Browsing by Author "Scrivani, John A."

Now showing 1 - 19 of 19
  • Thumbnail Image
    An analysis of correlated curve trend experiments in Eucalyptus grandis
    Bredenkamp, Brian Victor (Virginia Polytechnic Institute and State University, 1988)
    Correlated curve trend (C.C.T.) experiments in Eucalyptus grandis on the Zululand coast of South Africa were analyzed. Growth parameters were described as functions of age using Schnute’s generalized growth function and parameter estimates were described as functions of stand density. Growth attributes were used as moments of a probability density function to describe a diameter distribution model for the species. Time trends in the relationships between growth parameters and stand density were scrutinized with multiple comparisons of paired means. It was shown that diameter growth in lower size classes ceases under conditions of extreme suppression while growth continues unabated in the larger size classes, resulting in greater dispersion in diameter. Competition mortality was to a large extent confined to the lower size classes and severe mortality results in an apparent increase in mean diameter which precludes use of growth functions which impose an asymptote. Allometric growth was investigated on two different sites and growth trends were shown to be anamorphic between sites. This permits a ratio approach to the estimation of growth and yield on one site based on experimental evidence from another. Thinning effects in terms of diameter and height changes were estimated from simulated thinnings using data from unthinned stands while the results of long-term thinning studies were compared in terms of cumulative volume yields. The age at which mean annual increment culminates was determined and a model for the estimation of m.a.i. as a function of age and stand density was constructed. A critical examination of spacing indices revealed that the slopes thereof were much steeper than those for many other species. The better-known indices of Reineke and Yoda were found to be dependent on age.
  • Thumbnail Image
    The association and transmission of Leptographium procerum (Kendr.) wing., by root feeding insects in Christmas tree plantations
    Nevill, Ralph John Leslie (Virginia Tech, 1990)
    Procerum root disease (PRD), caused by Leptographium procerum (Kendr.) Wingf., is the most serious problem facing Christmas tree growers of eastern white pine, (Pinus strobus L.). Limited studies have shown an association between PRD affected trees and insect infestations, and L. procerum has been recovered from field collected insects. The objectives of this study were to demonstrate the association of L. procerum with the life cycle of potential insect vectors and determine if the insect associates could transmit the fungus to healthy trees. To study the association of PRD with potential insect vectors, PRD symptomatic trees from 4 Christmas tree plantations were excavated and examined monthly, June - September in 1988 and 1989, and April - September 1990. Potential insect vectors were collected weekly in baited pit-fall traps placed in: 1) paired plots placed in asymptomatic and symptomatic areas of PRD symptomatic plantations, 2) plots in plantations where PRD was absent, 3) plots in the headlands of plantations, 4) plots in forested areas and 5) one plot in an urban setting. Trees in the plots were also inspected for evidence of weevil feeding and for development of PRD. Larvae of two weevil species, Hylobius pales (Herbst.) and Pissodes nemorensis Germ., were recovered from 52, 42, and 43% of PRD symptomatic eastern white pine in 1988, 1989, and 1990, respectively. Hylobius pales and P. nemorensis contaminated with L. procerum were recovered from all plots. The proportion of H. pales contaminated with L. procerum was 73.0% in 1988, 86.5% in 1989 and 72.9% in 1990 while the proportion of P. nemorensis contaminated with the fungus was 17.8, 21.2 and 14.2% in 1988, 1989 and 1990, respectively. Over the three year period of the study, the proportion of PRD infected trees in the symptomatic paired plots rose from 3.6 to 29%. None of the trees in the asymptomatic plots became symptomatic. Transmission of L. procerum was determined by caging field collected and artificially infested H. pales and P. nemorensis on eastern white pine seedlings for 24 hours. To determine if transmission of the fungus during oviposition leads to contamination of the brood,field collected H. pales adults were allowed to feed and oviposit on fresh white pineee bolts. Feeding by artificially infested H. pales adults resulted in transmission of L. procerum 90 and 98% of eastern white pine seedlings in 1989 and 1990, respectively. Field collected H. pales adults transmitted the fungus to 58 and 68% of seedlings in 1989 and 1990, respectively. Artificially infested and field collected P. nemorensis adults transmitted L. procerum to 100 and 28% of the seedlings respectively. All bolts oviposited on by field collected H. pales became colonized by L. procerum and 100% of the weevils that emerged from them were contaminated with the fungus. The results from this study confirms the rules for insect transmission of a plant pathogen.
  • Thumbnail Image
    A comparison of potential agricultural and forestry investment returns for Virginia's marginal lands
    Deaton, Stuart A. (Virginia Tech, 1988-05-05)
    In the past five years, most agricultural producers in the United States have suffered from depressed conditions in the commodity markets. The supplies of basic food commodities have burgeoned, demand has fallen, and price levels have declined, despite $55 billion in U.S. Department of Agriculture (USDA) commodity support program expenditures since 1983 (FAPRI, 1988). Since the early 1980's, farm income and asset balances have declined and debt has risen, increasing the financial stress for many producers. The financial strain could be more acute for those operators with a significant proportion of marginal land in production. Conversely, the USDA Forest Service recently projected that during the next 30-to-40 years softwood forest product demand will increase, available supplies will decrease, and real price levels will increase (USDA Forest Service, 1987). These conditions create the option of converting marginal agricultural lands to forestry investments. A technique was developed to compare possible financial returns between prevalent cropping systems and forestry investments on marginal soil series throughout Virginia. Crops and tree species used in the study include the following: corn (Mays L.), soybeans (Glycine max), soft red winter wheat (Triticum L.), orchard grass (Festuca L.), clover (Trifolium L.), fescue (Festuca L.) pasture, and loblolly pine (Pinus taeda L.), and eastern white pine (Pinus strobus). With state-of-the-art management regimen for agricultural production, annual profit or loss estimates were computed for a variety of soil productivity classes and market price levels. Intensive management was also prescribed for the forestry investment analysis. Using current inputs and projected market price levels, a cash flow analysis program computed equal annual equivalent (EAE's) values for the investment analyses to derive results comparable to those for agriculture. Under the "base" assumptions and current and foreseen markets, forestry investment was determined to be competitive with agricultural production on the marginal soil series. However, government subsidies, benefiting both forest products and agricultural production, complicate the results. Further, the dependency of both markets on macroeconomic and other exogenous variables precludes any guarantee of investment performance over the 35-to-45 year investment horizon for either alternative. The study does provide a foundation for financial comparison, to which a landowner might add individual, subjective evaluations of land use (and financial criteria and assumptions) to reach a decision about the utilization of marginal agricultural lands.
  • Thumbnail Image
    Development of a taper equation for Pinus oocarpa Schiede in natural stands of central Honduras
    Regalado, Darlin Noe Perez (Virginia Tech, 1988-06-05)
    Nine taper equations were tested to predict diameters inside bark along the stem for Pinus oocarpa Schiede trees growing in natural stands of central Honduras. A five parameter submodel predicted as well as an eight parameter model proposed by Kozak, 1988. Taper variation was explored between two geographic regions from which trees with different taper were suspected. Results showed that different site classes, not fully accounted for in the model, might have an effect on the prediction of taper in each region. Also, the effect of crown class and live crown ratio on prediction was evaluated. The model selected exhibited different prediction patterns for dominant and suppressed trees. On the other hand, live crown ratio did not appear to affect prediction. A computer program was written to use the taper equation developed to compute total and merchantable volume to different top diameter limits.
  • Thumbnail Image
    Dominance/Suppression Competitive Relationships in Loblolly Pine (Pinus taeda L.) Plantations
    Dyer, Michael E. (Virginia Tech, 1997-11-13)
    Data from three long-term field studies with loblolly pine (Pinus taeda L.) plantations were used to examine inequality (Gini coefficient) trends in diameter and the relationship between diameter relative growth rate (r) and initial size. Analysis with two spacing studies shows inequality increases with increasing density. For a given initial density, inequality initially decreases and then begins to increase as trees compete for resources. The slope of the linear relationship between r and relative size also increases with increasing density. The slope is initially negative and switches to positive as competition intensifies. The switch in the slope of the r/size relationship occurs when the crown projection area exceeds 1.05 or when the crown ratio falls below 0.75. These results are consistent with the resource pre-emptive or dominance/suppression theory of intra-specific competition. The r/size trends are not evident when calculations are based on class means as opposed to individual trees. The slope of the r/size relationship is a function of stand height, density, and to a lesser extent, site quality. Density reduction through mid-rotation thinning tends to decrease the slope coefficient. The r/size trends are used to develop a disaggregation model to distribute stand-level basal area growth over an initial tree list. This approach compares well with two other disaggregation models but tends to over predict growth on the largest trees.
  • Thumbnail Image
    Economic comparisons of thinning from above and below in Loblolly Pine plantations using dynamic programming
    Arthaud, Greg John (Virginia Tech, 1986-06-15)
    Thinning from above and below were compared using an economic optimizing dynamic program, FORTE (Arthaud 1986). Economically optimal (net present value maximizing) thinning regime and rotation age were determined for benchmark economic and model inputs. Sensitivity of net present value and optimal management regime were tested for varying interest rates (6 or 8%), site indexes (50, 60 and 70, base 25 years), fixed and variable thinning costs, planting density (440, 680 and 910 trees per acre), stumpage prices and thinning type. Given the same assumptions, thinning from below consistently provided the higher net present value for the optimal regime than thinning from above. For the benchmark assumptions, both thinning types had two thinnings in their optimal regimes. Optimal rotation age and thinning timings occur later when thinning from above. Both thinning types provided higher net present values than not thinning under all conditions except pulpwood management.
  • Thumbnail Image
    Effect of manual digitizing error on the accuracy and precision of polygon area and line length
    Keefer, Brenton Jan (Virginia Tech, 1988-11-05)
    Manual digitizing has been recognized by investigators as a significant source of map error in GIS, but the error characteristics have not been well defined. This thesis presents a methodology for simulating manual digitizing error. Stream mode digitizing error was modeled using autoregressive moving average (ARMA) procedures, and point mode digitizing was stochastically simulated using an uniform random model. These models were developed based on quantification of digitizing error collected from several operators. The resulting models were used to evaluate the effect digitizing error had upon polygon size and total line length at varying map accuracy standards. Digitizing error produced no bias in polygon area. The standard deviation of polygon area doubled as the accuracy standard bandwidth doubled, but the standard deviation was always less than 1.6 percent of total area for stream mode digitizing. Smaller polygons (less than 10 square map inches) had more bias and more variance relative to their size than larger polygons. A doubling of the accuracy standard bandwidth caused a quadrupling of line length bias and a doubling to tripling of the line length standard deviation. For stream mode digitizing, reasonable digitizing standards produced line length biases of less than 2 percent of total length and standard deviations of less than 1 percent of total length. Bias and standard deviation both increased with increasing line length (or number of points), but the bias and standard deviation as a percent of total line length remained constant as feature size changed.
  • Thumbnail Image
    Effects of Mid-Rotation Release on Forest Structure, Wildlife Habitat, and Pine Yield
    Cheynet, Kyla Ingeborg (Virginia Tech, 1999-11-30)
    The effects of two forms of mid-rotation release on thinned, fertilized loblolly pine (Pinus taeda L.) plantations in Virginia were examined: aerial imazapyr and basal triclopyr application. Imazapyr measurement plots were installed in nine Piedmont and twelve Coastal Plain plantations operationally released with imazapyr, and triclopyr measurement plots were installed within a controlled fertilization/release study spanning both regions. No differences in volume were detected following triclopyr release. All release dates combined, Piedmont released areas averaged 0.06 m3/tree (18%) greater than the control and Coastal Plain released areas averaged 0.05 m3/tree (14%) greater than the control. Reductions in hardwood basal area, stem density, and shrub stratum cover were observed for both forms of release. Reductions in shrub stratum richness and diversity were also documented for imazapyr release; however, trends indicate that richness and diversity, as well as stem density and shrub stratum cover, may recover to pre-treatment levels. Herbaceous vegetation was increased on triclopyr sites, which was reflected in an elevated turkey (Meleagris gallopavo sylvestris L.) food/brood index. Following imazapyr release, habitat suitability index (HSI) values for pine warblers (Dendroica pinus L.) and black-capped chickadees (Parus atricapillus L.) increased due to reductions in canopy hardwoods and increases in snags. Reduced shrub stratum density resulted in a lower bobwhite quail (Colinus virginianus L.) cover index on imazapyr-released areas.
  • Thumbnail Image
    Effects of Shifting Populations and Preferences on Nonindustrial Landowner Behavior: An Example from Virginia
    Conway, M. Christine (Virginia Tech, 1998-09-11)
    The study was undertaken in response to a prediction by the Virginia Department of Forestry (VDOF) that current harvest levels cannot be sustained into the future given an increasing growth to removal insufficiency throughout the state of Virginia. The purpose of the study is to determine how market signals, land and owner characteristics, and owner preferences affect landowners' decisions concerning their forestland. Particular emphasis is placed on the effects of absenteeism and land fragmentation on landowner behavior. Such information is important for targeting policies that will successfully maintain commercial timber levels throughout the state.
  • Thumbnail Image
    Estimating Plot-Level Forest Biophysical Parameters Using Small-Footprint Airborne Lidar Measurements
    Popescu, Sorin Cristian (Virginia Tech, 2002-04-12)
    The main study objective was to develop robust processing and analysis techniques to facilitate the use of small-footprint lidar data for estimating forest biophysical parameters measuring individual trees identifiable on the three-dimensional lidar surface. This study derived the digital terrain model from lidar data using an iterative slope-based algorithm and developed processing methods for directly measuring tree height, crown diameter, and stand density. The lidar system used for this study recorded up to four returns per pulse, with an average footprint of 0.65 m and an average distance between laser shots of 0.7 m. The lidar data set was acquired over deciduous, coniferous, and mixed stands of varying age classes and settings typical of the southeastern United States (37° 25' N, 78° 41' W). Lidar processing techniques for identifying and measuring individual trees included data fusion with multispectral optical data and local filtering with both square and circular windows of variable size. The window size was based on canopy height and forest type. The crown diameter was calculated as the average of two values measured along two perpendicular directions from the location of each tree top, by fitting a four-degree polynomial on both profiles. The ground-truth plot design followed the U.S. National Forest Inventory and Analysis (FIA) field data layout. The lidar-derived tree measurements were used with regression models and cross-validation to estimate plot level field inventory data, including volume, basal area, and biomass. FIA subplots of 0.017 ha each were pooled together in two categories, deciduous trees and pines. For the pine plots, lidar measurements explained 97% of the variance associated with the mean height of dominant trees. For deciduous plots, regression models explained 79% of the mean height variance for dominant trees. Results for estimating crown diameter were similar for both pines and deciduous trees, with R2 values of 0.62-0.63 for the dominant trees. R2 values for estimating biomass were 0.82 for pines (RMSE 29 Mg/ha) and 0.32 for deciduous (RMSE 44 Mg/ha). Overall, plot level tree height and crown diameter calculated from individual tree lidar measurements were particularly important in contributing to model fit and prediction of forest volume and biomass.
  • Thumbnail Image
    Evaluating methods for characterizing slope conditions within polygons
    Weih, Robert C. (Virginia Tech, 1991)
    While the applications of Geographic Information Systems (GIS) have progressed from a descriptive tool to a decision making and modeling tool, the understanding of errors and variability of the components of a GIS has lagged behind. Slope is one of these components. This dissertation evaluates different methods for determining and characterizing slope values in polygons and how these methods affect natural resource models. Eight different previously used methods for determining cell slope values were compared using elevation data from the USGS Big Stone Gap, Virginia, Digital Elevation Model. The 28 pairwise comparisons were statistically different, but for practical applications six of the comparisons were similar with an average slope difference of less than one percent. In a decision model the effect of changing just the slope method used to determine cell slope values can influence the results of a model enough to cause almost a 10 fold difference. Since usually the smallest administered unit in natural resource management is the stand (polygon), nine ways of describing the slope of a polygon for 240 polygons using an aggregation of cell slope values were investigated. These polygon descriptors were mean, trim mean, median, mode, first quartile, third quartile, standard deviation, minimum and maximum cell slope value. Also, a new method of determining polygon slope was examined using trend surface techniques, which is not based on aggregation of single cell slope values. The distributions of cell slope values in a polygon cannot be assumed normal since few polygons had a normal distribution. The sensitivity of these polygon slope descriptors to polygon area and surface complexity, based on fractal dimension, was examined and found not to affect these polygon characteristics. The application and logical decisions required to choose an appropriate slope method and polygon slope descriptor(s) based on model objectives are shown in two examples, a harvesting and USLE model. Automating the process of choosing the appropriate polygon slope descriptor(s) and how to integrate these methods in an operational GIS using an Expert System is discussed.
  • Thumbnail Image
    Evaluation of photographic properties for area estimation
    Wiles, Steven Jay (Virginia Tech, 1988-05-15)
    From the known image positional errors on aerial photographs, this thesis computes and evaluates acreage estimation errors. Four hypothetical tracts were used in simulating aerial photographs with 104 different camera orientation combinations. Flying heights of 4000 and 6000 feet, focal lengths of 24 and 50 millimeters with and without lens distortion, and tilts of 0, 3, 6, and 12 degrees were simulated. The 416 photographs were all simulated with the camera exposure station centered above the midpoint of the respective tract's bounding rectangle. The topographic relief of the tracts ranged from 19 feet in the Coastal Plain to 105 feet in the Piedmont. It was found that lens focal length did not have an independent effect on the acreage estimates. Relief error, the lowest, averaged -0.080%. In comparison, small errors in calculating scale were shown to be larger than relief errors. Tilt was recommended to be limited to six degrees, averaging +1.6% error at six degrees tilt. Because of its positive exponential nature when the tracts are centered, tilt can induce large biases. including tilts from zero to six degrees,the average was 0.634%. Lens distortion error averaged -0.686%. Overall, the average acreage error was 0.363% for simulations up to and including six degrees of tilt with and without lens distortion. This result is for centered tracts, and it was felt many of the errors were compensating given this situation. In conclusion, the photographic images can estimate areas to $1%, however, additional errors are imparted during actual measurement of the photographs.
  • Thumbnail Image
    Forest soil and vegetation characteristics in two forest types following wildfire in the Shenandoah National Park, Virginia
    Groeschl, David A. (Virginia Tech, 1991-01-05)
    A wildfire of variable intensity occurred in mid-July of 1988 in the Shenandoah National Park and adjacent private lands. This study was established to measure post-fire forest soil and vegetation characteristics in both the mixed pine and mixed oak forest types occurring on Dovel Mountain in the Shenandoah National Park, Virginia. Vegetation and soil results represent one (1989) and two (1990) growing seasons following fire occurrence in the mixed pine forest type, whereas only second year results are reported for the mixed oak forest type. Forest floor and mineral soil parameters differed among burn levels and followed similar trends for both forest types. Forest floor depth and weight were significantly less on low and high burn areas as compared to unburned areas. Low intensity fires removed the Oi-Oe layer while leaving the Oa layer relatively intact. Conversely, high intensity fifes resulted in the consumption of the entire forest floor. Total carbon, nitrogen, and nutrient differences between low and unburned areas for the entire forest floor (Oi-Oe + Oa) were negligible. However, loss of these constituents were much greater following high intensity fifes. Differences in the surface 10 cm of mineral soil were also detected following following high intensity fifes. Differences in the sulface 10 cm of mineral soil were also detected following low and high intensity fires. Mineral soil acidity was lower while exchangeable cation concentrations were higher in burned areas compared to unburned areas. Total carbon and nitrogen levels were higher in low burn areas, whereas total carbon and nitrogen levels were lower on high bum areas. Inorganic nitrogen levels were higher in both low and high burn areas compared to unburned areas.
  • Thumbnail Image
    Increasing the Precision of Forest Area Estimates through Improved Sampling for Nearest Neighbor Satellite Image Classification
    Blinn, Christine Elizabeth (Virginia Tech, 2005-07-29)
    The impacts of training data sample size and sampling method on the accuracy of forest/nonforest classifications of three mosaicked Landsat ETM+ images with the nearest neighbor decision rule were explored. Large training data pools of single pixels were used in simulations to create samples with three sampling methods (random, stratified random, and systematic) and eight sample sizes (25, 50, 75, 100, 200, 300, 400, and 500). Two forest area estimation techniques were used to estimate the proportion of forest in each image and to calculate forest area precision estimates. Training data editing was explored to remove problem pixels from the training data pools. All possible band combinations of the six non-thermal ETM+ bands were evaluated for every sample draw. Comparisons were made between classification accuracies to determine if all six bands were needed. The utility of separability indices, minimum and average Euclidian distances, and cross-validation accuracies for the selection of band combinations, prediction of classification accuracies, and assessment of sample quality were determined. Larger training data sample sizes produced classifications with higher average accuracies and lower variability. All three sampling methods had similar performance. Training data editing improved the average classification accuracies by a minimum of 5.45%, 5.31%, and 3.47%, respectively, for the three images. Band combinations with fewer than all six bands almost always produced the maximum classification accuracy for a single sample draw. The number of bands and combination of bands, which maximized classification accuracy, was dependent on the characteristics of the individual training data sample draw, the image, sample size, and, to a lesser extent, the sampling method. All three band selection measures were unable to select band combinations that produced higher accuracies on average than all six bands. Cross-validation accuracies with sample size 500 had high correlations with classification accuracies, and provided an indication of sample quality. Collection of a high quality training data sample is key to the performance of the nearest neighbor classifier. Larger samples are necessary to guarantee classifier performance and the utility of cross-validation accuracies. Further research is needed to identify the characteristics of "good" training data samples.
  • Thumbnail Image
    Landsat TM-Based Forest Area Estimation Using Iterative Guided Spectral Class Rejection
    Wayman, Jared Paul (Virginia Tech, 2000-05-15)
    In cooperation with the USDA Forest Service Southern Research Station, an algorithm has been developed to replace the current aerial-photography-derived FIA Phase 1 estimates of forest/non-forest with a Landsat Thematic Mapper-based forest area estimation. Corrected area estimates were obtained using a new hybrid classifier called Iterative Guided Spectral Class Rejection (IGSCR) for portions of three physiographic regions of Virginia. Corrected area estimates were also derived using the Landsat Thematic Mapper-based Multi-Resolution Land Characteristic Interagency Consortium (MRLC) cover maps. Both satellite-based corrected area estimates were tested against the traditional photo-based estimates. Forest area estimates were not significantly different (at the 95% level) between the traditional FIA, IGSCR, and MRLC methods, although the precision of the satellite-based estimates was lower. The estimated percent forest area and the standard error (respectively) of the estimates for each region and method are as follows; Coastal Plain- Phase 1 66.06% and 1.08%, IGSCR 68.88% and 2.93%, MRLC 69.84% and 3.08%. Piedmont- Phase 1 63.87% and 1.91%, IGSCR 65.52% and 3.50%, MRLC 59.19% and 3.83%. Ridge and Valley- Phase 1 69.74% and 1.22%, IGSCR 70.02%, and 2.43%, MRLC 70.53% and 2.52%. Map accuracies were not significantly different (at the 95% level) between the IGSCR method and the MRLC method. Overall accuracies ranged from 80% to 89% using FIA definitions of forest and non-forest land use. Given standardization of the image rectification process and training data properties, the IGSCR methodology is objective and repeatable across users, regions, and time and outperforms the MRLC for FIA applications.
  • Thumbnail Image
    Modeling the impact of gypsy moth defoliation in individual tree mortality and basal area growth of northern hardwoods of central Pennsylvania
    Amrhein, John Francis (Virginia Tech, 1988-03-15)
    Data for this study were collected by the US Forest Service and the Pennsylvania Bureau of Forestry on nearly 600 plots in central Pennsylvania. Tree and stand characteristics recorded between 1978 and 1985 include estimates of percent defoliation on individual trees. Logistic regression using maximum likelihood estimation was employed to model individual-tree mortality of 15 species in central Pennsylvania that had been defoliated by the gypsy moth. Defoliation was estimated to the nearest ten percent for individual trees. Other variables used for prediction included stand basal area and an individual-tree relative basal area index. Success ranged from no fit for three of the species to an R value (a derivation of Akaike's information criterion) of .613 for white oak. The inclusion of defoliation in the models had a varied effect. For four of the species percent defoliation was not significant. For hickory and white oak respectively, percent defoliation raised the R value by .305 and .290 percentage points. As many as five models for each species were developed: one or two models with no defoliation measure in the model and one each for one, two or three consecutive years of defoliation measures. A beta and gamma function were used to model individual· tree basal area growth for the same 15 species. The models were fit using nonlinear least squares. Variables used include the relative basal area index, stand basal area, site index and a defoliation index that incorporated three years of individual-tree, percent defoliation. The beta and gamma functions fit equally well with values of (1 - relative mean square error) ranging from .1967 to .6290. Results for both models are presented for each species. The defoliation index was a significant variable for five of the fifteen species: white, chestnut, red, and black oak and sassafras.
  • Thumbnail Image
    Qualitative response models theory and its application to forestry
    Arabatzis, Alexandros A. (Virginia Tech, 1990-01-04)
    The focus of this dissertation is the theory of qualitative response models and its application to forestry related problems. Qualitative response models constitute a class of regression models used for predicting the result in one of a discrete number of mutually exclusive outcomes. These models, also known as discrete regression models, differ from the usual continuous regression models in that the response variable takes only discrete values. In forestry applications the use of such models has been largely confirmed to mortality studies where only the simplest kind of qualitative response models - a dichotomous (binary) dependent variable model - is applied. However, it is common in forestry to deal with many variables which are either discrete or recorded discretely and need to be formulated by more complex models involving polychotomous dependent variables. The estimation of such complex qualitative response models only recently has been made possible by the development of advanced computer technology. The first objective of this study was to specify dichotomous and polychotomous response models that appear to be suitable for forestry applications and present methods of statistical analysis for these models. The models considered in this study were: the linear probability model, binary logit and probit, ordered and unordered multinomiallogit and probit and McFadden's conditionallogit. Special attention was paid to the following problems: i) how to motivate a qualitative response model which is theoretically correct and statistically manageable, ii) how to estimate and draw inferences about the model parameters, iii) what criteria to use when choosing among competing models and iv) how to detect outlying, high leverage and highly influential observations. The second objective was to exemplify the utility of the above models by considering two, forestry related, case studies. Assessing the merchantability of loblolly pine trees growing on plantations in southern United States and modelling the incidence and spread of fusifonn rust on loblolly and slash pine plantations in east Texas. The results demonstrated the potential of qualitative response models for meaningful implementation in a variety of forestry applications and also, suggested topics for future research work.
  • Thumbnail Image
    Silvicultural Methods for Improving Hardwood Management on Non-Industrial Private Forest land in Virginia
    Matthews, Bonnie L. C. (Virginia Tech, 2005-03-18)
    Hardwood management has been discouraged because of long rotations, low stumpage values, expensive treatments, and an undependable market (Bechtold and Phillips 1983). Knowledge gaps on how various biological factors affect hardwood growth also exist. Stand improvement methods attempt to shift growth to desirable stems. Three different hardwood stand improvement treatments were evaluated. A pre-commercial chemical thin occurred in a twelve year old stand in 1990. In 1995, two of the treatments showed a significant increase in dbh over the control. However, the 2004 measurements of the stand did not find any significant differences between treatments. A case study examined paired plots throughout the state of Virginia where the crown touching crop tree release method was applied. In both the Piedmont and Ridge and Valley regions of the state an increase in dbh was observed. Finally, a timber stand improvement study examined different treatments in a 60-80 year old stand, but did not result in any significant increases in volume after three years. When oaks are harvested or a major disturbance occurs, the number of oaks that regenerates is less than there were previously (Smith 1992). Therefore, oak regeneration is a problem and methods are needed to facilitate oak regeneration (Smith 1992). One method of oak regeneration was examined. Burning five years after a deferment cut did not result in significantly more stems of oak regeneration. Various reductions in basal area also did not result in an increase in oak regeneration under our 60-80 year old timber stand improvement study. These studies attempt to close knowledge gaps in hardwood management and provide useful information for non-industrial private forest (NIPF) landowners. It is so important to target NIPF landowners because the 350 million acres of timberland they own will play a large part in the future of the United States timber supply (Haynes 2002).
  • Thumbnail Image
    Statistical characterization of area and distance in arc-node geographic information systems
    Prisley, Stephen P. (Virginia Polytechnic Institute and State University, 1989)
    While Geographic Information Systems (GIS) have proven to be effective tools for the management and analysis of forest resources data, estimates of the reliability of area and distance measures computed in GIS have been lacking. Using fairly weak assumptions regarding the variability of point location errors, expressions for computing the mean, variance and covariance of polygon area, and an approximate distribution for distance are derived. Assumptions about point location errors include unbiasedness, independence between X and Y coordinate errors, known and equal variance of errors in X and Y coordinates, and correlation between errors at adjacent points. For the derivation of distance from a point to a line, the assumption of normality of errors is added. Because the variance of polygon area that was derived depends on the location of the centroid, a centroid location which minimizes polygon variance was defined. After the mean and variance of polygon area errors were obtained, polygon area was shown to be approximately normally distributed in a simulation of errors in regular polygons. Distance between a point and a line consists of two cases: distance from the point to a vertex of the line, and perpendicular distance to a line segment. The square of vertex distance was shown to be distributed as a non-central chi-square random variable when normal errors are assumed. The normal distribution was demonstrated to be a reasonable approximation for perpendicular distance under similar assumptions. As an application of the polygon variance and covariance formulas, the variability of value of a tract of land was estimated, based upon fixed per-acre values and assumptions regarding variability of location errors. Under moderate assumptions of variability and correlation, the coefficient of variation of mean tract value was 8%. To demonstrate the application of the distribution of distance, a probabilistic point-in-polygon analysis was performed using timber cruise plot locations in a timber stand map. Over half of the plots were ambiguously located when evaluated using the most liberal set of assumptions tested. The advantages and disadvantages of the models developed herein are discussed.