Within-site adaptation: Growth and mortality of Norway spruce, Scots pine and Silver birch seedlings in different planting positions across a soil moisture gradient

Abstract

Adapting to site conditions is a central part of forest regeneration and can be done through selection of different planting positions. Requirements are tree species specific, and the use of soil moisture maps could be a way to support decision making in forest regeneration planning. At two experimental sites with varying soil moisture conditions in southern Sweden Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth) seedlings were planted in four different planting positions following mounding site preparation; Depression, Hinge, Mound and Unscarified. Soil moisture estimates were obtained from a high-resolution depth-to-water raster for each planting spot. The effect of soil moisture, planting position and their interactions on mortality, height and diameter was evaluated for each tree species. In wet conditions mounds proved to be the best option to minimize seedling mortality for conifers, but with decreasing soil moisture, differences between the planting positions decreased. Birch on the other hand had the greatest survival in the hinge. The coniferous species displayed increased height and diameter when planted in mounds independent of the soil moisture conditions, whereas silver birch was less dependent on a specific planting position. Results from this study shows that a soil moisture map can explain mortality, height and diameter and thus can be a useful tool when choosing planting position in different soil moisture conditions.