Light-dependent growth responses of young Abies alba to simulated ungulate browsing

Abstract

1. The shade-tolerant conifer Abies alba Mill. (European silver fir) is a favoured browse species of several large herbivores. We performed an experiment to investigate how sun or shade conditions influence the recovery of A. alba saplings following simulated browsing. The browsing treatment involved clipping the leader and/or lateral shoots. For two growing seasons after applying the treatment, we recorded various measures of plant growth including bud development and the origin of new leaders. We then harvested the trees to determine patterns of above-ground biomass allocation. In the discussion, we consider the physiological basis for the observed responses to damage and evaluate the hypothesis that they represent an evolved defensive strategy against herbivory. 2. After 2 years, most saplings in the sun had compensated for lost height due to leader clipping, whereas saplings in the shade lagged 1 year behind. The degree of height recovery was influenced less by the length of the new leader than by its mode of origin; thus, there was overcompensation if the new leader developed from a side-shoot turning upwards ('flagging'), while there was undercompensation if it was produced from an interwhorl or a secondary whorl bud. Leader origin, in turn, proved to be light-dependent, with flagging occurring twice as often in sun as in shade. Some trees, mostly in the shade, produced no leader in the first year but several leaders from secondary whorl buds in the second year (i.e. multiple stemming). Clipping of the leader also induced local bud development and led to greater allocation of resources to new leaders than in unclipped trees. Clipping of lateral shoots had no effect on either height increment or growth of lateral branches. 3. We conclude that, by influencing how the new leader is produced, light availability also affects the degree to which A. alba can compensate for loss in height from browsing. A possible interpretation of the overcompensation under high-light is as a defensive reaction to escape future browsing. However, an alternative explanation is that overcompensation and other morphological reactions, for example, multiple leaders, are merely contrasting physiological responses to release from apical dominance under different environmental conditions. © 2007 The Authors.