Diprion pini (L.) is one of the most harmful forest pests in Finland. A remarkable local outbreak happened in 1989 at Harjavalta (SW Finland) in an area of 2000 hectares, where some Scots pine (Pinus sylvestris L.) trees lost almost their entire needle biomass. After one year of defoliation, randomly selected mature trees (N = 210) were treated with six different fertilizer treatments in May 1990: NPK, P alone, N alone, and three vitality fertilization treatments. The trees were classified into three damage categories: no defoliation, medium defoliation, heavy defoliation.
Increased shoot elongation and length of current-year needles occurred in treated trees for two successive years following treatment. However, there were no significant differences in mean growth response among trees in the three damage categories. The N, NPK, and vitality fertilization with nitrogen had the most vigorous effect on shoot and needle elongation.
Fertilization affected the nutrient content of current-year needles. Shortly after defoliation, nutrient concentrations were highest in the trees that had suffered heavy defoliation. Elevated levels of most nutrients (N, P, K, Ca, Mn, Al, Cu, Ni) were still detected in 1992, two years following treatment. Defoliated trees had very low starch content in current-year needles in 1990, indicating low food reserves. However, starch levels of defoliated trees increased to levels similar to those of undamaged trees in the year following fertilization treatment, but this response did not appear to be related to the fertilizer treatments.
Our results indicate that fertilization is a useful method for promoting recovery of Scots pine following defoliation.
Key words: Pinus sylvestris, Diprion pini, defoliation, fertilization, tree vigor.
Correspondence: Päivi Lyytikäinen, Finnish Forest Research Institute, Joensuu Research Station, P.O. Box 68, FIN-80101 Joensuu, Finland
Telefax: +358-73-1514567
E-mail: paivi.lyytikainen@metla.fi