Showing papers in "Ecosystems in 2003"

Biogeochemical Hot Spots and Hot Moments at the Interface of Terrestrial and Aquatic Ecosystems

Abstract: Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that show disproportionately high reaction rates relative to the surrounding matrix, whereas hot moments are defined as short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the carbon (C) and nitrogen (N) cycles, we show that hot spots occur where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants. Hot moments occur when episodic hydrological flowpaths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flowpaths, we can forge a better mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary so that biogeochemical hot spots can be identified at broader spatiotemporal scales and factored into quantitative models. We specifically recommend that resource managers incorporate both natural and artificially created biogeochemical hot spots into their plans for water quality management. Finally, we emphasize the needs for further research to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements, improve our ability to predict their occurrence, assess their importance in landscape biogeochemistry, and evaluate their utility as tools for resource management.

Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes

Abstract: Although it is generally acknowledged that invasions by exotic plant species represent a major threat to biodiversity and ecosystem stability, little attention has been paid to the potential impacts of these invasions on nutrient cycling processes in the soil. The literature on plant–soil interactions strongly suggests that the introduction of a new plant species, such as an invasive exotic, has the potential to change many components of the carbon (C), nitrogen (N), water, and other cycles of an ecosystem. I have reviewed studies that compare pool sizes and flux rates of the major nutrient cycles in invaded and noninvaded systems for invasions of 56 species. The available data suggest that invasive plant species frequently increase biomass and net primary production, increase N availability, alter N fixation rates, and produce litter with higher decomposition rates than co-occurring natives. However, the opposite patterns also occur, and patterns of difference between exotics and native species show no trends in some other components of nutrient cycles (for example, the size of soil pools of C and N). In some cases, a given species has different effects at different sites, suggesting that the composition of the invaded community and/or environmental factors such as soil type may determine the direction and magnitude of ecosystem-level impacts. Exotic plants alter soil nutrient dynamics by differing from native species in biomass and productivity, tissue chemistry, plant morphology, and phenology. Future research is needed to (a) experimentally test the patterns suggested by this data set; (b) examine fluxes and pools for which few data are available, including whole-site budgets; and (c) determine the magnitude of the difference in plant characteristics and in plant dominance within a community that is needed to alter ecosystem processes. Such research should be an integral component of the evaluation of the impacts of invasive species.

Mobile Link Organisms and Ecosystem Functioning: Implications for Ecosystem Resilience and Management

Abstract: Current natural resource management seldom takes the ecosystem functions performed by organisms that move between systems into consideration. Organisms that actively move in the landscape and connect habitats in space and time are here termed “mobile links.” They are essential components in the dynamics of ecosystem development and ecosystem resilience (that is, buffer capacity and opportunity for reorganization) that provide ecological memory (that is, sources for reorganization after disturbance). We investigated the effects of such mobile links on ecosystem functions in aquatic as well as terrestrial environments. We identify three main functional categories: resource, genetic, and process linkers and suggest that the diversity within functional groups of mobile links is a central component of ecosystem resilience. As the planet becomes increasingly dominated by humans, the magnitude, frequency, timing, spatial extent, rate, and quality of such organism-mediated linkages are being altered. We argue that global environmental change can lead to (a) the decline of essential links in functional groups providing pollination, seed dispersal, and pest control; (b) the linking of previously disconnected areas, for example, the spread of vector-borne diseases and invasive species; and (c) the potential for existing links to become carriers of toxic substances, such as persistent organic compounds. We conclude that knowledge of interspatial exchange via mobile links needs to be incorporated into management and policy-making decisions in order to maintain ecosystem resilience and hence secure the capacity of ecosystems to supply the goods and services essential to society.

Effects of Land Cover on Stream Ecosystems: Roles of Empirical Models and Scaling Issues

Abstract: We built empirical models to estimate the effects of land cover on stream ecosystems in the mid-Atlantic region (USA) and to evaluate the spatial scales over which such models are most effective. Predictive variables included land cover in the watershed, in the streamside corridor, and near the study site, and the number and location of dams and point sources in the watershed. Response variables were annual nitrate flux; species richness of fish, benthic macroinvertebrates, and aquatic plants; and cover of aquatic plants and riparian vegetation. All data were taken from publicly available databases, mostly over the Internet. Land cover was significantly correlated with all ecological response variables. Modeled R 2 ranged from 0.07 to 0.5, but large data sets often allowed us to estimate with acceptable precision the regression coefficients that express the change in ecological conditions associated with a unit change in land cover. Dam- and point-source variables were ineffective at predicting ecological conditions in streams and rivers, probably because of inadequacies in the data sets. The spatial perspective (whole watershed, streamside corridor, or local) most effective at predicting ecological response variables varied across response variables, apparently in concord with the mechanisms that control each of these variables. We found some evidence that predictive power fell in very small watersheds (less than 1–10 km2), suggesting that the spatial arrangement of landscape patches may become critical at these small scales. Empirical models can replace, constrain, or be combined with more mechanistic models to understand the effects of land-cover change on stream ecosystems.

Dynamics of an Anthropogenic Fire Regime

Abstract: Human interaction with fire and vegetation occurs at many levels of human population density and cultural development, from subsistence cultures to highly technological societies. The dynamics of these interactions with respect to wildland fire are often difficult to understand and identify at short temporal scales. Dendrochronological fire histories from the Missouri Ozarks, coupled with human population data, offer a quantitative means of examining historic (1680-1990) changes in the anthropogenic fire regime. A temporal analysis of fire scar dates over the last 3 centuriqs indicates that the percent of sites burned and fire intervals of anthropogenic fires are conditioned by the following four limiting factors: (a) anthropogenic ignition, (b) surface fuel production, (c) fuel fragmentation, and (d) cultural behavior. During an ignition-dependent stage (fewer than 0.64 humans/km2), the percent of sites burned is logarithmically related to human population (r2 = 0.67). During a fuel-limited stage, where population density exceeds a threshold of 0.64 humans/km2, the percent of sites burned is independent of population increases and is limited by fuel production. During a fuel-fragmentation stage, regional trade allows population densities to increase above 3.4 humans/km2, and the percent of sites burned becomes inversely related to population (r2 = 0.18) as decreases in fuel continuity limit the propagation of surface fires. During a culture-dependent stage, increases in the value of timber over forage greatly reduce the mean fire interval and the percent of sites burned. Examples of the dynamics of these four stages are presented from the Current River watershed of the Missouri Ozarks.

Spatial and Temporal Variability in Growing-Season Net Ecosystem Carbon Dioxide Exchange at a Large Peatland in Ontario, Canada

Abstract: We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas—poor fen and beaver pond margin— had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities.

The Impact of Nutrient State and Lake Depth on Top-down Control in the Pelagic Zone of Lakes: A Study of 466 Lakes from the Temperate Zone to the Arctic

Abstract: Using empirical data from 466 temperate to arctic lakes covering a total phosphorus (TP) gradient of 2–1036 gL –1 , we describe how the relative contributions of resource supply, and predator control change along a nutrient gradient. We argue that (a) predator control on large-bodied zooplankton is unimodally related to TP and is highest in the most nutrient-rich and nutrient-poor lakes and generally higher in shallow than deep lakes, (b) the cascading effect of changes in predator control on phytoplankton decreases with increasing TP, and (c) these general patterns occur with significant variations—that is, the predation pressure can be low or high at all nutrient levels. A quantile regression revealed that the median share of the predatorsensitive Daphnia to the total cladoceran biomass was significantly related unimodally to TP, while the 10% and 90% percentiles approached 0 and 100%, respectively, at all TP levels. Moreover, deep lakes (more than 6 m) had a higher percentage of

Soil Carbon Dynamics after Forest Harvest: An Ecosystem Paradigm Reconsidered

Abstract: In one of the most influential studies in the recent history of forest ecology, W. W. Covington (1981) described a pattern in organic matter storage in the forest floors of northern hardwood stands as a function of date of harvest. We review the history of the use and misuse of Covington’s curve, describe the studies that tested and failed to support early interpretations of the curve, and provide some alternate interpretations. The curve suggested that forest floor organic matter declines by 50% within 20 years after harvest, and this decline was attributed to accelerated decomposition and changes in litter inputs after harvest. Subsequent studies showed that decomposition rates of surface litter generally decrease after clear-cutting, but accelerated decomposition remains possible in the Oe and Oa horizons. Changes in litter inputs are still difficult to evaluate, because the rate at which woody debris enters the forest floor is unknown. Although Covington attempted to minimize variation due to mechanical disturbance during logging, a reasonable alternative explanation for low organic matter in the forest floor of young stands is that surface material is mixed into mineral soil during harvesting operations. The pattern of forest floor organic matter in stands of different ages may be partly due to changes over time in logging technology and the intensity of biomass removal, in addition to successional effects. It is important to distinguish between mechanisms that release carbon to the atmosphere and those that transfer it to the mineral soil before making inferences about nutrient cycling and carbon sequestration.

Interactions between Carbon and Nitrogen Mineralization and Soil Organic Matter Chemistry in Arctic Tundra Soils

Abstract: We used long-term laboratory incubations and chemical fractionation to characterize the mineralization dynamics of organic soils from tussock, shrub, and wet meadow tundra communities, to determine the relationship between soil organic matter (SOM) decomposition and chemistry, and to quantify the relative proportions of carbon (C) and nitrogen (N) in tundra SOM that are biologically available for decomposition. In all soils but shrub, we found little decline in respiration rates over 1 year, although soils respired approximately a tenth to a third of total soil C. The lack of decline in respiration rates despite large C losses indicates that the quantity of organic matter available was not controlling respiration and thus suggests that something else was limiting microbial activity. To determine the nature of the respired C, we analyzed soil chemistry before and after the incubation using a peat fractionation scheme. Despite the large losses of soil C, SOM chemistry was relatively unchanged after the incubation. The decomposition dynamics we observed suggest that tundra SOM, which is largely plant detritus, fits within existing concepts of the litter decay continuum. The lack of changes in organic matter chemistry indicates that this material had already decomposed to the point where the breakdown of labile constituents was tied to lignin decomposition. N mineralization was correlated with C mineralization in our study, but shrub soil mineralized more and tussock soil less N than would have been predicted by this correlation. Our results suggest that a large proportion of tundra SOM is potentially mineralizable, despite the fact that decomposition was dependent on lignin breakdown, and that the historical accumulation of organic matter in tundra soils is the result of field conditions unfavorable to decomposition and not the result of fundamental chemical limitations to decomposition. Our study also suggests that the anticipated increases in shrub dominance may substantially alter the dynamics of SOM decomposition in the tundra.

Association between Plant Canopies and the Spatial Patterns of Infiltration in Shrubland and Grassland of the Chihuahuan Desert, New Mexico

Abstract: Shrubs have invaded extensive areas of grassland in the southwestern United States. The zones of nutrient-rich soil found beneath plant canopies, referred to as “islands of fertility,” are more intense and spaced farther apart in shrubland than in grassland. This difference in the spatial pattern of soil nutrients may reinforce shrub invasion. Changes in water availability in the soil could also influence shrub invasion. Here we compare the spatial patterns of infiltration, defined as the total equivalent water depth entering the soil following individual rainfall events or summed over many events, at adjacent grass- and shrub-dominated sites in the Sevilleta National Wildlife Refuge. We use two infiltration data sets. First, following four rainfall events, we measured soil moisture and wetting front depth at 10-cm intervals along 24-m transects. We estimate infiltration from these data. Second, we use vertical arrays of soil moisture probes to compare infiltration between adjacent canopies and interspaces following 31 storms. In both the grassland and shrubland, infiltration is typically greater beneath plant canopies than beneath interspaces. Canopies are oases where soil moisture is higher than in the surrounding areas. However, infiltration is not greater beneath canopies when surface runoff is limited. In the shrubland, the canopy–interspace infiltration ratio increases as storm size, and therefore runoff, increases. This relationship also exists in the grassland, but it is not as strong or clear. The magnitude of spatial variability of infiltration is similar in shrubland and grassland. In addition, the distance over which infiltration is correlated is approximately 50 cm in both environments. Most of the spatial variability exists between the stem and canopy margin in the shrubland and straddling the canopy margin in the grassland. The most notable difference is that subcanopy oases are spread farther apart in the shrubland because canopies are separated by larger interspaces in this environment.

Nitrogen Removal by Riparian Buffers along a European Climatic Gradient: Patterns and Factors of Variation

Abstract: Reference GEOLEP-ARTICLE-2003-008doi:10.1007/s10021-002-0183-8View record in Web of Science Record created on 2008-02-13, modified on 2016-08-08

Regime Shifts in the Sahara and Sahel: Interactions between Ecological and Climatic Systems in Northern Africa

Abstract: The Sahara and Sahel regions of northern Africa have complex environmental histories punctuated by sudden and dramatic “regime shifts” in climate and ecological conditions. Here we review the current understanding of the causes and consequences of two environmental regime shifts in the Sahara and Sahel. The first regime shift is the sudden transition from vegetated to desert conditions in the Sahara about 5500 years ago. Geologic data show that wet environmental conditions in this region—giving rise to extensive vegetation, lakes, and wetlands—came to an abrupt end about 5500 years ago. Explanations for climatic changes in northern Africa during the Holocene have suggested that millennial-scale changes in the Earth’s orbit could have caused the wet conditions that prevailed in the early Holocene and the dry conditions prevalent today. However, the orbital hypothesis, by itself, does not explain the sudden regime shift 5500 years ago. Several modeling studies have proposed that strong, nonlinear feedbacks between vegetation and the atmosphere could amplify the effects of orbital variations and create two alternative stable states (or “regimes”) in the climate and ecosystems of the Sahara: a “green Sahara” and a “desert Sahara.” A recent coupled atmosphere-ocean-land model confirmed that there was a sudden shift from the “green Sahara” to the “desert Sahara” regime approximately 5500 years ago. The second regime shift is the onset of a major 30-year drought over the Sahel around 1969. Several lines of evidence have suggested that the interactions between atmosphere and vegetation act to reinforce either a “wet Sahel” or a “dry Sahel” climatic regime, which may persist for decades at a time. Recent modeling studies have indicated that the shift from a “wet Sahel” to a “dry Sahel” regime was caused by strong feedbacks between the climate and vegetation cover and may have been triggered by slow changes in either land degradation or sea-surface temperatures. Taken together, we conclude that the existence of alternative stable states (or regimes) in the climate and ecosystems of the Sahara and Sahel may be the result of strong, nonlinear interactions between vegetation and the atmosphere. Although the shifts between these regimes occur rapidly, they are made possible by slow, subtle changes in underlying environmental conditions, including slow changes in incoming solar radiation, sea-surface temperatures, or the degree of land degradation.

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N2-fixing Red Alder

Abstract: Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r 2 = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha−1 y−1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r 2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

Abstract: In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.

Biomass, Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Abstract: Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pas- ture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of Mexico, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha -1 , whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha -1 . The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha -1 . Dead wood (standing and downed combined) comprised 27%-29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass av- eraged 32.0 Mg ha -1 in floodplain forest, 17.1 Mg ha -1 in dry forest, and 5.8 Mg ha -1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha -1 in pasture and reached 180 and 12.9 Mg ha -1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha -1 in pasture and reached 2623 and 264 kg ha -1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha -1 in floodplain forest, 141 Mg ha -1 in dry forest, and 124 Mg ha -1 in pasture. Soil C comprised 37%-90% of the total ecosystem C, whereas soil N comprised 85%-98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in Mex- ico, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmo- sphere from the burning of biomass in the dry tropical landscapes of Mexico may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.

Historical Food Web Structure and Restoration of Native Aquatic Communities in the Lake Tahoe (California–Nevada) Basin

Abstract: Plans for the restoration of aquatic ecosystems are increasingly focusing on the restoration and rehabilitation of self-sustaining native fish communities. Such efforts have not traditionally adopted an ecosystem-based perspective, which considers species as embedded within a broader food web context. In this study, we quantify food web changes in Lake Tahoe (California-Nevada) over the last century based on stable isotope analysis of museum-archived, preserved fish specimens collected during 4 historical periods and under present conditions. We also examine the contemporary food web of nearby Cascade Lake, which is free from most exotic species and contains a species assemblage resembling that of Lake Tahoe prior to historical species introductions. During the last century, the freshwater shrimp Mysis relicta and lake trout (Salvelinus namaycush) have been introduced and established in Lake Tahoe, and the native top predator, Lahontan cutthroat trout (Oncorhynchus clarki henshawi; hereafter LCT), has been extirpated. Isotope analysis indicates that lake trout now occupy a trophic niche similar to that of historical LCT. Fish production has shifted from benthic to pelagic, corresponding with the eutrophication of Lake Tahoe during recent decades. The current Cascade Lake food web resembles that of the historical Lake Tahoe food web. Our isotope-based food web reconstructions reveal long-term food web changes in Lake Tahoe and can serve as the basis for setting historically relevant restoration targets. Unfortunately, the presence of nonnative species, particularly Mysis and lake trout, have dramatically altered the pelagic food web structure; as such, they are barriers to native fish community restoration. Fish community restoration efforts should focus on adjacent ecosystems, such as Cascade Lake, which have a high likelihood of success because they have not been heavily affected by nonnative introductions.

Sources of carbon dioxide supersaturation in clearwater and humic lakes in northern Sweden

Abstract: Partial pressure (pCO(2)) and flux to the atmosphere of carbon dioxide (CO2) were studied in northern alpine and forest lakes along a gradient of dissolved organic carbon (DOC) content (0.4-9.9 mg ...

Spatial Patterns of Biomass and Aboveground Net Primary Productivity in a Mangrove Ecosystem in the Dominican Republic

Abstract: The objective of this study was to quantify spatial patterns in above- and belowground biomass, primary productivity, and growth efficiency along a tidal gradient in a 4700-ha mangrove forest in the Dominican Republic. We tested the hypothesis that spatial patterns of forest structure and growth following 50 years of development were associated with variations in the soil environment across the tidal gradient. Twenty-three plots were monitored from 1994 to 1998. Aboveground biomass and biomass accumulation were estimated by applying allometric regression equations derived from dimension analysis of trees harvested at our study site. Soil porewater salinity ranged from 5 to 38 g kg -1 across the tidal gradient, and most measurements of forest biomass and productivity were inversely related to salinity. Mean standing biomass (233 ± 16.0 Mg ha -1 ; range, 123.5-383.5), biomass increment (9.7 ± 1.0 Mg . ha -1 y -1 ; range, 3.7-18.1), annual litterfall rates (11.4 Mg . ha -1 yr -1 ; range, 10.2-12.8), leaf area index (LAI) (4.4 m 2 . m -2 ; range, 2.9-5.6), aboveground net primary productivity (ANPP) (19.7 Mg . ha -1 y -1 ; range, 15.6-25.0), and growth efficiency (1.6±0.2 kg . ha -1 y -1 ; range, 1.0-3.6) all showed an inverse linear relationship with salinity. Fine-root biomass (≤ 2 mm) (9.7 ± 1.2 Mg . ha -1 ; range, 2.7-13.8) showed a weak tendency to increase with salinity, and the ratio of root to aboveground biomass increased strongly with salinity. Our results suggest that physiological stresses associated with salinity, or with some combination of salinity and other covarying soil factors, control forest structure and growth along the tidal gradient. The higher allocation of carbon to belowground resources in more saline sites apparently contributed to reductions in ANPP along the tidal gradient.

Slow response of societies to new problems: causes and costs

Abstract: Human societies are confronted with a continuous stream of new problems. Many of these problems are caused by a limited sector of society but cause “spillover costs” to society as a whole. Here we show how a combination of mechanisms tends to delay effective regualtion of such situations. Obviously, problems may remain undetected for some time, especially if they are unlike those experienced in the past. However, it is at least as important to address the dynamics preceding the solution because societies that are systematically slow in suppressing problems in the early phases will pay a high overall cost. Here we show how a combination of mechanisms tends to delay effective regulation. Obviously, problems may remain undetected for some time, especially if it is unlike those experienced in the past. However, even if a problem is recognized by experts, the time lag before society in general recognizes that something should be done can be long because of the hysteresis in change of opinion. This causes abrupt but late shifts in opinion, much as described for Kuhn’s paradigm shifts. We use a mathematical model and review empirical evidence to show that this phenomenon will be particularly pronounced for complex problems and in societies that have strong social control, whereas key individuals such as charismatic leaders may catalyze earlier opinion shifts, reducing the time lag between problem and solution. An opinion shift may also be inhibited by downplay of a problem by a credible authority and by competition for attention by simultaneously occurring problems. Even if a problem is generally recognized, actual regulation may come late. We argue that this last phase of delay tends to be longer if a central decision-making authority is lacking and if disproportionately powerful stakeholders that benefit from the unregulated status quo are involved.

Spatial Variability of Soil Properties in a Floodplain Forest in Northwest Spain

Abstract: Floodplain forests are generally areas of high plant diversity compared with upland forests. Higher environmental heterogeneity, especially variation in belowground properties may help explain this high diversity. However, there is little information available on the spatial scale and pattern of belowground resources in floodplain forests. Geostatistics and coefficient of variation (CV) were used to describe the spatial variability of 20 soil properties ranging from essential plant nutrients, such as NH4 or PO4, to nonessential elements like Ti or V. The spatial variation of Si-to-(Al + Fe) ratio, an index of soil development, was also analyzed. Semivariograms and maps of selected properties were used to discriminate between the effect of flooding (and other mechanisms that may contribute to large scale trends in data) and local heterogeneity. The hypothesis that elements mainly cycled through biological processes (such as N) show different spatial properties than elements cycled through both biological and geological processes (such as P) or elements under strict geological control (such as Ti or V) is also presented. Redox potential was the most variable property (CV = 1.35) followed by mineral N, phosphate, organic matter, and carbon. Nonessential elements for organisms such as Si, Al, Ti, Rh, or V were less variable, supporting the hypothesis that biological control on soil properties leads to higher spatial variability. The range (the average distance within which the samples correlate spatially) varied between 3.89 m for water content to 18.5 m for the Si-to-(Al + Fe) ratio. The proportion of the total variance that can be modeled as spatial dependence (structural variance) was very variable, ranging between 0.34 for Fe and 0.96 for K. The addition of the large trend had a strong influence on the CV of most soil variables and created a gradient in C accumulation and the mineral weathering rate. The results suggest that flooding and other processes that are responsible for large spatial trends in the floodplain forest differentially affect biologically and geologically controlled variables with different turnover rates, thus providing a heterogeneous edaphic environment.

Forest Floor Decomposition Following Hurricane Litter Inputs in Several Puerto Rican Forests

Abstract: Hurricanes affect ecosystem processes by altering resource availability and heterogeneity, but the spatial and temporal signatures of these events on biomass and nutrient cycling processes are not well understood. We examined mass and nutrient inputs of hurricane-derived litter in six tropical forests spanning three life zones in northeastern Puerto Rico after the passage of Hurricane Georges. We then followed the decomposition of forest floor mass and nutrient dynamics over 1 year in the three forests that experienced the greatest litter inputs (moist, tabonuco, and palm forests) to assess the length of time for which litter inputs influence regeneration and nutrient cycling processes. The 36-h disturbance event had litterfall rates that ranged from 0.55 to 0.93 times annual rates among the six forests; forest floor ranged between 1.2 and 2.5 times prehurricane standing stocks. The upperelevation forest sites had the lowest nonhurricane litterfall rates and experienced the lowest hurricane litterfall and the smallest relative increase in forest floor standing stocks. In the three intensively studied forests, the forest floor returned to prehurricane values very quickly, within 2‐10 months. The palm forest had the slowest rate of decay (k 0.74 0.16 y ‐1 ), whereas the tabonuco forest and the moist forest had similar decay rates (1.04 0.12 and 1.09 0.14, respectively). In the moist forest, there were short-term increases in the concentrations of nitrogen (N), phosphorus (P), calcium (Ca), and magnesium (Mg) in litter, but in the other two forests nutrient concentrations generally decreased. The rapid disappearance of the hurricane inputs suggests that such pulses are quickly incorporated into nutrient cycles and may be one reason for the extraordinary resilience of these forests to wind disturbances.

An Unexpected Nitrate Decline in New Hampshire Streams

Abstract: Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO3 concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376 –381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996 –97, mean NO3 concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 mol/L), Ca 2 decreased by 24% (8 mol/L), and Mg 2 decreased by 22% (5 mol/L).

Ecological Forecasting and the Urbanization of Stream Ecosystems: Challenges for Economists, Hydrologists, Geomorphologists, and Ecologists

Abstract: The quantity and quality of freshwater resources are now being seriously threatened, partly as a result of extensive worldwide changes in land use, and scientists are often called upon by policy makers and managers to predict the ecological consequences that these alterations will have for stream ecosystems. The effects of the urbanization of stream ecosystems in the United States over the next 20 years are of particular concern. To address this issue, we present a multidisciplinary research agenda designed to improve our forecasting of the effects of land-use change on stream ecosystems. Currently, there are gaps in both our knowledge and the data that make it difficult to link the disparate models used by economists, hydrologists, geomorphologists, and ecologists. We identify a number of points that practitioners in each discipline were not comfortable compromising on—for example, by assuming an average condition for a given variable. We provide five instructive examples of the limitations to our ability to forecast the fate of stream and riverine ecosystems one drawn from each modeling step: (a) Accurate economic methods to forecast land-use changes over long periods (such as 20 years) are not available, especially not at spatially explicit scales; (b) geographic data are not always available at the appropriate resolution and are not always organized in categories that are hydrologically, ecologically, or economically meaningful; (c) the relationship between low flows and land use is sometimes hard to establish in anthropogenically affected catchments; (d) bed mobility, suspended sediment load, and channel form—all of which are important for ecological communities in streams—are difficult to predict; and (e) species distributions in rivers are not well documented, and the data that do exist are not always publicly available or have not been sampled at accurate scales, making it difficult to model ecological responses to specified levels of environmental change. Meeting these challenges will require both interdisciplinary cooperation and a reviewed commitment to intradisciplinary research in the fields of economics, geography, quantitative spatial analysis, hydrology, geomorphology, and ecology.

Erosion and the Rejuvenation of Weathering-derived Nutrient Supply in an Old Tropical Landscape

Abstract: Studies of long-term soil and ecosystem development on static geomorphic surfaces show that old soils become depleted in most rock-derived nutrients. As they are depleted, however, static surfaces also are dissected by fluvial erosion. This fluvial erosion leads to colluvial soil transport on the resulting slopes, which in turn can rejuvenate the supply of weathering-derived nutrients to plants. We evaluated the influence of erosion and consequent landscape evolution on nutrient availability along a slope on the Island of Kaua’i, near the oldest, most nutrient-depleted site on a substrate age gradient across the Hawaiian Islands. Noncrystalline minerals characteristic of younger Hawaiian soils increased from 3% of the soil on the static constructional surface at the top of the slope to 13% on the lower slope, and the fraction of soil phosphorus (P) that was occluded (and hence unavailable) decreased from 80% to 56% at midslope. Foliar nitrogen and P concentrations in Metrosideros polymorpha increased from 0.82% and 0.062% to 1.13% and 0.083% on the constructional surface and lower slope, respectively. The increase in foliar P over a horizontal difference of less than 250 m represents nearly half of the total variation in foliar P observed over 4.1 million years of soil and ecosystem development in Hawai’i. The fraction of foliar strontium (Sr) derived from weathering of Hawaiian basalt was determined using 87 Sr: 86 Sr; it increased from less than 6% on the constructional surface to 13% and 31% on lower slope and alluvial positions. Erosional processes increase both nutrient supply on this slope and the fine-scale biogeochemical diversity of this old tropical landscape; it could contribute to the relatively high level of species diversity observed on Kaua’i.

Pine Forest Floor Carbon Accumulation in Response to N and PK Additions: Bomb 14C Modelling and Respiration Studies

Abstract: The addition of nitrogen via deposition alters the carbon balance of temperate forest ecosystems by affecting both production and decomposition rates. The effects of 20 years of nitrogen (N) and phosphorus and potassium (PK) additions were studied in a 40-year-old pine stand in northern Sweden. Carbon fluxes of the forest floor were reconstructed using a combination of data on soil 14C, tree growth, and litter decomposition. N-only additions caused an increase in needle litterfall, whereas both N and PK additions reduced long-term decomposition rates. Soil respiration measurements showed a 40% reduction in soil respiration for treated compared to control plots. The average age of forest floor carbon was 17 years. Predictions of future soil carbon storage indicate an increase of around 100% in the next 100 years for the N plots and 200% for the NPK plots. As much as 70% of the increase in soil carbon was attributed to the decreased decomposition rate, whereas only 20% was attributable to increased litter production. A reduction in decomposition was observed at a rate of N addition of 30 kg C ha−1 y−1, which is not an uncommon rate of N deposition in central Europe. A model based on the continuous-quality decomposition theory was applied to interpret decomposer and substrate parameters. The most likely explanations for the decreased decomposition rate were a fertilizer-induced increase in decomposer efficiency (production-to-assimilation ratio), a more rapid rate of decrease in litter quality, and a decrease in decomposer basic growth rate.

Analyzing the Ecosystem Carbon Dynamics of Four European Coniferous Forests Using a Biogeochemistry Model

Abstract: This paper provides the first steps toward a regional-scale analysis of carbon (C) budgets. We explore the ability of the ecosystem model BIOME-BGC to estimate the daily and annual C dynamics of four European coniferous forests and shifts in these dynamics in response to changing environmental conditions. We estimate uncertainties in the model results that arise from incomplete knowledge of site management history (for example, successional stage of forest). These uncertainties are especially relevant in regional-scale simulations, because this type of information is difficult to obtain. Although the model predicted daily C and water fluxes reasonably well at all sites, it seemed to have a better predictive capacity for the photosynthesis-related processes than for respiration. Leaf area index (LAI) was modeled accurately at two sites but overestimated at two others (as a result of poor long-term climate drivers and uncertainties in model parameterization). The overestimation of LAI (and consequently gross photosynthetic production (GPP)), in combination with reasonable estimates of the daily net ecosystem productivity (NEP) of those forests, also illustrates the problem with modeled respiration. The model results suggest that all four European forests have been net sinks of C at the rate of 100 ‐300 gC/m 2 /y and that this C sequestration capacity would be 30%‐70% lower without increasing nitrogen (N) deposition and carbon dioxide (CO 2 ) concentrations. The magnitude of the forest responses was dependent not only on the rate of changes in environmental factors, but also on sitespecific conditions such as climate and soil depth. We estimated that the modeled C exchange at the study sites was reduced by 50%‐100% when model simulations were performed for climax forests rather than regrowing forests. The estimates of water fluxes were less sensitive to different initializations of state variables or environmental change scenarios than C fluxes.

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone, Green Lakes Valley, Colorado Front Range

Abstract: Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH4-N), nitrate-N (NO3-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO3-N decreased downstream from 4.3 kg ha 1 in the alpine to 0.75 kg ha 1 at treeline, while yields of DON were much less variable (0.40‐0.34 kg ha 1 ). Yields of NH4-N and PON were low and showed little variation with basin area. NO3-N accounted for 40%‐ 90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R 2 0.81, P 0.001) Concentrations of NO3-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon‐nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry.

Soil Diversity and Land Use in the United States

Abstract: Soils are dynamic components of terrestrial ecosystems that historically have been viewed as economic resources by government and private interests. The large-scale conversion of many sections of the United States to agriculture and urban land uses, combined with the growing awareness of the role of soils in global biogeochemistry and ecology, ultimately requires an assessment of the remaining distribution of undisturbed soils in the country. Here we conduct the first quantitative analysis of disturbed and undisturbed soil distribution in the USA using a GIS-based approach. We find that a sizable fraction (4.5%) of the nation’s soils are in danger of substantial loss, or complete extinction, due to agriculture and urbanization. In the agricultural belt of the country, up to 80% of the soils that were naturally of low abundance are now severely impacted (greater than 50% conversion to agricultural/urban uses). Undisturbed soils provide ecosystem services that warrant their preservation, including a somewhat complex relationship with rare or endangered plants. The known and unknown attributes of undisturbed soils suggests the need for an integrated biogeodiversity perspective in landscape preservation efforts.

Response of coral assemblages to the interaction between natural temperature variation and rare warm-water events

Abstract: We examined changes in coral assemblages in four back-reef locations across the warm 1998 E1 Nino–Southern Oscillation (ENSO) event based on annually collected line-transect data from 3 years before and after this event. The physical locations of the reefs differed such that there was a 120%–275% warm-season range in the SDs of seawater temperatures but only minor differences in mean temperatures, based on 2 non-ENSO years. We tested the predictions that (a) rare warm-water events would produce fewer changes in eurythermal than stenothermal coral assemblages; and (b) after the disturbance, the stenothermal assemblages would more closely resemble the eurythermal ones. The 1998 event produced fewer changes in coral cover and community similarity among the assemblages in the reefs with high variation in temperature than in those with low variation in temperature. Despite the initially lower taxonomic richness in the eurythermal assemblage, there was an additional loss of taxonomic richness in the high and none in the stenothermal reefs. There was some evidence for taxonomic convergence, of the stenothermal towards the eurythermal reefs and a general loss of some of the branching taxa, such as branching Porites, Pavona, and Stylophora, and a relative increase in massive Porites and Favia. There was, however, moderate site specificity that did not produce true convergence. The eurythermal assemblages maintained the basic community structure but lost taxonomic richness, whereas the opposite was true for the stenothermal assemblages.

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Abstract: Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO3 −) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO3 − concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO3 - concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO3 -. At the regional scale, we observed high variability in the magnitude of NO3 - losses: while six of the surveyed watersheds showed accelerated rates of NO3 − loss, three did not. Moreover, in contrast to the strong linear relationship between NO3 − loss and crown damage within HBEF watersheds [r 2: (W1 = 0.91, W6 = 0.85)], stream water NO3 − concentrations were weakly related to crown damage (r 2 = 0.17) across our regional sites. The efflux of NO3 − associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO3 − fluxes associated with forest harvesting. Because over one half of the entire year’s worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems.