Effects of small aggregates of mature tree retention on adjacent planted coastal Douglas-fir growth and survival

Abstract

Long-term retention of mature trees at the time of harvesting, called variable retention, has increased significantly since the 1990s as a silvicultural practice for maintaining stand-level biodiversity. A common approach is aggregated retention where groups of trees are left to increase structural diversity and provide forest influence over the adjacent harvested area. Despite the increased global application of aggregated retention, the growth and survival of adjacent trees has received less research effort than biodiversity studies. This knowledge gap was addressed with a study of small (0.01 to 0.58 ha, mean 0.3 ha) retained mature tree aggregates in harvested areas on southeast Vancouver Island, British Columbia, Canada. Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco.) seedlings were planted in ~10-m-wide belt transects (three planting rows), extending up to 10 m into and 50 m away from aggregate edges in four cardinal directions. Nitrogen fertilizer was applied at planting to some transects. Basal diameter, height and survival of the planted Douglas-fir were measured for an average of 11 years. We used nonlinear mixed-effects models to assess the relationships of planted-tree basal diameter and height growth to distance from aggregate edge, edge orientation, planted-tree age, fertilization and aggregate characteristics (basal area and height). We extended planting into the aggregates to model tree response on both sides of edges. Modeled growth rates were slower for 10-year-old planted trees under the aggregate canopy and along the edges of the harvested area compared to growth at about 10 m and greater distance from aggregate edges; i.e. trees achieved 90% of their potential height and diameter by about 10 m from the aggregate canopy. Impacts were greatest on north-facing edges and least on south-facing edges, although differences were very small. We examined planted-tree survival by proximity to aggregate edges using a Cox proportional hazards model. Survival was lower directly beneath the aggregate canopy and up to approximately 5 m into the harvested area and was also slightly lower adjacent to south-facing edges. Fertilized trees showed a minor increase in survival versus unfertilized trees. Structural characteristics of aggregates had only a minimal impact on planted-tree growth and survival. This study helps provide a better understanding of the relationships among planted-tree growth and survival and edge effects for implementation of aggregated retention.