Ecological restoration and fine-scale forest structure regulation in southwestern ponderosa pine forests

Abstract

Fine-scale forest patterns are an important component of forest ecosystem complexity and spatial pattern objectives are an increasingly common component of contemporary silviculture prescriptions in dry fire-adapted forests of North America. Despite their importance, questions remain regarding the assessment of silvicultural treatments designed to meet spatial objectives. We initiated a replicated silvicultural assessment of two forest management approaches commonly applied in dense ponderosa pine forests of the Southwest United States: historical evidence-based ecological restoration guidelines (ERG) and northern goshawk (Accipiter gentilis) foraging area management recommendations (GMR). We compared stand-level characteristics, global tree location point patterns and tree group-level attributes resulting from the marking of these approaches to current forest conditions and patterns of historical forest remnants in six, 2.02ha stem mapped plots. We also assessed group-level Vegetative Structural Stage (VSS; a classification of fine-scale forest structural development used to regulate fine-scale spatial patterns in these forests). ERG and GMR-based treatments significantly reduced densities and basal area from the current condition, but did not significantly differ in density from historical forest remnant estimates. GMR-based treatments retained greater stand level basal area than ERG-based treatments, primarily in large, 28–48cm tree diameter classes. GMR-based treatments approximated global tree location point patterns of forest remnants better than ERG-based treatments, primarily due to a 5–6m minimum spacing of residual trees, but also likely due to specific aspects of ERG-based marking techniques. Despite this difference, both treatments resulted in group-level characteristics similar to those exhibited by historical forest remnants. Both treatments significantly altered group-level VSS area and reduced variation of tree diameters within classified VSS groups.Our study provides insight how tree marking techniques using historical forest remnants may have important effects on resulting fine-scale forest structure patterns. We also demonstrate how the use of global point or group-level pattern assessment methods can affect the evaluation of fine-scale spatial pattern objectives. Our analysis of VSS group characteristics highlights implementation and assessment issues associated with group-level spatial pattern identification, classification and regulation. We conclude that group-level classification and regulation is not necessary for maintaining fine-scale spatial patterns in complex ponderosa pine ecosystems subject to frequent fire disturbances and changing future climate conditions and societal demands. Both ERG and GMR-based approaches applied in this study may have utility in maintaining fine-scale spatial heterogeneity and promoting resiliency in Southwest ponderosa pine forests.