The optimum size of gaps made in a forest stand in group shelterwood or selection cuttings is an important issue for foresters working with continuous cover forestry methods. In Latvia, shelterwood and selection cuttings are usually practised in forests where clear-cutting is forbidden or there are a high proportion of mature stands, as well as in private holdings.
To assess the influence of gap size on the growth and survival of seedlings, rectangular gaps (size 100 to 1600 m2) were created in two 100-year-old Scots pine (pine) dominated stands in Hylocomiosa forest type. Pine and Norway spruce (spruce) were planted in the gaps and arranged in a square pattern, so that the distance from the gap edge would be known for each tree planted.
For performance analysis of the planted trees, the survival and height increment over a period of three years was used. Significance of the test site, initial tree height, gap size and the distance from the closest gap edge on survival and growth of seedlings was tested using univariate analysis of variance. Survival rate of the pine 3 years after planting notably differed between test sites (66% and 24%), but did not differ regarding the gap size and the distance from the closest gap edge.
Survival rate of the spruce 3 years after planting was at least 86% regardless of gap size, and at least 80% regardless of the position within the gap.
Gap size had a significant (p < 0.05) influence on the growth in height for both species, but the distance to the closest gap edge only had a significant (p < 0.05) influence on the growth of spruce.
Based on these results, it can be concluded that the gap size must be larger than 10×10 m to ensure sufficient growth during the first years after planting pine and spruce.
Keywords: gap, edge effect, survival, growth, Pinus sylvestris, Picea abies.