Improving boreal forest regeneration in a variable climate

Abstract: Regeneration of boreal forests in Sweden mainly involves planting Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings on clear-cut areas, although there is an increasing interest in planting broadleaves and in applying continuous cover forestry (CCF). Seedling establishment is generally enhanced by mechanical soil preparation. With a warming climate, increases in drought periods and pests can reduce the regeneration success and further consideration of site conditions may be needed during planting. This thesis aims to better adapt boreal forest regeneration practices to local environments. My studies have focused on regeneration with seedlings of Scots pine (Papers I-IV), Norway spruce (Papers II & III) and silver birch (Betula pendula Roth.) (Paper II) by analyzing data collected from field experiments from 16 sites in Sweden. Based on this field data, I have evaluated the effects of planting positions (Papers I & III), environmental conditions (Paper I), seedling sizes (Paper III), arginine-phosphate addition (Papers I, II & III), and competition with mature trees (Paper IV). Pine survival was, overall, higher, and less variable over a wide range of site conditions when planted in mineral soil than in capped mounds, especially during dry weather (Paper I). Growth of pine was equal when planted in mineral soil and in capped mounds on most sites studied in Paper I, while growth of pine and spruce in general was lower in mineral soil than in hinge and capped mound positions in Paper III. Damage and mortality by pine weevil on pine and spruce was higher in planting positions where organic material was present (Paper III). Addition of arginine-phosphate enhanced seedling survival and growth on some, but not all sites (Papers I & II), and its effect varied between sites and tree species (Papers I & II) and, for pine, with seedling size (Paper III). For pine seedlings, root isolation and fertilization enhanced growth along the edge of a mature forest (Paper IV), while only root isolation enhanced growth beneath a closed canopy of mature trees (Paper IV). These results emphasize that adapting regeneration practices to the site conditions at the time of planting is a key to success.