Rehabilitation of monotonous exotic coniferous plantations: A case study of spontaneous establishment of different tree species

Conversion of plantations of exotic coniferous species, such as Norway spruce (Picea abies), Douglas fir (Pseudotsuga menziesii) and Sitka spruce (Picea sitchensis), into more natural woodland is intended in two national parks in the province of Drenthe, The Netherlands. For that purpose, artificial gaps in the plantations were made and natural regeneration of both indigenous and exotic species was investigated. A total of 87 sample plots were analysed, each 100 m2 in size, and located under canopy or in the gaps. The densities of naturally established seedlings and saplings of tree species were recorded. Norway spruce attained the highest regeneration among all species investigated, both in gaps and under canopy. However, as in the case of other exotics (except Japanese larch, Larix kaempferi), its abundance was higher under canopy than in gaps. Indigenous species generally regenerated better in gaps than under canopy, forming 28% of the total number of seedlings in gaps and only 3.8% under canopy. The most numerous indigenous species were rowan (Sorbus aucuparia) and silver birch (Betula pendula). Scots pine (Pinus sylvestris) and pedunculate oak (Quercus robur) were less common. The most important factors influencing the regeneration of indigenous species were: numbers of seed trees within a 50 m distance from the plot, the type of plot (gap or canopy), canopy cover and age and size of gaps. It was obvious that regeneration of indigenous species can be stimulated by suitable forestry management practices, such as thinning dense stands and creating gaps of various sizes. Mixed stands of Norway spruce and indigenous broadleaves represent a reasonable target resulting from the interventions