic insects, respectively. In contrast, the ant Manica rubida fed mainly on terrestrial sources. The proportion of aquatic insects in the diet of lycosid spiders changed seasonally, being related to the seasonal abundance of lycosid spiders along the stream edge. The degree of spatial and seasonal aggregation of riparian arthropods at the river edge coincided with their proportional use of aquatic subsidies. The results suggest that predation by riparian arthropods is a quantitatively important process in the transfer of aquatic secondary production to the riparian food web.

Effects of European earthworm invasion on soil characteristics in Northern Hardwood Forests of Minnesota, USA

Abstract: European earthworms are colonizing worm-free hardwood forests across North America. Leading edges of earthworm invasion in forests of northern Minnesota provide a rare opportunity to document changes in soil characteristics as earthworm invasions are occurring. Across leading edges of earthworm invasion in four northern hardwood stands, increasing total earthworm biomass was associated with rapid disappearance of the O horizon. Concurrently, the thickness, bulk density and total soil organic matter content of the A horizon increased, and its percent organic matter and fine root density decreased. Different earthworm species assemblages influenced the magnitude and type of change in these soil parameters. Soil N and P availability were lower in plots with high earthworm biomass compared to plots with low worm biomass. Decreases in soil nitrogen availability associated with high earthworm biomass were reflected in decreased foliar nitrogen content for Carex pensylvanica, Acer saccharum and Asarum canadense but increased foliar N for Athyrium felixfemina. Overall, high earthworm biomass resulted in increased foliar carbon to nitrogen ratios. The effects of earthworm species assemblages on forest soil properties are related to their feeding and burrowing habits in addition to effects related to total biomass. The potential for large ecosystem consequences following exotic earthworm invasion has only recently been recognized by forest ecologists. In the face of rapid change and multiple pressures on native forest ecosystems, the impacts of earthworm invasion on forest soil structure and function must be considered.

Building Resilience in Lagoon Social–Ecological Systems: A Local-level Perspective

Abstract: Building resilience in integrated human and nature systems or social–ecological systems (SES) is key for sustainability. Therefore, developing ways of assessing resilience is of practical as well as theoretical significance. We approached the issue by focusing on the local level and using five lagoon systems from various parts of the world for illustration. We used a framework based on four categories of factors for building resilience: (1) learning to live with change and uncertainty; (2) nurturing diversity for reorganization and renewal; (3) combining different kinds of knowledge; and (4) creating opportunity for self-organization. Under each category, the cases generated a number of items for building resilience, and potential surrogates of resilience, that is, variables through which the persistence of SES emerging through change can be assessed. The following factors were robust across all five lagoon SES cases: learning from crisis, responding to change, nurturing ecological memory, monitoring the environment, and building capacity for self-organization and conflict management.

A Systems Model Approach to Determining Resilience Surrogates for Case Studies

Abstract: Resilience theory offers a framework for understanding the dynamics of complex systems. However, operationalizing resilience theory to develop and test empirical hypotheses can be difficult. We present a method in which simple systems models are used as a framework to identify resilience surrogates for case studies. The process of constructing a systems model for a particular case offers a path for identifying important variables related to system resilience, including the slowly-changing variables and thresholds that often are keys to understanding the resilience of a system. We develop a four-step process for identifying resilience surrogates through development of systems models. Because systems model development is often a difficult step, we summarize four basic existing systems models and give examples of how each may be used to identify resilience surrogates. The construction and analysis of simple systems models provides a useful basis for guiding and directing the selection of surrogate variables that will offer appropriate empirical measures of resilience.

Postfire Soil N Cycling in Northern Conifer Forests Affected by Severe, Stand-Replacing Wildfires

Abstract: Severe, stand-replacing fires affect large areas of northern temperate and boreal forests, potentially modifying ecosystem function for decades after their occurrence. Because these fires occur over large extents, and in areas where plant production is limited by nitrogen (N) availability, the effect of fire on N cycling may be important for long-term ecosystem productivity. In this article, we review what is known about postfire N cycling in northern temperate and boreal forests experiencing stand-replacing fires. We then build upon existing literature to identify the most important mechanisms that control postfire N availability in systems experiencing severe, stand-replacing fires compared with fires of lower severity. These mechanisms include changes in abiotic conditions caused by the opening of the canopy (for example, decreased LAI, increased solar radiation), changes in ground layer quantity and quality (for example, nutrient release, permafrost levels), and postfire plant and microbial adaptations affecting N fixation and N uptake (for example, serotiny, germination cues). Based on the available literature, these mechanisms appear to affect N inputs, internal N cycling, and N outputs in various ways, indicating that severe fire systems are variable across time and space as a result of complex interactions between postfire abiotic and biotic factors. Future experimental work should be focused on understanding these mechanisms and their variability across the landscape.

The Use of Discontinuities and Functional Groups to Assess Relative Resilience in Complex Systems

Abstract: It is evident when the resilience of a system has been exceeded and the system qualitatively changed. However, it is not clear how to measure resilience in a system prior to the demonstration that the capacity for resilient response has been exceeded. We argue that self-organizing human and natural systems are structured by a relatively small set of processes operating across scales in time and space. These structuring processes should generate a discontinuous distribution of structures and frequencies, where discontinuities mark the transition from one scale to another. Resilience is not driven by the identity of elements of a system, but rather by the functions those elements provide, and their distribution within and across scales. A self-organizing system that is resilient should maintain patterns of function within and across scales despite the turnover of specific elements (for example, species, cities). However, the loss of functions, or a decrease in functional representation at certain scales will decrease system resilience. It follows that some distributions of function should be more resilient than others. We propose that the determination of discontinuities, and the quantification of function both within and across scales, produce relative measures of resilience in ecological and other systems. We describe a set of methods to assess the relative resilience of a system based upon the determination of discontinuities and the quantification of the distribution of functions in relation to those discontinuities.

Plant communities, soil microorganisms, and soil carbon cycling: Does altering the world belowground matter to ecosystem functioning?

Abstract: Soil microorganisms mediate many critical ecosystem processes. Little is known, however, about the factors that determine soil microbial community composition, and whether microbial community composition influences process rates. Here, we investigated whether aboveground plant diversity affects soil microbial community composition, and whether differences in microbial communities in turn affect ecosystem process rates. Using an experimental system at La Selva Biological Station, Costa Rica, we found that plant diversity (plots contained 1, 3, 5, or > 25 plant species) had a significant effect on microbial community composition (as determined by phospholipid fatty acid analysis). The different microbial communities had significantly different respiration responses to 24 labile carbon compounds. We then tested whether these differences in microbial composition and catabolic capabilities were indicative of the ability of distinct microbial communities to decompose different types of litter in a fully factorial laboratory litter transplant experiment. Both microbial biomass and microbial community composition appeared to play a role in litter decomposition rates. Our work suggests, however, that the more important mechanism through which changes in plant diversity affect soil microbial communities and their carbon cycling activities may be through alterations in their abundance rather than their community composition.

Forecasting Environmental Responses to Restoration of Rivers Used as Log Floatways: An Interdisciplinary Challenge

Abstract: Log floating in the 19th to mid 20th centuries has profoundly changed the environmental conditions in many northern river systems of the world. Regulation of flow by dams, straightening and narrowing of channels by various piers and wing dams, and homogenization of bed structure are some of the major impacts. As a result, the conditions for many riverine organisms have been altered. Removing physical constructions and returning boulders to the channels can potentially restore conditions for these organisms. Here we describe the history of log driving, review its impact on physical and biological conditions and processes, and predict the responses to restoration. Reviewing the literature on comparable restoration efforts and building upon this knowledge, using boreal Swedish rivers as an example, we address the last point. We hypothesize that restoration measures will make rivers wider and more sinuous, and provide rougher bottoms, thus improving land-water interactions and increasing the retention capacity of water, sediment, organic matter and nutrients. The geomorphic and hydraulic/hydrologic alterations are supposed to favor production, diversity, migration and reproduction of riparian and aquatic organisms. The response rates are likely to vary according to the types of processes and organisms. Some habitat components, such as beds of very large boulders and bedrock outcrops, and availability of sediment and large woody debris are believed to be extremely difficult to restore. Monitoring and evaluation at several scales are needed to test our predictions.

Net Primary Production and Canopy Nitrogen in a Temperate Forest Landscape: An Analysis Using Imaging Spectroscopy, Modeling and Field Data

Abstract: Understanding spatial patterns of net primary production (NPP) is central to the study of terrestrial ecosystems, but efforts are frequently hampered by a lack of spatial information regarding factors such as nitrogen availability and site history. Here, we examined the degree to which canopy nitrogen can serve as an indicator of patterns of NPP at the Bartlett Experimental Forest in New Hampshire by linking canopy nitrogen estimates from two high spectral resolution remote sensing instruments with field measurements and an ecosystem model. Predicted NPP across the study area ranged from less than 700 g m−2 year−1 to greater than 1300 g m−2 year−1 with a mean of 951 g m−2 year−1. Spatial patterns corresponded with elevation, species composition and historical forest management, all of which were reflected in patterns of canopy nitrogen. The relationship between production and elevation was nonlinear, with an increase from low- to mid-elevation deciduous stands, followed by a decline in upper-elevation areas dominated by evergreens. This pattern was also evident in field measurements and mirrored an elevational trend in foliar N concentrations. The increase in production from low-to mid-elevation deciduous stands runs counter to the generally accepted pattern for the northeastern U.S. region, and suggests an importance of moisture limitations in lower-elevation forests. Field measurements of foliar N, wood production and leaf litterfall were also used to evaluate sources of error in model estimates and to determine how predictions are affected by different methods of acquiring foliar N input data. The accuracy of predictions generated from remotely sensed foliar N approached that of predictions driven by field-measured foliar N. Predictions based on the more common approach of using aggregated foliar N for individual cover types showed reasonable agreement in terms of the overall mean, but were in poor agreement on a plot-by-plot basis. Collectively, these results suggest that variation in foliar N exerts an important control on landscape-level spatial patterns and can serve as an integrator of other underlying factors that influence forest growth rates.

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

Abstract: 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.

Plant Species Numbers Predicted by a Topography-based Groundwater Flow Index

Abstract: The lack of a clear understanding of the factors governing the often-great variation of species numbers over entire landscapes confounds attempts to manage biodiversity. We hypothesized that in a t ...

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

Abstract: 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.

N Retention in Urbanizing Headwater Catchments

Abstract: Urbanization can potentially alter watershed nitrogen (N) retention via combined changes in N loading, water runoff, and N processing potential. We examined N export and retention for two headwater catchments ( 4k m 2 ) of contrasting land use (16% vs. 79% urban) in the Plum Island Ecosystem (PIE-LTER) watershed, MA. The study period included a dry year (2001–2002 water year) and a wet year (2002–2003 water year). We generalized results by comparing dissolved inorganic nitrogen (DIN) concentrations from 16 additional headwater catchments (0.6–4.2 km 2 ) across a range of urbanization (6–90%). Water runoff was 25– 40% higher in the urban compared to the forested catchment, corresponding with an increased proportion of impervious surfaces (25% vs. 8%). Estimated N loading was 45% higher and N flux 6.5 times higher in the urban than in the forested catchment. N retention (1 ) measured stream export / estimated loading) was 65–85% in the urban site and 93–97% in the forested site, with lower retention rates during the wetter year. The mechanisms by which N retention stays relatively high in urban systems are poorly known. We show that N retention is related to the amount of impervious surface in a catchment because of associated changes in N loading (maximized at moderate levels of imperviousness), runoff (which continues to increase with imperviousness), and biological processes that retain N. Continued declines in N retention due to urbanization have important negative implications for downstream aquatic systems including the coastal zone.

Using Satellite Remote Sensing to Estimate the Colored Dissolved Organic Matter Absorption Coefficient in Lakes

Abstract: Given the importance of colored dissolved organic matter (CDOM) for the structure and function of lake ecosystems, a method that could estimate the amount of CDOM in lake waters over large geographic areas would be highly desirable. Satellite remote sensing has the potential to resolve this problem. We carried out model simulations to evaluate the suitability of different satellite sensors (Landsat, IKONOS, and the Advanced land Imager [ALI]) to map the amount of CDOM in concentration ranges that occur in boreal lakes of the Nordic countries. The results showed that the 8-bit radiometric resolution of Landsat 7 is not adequate when absorption by CDOM at 420 nm is higher than 3 m−1. On the other hand, the 16-bit radiometric resolution of ALI, a prototype of the next generation of Landsat, is suitable for mapping CDOM in a wider range of concentrations. An ALI image of southern Finland was acquired on 14, July 2002 and in situ measurements were carried out in 15 lakes (18 stations). The results showed that there is a high correlation (R2 = 0.84) between the 565 nm/660 nm ALI band ratio and the CDOM absorption coefficient in lakes. Analysis of 245 lakes in the acquired satellite image showed a normal distribution of CDOM concentration among the lakes. However, the size distribution of lakes was highly skewed toward small lakes, resulting in the CDOM concentration per unit lake area being skewed toward high values. We showed that remote sensing enables synoptic monitoring of the CDOM concentration in a large number of lakes and thus enables scaling up to the level of large ecosystems and biomes.

Global Scenarios: Background Review for the Millennium Ecosystem Assessment

Abstract: The long-range outlook for the world’s ecosystems depends on the course taken by global development in the coming decades. Current global trends and ecological dynamics are consistent with very different outcomes, defined by alternative assumptions about the technological, economic, demographic, geopolitical, and social aspects of development and the ways in which institutions, personal and public values, and natural systems may be expected to respond to historically novel stressors. Recent advances in scenario analysis have addressed the dual methodological challenge of exploring these uncertainties in an organized way and determining what would be needed to make the transition to sustainability. This paper reviews global scenario research, setting current efforts in a historical context. It focuses on seven recent studies that are comprehensive, regionally disaggregated, and narratively rich—and thus of greatest relevance to the Millennium Ecosystem Assessment (MA). It summarizes their social visions and the level of quantitative detail used in these exercises. Taken together, this suite of global scenario studies provides a useful platform for the MA by offering insight into the complex factors that drive ecosystem change, estimating the magnitude of regional pressures on ecosystems, sounding the alert on critical uncertainties that could undermine sustainable development, and understanding the importance of institutions and values. But these studies are only a point of departure. The integration of changing ecosystem conditions into global development scenarios, as both effects and causes, is at the cutting edge of scenario analysis. The paper concludes by identifying directions for this research program and suggesting ways that the MA can contribute to this effort.

Understanding the Interaction of Rural People with Ecosystems: A Case Study in a Tropical Dry Forest of Mexico

Abstract: The aim of this study was to help understand the interaction of rural people with tropical dry forests. It was based on social research conducted in the Chamela-Cuixmala region, on the Pacific coast of Mexico. The analytical tools used in the study included stakeholder identification, environmental history and social perceptions. The two main social groups in the study were ejidatarios, who own most of the territory, and avecindados, who possess no land but have high population numbers. Through an interpretative methodological approach we documented the vision and meaning that rural people give to their natural and social worlds. The agricultural development model promoted by the Mexican government for decades was identified as the main driver of ecosystem transformation. Rural people, who arrived recently in the region, were proud of the pasture-lands that were transformed from tropical forests. Conservation policies implemented during the last two decades were viewed as impositions although people recognized the value of services provided by ecosystems. This case study has helped to unravel the main dimensions of the human system and how it relates to structures of signification. The social panorama unveiled can be used as an initial basis to promote further research on the social-ecological system of the Chamela-Cuixmala region and to develop future participatory management schemes.

Forest Remnants Along Urban-Rural Gradients: Examining Their Potential for Global Change Research

Abstract: Over the next century, ecosystems throughout the world will be responding to rapid changes in climate and rising levels of carbon dioxide, inorganic N and ozone. Because people depend on biological systems for water, food and other ecosystem services, predicting the range of responses to global change for various ecosystem types in different geographic locations is a high priority. Modeling exercises and manipulative experimentation have been the principle approaches used to place upper and lower bounds on community and ecosystem responses. However, each of these approaches has recognized limitations. Manipulative experiments cannot vary all the relevant factors and are often performed at small spatio-temporal scales. Modeling is limited by data availability and by our knowledge of how current observations translate into future conditions. These weaknesses would improve if we could observe ecosystems that have already responded to global change factors and thus presage shifts in ecosystem structure and function. Here we consider whether urban forest remnants might offer this ability. As urban forests have been exposed to elevated temperature, carbon dioxide, nitrogen deposition and ozone for many decades, they may be ahead of the global change “response curve” for forests in their region. Therefore, not only might forests along urbanization gradients provide us with natural experiments for studying current responses to global change factors, but their legacy of response to past urbanization may also constitute space-for-time substitution experiments for predicting likely regional forest responses to continued environmental change. For this approach to be successful, appropriate criteria must be developed for selecting forest remnants and plots that would optimize our ability to detect incipient forest responses to spatial variation in global change factors along urbanization gradients, while minimizing artifacts associated with remnant size and factors other than those that simulate global change. Studying forests that meet such criteria along urban-to-rural gradients could become an informative part of a mixed strategy of approaches for improving forecasts of forest ecosystem change at the regional scale.

Impacts of Reservoir Creation on the Biogeochemical Cycling of Methyl Mercury and Total Mercury in Boreal Upland Forests

Abstract: The FLooded Upland Dynamics Experiment (FLUDEX) at the Experimental Lakes Area (ELA) in northwest Ontario was designed to test the hypothesis that methylmercury (MeHg) production in reservoirs is related to the amount, and subsequent decomposition, of flooded organic matter. Three upland forest sites that varied in the amounts of organic carbon stored in vegetation and soils (Low C, 30,870 kg C ha−1; Medium C, 34,930 kg C ha−1; and High C, 45,860 kg C ha−1) were flooded annually from May to September with low-organic carbon, low-MeHg water pumped from a nearby lake. Within five weeks of flooding, MeHg concentrations in the reservoir outflows exceeded those in reservoir inflows and remained elevated for the duration of the experiment, peaking at 1.60 ng L−1 in the Medium C reservoir. We estimated the net production of MeHg in each reservoir by calculating annual changes in pools of MeHg stored in flooded soils, periphyton, zooplankton, and fish. Overall, there was an initial pulse of MeHg production (range = 120–1590 ng m−2 day−1) in all FLUDEX reservoirs that lasted for 2 years, after which time net demethylation (range = 360–1230 ng MeHg degraded m−2 day−1) began to reduce the pools of MeHg in the reservoirs, but not back to levels found prior to flooding. Rates of MeHg production were generally related to the total amount of organic carbon flooded to create the reservoirs. Large increases in MeHg stores in soils compared to those in water and biota indicate that flooded soils were the main sites of MeHg production. This study should assist hydroelectric utilities and government agencies in making informed decisions about selecting sites for future reservoir development to reduce MeHg contamination of the reservoir fisheries.

Fine Root Production and Turnover in a Norway Spruce Stand in Northern Sweden: Effects of Nitrogen and Water Manipulation

Abstract: Fine root length production, biomass production, and turnover in forest floor and mineral soil (0–30 cm) layers were studied in relation to irrigated (I) and irrigated-fertilized (IL) treatments in a Norway spruce stand in northern Sweden over a 2-year period. Fine roots (<1 mm) of both spruce and understory vegetation were studied. Minirhizotrons were used to estimate fine root length production and turnover, and soil cores were used to estimate standing biomass. Turnover was estimated as both the inverse of root longevity (RTL) and the ratio of annual root length production to observed root length (RTR). RTR values of spruce roots in the forest floor in I and IL plots were 0.6 and 0.5 y−1, respectively, whereas the corresponding values for RTL were 0.8 and 0.9 y−1. In mineral soil, corresponding values for I, IL, and control (C) plots were 1.2, 1.2, and 0.9 y−1 (RTR) and 0.9, 1.1, and 1 y−1 (RTL). RTR and RTL values of understory vegetation roots were 1 and 1.1 y−1, respectively. Spruce root length production in both the forest floor and the mineral soil in I plots was higher than in IL plots. The IL-treated plots gave the highest estimates of spruce fine root biomass production in the forest floor, but, for the mineral soil, the estimates obtained for the I plots were the highest. The understory vegetation fine root production in the I and IL plots was similar for both the forest floor and the mineral soil and higher (for both layers) than in C plots. Nitrogen (N) turnover in the forest floor and mineral soil layers (summed) via spruce roots in IL, I, and C plots amounted to 2.4, 2.1, and 1.3 g N m−2 y−1, and the corresponding values for field vegetation roots were 0.6, 0.5, and 0.3 g N m−2 y−1. It was concluded that fertilization increases standing root biomass, root production, and N turnover of spruce roots in both the forest floor and mineral soil. Data on understory vegetation roots are required for estimating carbon budgets in model studies.

Long-term Decreases in Stream Nitrate: Successional Causes Unlikely; Possible Links to DOC?

Abstract: Huntington (2005) poses the hypothesis that ‘‘immobilization of inorganic N, in combination with region-wide recovery from past disturbances may explain the decrease in stream water nitrate in recent decades’’ that we observed in 28 White Mountain, New Hampshire streams (Goodale and others 2003). We focus our response on Huntington s suggestions, and note that a range of additional factors not discussed here can have marked impacts on long-term patterns of NO3 ) loss (for example, N deposition, insect outbreaks, climate variation and extreme events such as soil frost). Below, we address Huntington s suggestion in two parts, first considering the role of region-wide recovery from historical disturbances and then considering recent changes in environmental factors that might have increased soil N immobilization through increased belowground productivity. We then introduce a hypothesis that suggests that changes in stream NO3 ) might be partly linked to observed changes in dissolved organic carbon (DOC). We agree with Huntington that historical disturbances have had substantial impacts on forest carbon and nitrogen dynamics in the White Mountains of New Hampshire (see for example, Bormann and Likens 1979; Covington 1981; Thorne and Hamburg 1985). We have previously considered in some depth the role of the region s disturbance history as a major driver of patterns of organic matter accumulation and watershed N retention (see for example, Vitousek and Reiners 1975; Aber and Driscoll 1997; Goodale and others 2000; Goodale and Aber 2001). We agree that re-accumulation of soil organic matter in re-growing forests provides important sinks for atmospheric CO2 and N deposition across large parts of the eastern U.S. (see for example, Huntington 1995; Hooker and Compton 2003). However, we do not believe that forest re-growth and soil re-accumulation explain the approximately 70% decrease in stream NO3 ) concentration (mean decrease of 25 lmol/L) that we observed between 1973)4 and 1996–97 (Goodale and others 2003). First, we emphasize that we observed large decreases in stream NO3 ) concentration in both aggrading successional systems and in several oldgrowth watersheds that had no evidence of standlevel disturbance by historical logging or fire (see Goodale and others 2003 Figure 6; see also Martin and others 2000). Second, the observed decreases in stream NO3 ) between the 1970s and the 1990s would require a synchronous increase in the rate of soil re-accumulation during this time period across all watersheds. Few successional factors could cause an increase in soil re-accumulation rate, and such factors are unlikely to occur simultaneously in both old-growth and successional watersheds.

Biocomplexity in coupled natural-human systems : A multidimensional framework

Abstract: As defined by Ascher, biocomplexity results from a “multiplicity of interconnected relationships and levels.” However, no integrative framework yet exists to facilitate the application of this concept to coupled human–natural systems. Indeed, the term “biocomplexity” is still used primarily as a creative and provocative metaphor. To help advance its utility, we present a framework that focuses on linkages among different disciplines that are often used in studies of coupled human–natural systems, including the ecological, physical, and socioeconomic sciences. The framework consists of three dimensions of complexity: spatial, organizational, and temporal. Spatial complexity increases as the focus changes from the type and number of the elements of spatial heterogeneity to an explicit configuration of the elements. Similarly, organizational complexity increases as the focus shifts from unconnected units to connectivity among functional units. Finally, temporal complexity increases as the current state of a system comes to rely more and more on past states, and therefore to reflect echoes, legacies, and evolving indirect effects of those states. This three-dimensional, conceptual volume of biocomplexity enables connections between models that derive from different disciplines to be drawn at an appropriate level of complexity for integration.

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Abstract: Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha–1 y–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha–1 y–1 below the litter layer to around 50 kg C kg C ha–1 y–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO 3 − ). The dominance of NO 3 – relative to the total amount nitrate of N leaching from the soil shows that NO 3 – is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.

Regional Patterns in Carbon Cycling Across the Great Plains of North America

Abstract: The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m−2 y−1) to tallgrass prairie (180, 1221, and 208 g C m−2 y−1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.

Production of CO2 in Soil Profiles of a California Annual Grassland

Abstract: Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil–air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Nino event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Nino year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4–11 mg C– CO2 m−2 h−1), 20%–50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.