Impact of leaf damage on growth of mountain birch shoots

Canopies of heterophyllous trees expand by production of long shoots. We have previously shown in mountain birch (Betula pubescens ssp. czerepanovii) that damage to internode leaves within long shoots does not impede shoot growth, indicating that long‐shoot elongation occurs by means of external resources. To study to what extent leaves other than true long‐shoot leaves are necessary for the normal growth of mountain birch long shoots, we simulated herbivore damage to the two basal leaves of shoots (which flush simultaneously with short‐shoot leaves) and the short‐shoot leaves nearest to the long shoot within the branch. Damage to the two basal long‐shoot leaves significantly reduced long‐shoot growth. Additional damage to short‐shoot leaves, situated proximally to the long shoot, did not retard long‐shoot growth any more than damage to basal leaves alone. To determine the extent to which short‐shoot leaves within a large branch are responsible for the pooled long‐shoot production of the branch, we clipped differing proportions of short‐shoot leaves from such branches. We found small but significant reduction in the pooled length of the long shoots of the branch, presumably indicating a limited role in long‐shoot elongation of current photosynthates within the branch. Our experiments indicate that long shoots are not independent modular units in their carbon economy.