Impact of leaf damage on growth of mountain birch shoots

Canopies of heterophyllous trees expand by production of long shoots. We ha ve previously shown in mountain birch (Betula pubescens ssp. czerepanovii) that damage to internode leaves within long shoots does not impede shoot gr owth, indicating that long-shoot elongation occurs by means of external res ources. To study to what extent leaves other than true long-shoot leaves ar e necessary for the normal growth of mountain birch long shoots, we simulat ed herbivore damage to the two basal leaves of shoots (which flush simultan eously with short-shoot leaves) and the short-shoot leaves nearest to the l ong shoot within the branch. Damage to the two basal long-shoot leaves sign ificantly reduced long-shoot growth. Additional damage to short-shoot leave s, 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 whic h short-shoot leaves within a large branch are responsible for the pooled l ong-shoot production of the branch, we clipped differing proportions of sho rt-shoot leaves from such branches. We found small but significant reductio n in the pooled length of the long shoots of the branch, presumably indicat ing a limited role in long-shoot elongation of current photosynthates withi n the branch. Our experiments indicate that long shoots are not independent modular units in their carbon economy.