Affect Community Composition Research Articles

Forest Models Defined by Field Measurements: Estimation, Error Analysis and Dynamics

A spatial and mechanistic model is developed for the dynamics of transition oak—northern hardwoods forests in northeastern North America. The purpose of the model is to extrapolate from measurable fine—scale and short—term interactions among individual trees to large—scale and long—term dynamics of forest communities. Field methods, statistical estimators, and model structure were designed simultaneously to ensure that parameters could be estimated from data collected in the field. This paper documents eight aspects of a three—year study to calibrate, test, and analyze the model for the nine dominant and subdominant tree species in transition oak—northern hardwoods forests: 1) Design and structure of the model. The model makes population dynamic forecasts by predicting the fate of every individual tree throughout its life. Species—specific functions predict each tree's dispersal, establishment, growth, mortality, and fecundity. Trees occupy unique spatial positions, and individual performance is affected by the local availability of resources. Competition is mechanistic; resources available to each tree are reduced by neighbors. Although the model was developed to include light, water, and nitrogen, the version described here includes only competition for light (shading and light—dependent performance) because the field data provide little evidence of competition for nitrogen and water over the range of sites examined. 2) Estimates of the model's parameters for each species. The estimates reveal a variety of "strategic trade—offs" among the species. For example, species that grow quickly under high light tend to cast relatively little shade, have low survivorship under low light, and have high dispersal. In contrast, species that grow slowly under high light tend to cast relatively dark shade, and to have high survivorship under low light and low dispersal. These trade—offs define one of two dominant "axes" of strategic variation. 3) Community level predictions of the model. The model predicts succession from early dominance by species such as Quercus rubra and Prunus serotina, to late dominance by Fagus grandifolia and Tsuga canadensis, with Betula alleganiensis present as a gap phase species in old—growth stands. The model also predicts that old—growth communities will have intraspecifically clumped and interspecifically segregated spatial distributions. 4) An error analysis that identifies community level predictions that are robust given the level of sampling uncertainty in the study. This analysis translates the statistical uncertainty associated with each parameter estimate into statistical uncertainty in the model's predictions. The robust predictions include those mentioned in aspect (3) above. 5) Sensitivity of the model to changes in initial conditions and to changes in the three parameters not included in the error analysis. For example, the model predicts that initial abundances continue to affect community composition well into succession (> 300 yr for some species). 6) Tests of the system— and community—level predictions of the model against independent data gleaned from other studies. These tests support the predictions found to be robust in the error analysis, including those predictions mentioned in aspect (3) above. 7) Modeling experiments that determine which aspects of individual performance and inter—neighbor competition are responsible for each of the robust predictions identified in aspect (4) above and tested in aspect (6) above. This analysis reveals a wide variety of causal relationships, with most parameters contributing to at least one community level phenomenon. 8) An explanation of the diversity of individual level causes identified in aspect (7). The two "axes" describing most of the strategic variation among the species (see [2]), provide a simple explanation of community level pattern in terms of individual level processes.

Effects of Urine Deposition on Small‐Scale Patch Structure in Prairie Vegetation

Large grazing mammals contribute to the species diversity of grasslands via direct and indirect effects of defoliation and urine deposition. We examined the influence of one and two applications of simulated bovine urine on vegetation structure on (1) tallgrass prairie burned either every 2 or 4 yr at Konza Prairie Research Natural Area (KPRNA), Kansas, and (2) unburned sandhills prairie at the Niobrara Valley Preserve, Nebraska. We also examined the influence of urine and the clipping of graminoids on an annually burned site at KPRNA. Plant abundance in general increased on urine patches but the response appeared dependent on litter accumulation. C4 grasses increased at the annual burn and Niobrara sites where litter levels were low. C3 forbs increased at the 2— and 4—yr burn sites where litter levels were high. Urine treatment significantly affected community composition at all but the 2—yr burn site. Alpha—diversity decreased on urine patches at the annual and 4—yr burn sites but increased on urine patches at the Niobrara site. Beta—diversity increased on urine patches at the annual burn and Niobrara site but decreased on urine patches at the 4—yr burn site. The clipping of graminoids at the annual burn site reduced both α— and β—diversity and graminoid abundance while forb abundance was not affected.

Heterotrophic denitrification potential as an adaptive response in groundwater bacteria

The denitrification rate of bacteria indigenous to aquifers with different nitrate concentrations was investigated. The objective was to test how high nitrate loads affect community composition and select for enhanced nitrate reduction rates among denitrifying strains. Groundwater bacteria were isolated from two aquifers with high in situ concentrations of nitrate (24.1 and 35.2 mg NO 3 −-N 1 −1) and one aquifer with a lower in situ concentration (6.3 mg NO 3 −-N 1 −1). The isolates were characterized with respect to 15 colony and cell morphological and 8 physiological features. The frequency of heterotrophic denitrifiers of all isolated strains was 45–50% in the nitrate contaminated aquifers compared to slightly more than 20% in the uncontaminated aquifer. Kinetics of nitrate reduction were measured by using progress curves ot nitrous oxide production. Bacterial respiration was measured to determine whether proposed differences in denitrification rates were due to differences in carbon utilization. Strains from the aquifers with enhanced nitrate concentrations showed shorter lag periods and higher growth rates than strains from the aquifer with low nitrate concentration. Denitrification rates for unamended incubations varied between 0.10 and 0.42 ml cells −1 day −1 and between 0.42 and 2.10 ml cells −1 day −1 for the glucose amended incubations. The average denitrification rate for the strains from the contaminated sites was 2-fold higher than for the strains from the uncontaminated site, and glucose amendment increased the difference to 2.5-fold. The differences in rates were not explained by differences in carbon utilization. The strains from the contaminated sites reduced more nitrate per unit of carbon dioxide produced than the strains from the uncontaminated aquifer. This suggests that strains in soils which have been heavily N-fertilized may have developed denitrification reductases with optimum activities at high in situ concentrations of nitrate.

Heterotrophic denitrification potential as an adaptive response in groundwater bacteria

The denitrification rate of bacteria indigenous to aquifers with different nitrate concentrations was investigated. The objective was to test how high nitrate loads affect community composition and select for enhanced nitrate reduction rates among denitrifying strains. Groundwater bacteria were isolated from two aquifers with high in situ concentrations of nitrate (24.1 and 35.2 mg NO3−-N 1−1) and one aquifer with a lower in situ concentration (6.3 mg NO3−-N 1−1). The isolates were characterized with respect to 15 colony and cell morphological and 8 physiological features. The frequency of heterotrophic denitrifiers of all isolated strains was 45–50% in the nitrate contaminated aquifers compared to slightly more than 20% in the uncontaminated aquifer. Kinetics of nitrate reduction were measured by using progress curves ot nitrous oxide production. Bacterial respiration was measured to determine whether proposed differences in denitrification rates were due to differences in carbon utilization. Strains from the aquifers with enhanced nitrate concentrations showed shorter lag periods and higher growth rates than strains from the aquifer with low nitrate concentration. Denitrification rates for unamended incubations varied between 0.10 and 0.42 ml cells−1 day−1 and between 0.42 and 2.10 ml cells−1 day−1 for the glucose amended incubations. The average denitrification rate for the strains from the contaminated sites was 2-fold higher than for the strains from the uncontaminated site, and glucose amendment increased the difference to 2.5-fold. The differences in rates were not explained by differences in carbon utilization. The strains from the contaminated sites reduced more nitrate per unit of carbon dioxide produced than the strains from the uncontaminated aquifer. This suggests that strains in soils which have been heavily N-fertilized may have developed denitrification reductases with optimum activities at high in situ concentrations of nitrate.

Seedling Recruitment in Forests: Calibrating Models to Predict Patterns of Tree Seedling Dispersion

Recruitment, the addition of new individuals into a community, is an important factor that can substantially affect community composition and dynamics. We present a method for calibrating spatial models of plant recruitment that does not require identifying the specific parent of each recruit. This method calibrates seedling recruitment functions by comparing tree seedling distributions with adult distributions via a maximum likelihood analysis. The models obtained from this method can then be used to predict the spatial distributions of seedlings from adult distributions. We calibrated recruitment functions for 10 tree species characteristics of transition oak—northern hardwood forests. Significant differences were found in recruit abundances and spatial distributions. Predicted seedling recruitment limitation for test stands varied substantially between species, with little recruitment limitation for some species and strong recruitment limitation for others. Recruitment was limited due to low overall recruit production or to restricted recruit dispersion. When these seedlings recruitment parameters were incorporated into a spatial, individual—based model of forest dynamics, called SORTIE, alterations of recruitment parameters produced substantial changes in species abundance, providing additional support for the potential importance of seedling recruitment processes in community structure and dynamics.

The Response of Temperate Tree Seedlings Grown in Elevated CO"2 to Extreme Temperature Events

Mean global temperatures have been predicted to increase in the next century; if so the frequency of extreme temperature events may also increase. Extreme temperatures may damage plant tissue and consequently limit the survival of certain plant species in a region. Elevated concentrations of CO2 in the atmosphere alter plant allocation, physiology, and growth, and may accentuate or ameliorate the damage from extreme temperatures. In this paper we explore the interactive effects of atmospheric CO2 concentration, nutrient levels, and exposure to extreme temperatures on seedlings of three species of temperate deciduous trees. A 1—d exposure to extreme heat (45°C) significantly decreased conductance the following day and decreased biomass as measured at both 35 and 105 d following the extreme temperature event, regardless of atmospheric CO2 concentration. The most shade—tolerant species, striped maple, was most severely impacted by the extreme heat event in both CO2 environments. Furthermore, striped maple seedlings grown in elevated CO2 concentrations had a significantly greater decrease in biomass due to the extreme heat event as compared with striped maple plants grown in ambient CO2 concentrations at 35 d after the heat event; however, at the end of the growing season at 105 d post treatment, this difference was not significant. A one—night exposure to low temperatures (4°C) did not affect biomass for any of these species. With an increase in global mean temperatures, the frequency of extreme temperature events, particularly hot weather events, may increase and may extend to shaded understory sites. If the frequency of extremely high temperatures increases, the role that temperature extremes may play in changing competitive interactions and thus affecting community composition may increase in importance, as these temperatures appear to severely alter plant survival and growth in some species.

Pattern of species dominance and factors affecting community composition in areas deforested due to air pollution

Vegetation of mountain areas affected by SO2 pollution (Krusne hory Mts., Czech Republic) was analysed using multivariate methods. Communities with prevailing species Calamagrostis villosa, a rhizomatous grass expanding into deforested sites, were sampled by Braun-Blanquet releve method. Canonical correspondence analysis was used to assess the effect of environmental variables (soil removal, deforestation, and shading). To test the effects of light, moisture, soil acidity and nitrogen, mean sample indicator values were correlated a posteriori with sample axes on ordination scores. Light, soil acidity, moisture, and site history (in terms of past deforestation and soil removal applied in reclamation procedures) were found to be the main factors responsible for the community composition. Nitrogen level had not a significant effect on the community composition. When analyzing the whole data set, i.e. including also remnants of natural spruce forests, light was the factor affecting at most the composition of communities. Within the bare spot vegetation, if treated separately, the highest variation was found along the soil acidity/moisture gradient. The effect of soil removal was only obvious at early successional stages. Species diversity increased with moisture and decreased with soil acidity. Species exhibiting S- and/or R-strategy are successful on extremely acid soils whereas forbs present in bare spots appear to be supported by disturbances.

Shorebird Communities on British Estuaries: Factors Affecting Community Composition

Environmental gradients across 109 British estuaries were examined in relation to the communities of shorebirds in winter. Ordination and classification techniques were employed on the 5-year means of peak-winter high-tide counts for the 13 most common species, converted to densities, for the two periods, 1969-75 (first period) and 1981 -85 (second period), in order to quantify changes in community composition between the two periods. Multiple regression within GLIM was then used to relate the DCA ordination scores to physical, climatic, geographic and water chemistry variables. In an ordination analysis of the combined data for the two periods, axis 1 had a left-right gradient of species favouring coastal lagoons (avocet) to those favouring sandy estuaries (bar-tailed godwit, sanderling, knot)

Influence of Grazing History on the Community Structure of Grasshoppers of a Mixed-Grass Prairie

A study was conducted to determine if grazing history affected community composition of plants and grasshoppers by comparing communities in an undisturbed grassland that has not been grazed by livestock for approximately 60 yr (Badlands National Park, South Dakota) with communities on adjacent grasslands that are managed for grazing (Buffalo Gap National Grasslands). Grasshoppers were sampled with sweep nets in August 1986 at 29 sites within the Badlands National Park and Buffalo Gap National Grasslands. Concurrently, plant communities were sampled at the sites to determine species richness of grasses and percentage of coverage of all grass species, forbs, sedges, and bare ground. Detrended correspondence analysis and Spearman correlation analysis of insect abundance and incidence data were used to assess spatial changes in grasshopper community structure and to relate vegetation variables to grasshopper community composition. Results indicated that grasshopper communities changed along habitat gradients defined by grazing history and coverage and species richness of grasses. Sites within the undisturbed grassland habitat had similar community compositions of grasshoppers and were dominated by mixed forb- and grass-feeding species. Grazed sites supported higher populations of obligate grass-feeding grasshoppers. Differences in grasshopper communities between the grazed and undisturbed sites were, in part, due to differences in plant community composition; grazed sites were dominated by short grasses, whereas ungrazed sites were composed mainly of tallgrass species. In general, the mixed forb- and grass-feeding species of grasshoppers increased along vegetation gradients defined by increasing coverage of Agropyron smithii Rydb., Bromus spp., Poa spp., Stipa viridula Trin., and Carex spp., and decreasing coverage of bare ground and Bouteloua gracilis (H.B.K.) Lag. ex Steud. Communities dominated by obligate grass-feeding species were associated with high coverage of bare ground and B. gracilis . Grasshopper species richness was greatest at sites with high richness of grasses and total coverage of grasses and sedges.

Gradient analysis of forest vegetation in the Maungataniwha Range, western Northland, New Zealand

Abstract Phytosociological data from 51 forest study plots in the Maungataniwha Range, New Zealand, were analysed by numerical techniques including ordination, classification, and direct gradient analysis to identify vegetation/ environmental patterns. Nine community types segregated by specific ranges of environmental conditions were identified. Interpretations of ordination axes suggest temperature and moisture are prime factors affecting community composition. Gradient analysis is seen as an important step in evaluating the representativeness of existing and potential nature reserves. Previously, the techniques have been shown to be useful in an area where vegetation/environmental patterns are obvious. The present study demonstrates the utility where patterns are not obvious.