Showing papers in "Ecological Applications in 1995"

Relationships Between NDVI, Canopy Structure, and Photosynthesis in Three Californian Vegetation Types

Abstract: In a range of plant species from three Californian vegetation types, we examined the widely used "normalized difference vegetation index" (NDVI) and "simple ratio" (SR) as indicators of canopy structure, light absorption, and photosynthetic activity. These indices, which are derived from canopy reflectance in the red and near-infrared wavebands, highlighted phenological differences between evergreen and deciduous canopies. They were poor indicators of total canopy biomass due to the varying abundance of non-green standing biomass in these vegetation types. However, in sparse canopies (leaf area index [LAI]°0-2), NDVI was a sensitive indicator of canopy structure and chemical content (green biomass, green leaf area index, chlorophyll content, and foliar nitrogen content). At higher canopy green LAI values (>2; typical of dense shrubs and trees), NDVI was relatively insensitive to changes in canopy structure. Compared to SR, NDVI was better correlated with indicators of canopy structure and chemical content, but was equivalent to the logarithm of SR. In agreement with theoretical expectations, both NDVI and SR exhibited near-linear correlations with fractional PAR intercepted by green leaves over a wide range of canopy densities. Maximum daily photosynthetic rates were positively correlated with NDVI and SR in annual grassland and semideciduous shrubs where canopy development and photosynthetic activity were in synchrony. The indices were also correlated with peak springtime canopy photosynthetic rates in evergreens. However, over most of the year, these indices were poor predictors of photosynthetic performance in evergreen species due to seasonal reductions in photosynthetic radiation-use efficiency that occurred without substantial declines in canopy greenness. Our results support the use of these vegetation indices as remote indicators of PAR absorption, and thus potential photosynthetic activity, even in heterogeneous landscapes. To provide accurate estimates of vegetation-atmosphere gas fluxes, remote NDVI and SR measurements need to be coupled with careful estimates of canopy photosynthetic radiation-use efficiency.

Juvenile Tree Survivorship as a Component of Shade Tolerance

Abstract: With a view toward understanding species-specific differences in juvenile tree mortality and the community-level implications of these differences, we characterized juvenile survivorship of 10 dominant tree species of oak transition-northern hardwood forests using species-specific mathematical models. The mortality models predict a sapling's probability of dying as a function of its recent growth history. These models and species-specific growth functions (published elsewhere), characterize a species' shade tolerance. Combined growth and mortality models express a sapling's probability of mortality as a function of light availability. We describe the statistical bases and the field methods used to calibrate the mortality models. We examined inter- and intraspecific variation in juvenile mortality across three sites: Great Mountain Forest (low pH, nutrient poor soils) in northwestern Connecticut, a calcareous bedrock region (neutral pH, nutrient rich soils) also in northwestern Connecticut, and a site in central-western Michigan (low pH, nutrient poor soils). Interspecific differences in juvenile mortality have profound effects on community dynamics and composition; the importance of these effects is demonstrated through a spatially explicit simulator of forest dynamics (SORTIE). The 10 species we examined occupy a continuum of survivorship levels at 1% of full sun. There was surprisingly little intraspecific variation in mortality functions for sugar maple, American beech, eastern hemlock, and white ash between the Great Mountain and Michigan sites. However, there was a striking increase in survivorship for sugar maple in the calcareous site. Differences in survivorship among the sites are correlated with soil pH and presumably nutrient availability. Growth rates in high-light and low-light survivorship are inversely correlated across species; as level of shade tolerance increases, a species grows more slowly in high light and exhibits increased survivorship under low light. Our results indicate that interspecific differences in sapling mortality are critical components of forest community dynamics.

Growing‐Season Microclimatic Gradients from Clearcut Edges into Old‐Growth Douglas‐Fir Forests

Abstract: Edge is an important landscape feature of fragmented forest landscapes in the Pacific Northwest, USA. Our primary objective of this study is to characterize the changes in microclimatic variables from recent clearcut edges into the old-growth Douglas- fir forests as influenced by edge exposures and local weather conditions. Microclimatic gradients are described along transects extending from recently clearcut edges 240 m into stands of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest west of the Cascade Range in the U.S. Pacific Northwest. Data for air temperature, soil temperature, relative humidity, short-wave radiation, and wind speed were collected over the course of the day from 16 different edges representing a range of edge orientations and local weather conditions over two growing seasons (1989-1990). Data for soil moisture were collected over three consecutive days in September 1990. Two indices, significance of edge influence (SEI) and depth of edge influence (DEI), were used to evaluate the effects of edges on microclimatic variables. Edge effects typically extended 30 to >240 m into the forest. From the edge into the forest, air temperatures decreased during the day and increased at night; the reversal produced mid-morning and late-afternoon periods when a gradient was absent. Changes in soil temperature from the edge into the forest were comparable to those for air temperature, except that edge effects did not extend as deeply into the forest. The gradient for relative humidity increased from the edge and was steepest in mid-afternoon. Humidity effects sometimes extended >240 m into the forest. Short-wave radiation decreased rapidly with distance from the edge, reaching interior forest levels by 30-60 m. Wind speed de- creased exponentially from the edge into the forest, depending on the relationship of edge orientation to wind direction; stronger winds influenced conditions deeper inside the forest, sometimes >240 m from the edge. Edge orientation played a critical role for all variables; for air and soil temperature and humidity, it affected the times of day at which maximum and minimum values peaked. Influence of local weather conditions on gradients was highly variable. Overall, however, gradients generally were longest and steepest on partially clear, warm, dry days, at southwest-facing edges, and for air temperature, soil temperature, and relative humidity. SEI and DEI were found to be necessary measurements for evaluating edge effects on microclimatic variables, which responded differently depending on time of day, edge orientation, and local weather. No single value could be calculated for DEI. Because many ecological features near edges, such as tree stocking and regeneration, dis- persal of flying insects, and decomposition of woody debris, seem related to microclimatic gradients, forest management to protect interior conditions should shift from the traditional charge ("create as much edge as possible") to a new charge in which the amount of edge is reduced at both the stand and landscape levels.

Plant Species Diversity in Natural and Managed Forests of the Pacific Northwest

Abstract: With the exception of the tropics, nowhere has the relationship between resource management and conservation of biological diversity been more controversial than in the Pacific Northwest region of the United States. Widespread loss and fragmentation of old-growth ecosystems have stimulated critical review and revision of existing forest management policies. However, studies of the consequences of forest management for plant species diversity are sorely lacking. We present data from permanent-plot and chronose- quence studies in managed and unmanaged forests of western Oregon and Washington to describe the early responses of understory communities to forest harvest, and to suggest how post-harvest practices that alter natural successional processes may influence long- term patterns of diversity and species occurrence. Permanent-plot studies of early succession in old-growth Pseudotsuga forests suggest that changes in understory diversity are fairly short-lived following clear-cut logging and slash burning. Populations of most vascular plant species recover to original levels prior to canopy closure. However, diversity may remain depressed for more than two decades on severely burned sites, and some species may experience local extinction. Evidence of the effects of post-harvest practices on vascular plant diversity is limited by an absence of community-level studies in older, managed forests. Chronosequence studies of natural forest stands indicate that, following canopy closure, vascular plant species diversity tends to increase with time, peaking in old growth. Few understory species are restricted to, or absent from, any stage of stand development (i.e., young, mature, or old growth). However, many species differ significantly in their abundance among stages. A majority of these showed greatest abundance in old growth. Changes in levels of resources (increased shade), changes in the spatial variability of resources and environments (increased horizontal and vertical heterogeneity), and species' sensitivity to fire and slow rates of reestablishment/growth may drive these trends during natural stand development. Silvicultural prescriptions that maintain or foster spatial and temporal diversity of re- sources and environments will be most effective in maintaining plant species diversity. Practices associated with intensive, short-rotation plantation forestry, that preclude or delay the development of old-growth attributes, may result in long-term loss of diversity. Ulti- mately, it may be necessary to manage some stands on long rotations (150-300 yr) to maintain understory species that require long periods to recover from disturbance.

A Model for Predicting Continental‐Scale Vegetation Distribution and Water Balance

Abstract: A Mapped atmosphere-Plant-Soil System (MAPSS) has been constructed for simulating the potential biosphere impacts and biosphere-atmosphere feedbacks from climatic change The system calculates the potential vegetation type and leaf area that could be supported at a site, within the constraints of the abiotic climate Both woody vegetation and grass are supported and compete for light and water The woody vegetation can be either trees or shrubs, evergreen or deciduous, and needleleaved or broadleaved A complete site water balance is calculated and integrates the vegetation leaf area and stomatal conductance in canopy transpiration and soil hydrology The MAPSS model accurately simulates the distributions of forests, grasslands, and deserts and reproduces observed monthly runoff The model can be used for predictions of new vegetation distribution patterns, soil moisture, and runoff patterns in alternative climates 112 refs, 11 figs, 4 tabs

Spatially explicit population models: current forms and future uses

Abstract: Spatially explicit population models are becoming increasingly useful tools for population ecologists, conservation biologists, and land managers. Models are spatially explicit when they combine a population simulator with a landscape map that describes the spatial distribution of landscape features. With this map, the locations of habitat patches, individuals, and other items of interest are explicitly incorporated into the model, and the effect of changing landscape features on population dynamics can be studied. In this paper we describe the structure of some spatially explicit models under development and provide examples of current and future research using these models. Spatially explicit models are important tools for investigating scale-related questions in population ecology, especially the response of organisms to habitat change occurring at a variety of spatial and temporal scales. Simulation models that incorporate real-world landscapes, as portrayed by landscape maps created with geographic information systems, are also proving to be crucial in the development of management strategies in response to regional land-use and other global change processes. Spatially explicit population models will increase our ability to accurately model complex landscapes, and therefore should improve both basic ecological knowledge of landscape phenomena and applications of landscape ecology to conservation and man- agement.

A Carbon Budget for Forests of the Conterminous United States

Abstract: The potential need for national-level comparisons of greenhouse gas emis- sions, and the desirability of understanding terrestrial sources and sinks of carbon, has prompted interest in quantifying national forest carbon budgets. In this study, we link a forest inventory database, a set of stand-level carbon budgets, and information on harvest levels in order to estimate the current pools and flux of carbon in forests of the conterminous United States. The forest inventory specifies the region, forest type, age class, productivity class, management intensity, and ownership of all timberland. The stand-level carbon bud- gets are based on growth and yield tables, in combination with additional information on carbon in soils, the forest floor, woody debris, and the understory. Total carbon in forests of the conterminous U.S. is estimated at 36.7 Pg, with half of that in the soil compartment. Tree carbon represents 33% of the total, followed by woody debris (10%), the forest floor (6%), and the understory (1%). The carbon uptake associated with net annual growth is 331 Tg, however, much of that is balanced by harvest-related mortality (266 Tg) and decomposition of woody debris. The forest land base at the national level is accumulating 79 Tg/yr, with the largest carbon gain in the Northeast region. The similarity in the mag- nitude of the biologically driven flux and the harvest-related flux indicates the importance of employing an age-class-based inventory, and of including effects associated with forest harvest and harvest residue, when modeling national carbon budgets in the temperate zone.

Fundamental Differences Between Conventional and Organic Tomato Agroecosystems in California

Abstract: In an integrated, multidisciplinary study we compared ecological characteristics and productivity of commercial farms categorized as either organic (ORG) or conventional (CNV) based on their use of synthetic fertilizers and pesticides or reliance on organic soil amendments and biological pest control. We measured belowground parameters: various soil chemical and biological properties and root disease severity; common agronomic indicators: biomass, fruit yield and insect pest damage; and community level indicators, including arthropod diversity and soil microbial activity and diversity. CNV and ORG production systems could not be distinguished based on agronomic criteria such as fruit yield and arthropod pest damage levels. However, differences were demonstrated in many soil, plant, disease, and diversity indicators suggesting that the ecological processes determining yields and pest levels in these two management systems are distinct. In particular, nitrogen mineralization potential and microbial and parasitoid abundance and diversity were higher in ORG farms. Differences between the agroecosystems were sufficiently robust to be distinguished from environmental variation and suggest that biological processes compensated for reductions in the use of synthetic fertilizers and pesticides.

Fire, Global Warming, and the Carbon Balance of Boreal Forests

Abstract: Fire strongly influences carbon cycling and storage in boreal forests. In the near-term, if global warming occurs, the frequency and intensity of fires in boreal forests are likely to increase significantly. A sensitivity analysis on the relationship between fire and carbon storage in the living-biomass and ground-layer compartments of boreal forests was performed to determine how the carbon stocks would be expected to change as a result of global warming. A model was developed to study this sensitivity. The model shows if the annual area burned in boreal forests increases by 50%, as predicted by some studies, then the amount of carbon stored in the ground layer would decrease between 3.5 and 5.6 kg/M2, and the amount of carbon stored in the living biomass would increase by 1.2 kg/M2. There would be a net loss of carbon in boreal forests between 2.3 and 4.4 kg/M2, or 27.1- 51.9 Pg on a global scale. Because the carbon in the ground layer is lost more quickly than carbon is accumulated in living biomass, this could lead to a short-term release of carbon over the next 50-100 yr at a rate of 0.33-0.8 Pg/yr, dependent on the distribution of carbon between organic and mineral soil in the ground layer (which is presently not well-under- stood) and the increase in fire frequency caused by global warming.

Environmental and substrate controls over carbon and nitrogen mineralization in northern wetlands

Abstract: Northern wetlands may be a potential carbon source to the atmosphere upon global warming, particularly with regard to methane. Three wetland soils were incubated for 80 wk in the laboratory under both aerobic and anaerobic conditions at 15{degrees} and 30{degrees}C. The soils were obtained from a Scirpus-Carex-dominated meadow in an abandoned beaver pond and from the surface and at 1 m depth of a spruce (Picea)-Sphagnum bog in Voyageurs National Park, Minnesota. The responses of nitrogen mineralization, carbon mineralization, and trace gas partitioning to both temperature and aeration depended strongly on the substrate quality of the soils. Sedge meadow soil had the highest nitrogen and carbon mineralization rates and methane production under anaerobic conditions, and carbon mineralization under aerobic conditions, but the surface peats had the highest nitrogen mineralization rates under aerobic conditions. Methanogenesis was highest in sedge soil but less sensitive to temperature than the peats. Most of the variation in nitrogen and carbon mineralization among the soils and treatments was accounted for by differences in the size and kinetics of a relatively small labile pool. The kinetics of this pool were more sensitive to changes in temperature and aeration than that of the larger recalcitrant pool. Principal componentsmore » analysis separated the soils on the basis of labile and recalcitrant carbon fractions. Total C and N mineralization correlated positively with the factor representing labile elements, while methanogenesis also showed a negative correlation with the factor representing recalcitrant elements. Estimates of atmospheric feedbacks from northern wetlands upon climatic change must account for extreme local variation in substrate quality and wetland type; global projections based on extrapolations from a few field measurements do not account for this local variation and may be in error. 52 refs., 4 figs., 6 tabs.« less

Isohaline Position as a Habitat Indicator for Estuarine Populations

Abstract: Populations of native and introduced aquatic organisms in the San Francisco Bay/Sacramento- San Joaquin Delta Estuary ("BayJDelta") have undergone significant declines over the past two decades. De- creased river inflow due to drought and increased freshwater diversion have contributed to the decline of at least some populations. Effective management of the estuary's biological resources requires a sensitive indicator of the response to freshwater inflow that has ecological significance, can be measured accurately and easily, and could be used as a "policy" variable to set standards for managing freshwater inflow. Positioning of the 27~0 (grams of salt per kilogram of seawater) bottom salinity value along the axis of the estuary was examined for this purpose. The 2% bottom salinity position (denoted by X,) has simple and significant statistical relationships with annual measures of many estuarine resources, including the supply of phytoplankton and phytoplankton-derived detritus from local production and river loading; benthic macroinvertebrates (molluscs); mysids and shrimp; larval fish survival; and the abundance of planktivorous, piscivorous, and bottom-foraging fish. The actual mechanisms are understood for only a few of these populations. X, also satisfies other recognized requirements for a habitat indicator and probably can be measured with greater accuracy and precision than alternative habitat indicators such as net freshwater inflow into the estuary. The 2%~ value may not have special ecological significance for other estuaries (in the BayJDelta, it marks the locations of an estuarine turbidity maximum and peaks in the abundance of several estuarine organisms), but the concept of using near-bottom isohaline position as a habitat indicator should be widely applicable. Although X, is a sensitive index of the estuarine community's response to net freshwater inflow, other hydraulic features of the estuary also determine population abundances and resource levels. In particular, di - version of water for export from or consumption within the estuary can have a direct effect on population abundance independent of its effect on X,. The need to consider diversion, in addition to X,, for managing certain estuarine resources is illustrated using striped bass survival as an example. The striped bass survival data were also used to illustrate a related important point: incorporating additional explanatory variables may decrease the prediction error for a population or process, but it can increase the uncertainty in parameter estimates and management strategies based on these estimates. Even in cases where the uncertainty is currently too large to guide management decisions, an uncertainty analysis can identify the most practical direction for future data acquisition.

Small mammals in managed, naturally young, and old-growth forests.

Abstract: Forest managers in the Pacific Northwest are faced with new challenges of providing for all wildlife in managed forests. Our objective was to elucidate the factors governing the composition and biomass of forest floor mammal communities that are ame- nable to management. We sampled small mammal communities in forests of various man- agement histories on the Olympic Peninsula and contrasted our results with those of other large studies in the Pacific Northwest. Forest floor mammal communities in forests >35 yr old in the Western Hemlock Zone of Washington and Oregon are composed of 5-8 characteristic species. These include Sorex trowbridgii (numerically the most dominant); one species each of Clethrionomys, the Sorex vagrans complex, and Peromyscus; and Neurotrichus gibbsii. Species composition changes from south to north, and the communities on the Olympic Peninsula contain two or three additional species compared to communities to the south. Communities in naturally re- generated and clearcutting regenerated (managed) young forests are similar in composition to those in old growth; old growth, however, supports 1.5 times more individuals and biomass than managed forest. Community diversity seems related to the south-north mois- ture-temperature gradient that is reflected in increased diversity of canopy conifers, de- velopment of forest floor litter layers, accumulation of coarse woody debris, and abundance of herbs, deciduous shrubs, and shade-tolerant seedlings (as opposed to understories dom- inated by evergreen shrubs). Previous work found few habitat variables that were good predictors of species abundance in natural young and old-growth stands. Naturally regen- erated young stands had higher levels of coarse woody debris than old growth. Managed stands had much lower abundance of coarse woody debris and tall shrubs than old growth. Understory vegetation (herbs and shrubs) and coarse woody debris accounted for a major part of the variation in abundance of six of eight species in managed stands, but only two species in old growth. Management of Western Hemlock Zone forest for conservation of biodiversity and restoration of old-growth conditions should concentrate on providing mul- tispecies canopies, coarse woody debris, and well-developed understories.

Patterns and Mechanisms of Plant Diversity in Forested Ecosystems: Implications for Forest Management

Abstract: The objectives of this paper are to (1) review existing diversity models, (2) identify principles that explain patterns of plant species diversity, (3) discuss implications for forest management, and (4) identify research needs. Many current theories cast distur- bance as the key player in maintaining species diversity by preventing competitive dom- inance of one or a few species. Equilibrium and nonequilibrium theories alike agree that maximum diversity should occur at intermediate size, frequency, and intensity of distur- bance. These models do not adequately predict patterns at all spatial scales or across community types. A mechanistic theory is needed to explain diversity patterns at the patch, stand, and landscape scales, as well as across site quality and successional gradients. Such a theory should be based upon the interaction between species' life-history characteristics and the nature of disturbance. New research is needed in the following areas: identifying disturbance-life-history interactions, particularly with respect to the short-term and long- term effects of disturbance; quantifying patch diversity and determining its relationship with species diversity; determining relationships between species diversity and structural and functional diversity; and identifying appropriate standards of comparison for managed stands. Comparative studies in different ecosystem types, such as boreal and tropical forests, should be encouraged to help clarify the relative importance of processes that influence diversity.

Scalable Decision Rules for Environmental Impact Studies: Effect Size, Type I, and Type II Errors

Abstract: Assessments of environmental impacts are being subject to greater scientific and legal scrutiny than ever before. The application of traditional statistical decision-making criteria to questions of environmental impacts has become increasingly inadequate as society demands greater environmental accountability from economic development. In particular, impact assessment has inherited a preoccupation with Type I error rates that has pervaded ecological research, even though Type II errors are often equally severe in impact assess- ment. Estimation of Type II error rates and specification of critical effect sizes-or the magnitudes of impacts considered important-are mutually dependent. Consideration of Type II errors, therefore, requires the exact specification of an hypothesized impact, which is often difficult. Insistence on low rates of Type I error (e.g., at = 0.05) typically means that equivalent rates of Type II error can be realized only when effect sizes (ES) are very large or when very many samples are taken. Rather than adhering to a fixed, arbitrary, critical, Type I error rate, I propose a procedure by which the critical ES is given primacy. Statistical decision criteria are then selected according to the relative weighting of the perceived consequences of Type I or Type II errors. The critical Type I error rate is set by iteration to some multiple (k) of the estimated

Global Warming and Soil Microclimate: Results from a Meadow-Warming Experiment

Abstract: We used overhead infrared radiators to add a constant increment of -15 W/m2, over 2 yr, to the downward heat flux on five 30-M2 montane meadow plots in Gunnison County, Colorado, USA. Heating advanced snowmelt by -1 wk, increased summer soil temperatures by up to 3?C, and reduced summer soil moisture levels by up to 25% compared to control plots. Soil microclimate response to heating varied with season, time of day, weather conditions, and location along the microclimate and vegetation gradient within each plot, with the largest temperature increase observed in daytime and in the drier, more sparsely vegetated zone of each plot. Day-to-day variation in the daily-averaged temperature response to heating in the drier zone was negatively correlated with that in the wetter zone. Our experimental manipulation provides a novel and effective method for investigating feedback processes linking climate, soil, and vegetation.

Soil Organic Matter Recovery in Semiarid Grasslands: Implications for the Conservation Reserve Program

Abstract: Although the effects of cultivation on soil organic matter and nutrient supply capacity are well understood, relatively little work has been done on the long-term recovery of soils from cultivation. We sampled soils from 12 locations within the Pawnee National Grasslands of northeastern Colorado, each having native fields and fields that were his- torically cultivated but abandoned 50 yr ago. We also sampled fields that had been cultivated for at least 50 yr at 5 of these locations. Our results demonstrated that soil organic matter, silt content, microbial biomass, po- tentially mineralizable N, and potentially respirable C were significantly lower on cultivated fields than on native fields. Both cultivated and abandoned fields also had significantly lower soil organic matter and silt contents than native fields. Abandoned fields, however, were not significantly different from native fields with respect to microbial biomass, po- tentially mineralizable N, or respirable C. In addition, we found that the characteristic small-scale heterogeneity of the shortgrass steppe associated with individuals of the dom- inant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200 g/m2 more C than between-plant locations. We suggest that 50 yr is an adequate time for recovery of active soil organic matter and nutrient availability, but recovery of total soil organic matter pools is a much slower process. Plant population dynamics may play an important role in the recovery of shortgrass steppe ecosystems from disturbance, such that establishment of perennial grasses determines the rate of organic matter recovery.

Disturbance and Diversity in Tropical Rain Forests: The Density Effect

Abstract: A null model for the effects of treefall gaps on tree species diversity of tropical rain forests is based on the well-known relationship between abundance and diversity: habitats supporting larger numbers of individuals can support more populations and more species than habitats supporting small numbers of individuals. Because seedling establishment and sapling density increase following canopy opening, gaps are often also sites of high species diversity. Both tree density and diversity are likely to be greater in areas of high stand-turnover rates. Thinning and non-catastrophic mortality reduce both density and diversity in areas of low gap frequency. Nevertheless, natural and anthropogenic disturbances, such as logging, have been hypothesized to both enhance and limit species diversity through changes in habitat heterogeneity, shifts in competitive balances among species, and creation of otherwise-rare habitats. Species-accumulation curves can be used to evaluate these alternatives by comparing observed species-abundance patterns with expected changes in diversity due to density effects alone. Topographic variation in density and diversity of shrubs and small saplings at the La Selva (Costa Rica) Biological Station preserve was analyzed as an example. The total number of species for each of 18 sites was estimated by fitting a two-parameter hyperbola (similar to the Michaelis-Menten equation) to sample data using a maximum-likelihood method. Although both density and diversity were higher on slopes than on ridges, total species richness was nevertheless higher on slopes than is predictable from density effects alone. High stand-turnover rates on slopes may enhance tree species diversity through increases in tree density and in habitat heterogeneity.

Remote Sensing of Forest Biophysical Structure Using Mixture Decomposition and Geometric Reflectance Models

Abstract: Using geometric shadow and linear mixture models we develop and evaluate an algorithm to infer several important structural parameters of stands of black spruce (Picea mariana), the most common boreal forest dominant. We show, first, that stand reflectances for this species can be represented as linear combinations of the reflectances of more elemental radiometric components: sunlit crowns, sunlit background, and shadow. Secondly, using a geometric model, we calculate how the fractions of these radiometric elements covary with each other. Then, using hand-held measurements of the reflectances of the sunlit background, sphagnum moss (Sphagnum spp.), and assuming shadow reflectance to be that of deep, clear lakes, we infer the reflectance of sunlit crowns from the geometric shadow model and low- altitude reflectance measurements acquired by a helicopter-mounted radiometer. Next, we as- sume that the reflectance for all black spruce stands is simply a linear combination of shadow, sunlit crown, and sunlit background reflectance, weighted in proportion to the relative areal fractions of these pixel elements. We then solve a set of linear equations for the areal fractions of these elements using as input helicopter observations of total stand reflectance. Using this algorithm, we infer the values for the areal proportions of sunlit canopy, sunlit background, and shadow for 31 black spruce stands of varying biomass density, net primary productivity, etc. We show empirically and theoretically that the areal proportions of these radiometric elements are related to a number of stand biophysical characteristics. Specifically, the shadow fraction is increasing with increasing biomass density, average diameter at breast height, leaf area index (LAI), and aboveground net primary productivity (NPP), while sunlit background fraction is decreasing. We show that the end member fractions can be used to estimate biomass with a standard error of -2 kg/M2, LAI with a standard error of 0.58, dbh with a standard error of -2 cm, and aboveground NPP with a standard error of 0.07 kg . m-2. yr- I. We, also show that the fraction of sunlit canopy is only weakly correlated with the biophysical variables and are thus able to show why a popular vegetation index, Normalized Difference Vegetation Index (NDVI), does not provide a useful measure of these biophysical characteristics. We do show, however, that NDVI should be related to the fraction of photosynthetically active radiation incident upon and absorbed by the canopy. This work has convinced us that a paradigm shift in the remote sensing of biophysical characteristics is in order-a shift away from direct inference of biophysical characteristics from vegetation indices and toward a two-step process, in which (1) stand-level reflectance is approximated in terrns of linear combinations of reflectance-invariant, spectrally distinct com- ponents (spectral end members) and mixture decomposition used to infer the areal fractions of these components, e.g., shadow, sunlit crown, and sunlit background, followed by (2) the use of radiative transfer models to compute biophysical characteristic values as a function of the end member fractions.

Inertia in Plant Community Structure: State Changes After Cessation of Nutrient-Enrichment Stress

Abstract: Water, nitrogen, and water-plus-nitrogen at levels beyond the range normally experienced by shortgrass steppe communities were applied from 1971 through 1975, plant populations were sampled through 1977, and the results of the experiment were published. Upon revisiting the plots in 1982, we found it apparent that large changes had occurred since 1977. Sampling was re-established in 1982 to follow trajectories of recovery. Our purposes in this paper are to examine how conclusions from this study changed through time, and discuss implications of these changes for monitoring potentially stressed eco- systems. Although productivities increased, dissimilarities in plant species composition at the end of the 5 yr of nutrient treatments were not significantly different from controls. Two years after cessation of the treatments exotic "weed" species were increasing in water- plus-nitrogen treated communities, and community dissimilarities were diverging in water and water-plus-nitrogen treated communities. Seven years after cessation of treatments all communities were significantly different from controls. Exotics were more than ten times more abundant in water-plus-nitrogen and nitrogen treated communities than they had been 2 yr post-treatment. A consistent trend in recovery of all treated communities was evident over the next 5 yr. However, the trend towards recovery reversed over the next four con- secutive years in the previously water-plus-nitrogen and water treated communities. The four-to-five year cycles in species composition and abundance of exotics towards, and then away from, conditions in undisturbed control communities were not related to weather, but large accumulations of litter suggested biotic regulation. Inertia of existing plant populations, or the tendency to continue to occupy a site when conditions become unfavorable, can mask both future deterioration in ecosystem condition and unstable behavior resulting from environmental stressors. Time lags in initial response means that an ecosystem can pass a threshold leading to transitions to alternate states before it is evident in structural characteristics such as species composition. Global climate change and sulfur and nitrogen oxide pollutants also have the potential to act as enrichment-stressors with initial time lags and/or positive effects and cumulative, subsequent negative effects, rather than as disturbance forces with immediate negative impacts. Sociopolitical systems, however, often require change in biological variables or negative impacts before acting to ameliorate environmental problems. The manner in which conclusions changed at various periods in time, and the potential for time lags in responses of species populations, raises questions about which variables are most useful for detection of stress and how long studies must last to be useful.

Change in Arctic CO2Flux Over Two Decades: Effects of Climate Change at Barrow, Alaska

Abstract: A significant difference in net ecosystem carbon balance of wet sedge eco- systems in the Barrow, Alaska region was observed between CO2 flux measurements ob- tained during the International Biological Program in 1971 and measurements made during the 1991-1992 growing seasons. Currently, high-center polygons are net sources of CO2 to the atmosphere of -14 gC-m-2.yr-', while low-center polygons are losing -3.6 gC.m-2.yr-', and ice wedge habitats are accumulating 4.0 gC.m-2*yr-t. On average, moist meadow habitats characteristic of the IBP-II site are currently sources of -1.3 gC.m-2.yr-t to the atmosphere compared to the reported accumulation of -25 gC_m-2.yr-t determined in 1971. This difference in ecosystem function over the last two decades may be due to the recently reported increase in surface temperatures resulting in decreases in the soil moisture status. These results point to the importance of long-term research sites and databases for determining the potential effects of climate change on ecosystem function.

Biodiversity and the Transformation of a Tropical Agroecosystem: Ants in Coffee Plantations

Abstract: Studies of biological diversity have focused mainly on undisturbed ecosystems, effectively neglecting potential losses due to changes in areas already altered by human intervention. In this study we test the hypothesis that measures of biological diversity change significantly with changes in agricultural practices. In particular, we examine differences in measures of ant species diversity correlated with changes in vegetational complexity associated with the modernization of Costa Rica's coffee agroecosystem. We examine patterns of within-habitat α and between-habitat β diversity in the ant community. Ants were sampled in 16 coffee farms falling on a gradient of vegetational and structural complexity. Percentage of shade created by the canopy was used as an index of vegetational complexity. As a partial indicator of the food resource base, arthropods were sampled using pitfall traps. The diversity (S, H', and E) of ground-foraging ants decreased significantly with the reduction of vegetational diversity. However, no significant changes were recorded for the diversity of the ants on the surface of coffee bushes. Similarity indices (I) showed a high degree of similarity among ant communities in coffee monocultures but a low degree of similarity among farms with high vegetational diversity. We discuss several possible mechanisms leading to reduced ant diversity.

Analyzing the Effects of Accidental Environmental Impacts: Approaches and Assumptions

Abstract: Because studies of environmental accidents must be initiated after the fact and because the accidents cannot (or should not) be replicated, sampling cannot be entirely randomized, and investigations involve some degree of confounding and pseudoreplication. The methodological issues relate to consistency in sampling methods and to the adequacy of sampling of levels of environmental disturbance or contamination, while the ecological assumptions derive from spatial and temporal variation in biological resources and the factors that affect them. These methodological issues and ecological assumptions affect study designs based on before-after comparisons and on single-time or multiple-time sampling after an accident. Designs that rest on the assumption of a steady-state equilibrium in resource-environment relationships (baseline and time-series designs) must be interpreted with particular care. Other designs (pre/post paired samples, impact level-by-time, impact trend-by-time) assume only that environmental variations are equivalent among areas and/or contamination levels (a dynamic equilibrium) and are less affected by pseudoreplication. Single-time designs (comparisons between impact and reference sites, between matched paired sites, or over a contamination gradient) have fewer methodological limitations, but assume that other natural factors that may influence the response of a resource are equal among all samples. If measurements of other factors are included in themore » design, covariance analysis may help to reduce this problem. In evaluating the effects of unplanned environmental impacts, post facto study designs that document both initial effects and subsequent recovery (impact level-by-time, impact trend-by-time) or that treat effects as continuous rather than categorical variables (gradient or trend designs) may be more useful than before-after comparisons. 54 refs., 6 figs., 2 tabs.« less

Herbaceous-Layer and Overstory Species in Clear-cut and Mature Central Appalachian Hardwood Forests

Abstract: The current interest among resource managers in ecosystem management necessitates a better understanding of the response of plant species diversity to forest management practices. This study attempted to assess the effects of one forest management practice-clear-cutting-on plant biodiversity in a mid-Appalachian hardwood forest by comparing species composition and diversity between two young (a20 yr following clear- cutting) and two "mature" (>70 yr following selective cutting) watersheds. Sampling was confined to the herbaceous layer (vascular plants l 1 -m in height) and woody overstory (stems ?2.5 cm diameter at 1.5-m height). The important tree species occurred on all watersheds, but the relative importance of these species varied greatly with stand age. Less shade-tolerant species, such as black cherry (Prunus serotina) and tulip poplar (Lirioden- dron tulipifera), are replaced by more-tolerant species, such as northern red oak (Quercus rubra) and sugar maple (Acer saccharum), as the stand matures. Analysis of tree species richness per plot suggests that the competitive thinning process decreases species evenness as the stand develops. Important herb-layer species included stinging nettle (Laportea canadensis), violets (Viola spp.), seedlings of striped maple (A. pensylvanicum), and several fern species. In sharp contrast with the trees, these species appeared to vary little with stand age. Species diversity (H') did not vary with stand age for either the overstory or the herbaceous layer. Detrended correspondence analysis showed a significant correlation between species composition of the two strata for the mature watersheds, but not the young, clear-cut watersheds. Thus, we suggest a temporal shift in processes influencing species composition following disturbance from allogenic factors (e.g., soil characteristics) to au- togenic factors (e.g., stand characteristics), which lead to a linkage between forest strata later in succession. The degree to which forest management alters species composition in these central Appalachian forest ecosystems may be tied to the degree of alteration of the link between strata.

Usefulness of spatially explicit population models in land management

Abstract: Land managers need new tools, such as spatial models, to aid them in their decision-making processes because managing for biodiversity, water quality, or natural disturbance is challenging, and landscapes are complex and dynamic. Spatially explicit population models are helpful to managers because these models consider both species - habitat relationships and the arrangement of habitats in space and time. The visualizations that typically accompany spatially explicit models also permit managers to {open_quotes}see{close_quotes} the effects of alternative management strategies on populations of interest. However, the expense entailed in developing the data bases required for spatially explicit models may limit widespread implementation. In addition, many of the models are developed for one or a few species, and dealing with multiple species in a landscape remains a significant challenge. To be most useful to land managers, spatially explicit population models should be user friendly, easily portable, operate on spatial and temporal scales appropriate to management decisions, and use input and output variables that can be measured affordably. 20 refs.

Chesapeake Bay Watershed Historical Land Use: Impact on Water Quality and Diatom Communities

Abstract: Stratigraphic records preserved in the sediments of the mesohaline Chesapeake Bay were used to reconstruct a 2000-yr history of sedimentation, eutrophication, anoxia, and diatom community structure over time. Diatoms, pollen, total and organic carbon (TOC), total and organic nitrogen, total sulfur, acid-soluble iron, an estimate of the degree of pyritization of iron (DOP), and biogenic silica (BSi) were used as paleoecological indicators in four cores collected from a transect across the Chesapeake Bay from the Choptank River to Plum Point, Maryland. This paper covers results for diatoms, pollen, and BSi. Sediments were dated using radiocarbon and pollen techniques, and sedimentation rates were determined (0.2-5.8 mm/yr) using pollen methods. Geochemical indicators were measured and diatom species identified at subsampled intervals within each core. More than 400 diatom species, primarily marine and estuarine taxa, were identified in the sediments, some for the first time. Analysis of the data indicates that sedimentation rates, eutrophication, turbidity, and anoxia have increased in the Chesapeake Bay since the time of European settlement of the watershed. There is also evidence that freshwater input to the mesohaline Chesapeake Bay has increased. Changes in diatom community structure and geochemical indicators reflect major changes in land use patterns of the watershed and increasing population. Diatom community diversity exhibits a continuing decline, while centric/pennate ratios rise dramatically in most recent sediments.

Possible Ecological Mechanisms for Loss of Vernal‐Herb Diversity in Logged Eastern Deciduous Forests

Abstract: The ecological literature on eastern forest-floor herbs and data collected in the southern Appalachians in Tennessee and North Carolina suggest five possible ecological mechanisms for reducing or limiting alpha diversity of vernal herbs in logged stands, three of which may also account for the slow recovery of some herbaceous species: (1) logging reduces populations of rarer herbs; (2) populations of forest-floor species are further reduced during the successional stages following logging, either by inability to adapt to changed microclimate or by competition with r-selected species that are better dispersers and better able to tolerate desiccation and increased radiation; (3) forest-floor herbs have slow growth and reproduction rates, thus population densities increase slowly; (4) many forest-floor herbs are clonal, ant-dispersed, or gravity-dispersed, thus they are slow to reoccupy suitable habitat once extirpated or greatly reduced in population numbers; and (5) logging results in less-than-optimal conditions for forest-floor herb reproduction by modifying microhab- itats on the forest floor and by temporarily eliminating gap-phase succession. The data indicate some species of vernal herbs are far more tolerant of disturbance than others, and that sensitive species can be identified and utilized as indicators of community integrity and diversity.

Sciurids in pacific northwest managed and old-growth forests'

Abstract: An understanding of the factors governing sciurid abundance in the Pacific Northwest is essential for prescribing forest management practices for second-growth forests where recovery of Spotted Owl (Strix occidentalis) populations and enhancement of biodiversity are objectives. We compared results of companion studies of sciurids in western Washington and Oregon and examined patterns of abundance in relation to habitat elements on the Olympic Peninsula to elucidate governing factors and make recommendations for forest management. Regional contrasts show that Glaucomys sabrinus and Tamias townsendii in Douglas-fir forests in Oregon are 4 times more abundant than in western hemlock forests in Washington, and dietaries of Glaucomys, and the fungal communities that provide its food, are more diverse in Oregon than in Washington. Glaucomys sabrinus in old forests are 2 times more abundant than in young, managed forests without old-forest legacies (large live trees, large snags and large, decaying fallen trees): populations in young forests with old-forest legacies and with understory development may equal those in old Growth. On the Olympic Peninsula. Glaucomys sabrinus abundance can be predicted by density of large snags and abundance of ericaceous shrubs. At least seven large snags/ha and well-distributed patches of dense shrubs (cover within patches >24% and patches covering 40% of the total area) are necessary for high densities of Glaucomys sabrinus. Abundance of Tamias townsendii reflects size of dominant tree and well-developed understories. Abundance of Ta,niasciurus douglasii seems to reflect territoriality in concordance with food supply and was greatest where Glaucomys and Tamias were low in abundance. Patterns of abundance of the sciurids in old- and managed forests suggests that silvicultural manipulation of vegetation and creative snag or den-tree management could be used in a management strategy to accelerate the development of Spotted Owl habitat in areas where old Growth is lacking.

Phosphorus Retention in Constructed Freshwater Riparian Marshes

Abstract: Four constructed freshwater riparian marshes in northeastern Illinois, USA, each experimentally subjected to either high or low loadings of turbid river water over a 3-yr period, were investigated for phosphorus retention and fate. Average phosphorus con- centrations decreased by 64-92% in low-flow wetlands (to 11-40 Kg P/L) and by 53-90% (to 12-57 Kg P/L) in high-flow wetlands. Intensive sampling at 4-h intervals during spring and summer of the final year showed phosphorus mass decreases of 81-74% for low- and high-flow wetlands, respectively. During this frequent sampling, inflow averaged 176 Kg P/L, while outflows were 34 Kg P/L for the low-flow wetland and 45 Kg P/L for the high- flow wetland. Spatial patterns of isopleths showed clear gradients in high-flow wetlands and near-homogeneous concentrations in low-flow wetlands. Annual budgets illustrated that most inflowing phosphorus was retained through sedimentation with some capacity for phosphorus retention by macrophytes and a lesser amount by microbial (periphyton and planktonic) communities. A simple Vollenweider-type model predicted well the retention of phosphorus in these wetlands for low- and high-flow wetlands with 3 yr of data, i.e., the model was calibrated and verified with six independent data sets. A mean deviation of only 9.3% between measured and simulated retention was found when the retention co- efficient was k = 6.51 wk-'. Adding a factor for seasonal temperature fluctuation did not improve the fit, whereas using a model where the retention is a function of flow provided the best overall fit to the data (mean deviation of 6.3% between data and predictions). These constructed wetlands retained about the same amount of phosphorus per unit area (0.5-3 g P.m-2.yr-') as have several other natural and constructed wetlands receiving similar concentrations of phosphorus.

Herbivore-Induced Species Replacement in Grasslands: Is it Driven by Herbivory Tolerance or Avoidance?

Abstract: Herbivore-induced shifts in species composition have been documented from grasslands throughout the world, but the mechanism(s) of species replacement remains largely unexplored. An experiment was conducted in a transplant garden, on the campus of Texas AM however, tiller number per plant was suppressed by B. saccharoides, but not by S. leucotricha or conspecific neighbors. Defoliation of S. scoparium plants, but not neighbors, negatively impacted the late-seral plants. Selective defoliation of S. sco- parium plants significantly reduced tiller variables of mean mass, leaf blade area, and leaf number, but did not significantly reduce plant variables including mean basal area, tiller number, or annual shoot production. Defoliation of both S. scoparium plants and neighbors increased annual shoot production, mean basal area per plant, mean tiller leaf area, leaf number, tiller mass, stomatal conductance to H20 vapor, and plant xylem pressure potential in comparison with S. scoparium plants grown with comparable, nondefoliated neighbors. An increase in both plant and tiller variables in defoliated S. scoparium plants grown with uniformly defoliated neighbors establishes that replace- ment of a late-seral dominant is not driven by a greater relative expression of herbivory tolerance of mid-seral species. These results collectively suggest that the late-seral dominant, S. scoparium, pos- sesses a greater competitive ability and a comparable or greater degree of herbivory tolerance than the mid-seral species that comprise the community. Therefore, the initial hypothesis was rejected. It can be inferred that the alternative mechanism, selective herbivory of the late-seral dominant, is the dominant mechanism contributing to species replacement. Herbivore-induced modifications of competitive interactions are most like- ly to drive species replacement in grasslands characterized by high and consistent resource availability. This may partially explain why condition and trend analysis was developed and initially implemented in the true and mixed prairie associations of North America and why it is widely used by rangeland managers in these grasslands.

Impacts of Pastoralists on Woodlands in South Turkana, Kenya: Livestock- Mediated Tree Recruitment

Abstract: Since the turn of the century, African pastoralists have been held responsible for overuse of woody plants and for the degradation and desertification of many arid and semiarid lands. We analyzed the impacts of pastoral nomads and their livestock on the recruitment (establishment to first reproduction) of Acacia tortilis, a dominant tree in the dry woodlands of South Turkana, Kenya, where Acacia seedpods make up an important part of livestock diets. Seed density averaged over 85 times higher in bush-fenced livestock corrals than in the surrounding environment. The survival and growth of 14 cohorts of trees ranging in age from 1 to 39 yr were investigated comparing tree stands originating inside livestock corrals with those originating outside. Corral soils contained nine times more C, three times more N, and six times more P than adjacent noncorral soils immediately following corral abandonment. Corral soils also retained more moisture than noncorral soils after rainfall. These soil conditions accelerated seedling emergence in corrals, and enhanced survival and growth of 1st-yr seedlings. Survival of older trees in corral stands was not significantly different from those established outside corrals during this study. However, comparison of tree densities over time suggests that corral stands thin more rapidly than noncorral stands, probably because of crowding. The early survival and growth advantages of the corral environment appear to stabilize the reproductive patterns of A. tortilis in this arid ecosystem, where successful recruitment in noncorral sites may be restricted to the few years with high rainfall. Contrary to the conventional wisdom, pastoralists may be improving rangelands in South Turkana by enhancing recruitment reliability in this im- portant tree species.