Modeling dominant height using stand and water balance variables for loblolly pine in the Western Gulf, US

Abstract

Site-index models based on dominant height and age are commonly accepted measures of site productivity in forest plantations. Therefore, reliable estimates of dominant height over time are critical, as they provide the foundations for site productivity estimates used in growth and yield models. Forest site productivity depends on the combination of many environmental factors, whereas tree height is only a general proxy indicator of these factors interacting over time, and as such, subject to large errors. Hence, selection of appropriate environmental variables along with field-based data is expected to result in more consistent estimates. Although previous studies have incorporated climatic variables into site-index models, there is still a lack of understanding among the forest research community with respect to the selection of suitable climatic variables. Models including climatic water balance components such as water-deficit and excess-water are scarce. These components are especially important to address the influence of drought on plant growth. The main objective of this study is to evaluate the change in dominant height estimates of forest stands with respect to changes in water balance components. High-resolution actual evapotranspiration (AET) and potential evapotranspiration (PET) were combined to determine water excess/deficit of sites. These were incorporated into a loblolly pine polymorphic site index model using a generalized algebraic difference approach and maximum likelihood calibration methods that accounted for long-term uncertainty in the results. Our model further improves the precision in dominant height estimates for loblolly pine in the southeast US.