Interpreting the results from multiple regression and structural equation models
No abstract available.
Bulletin of the Ecological Society of America↗
Geology topics
James B. Grace
Publications and source records attributed to James B. Grace.
Effects of prescribed fire in the coastal prairies of Texas
Prescribed fire is widely applied for habitat management in coastal ecosystems. Fire management plans typically list a variety of objectives for prescribed burning, including succession management, promotion of native flora and fauna, providing habitat for species of importance, wildfire risk reduction (fuels management), as well as reduction and/or prevention of invasive species. In most cases, the information needed to determine the degree to which management objectives are met is not available. This study sought to provide an assessment of key objectives of fire management at the U.S. Fish and Wildlife Service (USFWS) Texas Mid-coast National Wildlife Refuge Complex. The main purpose of this work was to provide information and recommendations that will support Region 2 of the USFWS in the conduct of their fire and habitat management activities in the Western Gulf coast region. There were four main components of this project: (1) a historical analysis of the role of fire in this ecosystem, (2) the development of standard methodology for assessing and monitoring fire effects in this system, (3) an evaluation of the effects of prescribed burning on the habitat being managed, and (4) an evaluation of the effects of burning on select fauna of special concern. A team of researchers, including some from the U.S. Geological Survey (USGS), Southeast Missouri State University, and Louisiana State University were involved in the various components of this project. Extensive support by USFWS personnel, both at the Texas Mid-coast National Wildlife Refuge Complex and in the Regional Office (Region 2, USFWS), was a key component in this work. Data from the three years of this study were combined with the results of previous USGS studies performed at the site to strengthen our conclusions.
Open-File Report↗
Biodiversity and ecosystem functioning: Synthesis and perspectives
Does diversity matter in restored communities? This edited book explores the diversity–function debate, which has raged in ecology during the past several decades. The diversity–function question is relevant in restoration, but the question has not yet received a straightforward hearing in the field of restoration science. None of the authors of this book explicitly relate their ideas to restoration, but the book can educate restorationists about the most recent developments in the diversity–function debate. It is also an excellent book for students to learn about the recent developments and historical background of the diversity–function debate and can be used for graduate-level seminars in restoration or plant ecology.
Restoration Ecology↗
A Floristic Quality Assessment system for the coastal prairie of Louisiana
Evaluation systems to assess the biotic integrity of plant communities exist for some ecosystems, but not the increasingly rare coastal prairies of Louisiana. A list of plant species occurring in Louisiana's coastal prairie was created and coefficients of conservatism (C) were assigned for each species. A Floristic Quality Index (FQI), which is calculated using the C values provided by a panel of experts, can be used to evaluate prairie remnants and restorations. We assigned C values from 0-10 based on their estimated degree of association with prairies of various levels of natural quality and their tolerance of disturbance. Those species given a rank of 0-3 are deemed to be colonizing species found in a variety of habitats and are adapted to fairly severe disturbance. Species with C values of 4-6 are those that are often common in fairly high quality coastal prairie, occur in other community types and are moderately tolerant of disturbance. Species with rankings of 7-8 are associated with high quality natural prairie habitat and slight disturbance. Those species ranking 9-10 are those restricted to very high-quality habitat and have a high fidelity to coastal prairie. Unlike FQI systems devised for other areas, we also weight the coefficients assigned to nonnative species found in coastal prairie. We believe that the presence of exotic species in a native plant community lowers the conservation value of that community. Consequently, we assigned C values from -1 to -3 to nonnative species based on the perceived threat of their invasive potential and ability to exclude native species. Including the C values of exotic species allows the calculation of an adjusted floral quality index that provides an additional dimension to floristic quality analysis. This index will be of value to restorationists, managers and others involved in assessing the integrity of natural areas and developing management strategies based on these criteria.
Louisiana↗
Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh
1 Submergence of coastal wetlands in Louisiana is currently rapid and widespread. A number of factors contribute to this loss of habitat, including the activities of herbivores. The objective of this study was to examine the effects of large mammals, predominantly nutria and wild boar, on processes controlling soil elevation in coastal marshes. 2 Effects of herbivores on soil and vegetation were assessed by the use of paired fenced and unfenced plots over two successive growing seasons. Above‐ground biomass, litter production, changes in soil elevation, vertical soil accretion, shallow subsidence, below‐ground production of roots and rhizomes, the thickness of the root zone, soil bulk density, and soil organic matter were measured. 3 Above‐ground biomass, below‐ground production, soil elevation and the expansion of the root zone decreased due to herbivore activity. Litter production, the rate of soil surface accretion and shallow soil subsidence were all higher in grazed compared to ungrazed plots, while soil organic matter and bulk density did not differ significantly between treatments. 4 The results indicate that herbivores can have a negative effect on soil building processes, primarily by reducing below‐ground production and expansion of the root zone. Where natural rates of mineral sediment deposition are high, coastal marshes are expected to persist, despite herbivore activities. However, where sediment inputs are substantially less, herbivores may lead to destruction of habitat.
Louisiana↗
The importance of competition in regulating plant species abundance along a salinity gradient
Current theories differ in their predictions concerning the effects of interspecific interactions on species growth and distribution along environmental gradients. In this study, we examined the influence of competition on species composition across a salinity gradient. This work involved three common fresh and brackish marsh species. The three species, Spartina patens, Sagittaria lancifolia, and Panicum hemitomon, differ in salt tolerances but are commonly found in overlapping zones across coastal marsh gradients in the southeastern United States. Plants were grown in a greenhouse under four salinity treatments (0, 2, 4, and 8 g/kg), in monocultures and in three‐species mixtures. Data from the monocultures and mixtures were used to examine the importance of competition in regulating community composition and species' growth. Results for individual species were significantly different when grown in monoculture vs. the three‐species mixture. While with increasing salinity community dominance shifted toward the most salt‐tolerant species, Spartina patens, competition was found to alter community composition equally over all salinities. However, the responses of individual species demonstrated a very different pattern; the relative importance of competition in influencing individual growth differed substantially depending on the salinity tolerances of species. These results support the contention that, as abiotic stress increases, competition becomes less of a limiting factor in regulating the abundance of a species. However, the role of competition at the community level is the sum of the individual species' responses and may or may not vary with the level of abiotic stress.
Ecology↗
Interactions between fire and invasive plants in temperate grasslands of North America
A substantial number of invasive grasses, forbs and woody plants have invaded temperate grasslands in North America. Among the invading species are winter annuals, biennials, cool-season perennials, warm-season perennials, vines, shrubs, and trees. Many of these species have been deliberately introduced and widely planted; some are still used for range improvement, pastures, lawns, and as ornamentals, though many are listed as state or federal noxious weeds. Others have been greatly facilitated by widespread land disturbance. Historically, fire has been a major selective force in the evolution of temperate grasslands. Further, prescribed fire is commonly used as a method of ecological management for native grassland communities as well as in conjunction with restoration efforts. Within this context, it is important to understand how invasive species will interact with natural and prescribed fire regimes. In this paper, we consider what is known about how exotic species that invade temperate grasslands relate to fire. The primary issues addressed for each species are (1) Does fire appear to enhance colonization? (2) To what degree does fire affect the survival of plants? (3) Are plants that are burned able to regrow following fire and, if so, how rapidly can they recover? (4) How important is competition with native species to the response to fire? and (5) What effect does an invasive species have on the characteristics of the fire regime? For many species, results are preliminary, incomplete, or inconsistent among studies. For this reason, many of the conclusions drawn for individual species must be considered preliminary. Based on analyses of individual species, a conceptual framework is presented for considering how invasive plants may interact with fire when they invade an ecosystem. The major categories of influences are the native community, the fire regime, growth conditions for both invasive and native species, and influences that disturbances, human impacts, and landscape characteristics have had in the past and will have in the future. The examples considered in this paper provide support for a few, tentative generalizations. First, among our current worst invaders of temperate grasslands, adaptation to fire is quite variable. Some species are not well adapted to burning and can be easily eliminated; other species are better adapted but can still be eliminated if fire occurs during periods of particular vulnerability and/or at high frequency. There is a set of species that is extremely well adapted to fire and will not be eliminated through burning alone. Second, competitive interactions with native species play a crucial role in the success of nonnative invaders. In cases where differential burn responses between invasive and native species can be exploited, and adequate populations of native dominant species are present, fire can sometimes tip the competitive balance away from invasives. Third, there are a few invasive species that have exceptional attributes and for which there are no easy solutions. The ability of cheatgrass ( Bromus tectorum ) to enhance fire, the ability of Chinese tallow ( Triadica sebifera ) to suppress fire, and the ability of leafy spurge ( Euphorbia esula ) to resprout from repeated injury make these species exceptional threats to native diversity. Finally, the available information for many invasive species is very incomplete, partic- ularly with regard to how fire affects competitive interactions with the native community. There is much more we need to know if we are to consistently predict how invasive species will respond to fire and how burning can best be used to manage for natural diversity.
Conference Paper↗
Factors associated with plant species richness in a coastal tall-grass prairie
In this study we examine the factors associated with variations in species richness within a remnant tall-grass prairie in order to gain insight into the relative importance of controlling variables. The study area was a small, isolated prairie surrounded by wetlands and located within the coastal prairie region, which occurs along the northwestern Gulf of Mexico coastal plain. Samples were taken along three transects that spanned the prairie. Parameters measured included micro-elevation, soil characteristics, indications of recent disturbance, above-ground biomass (including litter), light penetration through the plant canopy, and species richness. Species richness was found to correlate with micro-elevation, certain soil parameters, and light penetration through the canopy, but not with above-ground biomass. Structural equation analysis was used to assess the direct and indirect effects of micro-elevation, soil properties, disturbance, and indicators of plant abundance on species richness. The results of this analysis showed that observed variations in species richness were primarily associated with variations in environmental effects (from soil and microtopography) and were largely unrelated to variations in measures of plant abundance (biomass and light penetration). These findings suggest that observed variations in species richness in this system primarily resulted from environmental effects on the species pool. These results fit with a growing body of information that suggests that environmental effects on species richness are of widespread importance.
Journal of Vegetation Science↗
Vegetation associations in a rare community type - Coastal tallgrass prairie
The coastal prairie ecoregion is located along the northwestern coastal plain of the Gulf of Mexico in North America. Because of agricultural and urban development, less than 1% of the original 3.4 million ha of this ecosystem type remains in native condition, making it one of the most endangered ecosystems in North America. The objective of this study was to characterize the vegetation and environmental relationships in a relatively pristine example of lowland coastal prairie in order to provide information for use in conservation and restoration. The study area was a small, isolated prairie located near the southern boundary of the coastal prairie region. Samples were taken along three parallel transects that spanned the prairie. Parameters measured included species composition, elevation, soil characteristics, indications of recent disturbance, above-ground biomass, and light penetration through the plant canopy. Fifty-four species were found in the 107 0.25-m2 plots and a total of 96 species were found at the site. Only two non-native species occurred in sample plots, both of which were uncommon. Cluster analysis was used to identify six vegetation groups, which were primarily dominated by members of the Poaceae or Asteraceae. A conspicuous, natural edaphic feature of the prairie was the presence of 'mima' mounds, which are raised areas approximately 0.5 to 1 m high and 5 to 10 m across. Indicator species analysis revealed a significant number of species that were largely restricted to mounds and these were predominately upland and colonizing species. Ordination was performed using nonmetric, multidimensional scaling. The dominant environmental influence on species composition was found to be elevation and a host of correlated factors including those associated with soil organic content. A secondary group of factors, consisting primarily of soil cations, was found to explain additional variance among plots. Overall, this prairie was found to contain plant associations that are now rare in the surrounding landscape. Within the prairie, plant groups were largely separated by a suite of environmental conditions associated with topography. These results suggest that conservation and restoration efforts will need to carefully consider local topographic influences in order to be successful.
Plant Ecology↗
Fire in eastern ecosystems
Prior to Euro-American settlement, fire was a ubiquitous force across most of the Eastern United States. Fire regimes spanned a time-scale from chronic to centuries. Fire severity varied from benign to extreme (fig. 1-2). Today, fire is still a major force on the landscape. In some ecosystems fire stabilizes succession at a particular sere, while in others, succession is set back to pioneer species. The wide range in fire regimes coupled with elevation and moisture gradients produce a myriad of plant communities that continually change over time in both stature and composition, although it is not uncommon for the major species to remain dominant. Discussion is primarily about major vegetation types, for example, oak-hickory. However, some minor types such as spruce-fir and Table Mountain pine are also covered. Vegetation types are discussed under the most representative fire regime type, recognizing that some vegetation types overlap two fire regime types (table 4-1).
General Technical Report↗
The relationship between species density and community biomass in grazed and ungrazed coastal meadows
Previous studies have indicated that the relationship between community biomass and species density can be represented by a multivariate model in which abiotic variables influence species density both through effects on biomass and through effects on the species pool. In this paper, we use data from grazed and ungrazed coastal meadows in Finland to evaluate and extend this general conceptual model of the factors controlling species density. Structural equation analysis was used to evaluate a model for all meadows and then to perform a multigroup analysis to determine how grazed and ungrazed meadows differ. By itself, biomass could explain only 12% of the variation in species density while the multivariate model was able to explain 47% using five types of predictor variables: site, soil, flooding, grazing, and biomass. Analyses found that flooding explained the greatest amount of variability in species density, primarily through negative effects on the species pool. Grazing was also found to have a strong effect on species density and results suggest that its negative influence may be largely through reductions in the species pool in grazed meadows. The most important difference found between grazed and ungrazed meadows was that species density had a strong negative relationship to biomass in the ungrazed meadows but no significant relationship in the grazed ones. Thus, it appears that the influence of competition on species density was much greater in ungrazed meadows compared to grazed ones.
Oikos↗
Can prescribed fire save the endangered coastal prairie ecosystem from Chinese Tallow invasion?
No abstract available.
Louisiana, Texas↗
Factors influencing cattail abundance in the northern Everglades
Since the early 1900s, the Everglades have been influenced by anthropogenic actions including altered hydrology and increased nutrient loading. In the northern Everglades an apparent effect of these disturbances has been the development and proliferation of dense cattail ( Typha spp. ) stands in areas previously dominated by sawgrass ( Cladium jamaicense Crantz) and sloughs. Cattail cover, soil nutrient concentrations, topography and fire history were determined for the Holey Land and Rotenberger Wildlife Management Areas, located in the northern Everglades. These data were analyzed using multiple regression to assess the relative influence of fire, hydrology and soil nutrients on cattail abundance. Holey Land and Rotenberger were overdrained over recent decades which resulted in soil compaction and nutrient accumulation, illustrated by increased soil bulk densities and elevated nutrient storage. Average bulk densities were 0.13 g cm −3 for Holey Land and 0.22 g cm −3 for Rotenberger. Average total P (TP) stored in the surface 10 cm of soil in Holey Land and Rotenberger were 7 and 13 g m −2 , respectively. In contrast, Everglades soils uninfluenced by nutrient enrichment and with less severe overdrainage have bulk densities of 0.07 g cm −3 and TP storage of 4 g m −2 . Typically, elevated soil P concentrations have been considered a primary factor influencing cattail growth and distribution in the Everglades. With the apparent absence of P limitation in Holey Land and Rotenberger, cattail abundance was influenced by either fire or hydrology. Forty-six percent of the variation of cattail cover in Holey Land was explained by elevation, indicating that increased water depth and duration of flooding have a significant impact on cattail expansion. In Rotenberger, fire was the most influential factor, explaining 57% of the variation in cattail cover. Hydrology was the second most important factor limiting cattail abundance.
Florida↗
On the use of path analysis and related procedures for the investigation of ecological problems
No abstract available.
American Naturalist↗
Can prescribed fire save the endangered coastal prairie ecosystem from the Chinese Tallow invasion?
No abstract available.
Louisiana, Texas↗