Growth responses of Scots pine (Pinaceae) to artificial and sawfly (Hymenoptera: Diprionidae) defoliation

The influence of defoliation type (artificial versus natural), timing (early versus late), and intensity (25%, 50%, and 75% of needle mass removed) on leader growth of Scots pine, Pinus sylvestris (Linnaeus), was assessed for 2 years after treatment on an even-aged stand located in southeastern Finland. Trees were defoliated simultaneously, either artificially with a pair of scissors or naturally with larvae of Neodiprion sertifer (Geoffroy) and Diprion pini (Linnaeus) for the early- and late-season treatments, respectively. After 1 year, early-season artificial defoliation generally caused greater growth reduction than natural defoliation. Late-season defoliation yielded opposite results. Trees defoliated artificially in early-season treatments were significantly shorter than control trees irrespective of defoliation intensity, whereas those defoliated late in the season did not differ from controls, except at the highest intensity. Trees defoliated by sawflies, either early or late in the season, were significantly shorter than control trees only at the highest defoliation intensity. The pattern of growth loss in the second year appeared similar to that in the first year. The impact of defoliation was either prolonged neutral or negative, as no compensatory responses on height growth in Scots pine were observed. Timing of the treatment in relation to completion of leader growth, differences in defoliation types, alteration of the photosynthetic capacity due to biomass loss, and the functional role of plant parts defoliated may explain the results observed.