Browsing by Author "Westfall, James A."

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    Effects of uncertainty in upper-stem diameter information on tree volume estimates
    Westfall, James A.; McRoberts, Ronald E.; Radtke, Philip J.; Weiskittel, Aaron R. (2016-10)
    Almost all relevant data in forestry databases arise from either field measurement or model prediction. In either case, these values have some amount of uncertainty that is often overlooked when doing analyses. In this study, the uncertainty associated with both measured and predicted data was quantified for upper-stem diameter at 5.27 m. This uncertainty was propagated through a tree taper model into predictions of individual-tree volume. The effects of uncertainty on individual-tree volume predictions and population estimates of total volume were assessed. Generally, when little or no systematic measurement deviation was present, less uncertainty was associated with field-measured diameters compared to model predictions. However, diameters predicted from a model were preferred when systematic deviations in field measurement exceeded approximately 0.2 cm. Comparisons of results obtained from an alternative taper model showed that more precise estimates of population totals might be obtained without upper-stem diameter information. Upper-stem diameter information increases the prediction accuracy of individual-tree volume, and thus, models using this information may be preferable in applications such as timber sales containing high-value trees. Due to the various factors that influence measurement and modeling uncertainty, foresters are encouraged to make similar evaluations in the context of their specific activities.
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    Growth Models and Mortality Functions for Unthinned and Thinned Loblolly Pine Plantations
    Westfall, James A. (Virginia Tech, 1998-09-04)
    Effects of thinning, such as increased diameter growth and decreased mortality in the residual stand, have been recognized by foresters for many years. These effects are largely the result of increased tree vigor which is induced by a decreased level of competition. These relationships are reflected in many of the models that are central to PTAEDA2, a growth and yield simulator which was developed for use with loblolly pine (Pinus taeda) plantations established on cut-over, site prepared lands. Data from a long-term thinning study served as a basis for attempting to improve the predictive output of PTAEDA2. Assessment of differences in model parameter estimates between three levels of thinning intensity led to various approaches to reach this goal. Height increment and mortality models were found to need no additional refinement and were re-fit using all available data. Diameter increment and crown ratio model forms could not account for thinning effects in their present form and thinning response functions that could provide the proper behavioral response were added to these models.Models were evaluated individually and in combinations in a reduced growth simulator. This reduced simulator is a modified form of the growth subroutines in PTAEDA2 and is designed to utilize external data. Results of growth simulation runs show improvements in predictive ability for the crown ratio model fit to all data and for the re-fit height increment model/crown ratio model with thinning response variable combination. The diameter increment model with a thinning response variable significantly improved diameter prediction within the simulator, but predicted stand volumes were poor. The re-fit mortality function resulted in greater prediction error for mortality than the original PTAEDA2 mortality function.
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    Improved accuracy of aboveground biomass and carbon estimates for live trees in forests of the eastern United States
    Radtke, Philip J.; Walker, David; Frank, Jereme; Weiskittel, Aaron R.; DeYoung, Clara; MacFarlane, David W.; Domke, Grant M.; Woodall, Christopher W.; Coulston, John W.; Westfall, James A. (2017-01)
    Accurate estimation of forest biomass and carbon stocks at regional to national scales is a key requirement in determining terrestrial carbon sources and sinks on United States (US) forest lands. To that end, comprehensive assessment and testing of alternative volume and biomass models were conducted for individual tree models employed in the component ratio method (CRM) currently used in the US' National Greenhouse Gas Inventory. The CRM applies species-specific stem volume equations along with specific gravity conversions and component expansion factors to ensure consistency between predicted stem volumes and weights, and additivity of predicted live tree component weights to match aboveground biomass (AGB). Data from over 76 600 stem volumes and 6600 AGB observations were compiled from individual studies conducted in the past 115 years - what we refer to as legacy data - to perform the assessment. Scenarios formulated to incrementally replace constituent equations in the CRM with models fitted to legacy data were tested using cross-validation methods, and estimates of AGB were scaled using forest inventory data to compare across 33 states in the eastern US. Modifications all indicated that the CRM in its present formulation underestimates AGB in eastern forests, with the range of underestimation ranging from 6.2 to 17 per cent. Cross-validation results indicated the greatest reductions in estimation bias and root-mean squared error could be achieved by scenarios that replaced stem volume, sapling AGB, and component ratio equations in the CRM. A change in the definitions used in apportioning biomass to aboveground components was also shown to increase prediction accuracy. Adopting modifications tested here would increase AGB estimates for the eastern US by 15 per cent, accounting for 1.5 Pg of C currently unaccounted for in live tree aboveground forest C stock assessments. Expansion of the legacy data set currently underway should be useful for further testing, such as whether similar gains in accuracy can be achieved in estimates of regional or national-scale C sequestration rates.
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    Simulation of Early Stand Development in Intensively Managed Loblolly Pine Plantations
    Westfall, James A. (Virginia Tech, 2001-11-26)
    A system of equations was developed and incorporated into the PTAEDA2 loblolly pine stand simulator to provide growth projections from time of planting. Annual height growth is predicted using a two-parameter Weibull function, where distribution parameters are estimated from equations that utilize site index and age as predictor variables. Allometric equations are employed to estimate tree diameter and height-to-crown attributes. First year after planting mortality estimates are based on physiographic region and drainage class, with adjustments for bedding or discing site preparation treatments. Thereafter, a simple mortality function is used. The onset of competition is defined through a point density measure, which was conditioned to correspond with inflection points of basal area growth curves from observed data. Early silvicultural treatment response functions were also developed. These equations modify growth for shearing and piling, discing, and bedding site preparation methods, fertilization with phosphorous, nitrogen, and/or potassium, and 1-year or 2-year herbaceous weed control treatments. Differential responses due to drainage class and physiographic region are included in the response functions where necessary. Equations that account for interactions between certain treatments are used to adjust response levels where treatments have similar effects site conditions. Analyses of pre-competitive growth projections where no treatments are specified reveal that a small amount of over-prediction is present when compared with observed data. Predicted values in the post-competitive growth phase confirm that the addition of the pre-competitive growth system did not significantly affect the predictive behavior of the PTAEDA2 model. The simulated growth responses attributed to early silvicultural treatments are consistent with response levels reported in other studies.