Nonlinear models of height growth for Douglas-fir in southwestern Oregon

A review is made of methods which assess the bias and non-normality of parameter estimates and predictions obtained with nonlinear regression. Particular emphasis is placed upon curvature measures of nonlinearity, related measures of parameter and prediction bias, and the effects of reparameterizations. Alternate models of individual tree height growth are compared on the basis of mean square error, intrinsic nonlinearity, parameter effects nonlinearity, and estimated bias. While results are specific to the data examined, some general conclusions are made concerning appropriate models for individual tree height growth. Both the Richards and a Weibull-type growth model are found to adequately describe individual tree height growth, with low levels of intrinsic nonlinearity, and acceptable parameter effects nonlinearity following repararneterization. Some evidence is found for a modification of either the Richards or Weibull model to include an asymptotic linear growth rate when modeling the height growth of some western conifers past the age of 200. Stem analysis data on Douglas-fir height growth in mixed confier stands located in southwestern Oregon are used to develop a system of dominant height growth and site index prediction. The Weibull model is used successfully to develop a polymorphic height growth prediction equation. A linear model, estimated with site index as the dependent variable, is used to predict site index. A comparision is made of pooled least squares and random coefficient estimation methods. The random coefficient method is found to more closely model the shape of early height growth, but appears to result in more biased predictions and performs very poorly on older height growth, with both the estimation and validation data. Alternative error assumptions are examined with the pooled data method. The best performance in validation is obtained with assumption of independent errors, heteroscedastic across trees

Consequences of Shifts in Abundance and Distribution of American Chestnut for Restoration of a Foundation Forest Tree

Restoration of foundation species, such as the American chestnut (Castanea dentata) that was devastated by an introduced fungus, can restore ecosystem function. Understanding both the current distribution as well as biogeographic patterns is important for restoration planning. We used United States Department of Agriculture Forest Service Forest Inventory and Analysis data to quantify the current density and distribution of C. dentata. We then review the literature concerning biogeographic patterns in C. dentata. Currently, 431 +/- 30.2 million stems remain. The vast majority (360 +/- 22 million) are sprouts \u3c 2.5 cm dbh. Although this number is approximately 10% of the estimated pre-blight population, blight has caused a major shift in the size structure. The current-day population has a larger range, particularly west and north, likely due to human translocation. While climate change could facilitate northward expansion, limited seed reproduction makes this unlikely without assisted migration. Previous research demonstrates that the current, smaller population contains slightly higher genetic diversity than expected, although little information exists on biogeographic patterns in the genetics of adaptive traits. Our research provides a baseline characterization of the contemporary population of C. dentata, to enable monitoring stem densities and range limits to support restoration efforts

Consequences of Shifts in Abundance and Distribution of American Chestnut for Restoration of a Foundation Forest Tree

Restoration of foundation species, such as the American chestnut (Castanea dentata) that was devastated by an introduced fungus, can restore ecosystem function. Understanding both the current distribution as well as biogeographic patterns is important for restoration planning. We used United States Department of Agriculture Forest Service Forest Inventory and Analysis data to quantify the current density and distribution of C. dentata. We then review the literature concerning biogeographic patterns in C. dentata. Currently, 431 ± 30.2 million stems remain. The vast majority (360 ± 22 million) are sprouts \u3c 2.5 cm dbh. Although this number is approximately 10% of the estimated pre-blight population, blight has caused a major shift in the size structure. The current-day population has a larger range, particularly west and north, likely due to human translocation. While climate change could facilitate northward expansion, limited seed reproduction makes this unlikely without assisted migration. Previous research demonstrates that the current, smaller population contains slightly higher genetic diversity than expected, although little information exists on biogeographic patterns in the genetics of adaptive traits. Our research provides a baseline characterization of the contemporary population of C. dentata, to enable monitoring stem densities and range limits to support restoration efforts