Defoliation reduces growth but not carbon reserves in Mediterranean Pinus pinaster trees

Key message : Reduced growth but high NSC after severe defoliation of evergreen trees can be explained by three, non-exclusive processes: critical loss of non-C reserves, hormonal changes, and prioritisation of C storage over growth. Abstract: In an attempt to simulate processionary moth impact on pines, we explored the extent to which late winter defoliation affects growth and carbon reserves in the following season. In separate treatments we removed 100, and 50% of needles of whole trees and defoliated single branches in naturally grown, 2-3-m-tall Pinus pinaster trees in Italy. Shoot and stem growth (lateral shoot length and basal area increment, respectively) were substantially reduced after 100% defoliation (−45% for shoots, −84% for stems). In 50% defoliated trees only stem growth was reduced (−37%), and in trees with a single branch defoliated, growth remained unaffected. Although substantial carbon and nitrogen reserves were removed from defoliated trees prior to bud break, non-structural carbohydrates (NSC) concentrations in branches and needles fell below control values only during the first half of the growing season, and considerable amounts of NSC persisted throughout the year. By the end of the dry and hot Mediterranean summer, NSC concentrations in branch xylem, branch phloem, previous year needles, stem sapwood and root xylem were similar among all treatments. Reduced growth and high late season NSC after defoliation can be explained by (1) a critical loss of reserves other than C (e.g. N and P), (2) hormonal changes which affected cambial activity, or (3) a prioritisation of carbon storage over growth, with all three mechanisms potentially contributing to the observed growth and NSC response