Stand dynamics 11 years after retention harvest in a lodgepole pine forest

摘要

Structurally diverse forests provide resilience to an array of disturbances and are a mainstay of multiple-resource management. Silviculture based on natural disturbance can increase structural heterogeneity while providing other ecological and economic benefits. One useful silvicultural tool for promoting structural heterogeneity is retention harvesting, whereby a portion of forest stands are left unlogged, transitioning even-aged stands to multi-aged. We report stand and tree dynamics 11 years after retention harvest in a central Montana Rocky Mountain lodgepole pine forest with evidence for a mixed-severity fire regime. Treatments were implemented on 16 experimental units with prescriptions for two 50% overstory basal area retention patterns (Aggregated and Dispersed) crossed with two levels of prescribed fire use (Burned and Unburned). The aim of this study was to identify (1) how retention harvest spatial pattern affects stand dynamics, (2) if stand dynamics after retention treatments are more simply attributable to tree size and competition, or if retention pattern affects dynamics beyond those measures, and (3) how stem and basal area heterogeneity varied over the 11-year measurement period. Retention pattern affected overstory density, growth, mortality, and regeneration density and stocking. After controlling for the fine-scale factors of tree size and competition, overstory mortality, regeneration stocking, and regeneration height growth did not vary by treatment-scale factors. Fine-scaled factors explained significant variation in overstory basal area growth, but at the scale of experimental units, growth was also greater in Dispersed treatments. Prescribed burning interacted with retention pattern to influence overstory tree growth, increased overstory mortality, and increased regeneration height growth. Overstory heterogeneity (e.g., in basal area) degraded more rapidly in treatments with the Dispersed spatial pattern than Aggregated. This study evaluates novel silvicultural treatments in a lodgepole pine forest and highlights the tradeoffs between retention patterns combined with broadcast burning on forest change. Our results are useful for planning silvicultural treatments in multiple-use forests designed to promote structural complexity and resilience to disturbances.