Effects on Growth of Simulated and Induced Shoot Pruning by Tomicus piniperda as Related to Carbohydrate and Nitrogen Dynamics in Scots Pine

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Abstract
In a field experiment in central Sweden, shoots of 20-yr old Scotch pine (P. sylvestris L.) were hand-pruned in 2 series, one with non-fertilized trees and another with fertilized trees, and in a third series, by inducing T. piniperda attacks on non-fertilized trees. The prunings, in four degrees of severity, were made in late summer; pruning by hand simulated attacks of the beetles. Starch and N concentrations in remaining and new needles were monitored. Two and 4 yr later, trees were felled and the effect on growth was recorded. Negative but small effects on radial stem growth and stem volume growth, significantly correlated with pruning level, were found up to 2 yr after the treatment. These were parallel with significant positive effects on leader length and needle biomass per remaining higher-order shoots. The response of radial stem growth differed along the stem, being significantly positive in the first year after pruning for the 1-yr old internode, nil for the internode below, and significantly negative further down. There was no difference in growth effects between the three pruning series. No significant effects on growth were found after 4 yr and no significant effect in total needle biomass could be demonstrated. The starch reserves in remaining needles of non-fertilized, pruned trees were significantly lowered compared to the control, while at the same time the N concentration was significantly raised. Needle N increased but there was no effect on starch concentration in fertilized, pruned trees. The small and transient effect on stem growth of even severe prunings is discussed in terms of compensatory mechanisms. These may be the result of an improvement in N status and an increased photosynthetic capacity of remaining and new needles, making possible a rapid restoration of needle biomass and, hence, growth.