Showing papers in "Ecological Applications in 1993"

The Role of Riparian Corridors in Maintaining Regional Biodiversity

Abstract: Riparian corridors possess an unusually diverse array of species and environmental processes. This "ecological" diversity is related to variable flood regimes, geomorphic channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. This dynamic environment results in a variety of life history strategies, and a diversity of biogeochemical cycles and rates, as organisms adapt to disturbance regimes over broad spatio-temporal scales. These facts suggest that effective riparian management could ameliorate many ecological issues related to land use and environmental quality. We contend that riparian corridors should play an essential role in water and landscape planning, in the restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.

Preserving Biodiversity: Species, Ecosystems, or Landscapes?

Abstract: Efforts to preserve biological diversity must focus increasingly at the ecosystem level because of the immense number of species, the majority of which are currently unknown. An ecosystem approach is also the only way to conserve processes and habitats (such as forest canopies, belowground habitats, and hyporheic zones) that, with their constituent species, are poorly known. Continued concern with species is essential, however. Landscape-level issues also need much greater attention. Designing an appropriate system of habitat reserves is one landscape- level concern. Understanding and appropriately manipulating the landscape matrix is at least equal in importance to reserves issues, however, since the matrix itselfis important in maintaining diversity, influences the effectiveness of reserves, and controls landscape connectivity.

Crop Rotation and Intercropping Strategies for Weed Management

Abstract: Results of a literature survey indicate that weed population density and biomass production may be markedly reduced using crop rotation (temporal diversification) and intercropping (spatial diversification) strategies. Crop rotation resulted in emerged weed densities in test crops that were lower in 21 cases, higher in 1 case, and equivalent in 5 cases in comparison to monoculture systems. In 12 cases where weed seed density was reported, seed density in crop rotation was lower in 9 cases and equivalent in 3 cases when compared to monocultures of the component crops. In intercropping systems where a main crop was intersown with a "smother" crop species, weed biomass in the intercrop was lower in 47 cases and higher in 4 cases than in the main crop grown alone (as a sole crop); a variable response was observed in 3 cases. When intercrops were composed of two or more main crops, weed biomass in the intercrop was lower than in all of the component sole crops in 12 cases, intermediate between component sole crops in 10 cases, and higher than all sole crops in 2 cases. It is unclear why crop rotation studies have focused on weed density, whereas intercropping studies have focused on weed biomass. The success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable and frequently inhospitable environment that prevents the proliferation of a particular weed species. The relative importance and most effective combinations of these weed control tactics have not been adequately assessed. In addition, the weed-suppressive effects of other related factors, such as manipulation of soil fertility dynamics in rotation sequences, need to be examined. Intercrops may demonstrate weed control advantages over sole crops in two ways. First, greater crop yield and less weed growth may be achieved if intercrops are more effective than sole crops in usurping resources from weeds or suppressing weed growth through allelopathy. Alternatively, intercrops may provide yield advantages without suppressing weed growth below levels observed in component sole crops if intercrops use resources that are not exploitable by weeds or convert resources to harvestable material more efficiently than sole crops. Because of the difficulty of monitoring the use of multiple resources by intercrop/weed mixtures throughout the growing season, identification of specific mechanisms of weed suppression and yield enhancement in intercrop systems has so far proven elusive. Significant advances in the design and improvement of weed-suppressive crop rotation and intercropping systems are most likely to occur if three important areas of research are addressed. First, there must be continued attention to the study of weed population dynamics and crop-weed interference in crop rotation and intercropping systems. More information is needed concerning the effects of diversification of cropping systems on weed seed longevity, weed seedling emergence, weed seed production and dormancy, agents of weed mortality, differential resource consumption by crops and weeds, and allelopathic interactions. Second, there needs to be systematic manipulation of specific components of rotation and intercropping systems to isolate and improve those elements (e.g., interrow cultivation, choice of crop genotype) or combinations of elements that may be especially important for weed control. Finally, the weed-related impacts of combining crop rotation and intercropping strategies should be assessed through careful study of extant, complex farming systems and the design and testing of new integrated approaches. Many aspects of crop rotation and intercropping are compatible with current farming practices and could become more accessible to farmers if government policies are restructured to reflect the true environmental costs of agricultural production.

Can Fertilization of Soil Select Less Mutualistic Mycorrhizae

Abstract: It has been noted previously that nutrient-stressed plants generally release more soluble carbohydrate in root exudates and consequently support more mycorrhizae than plants supplied with ample nutrients. Fertilization may select strains of vesicular-arbuscular mycorrhizal (VAM) fungi that are inferior mutualists if the same characteristics that make a VAM fungus successful in roots with a lowered carbohydrate content also reduce the benefits that the fungus provides a host plant. This two-phase study experimentally tests the hypothesis that fertilizing low-nutrient soil selects VAM fungi that are inferior mutualists. The first phase examines the effects of chemical fertilizers on the species composition of VAM fungal communities in long-term field plots. The second phase measures the effects of VAM fungal assemblages from fertilized and unfertilized plots on big bluestem grass grown in a greenhouse. The field results indicate that 8 yr of fertilization altered the species composition of VAM fungal communities. Relative abundance of Gigaspora gigantea, Gigaspora margarita, Scutellispora calospora, and Glomus occultum decreased while Glomus intraradix increased in response to fertilization. Results from the greenhouse experiment show that big bluestem colonized with VAM fungi from fertilized soil were smaller after 1 mo and produced fewer inflorescences at 3 mo than big bluestem colonized with VAM fungi from unfertilized soil. Fungal structures within big bluestem roots suggest that VAM fungi from fertilized soil exerted a higher net carbon cost on their host than VAM fungi from unfertilized soil. VAM fungi from fertilized soil produced fewer hyphae and arbuscules (and consequently provided their host with less inorganic nutrients from the soil) and produced as many vesicles (and thus provisioned their own storage structures at the same level) as fungi from unfertilized soil. These results support the hypothesis that fertilization selects VAM fungi that are inferior mutualists.

Uncertainty, Resource Exploitation, and Conservation: Lessons from History.

Abstract: There are currently many plans for sustainable use or sustainable development that are founded upon scientific information and consensus. Such ideas reflect ignorance of the history of resource exploitation and misunderstanding of the possibility of achieving scientific consensus concerning resources and the environment. Although there is considerable variation in detail, there is remarkable consistency in the history of resource exploitation: resources are inevitably overexploited, often to the point of collapse or extinction. We suggest that such consistency is due to the following common features: (i) Wealth or the prospect of wealth generates political and social power that is used to promote unlimited exploitation of resources. (ii) Scientific understanding and consensus is hampered by the lack of controls and replicates, so that each new problem involves learning about a new system. (iii) The complexity of the underlying biological and physical systems precludes a reductionist approach to management. Optimum levels of exploitation must be determined by trial and error. (iv) Large levels of natural variability mask the effects of overexploitation. Initial overexploitation is not detectable until it is severe and often irreversible.

Comparing Spatial Pattern in Unaltered Old-Growth and Disturbed Forest Landscapes.

Abstract: We used geographic information systems (GIS) to analyze the structure of a second-growth forest landscape (9600 ha) that contains scattered old-growth patches. We compared this landscape to a nearby, unaltered old-growth landscape on comparable land- forms and soils to assess the effects of human activity on forest spatial pattern. Our objective is to determine if characteristic landscape structural patterns distinguish the primary old- growth forest landscape from the disturbed landscape. Characteristic patterns of old-growth landscape structure would be useful in enhancing and restoring old-growth ecosystem functioning in managed landscapes. Our natural old-growth landscape is still dominated by the original forest cover of eastern hemlock (Tsuga canadensis), sugar maple (Acer saccharum), and yellow birch (Betula allegheniensis). The disturbed landscape has only scattered, remnant patches of old-growth ecosystems among a greater number of early successional hardwood and conifer forest types. Human disturbances can either increase or decrease landscape heterogeneity depending on the parameter and spatial scale examined. In this study, we found that a number of important structural features of the intact old-growth landscape do not occur in the dis- turbed landscape. The disturbed landscape has significantly more small forest patches and fewer large, matrix patches than the intact landscape. Forest patches in the fragmented landscape are significantly simpler in shape (lower fractal dimension, D) than in the intact old-growth landscape. Change in fractal dimension with patch size, a relationship that may be characteristic of differing processes of patch formation at different scales, is present within the intact landscape but has been obscured by human activity in the disturbed landscape. Important ecosystem juxtapositions of the old-growth landscape, such as hem- lock with lowland conifers, have been lost in the disturbed landscape. In addition, significant landscape heterogeneity in this glaciated region is produced by landforms alone, without natural or human disturbances. The features that distinguish disturbed and old-growth forest landscape structure that we have described need to be examined elsewhere to determine if such features are char- acteristic of other landscapes and regions. Such forest landscape structural differences that exist more broadly could form the basis of landscape principles to be applied both to the restoration of old-growth forest landscapes and the modification of general forest man- agement for enhancing biodiversity. These principles may be particularly useful for con- structing integrated landscapes managed for both commodity production and biodiversity protection.

Biomass Production in a Tallgrass Prairie Ecosystem Exposed to Ambient and Elevated CO"2

Abstract: Responses to elevated CO"2 have not been measured for natural grassland ecosystems. Global carbon budgets will likely be affected by changes in biomass production and allocation in the major terrestrial ecosystems. Whether ecosystems sequester or release excess carbon to the atmosphere will partly determine the extent and rate that atmospheric CO"2 concentration rises. Elevated CO"2 also may change plant community species composition and water status. We determined above- and belowground biomass production, plant community species composition, and measured and modeled water status of a tallgrass prairie ecosystem in Kansas exposed to ambient and twice-ambient CO"2 concentrations in open-top chambers during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii, A. scoparius, and Sorghastrum nutans (C"4 metabolism) and Poa pratensis (C"3). Aboveground biomass and leaf area were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, peak biomass and leaf area were estimated by an early August harvest. Relative root production among treatments was estimated using root ingrowth bags which remained in place throughout the growing season. Latent heat flux was simulated with and without water stress. Botanical composition was estimated annually. Compared to ambient CO"2 levels, elevated CO"2 increased production of C"4 grass species, but not of C"3 grass species. composition of C"4 grasses did not change, but Poa pratensis (C"3) declined, and C"3 forbs increased in the stand with elevated CO"2 compared to ambient. Open-top chambers appeared to reduce latent heat flux and increase water-use efficiency similar to the elevated CO"2 treatment when water stress was not severe, but under severe water stress, the chamber effect on water-use efficiency was limited. In natural ecosystems with periodic moisture stress, increased water-use efficiency under elevated CO"2 apparently would have a greater impact on productivity irrespective of photosynthetic pathway.

A Spatial Model of Atmospheric Deposition for the Northeastern U.S.

Abstract: Spatial patterns of atmospheric deposition across the northeastern United States were evaluated and summarized in a simple model as a function of elevation and geographic position within the region. For wet deposition, 3—11 yr of annual concentration data for the major ions in precipitation were obtained from the National Atmospheric Deposition Program/National Trend Network (NADP/NTN) for 26 sites within the region. Concentration trends were evaluated by regression of annual mean concentrations against latitude and longitude. For nitrate, sulfate, and ammonium concentrations, a more than twofold linear decrease occurs from western New York and Pennsylvania to eastern Maine. These trends were combined with regional and elevational trends of precipitation amount, obtained from 30—yr records of annual precipitation at >300 weather stations, to provide long—term patterns of wet deposition. Regional trends of dry deposition of N and S compounds were determined using 2—3 yr of particle and gas concentration data collected by the National Dry Deposition Network (NDDN) and several other sources, in combination with estimates of deposition velocities. Contrary to wet deposition trends, the dominant air concentration trends were steep decreases from south to north, creating regional decreases in total deposition (wet + dry) from the southwest to the northeast. This contrast between wet and dry deposition trends suggests that within the northeast the two deposition forms are received in different proportions from different source areas, wet deposited materials primarily from areas to the west and dry deposited materials primarily from urban areas along the southern edge of the region. The equations generated describing spatial patterns of wet and dry deposition within the region were entered into a geographic information system (GIS) containing a digital elevation model (DEM) in order to develop spatially explicit predictions of atmospheric deposition for the region.

Long-Term Response of Nutrient-Limited Forests to CO"2 Enrichment; Equilibrium Behavior of Plant-Soil Models.

Abstract: Established process-based models of forest biomass production in relation to atmospheric CO"2 concentration (McMurtrie 1991) and soil carbon/nutrient dynamics (Parton et al. 1987) are integrated to derive the @'Generic Decomposition and Yield@' model (G'DAY). The model is used to describe how photosynthesis and nutritional factors interact to determine the productivity of forests growing under nitrogen-limited conditions. A simulated instantaneous doubling of atmospheric CO"2 concentration leads to a growth response that is initially large (27% above productivity at current CO"2) but declines to <10% elevation within 5 yr. The decline occurs because increases in photosynthetic carbon gain at elevated CO"2 are not matched by increases in nutrient supply. Lower foliar N concentrations at elevated CO"2 have two countervailing effects on forest production: decreased rates of N cycling between vegetation and soils (with negative consequences for productivity), and reduced rates of N loss through gaseous emission, fire, and leaching. Theoretical analysis reveals that there is an enduring response to CO"2 enrichment, but that the magnitude of the long-term equilibrium response is extremely sensitive to the assumed rate of gaseous emission resulting from mineralization of nitrogen. Theory developed to analyze G'DAY is applicable to other published production-decomposition models describing the partitioning of soil carbon among compartments with widely differing decay-time constants.

Plant and Soil Responses to Chronic Nitrogen Additions at the Harvard Forest, Massachusetts

Abstract: Data are presented on changes in plant and soil processes in two forest types (red pine plantation and oak-maple forest) at the Harvard Forest, Petersham, Massachusetts, in response to 3 yr of chronic N fertilization. The hardwood stand exhibited greater N limitation on biological function than the pine stand prior to fertilization as evidenced by a lower net N mineralization rate, nearly undetectable rates of net nitrification, and very low foliar N content. N additions were made in six equal applications throughout the growing season, and consisted of 5 and 15 g°m-2 °yr-1 of N as ammonium nitrate. The pine stand showed larger changes than the hardwood stand for extractable N, foliar N, nitrification, and N leaching loss. Retention of added N was essentially 100% for all but the high application pine plot from which significant N leaching occurred in the 3rd yr of application. From 75 to 92% of N added to fertilized plots was retained in the soil, with larger fractions retained in the hardwood stand than the pine stand for all treatments. As hypothesized, the stands are exhibiting highly nonlinear patterns of nitrogen output in response to continuous nitrogen inputs. The implications of this nonlinearity for regional eutrophication of surface waters and atmospheric deposition control policy are discussed.

Invasion resistance to introduced species by a native assemblage of california stream fishes

Abstract: Assemblages of native stream fishes in California show a remarkable ability to resist invasion by introduced fishes as long as the streams are relatively undisturbed by human activity. Previous studies had indicated a high degree of spatial (microhabitat) segregation among the native fishes, which was confirmed by a principal components analysis of microhabitat use data from Deer Creek, a tributary of the Sacramento River. A null modelling study using the same data set was performed to see if competition was a major force structuring the assemblage, because theoretical studies had indicated that a competitively structured assemblage should be most able to resist invasions. The null models indicated that competition was not the major structuring force, so it is likely the assemblages are structured through a combination of morphological specialization (reflect- ing evolutionary history), predation, and some competition. The assemblages resist invasion through both environmental and biotic factors. Predation seems to be an especially im- portant biotic factor.

Breeding success of hatchery and wild coho salmon (oncorhynchus kisutch) in competition

Abstract: The divergence of hatchery fish in traits important for reproductive success has raised concerns about their ability to rehabilitate wild populations, and the threat that their inevitable straying poses to biological diversity through introgression. We therefore undertook a study of the breeding competition and success of sea-ranched hatchery fish placed in direct competition with wild fish. Experiments using wild and hatchery coho salmo (Oncorhynchus kisutch) were conducted within a controlled stream channel, allowing selective manipulation of breeding competition and density. Hatchery fish, particularly males, were competitively inferior to wild fish, being less aggressive and more submissive. As a consequence, hatchery males were denied access to ovipositing females; they partook in fewer spawnings, held more distal positions in spawning hierarchies, and attained only an estimated 62% of the breeding success of wild males. By contrast, competition did not appear to inhibit hatchery females as overtly as males. Hatchery and wild females exhibited similar levels of aggressive behavior, however hatchery females did suffer greater delays in the onset of breeding, failed to spawn larger proportions of their eggs, and lost more eggs to nest destruction by other females. They averaged an estimated 82% of the breeding success of wild females. There was thus a sex bias in the breeding disadvantage of hatchery fish, with males suffering more than females. Furthermore, the breeding disadvantage was density dependent with the relative success of hatchery to wild fish declining with increasing density. These results imply that hatchery fish have restricted abilities to rehabilitate wild populations, and may pose ecological and genetic threats to the conservation of wild populations.

Transient Ecotone Response to Climatic Change: Some Conceptual and Modelling Approaches

Abstract: Accurate prediction of the ecological impacts of climatic change is a pressing challenge to the science of ecology. The current state of the art for broad-scale estimates of change in biomes and ecotones between biomes is limited to equilibrium estimates of ecological change under some future equilibrium climate. Uncertainties in these estimates abound, ranging from uncertainties in future climate scenarios to uncertainties in our ecological models and finally to uncertainties in modelling the feedbacks between the climate and the biosphere. Ecologists and policymakers need to go beyond equilibrium estimates of biosphere change to transient responses of the biosphere as the climate changes. Ecotones between biomes have been suggested as sensitive areas of change that could be effectively modelled and monitored for future change. Ecotones are also important in influencing local and regional biodiversity patterns and ecological flows. The ecological processes that could affect change at ecotones and within biomes are discussed; they include internal ecosystem processes, such as competition, and external abiotic processes, most notably drought and related disturbances. Drought followed by infestations and fire appears to be the most likely process that could mediate ecological change under a rapidly changing climate. The impacts would be apparent all across biomes, not just at ecotones. However, specific predictions about the dynamics of ecotones can be made qualitatively, based on a theory of patch scaling and diversity in relation to abiotic stressors. Under current conditions, the size of homogeneous patches is expected to be small at ecotones, but to enlarge with distance from the ecotone. Directional climatic change should promote a coalescence of patches on one side of the ecotone and increased fragmentation on the other side. Ecotones should begin to blur as viewed from a satellite only to re-form at some later date in a new location. This view is in contrast to the notion that ecotones would retain sharp distinction and simply move across the landscape. These changes are presented as hypotheses based on theory and should be testable in a mechanistic modeling framework that is only now being developed.

Fish Assemblage Recovery Along a Riverine Disturbance Gradient.

Abstract: Artificial fluctuations in streamflow have been documented to alter the composition and structure of stream communities. This study tests the hypothesis that a spatial recovery gradient in fish assemblage structure exists downstream of a hydroelectric dam, and that recovery can be identified by the presence and abundance of species largely restricted to flowing-water habitats (fluvial specialists). A longitudinal gradient of change in a shoreline fish assemblage was quantified in a 66-km reach of a mid-sized, species-rich river (Tallapoosa River, Alabama) with daily flow fluctuations from hydropower generation. The shoreline fish assemblage in a nearby and similar river (Cahaba River, Alabama) was quantified as a regional reference for the occurrence of fish assemblage gradients. Fish were collected with prepositioned area electrofishers in 240 randomly located sampling sites, and physical habitat was quantified. Using distributional and habitat use information, fish species were categorized as fluvial specialists or macrohabitat generalists (species that occur in a wide variety of aquatic systems). Sampled habitats were similar between rivers and along each study reach. The longitudinal pattern of species occurrence and fish abundance was consistent in the free-flowing river. A longitudinal gradient of increasing abundance and richness of only fluvial specialist species existed downstream of the hydroelectric dam. No similar spatial gradient existed for macrohabitat generalists in either river. Although a fish community recovery gradient was identified, a recovery endpoint was not evident because assemblage change was gradual and possibly incomplete. The preservation and management of riverine fish faunas will partly depend on incorporating spatial recovery into decisions about permitting and siting of anthropogenic changes like hydroelectric dams.

Quantifying Dispersal of Southern Pine Beetles with Mark-Recapture Experiments and a Diffusion Model.

Abstract: Pest management decisions should take into consideration quantitative information on dispersal of insect pests, but such information is often lacking. The goal of this study was to measure intraforest dispersal in the southern pine beetle (SPB). We developed an analytical formula for interpreting data from mark-recapture studies of insect dispersal. The formula is obtained by postulating a simple model of diffusion with disappearance (e.g., as a result of death) for the spread of marked insects from the release point. Parameters of the model (assumed to be invariant in space) are estimated by fitting the curve to the cumulative number of recaptured insects as a function of the distance from release. We also derived an expression for the distribution of dispersal distances in terms of the fitted parameters. The proposed methodology was applied to a mark-recapture study of SPB dispersal Statistical analysis of recaptures-with-distance curves obtained in 11 replicate releases indicated that the proposed formula provided an accurate description of the data. There were no systematic departures from the functional relationship prescribed by the formula, and the model consistently outperformed another commonly used formula for fitting data on dispersal distances, the exponential curve. We explored the effect of spatial heterogeneity in the host distribution on SPB movement by regressing the deviation from the recapture rate predicted by the model in each trap on the pine basal area around the trap. This correlation was significantly greater than zero, indicating that beetles tended to aggregate in localities where pines were dense. This result suggests that a diffusion model with spatially varying parameters may provide a more accurate description of the redistribution process in the SPB. Quantitative results on SPB intraforest dispersal were summarized by calculatingradii of circles enclosing a given proportion of SPB dispersal distances. For example, we estimated that one half of released beetles dispersed >0.69 km. This result has important implications for evaluating the area-wide consequences of current or novel control tactics that rely on disrupting SPB movements, e.g., cut-and-leave and treatments with anti-congregation pheromone.

Peat Accretion and N, P, and Organic C Accumulation in Nutrient-Enriched and Unenriched Everglades Peatlands.

Abstract: Recent (1964-1989) rates of peat accretion and nitrogen (N), phosphorus (P), and organic carbon (C) accumulation were measured in Everglades soils to characterize the effects of altered hydroperiod and nutrient regimes on the nutrient storage capacity of the Everglades ecosystem. Peat accretion was related to hydroperiod and phosphorus load- ing. Accretion rates were highest in areas of extended hydroperiod (2.8-3.2 mm/yr) and/or phosphorus enrichment (4.0 mm/yr) and lowest in areas of reduced hydroperiod (1.6-2.0 mm/yr). Rates of accumulation of nitrogen were 3.8-11.6 g-m-2 yr-' (K = 8.2 g.m-2 yr-') and those of organic C were 54-161 gm-2 yr-' (X = 104 g m-3 yr-'). Ac- cumulation rates of N and organic C were primarily a function of peat accretion rates. Phosphorus accumulation was controlled by both peat accretion and increased soil P content. Soil P concentrations (1248 ,g/g) in an area receiving N and P enriched agricultural runoff were 2-3 times higher than P levels at unenriched locations (432-764 ,g/g). As a result, rates of P accumulation at this site (0.46 g m- 2yr- ') were 2-8 times greater com- pared to unenriched Everglades soils (0.06-0.23 g.m-2 yr-'). Inputs of P (0.53 g.m-2 yr-') and N (15.5 g m-2 yr-') to the nutrient-enriched area of Water Conservation Area (WCA) 2A (via rainfall and surface flow) were nine and twelve times higher than inputs to the unenriched part of WCA 2A. As a result, the efficiency of P removal was lower at the enriched site (87%) than at the unenriched location (100%). Nitrogen removal effi- ciencies also were lower in the enriched area (75%) as compared to the unenriched area, where N accumulation in peat was 290-450% of the input. The difference in N storage efficiencies between the two sites may reflect differences in N fixation and denitrification in nutrient-enriched and unenriched Everglades peatlands. Our findings suggest that nu- trient-enriched agricultural drainage has contributed to increased rates of peat accretion and phosphorus accumulation in areas of the northern Everglades that have been receiving agricultural drainage for the past 25-30 yr. The affected area has functioned effectively as a phosphorus sink, primarily due to increased organic P storage. However, the effects of nutrient loading, especially P, on the long-term stability of the Everglades ecosystem and on the long-term P storage potential of Everglades peats are poorly understood and are currently under investigation. Key wvords: agricultural runoff Florida Everglades; freshivater wetland; hydrology; organic soil.

A Model of the Effects of Tillage on Emergence of Weed Seedlings.

Abstract: A simple model is developed in which the density of weed seedlings emerging in a field is related to (1) the ability of seedlings to emerge from various depths in the soil, (2) the survival of seeds at different depths, and (3) the depth of seed burial in no tillage, rotary tillage, and plow tillage. Other tillage regimes are considered by analogy. Literature is reviewed to determine biologically reasonable functions describing seedling emergence, seed survival, and distribution of seeds with depth, and parameters of these equations are estimated from data in the literature. Problems related to the mathematical description of these phenomena are discussed, and it is noted that some commonly held beliefs regarding survival of seeds in the soil are mutually incompatible. Although many studies have investigated the persistence of seeds as a function of depth in the soil, few have distinguished death from the production of seedlings. The model indicates that in the first year following input of seeds to the soil, no tillage will have more seedlings than tillage, but in later years no tillage will likely have fewer seedlings unless innate or induced dormancy is high or seed survival near the soil surface is unusually good. If seed return is allowed, no tillage or minimum tillage will have more seedlings perennially. Recovery of good weed control following a year with substantial seed input may be easiest if the soil is plowed deeply to bury the seeds, and then shallow or no tillage is used in subsequent years to avoid returning seeds to the surface. Much of the literature on the effects of tillage on weed density is difficult to interpret because little indication is given of the vertical distribution of seeds in the soil at the beginning of the experiment.

Management of Weed Seed Banks with Microorganisms

Abstract: Successful weed management in agroecosystems centers on manipulating the weed seed bank in soil, the source of annual weed infestations. Despite advances in aboveground weed control and decreases in the production of new seed, weed infestations continue to be generated from a small portion of the seed bank that persists as a result of dormancy and resistance to decay. Depletion of the persistent seeds using soil-applied chemicals to stimulate germination has received much attention while the search for microorganisms selective for seed decay has been largely overlooked. This paper provides an overview of the effects of microorganisms on weed seed viability relative to seed bank depletion, and how this information can be applied to weed management. Limited studies indicate that microorganisms associated with weed seeds can contribute to seed bank depletion through attraction to seeds by chemotaxis, rapid colonization of the spermosphere and production of enzymes and/or phytotoxins to kill seeds prior to germination. It is recognized, however, that the best opportunity for success will be through integration of selected microorganisms or microbial products with other approaches including germination stimulation, application of low rates of herbicides, manipulation of the soil environment (e.g., solarization), and biological control agents for effectively eliminating dormant, persistent seeds from soil. To achieve success, more in-depth research on microbial factors affecting weed seed banks is required.

Maximization of Aboveground Grassland Production: The Role of Defoliation Frequency, Intensity, and History.

Abstract: Production of tallgrass prairie vegetation was measured on experimental plots in which defoliation intensity and frequency were manipulated by mowing and using movable exclosures on areas chronically grazed by cattle. Defoliation history largely controlled whether or not defoliated plants overcompensated (exhibited enhanced production compared to undefoliated controls) for tissue removal. Plants on chronically grazed sites only compensated for foliage removed by grazers. Production on plots mowed prior to the year of measurement was similar to that on chronically grazed sites, while previously unmowed plots exhibited substantial aboveground overcompensation. Aboveground production was maximized by the most frequent mowing treatment and by intermediate mowing heights. Nitrogen and phosphorus concentrations and amounts in aboveground tissues were increased by mowing and grazing. Current mowing regime was more important than mowing history in determining nitrogen concentrations except very early in the growing season. Effects of grazing and mowing on belowground biomass were inconsistent, but frequent mowing appeared to limit accumulation of belowground N reserves and biomass. In North American grasslands, overcompensation is a nonequilibrium plant response to grazing. Photosynthate that would be stored as reserves and used for root growth and flower and seed production instead is used to replace lost leaf area, thereby resulting in higher foliage productivity. However, under chronic grazing or mowing, vegetation is prevented from maintaining high nutrient and water uptake capacity (large root biomass) and accumulating reserves that allow overcompensation responses.

Prospects for Weed Control Through Crop Interference.

Abstract: Crop interference with weed growth is a fundamental method of nonchemical weed control. Crop interference, entailing weed suppression, should be distinguished from crop tolerance of weeds, i.e., relatively small yield loss due to the presence of weeds. Tolerance is less desirable than interference in that it may lead to increases in weed seed populations that can cause future yield losses. Ideally, interference should occur as early as possible in growth to prevent resource consumption by weeds. A path analysis of crop-weed interference is presented. The model can be used to estimate the relative magnitude of early vs. later crop interference with weed growth, and to identify crop traits that contribute to the interference with weed growth. Such analysis can identify crop varieties that show strong early interference with weed growth, and traits associated with strong early interference. For illustrative purposes, the model is applied to characterize differences among soybean varieties in interference with common cocklebur. Weed control by crop interference could be enhanced by crop breeding and management. However, several benefits and costs of interference should be evaluated before undertaking such approaches. First, the degree to which crop interference can substitute for other forms of weed control must be determined. Second, yield costs associated with interference should be measured; these may occur in crop varieties that interfere strongly with weeds, particularly when water, temperature, and mineral nutrients are limiting. A more mechanistic understanding of crop interference is needed so that interference can be enhanced through crop breeding and management while minimizing yield costs.

Modeling the Dynamics of Snags.

Abstract: Many wildlife species required standing dead trees (i.e., snags) as part of their habitat. Therefore, the ability to predict future density, distribution, and condition of snags can assist resource managers in making land-use decisions. Here we present methods for modeling the dynamics of snags using data from a 10-yr study on the rates of decay, falling, and recruitment of snags on burned and unburned plots in the Sierra Nevada, California. Snags (all species) in advanced stages of decay usually fell within 5 yr, and snags created by fire decayed rapidly and fell quicker (within 10 yr) than those on unburned plots. Pine (Pinus spp.) snags decayed more rapidly than fir (Abies spp.). Although there was an overall net increase in snag density on unburned plots, most of this increase was in the smaller (>13-38 cm diameter at breast height [dbh]) size classes; there was a net decrease in the larger (>38 cm dbh) snags preferred by many birds for nesting and feeding. Overall, snags remained standing the longest that were larger in diameter, shorter in height, less decayed, fir rather than pine, and lacking tops. A Leslie matrix model of snag dynamics predicted changes in snag decay and density only when adjusted for the specific environmental factors(s) causing initial tree mortality. Many snags are created by episodic events, such as fire, disease, drought, and insects. Models of snag dynamics must include the species and condition of trees becoming snags, as well as the factor(s) causing the tree to die. Forest managers must consider this episodic creation of snags when developing snag-management guidelines, and when planning tree-salvage programs based on short-term inventories.

A model of the responses of ecotones to climate change

Abstract: It has been suggested that global climatic change may be detected by monitoring the positions of ecotones. I built a model of the dynamics of ecotones similar to those found in altitudinal or latitudinal treelines, where a slow tendency for the ecotone to advance is counterbalanced by disturbances such as fire or landslides. The model showed that the response of such ecotones to a wide range of simulated climate changes was slow and that the ecotone front was dissected. It would appear that such ecotones would not make suitable sites for monitoring climate change.

Sampling to Detect Rare Species

Abstract: Often a sampling program has the objective of detecting the presence of one or more species. One night wish to obtain a species list for the habitat, or to detect the presence of a rare and possibly endangered species. How can the sampling effort necessary for the detection of a rare species can be determined? The Poisson and the negative binomial are two possible spatial distributions that could be assumed. The Poisson assumption leads to the simple relationship n = -(1/m)log @b, where n is the number of quadrats needed to detect the presence of a species having density m, with a chance @b (the Type 2 error probability) that the species will not be collected in any of the n quadrats. Even if the animals are not randomly distributed the Poisson distribution will be adequate if the mean density is very low (i.e., the species is rare, which we arbitrarily define as a true mean density of 0.95. Only 8 of the 273 cases represented rare species that failed this requirement. Thus we conclude that a Poisson-based estimate of necessary sample size will generally be adequate and appropriate.

Foraging Ecology of Bison and Cattle on a Mixed Prairie: Implications for Natural Area Management

Abstract: There exists considerable uncertainty whether differences between bison () and cattle (Bos taurus) should be exploited in natural area stewardship. Because foraging ecology may prescribe the fundamental relevance of either herbivore, we describe the foraging of bison and cattle on a northern mixed prairie natural area and discuss management implications. As generalist foragers, bison and cattle exhibited seasonal differences in selection for or against C"3 graminoids, C"4 grasses, forbs, and browse. Forty-eight hour in vitro dry matter digestibilities of C"3 graminoids and C"4 grasses were greater by bison than cattle, but equal digestive efficiencies were exhibited for forbs and browse. Dietary in vitro dry matter digestibility and crude protein differed little between herbivores. Cattle allocated more time to grazing than bison did during summer. Our study suggests that the relationship between feeding-time investment and forage patchiness is important in determining differences in diet choice between bison and cattle. In contrast to cattle, it appears that bison balance nutrient and time demands during the rut by consuming almost exclusively graminoids. Apparently, bison and cattle may achieve similar dietary quality through different foraging behavior. We suggest that specific scale-dependent programmatic conditions exist where either herbivore may be the most appropriate for natural area management.

Grasses and Grazers, Science and Management

Abstract: The evidence that pure ecological science has influenced grazing management practices employed in livestock husbandry on the North American Great Plains is tenuous at best. Those practices arose from critical technological innovations in the 19th century that modified the essential properties of grazing ecosystems when applied to livestock husbandry. Ecological research has identified many significant differences as well as similarities between natural and human—defined grazing systems, and how husbandry can lead to overgrazing. There is considerable evidence that moderate grazing can increase primary productivity and unequivocal evidence that some plants benefit from the presence of grazing animals. See full-text article at JSTOR

An Approach for Managing Vertebrate Diversity Across Multiple-Use Landscapes.

Abstract: There is widespread agreement that biological diversity is valuable and that it is rapidly being lost (Myers 1979, Wilson 1988, Soule 1991). Consequently, the conservation of biodiversity has emerged as a major international issue, and numerous laws, research initiatives, and management strategies have been enacted. Yet, biological diversity continues to decline even in wealthy and technologically advanced countries (Ehrlich and Wilson 1991). The “endangered species” approach of protecting species after they are at risk is insufficient for several reasons (Rohlf 1991, Mann and Plummer 1992). Nature preservation is also failing because reserves are often too few, too small, and too isolated to maintain natural processes and species (Pickett and Thompson 1978, Noss and Harris 1986, Newmark 1987, Hunter 1991).

Canopy Architecture of Natural and Planted Cordgrass Marshes: Selecting Habitat Evaluation Criteria.

Abstract: In order to set standards for restoration or mitigation "success" of salt marsh habitat designed for an endangered southern California bird, I explored the bird's nesting requirements, compared constructed habitats that lack the bird with natural habitats that support it, and selected habitat assessment criteria that best distinguish suitable and un- suitable nesting habitats. Selected attributes of canopy architecture are recommended for assessing the suitability of intertidal cordgrass (Spartina foliosa) marshes for the endangered Light-footed Clapper Rail (Rallus longirostris levipes) of southern California. Nests are built at Z 145 cm above Mean Lower Low Water (MLLW = 0 tidal datum), with nest rims at 165-170 cm MLLW; however, extreme high water is 232 cm MLLW during the March-July nesting season, and tidal inundation is a major cause of nest failure. Cordgrass that is >60 cm tall appears necessary for birds to weave a canopy that allows the nest to float upward, but not away, as the tide rises. Cordgrass height distributions and density data describe attributes of canopy architec- ture that assess Clapper Rail habitat value better than previously used measures (i.e., cover, biomass, mean height, maximum height). Height histograms differ for planted marshes (which do not support Clapper Rails) and natural marshes. The constructed marshes have few plants over 60 cm, while most stems in natural marshes exceed 60 cm. In natural marshes, cordgrass heights increase with freshwater flooding and nitrogen enrichment. Reference data from natural marshes that are used by Clapper Rails indicate that the standard for "suitable habitat" should be a density of at least 100 stems/M2 with at least 90 stems/M2 > 60 cm, of which at least 30 stems/M2 are > 90 cm in height. High interannual and spatial variability indicates the need for several (e.g., 20) years of data for assessment purposes and a large data base for reference wetlands.