Effects of forest fertilization on the radial growth and resin exudation of insect-defoliated Scots pines.
In studies in a 60- to 90-year-old stand on dry sandy soil in western Finland, Scots pine (Pinus sylvestris) trees that had been defoliated by the pine sawfly (Diprion pini) in 1989 were fertilized with various nutrient combinations in the growing season following defoliation in order to study the possibility of accelerating tree recovery. Granular fertilizers included N and NPK treatments with soluble N (urea and ammonium nitrate, respectively) and a vitality fertilizer with nitrogen in the form one-third ammonium nitrate and two-thirds slow-release methylene urea. Resin exudation, vertical resin duct density and vigour index of the trees were measured 3 years after defoliation, and radial growth 6 years after defoliation. Defoliation reduced stem growth, and annual rings were missing particularly in the severely defoliated trees. Nitrogen fertilizer enhanced radial growth even in the severely defoliated trees, and increased the number of vertical resin ducts in both defoliated and undefoliated trees, but resin duct density tended to be lower in the nitrogen-fertilized trees than in trees that had not received nitrogen fertilizer. Resin duct density did not correlate with resin flow rate. Resin exudation was strongest from trees that had been almost totally defoliated, intermediate from trees that had lost half of their foliage, and weakest from undefoliated trees. Resin flow was not affected by fertilizer application. There was a weak negative correlation between vigour index and resin flow. No trees with a high vigour index had strong resin exudation, while resin exudation varied from minimal to very strong in trees with a low vigour index. It is concluded that nitrogen fertilizer can be used to stimulate growth of defoliated trees without affecting resistance traits on infertile sites.