Vegetation response to canopy gap creation and herbivore exclusion in exotic Pinus radiata plantation forest

Introduction: Globally, there is significant potential to transition exotic Pinus plantation to native forest for ecosystem restoration and to retire plantations established in locations unsuitable for harvest. The possibility of transition is suggested by the frequent occurrence of understories of native vegetation. Transitions may be promoted by the creation of canopy gaps, however more research is required on the optimal size of gaps and how other interventions such as herbivore exclusion influence transitions.Objectives: This research explored how different canopy gap treatments and herbivore exclusion in exotic plantation forest influenced regenerating communities of native and exotic vegetation. Methods: We compared natural vegetation regeneration using a randomised block design with three replicates of three different widths of strip harvesting and an unharvested control in a mid-rotation (14-years-old) Pinus radiata plantation in Aotearoa New Zealand. Strips were 50 m long, and the strip widths ranged from 9 – 27 m. Herbivores were excluded from half of each strip via the installation of deer fences.Results: Pioneering vegetation quickly established following removal of canopy trees. The narrow strip produced the greatest species richness and total importance value of native vegetation regeneration, primarily composed of herbaceous species. However, there was no significant difference among the treatments for native trees and shrubs. Exotic grasses grew rapidly following canopy removal; however, their importance value was significantly lower in the narrow strip when compared to the medium and wide strips. Herbivore exclusion led to an increase in exotic grasses and had a negative effect on regeneration of native vegetation in the medium and wide strips. Abiotic variables had a significant influence on exotic regeneration but did not have an influence on native regeneration. While mean annual temperature and diurnal temperature range had varying effects on exotic species richness, importance value, and exotic grass importance value, available light had a strong positive effect on all exotic vegetation. Conclusions: The narrow strip encouraged the most native vegetation regeneration while lower available light compared to the medium and wide strips reduced competition with vigorous exotic grasses.