LEAF VALUE: EFFECTS OF DAMAGE TO INDIVIDUAL LEAVES ON GROWTH AND REPRODUCTION OF MOUNTAIN BIRCH SHOOTS

Most damage caused by invertebrate herbivores is local and limited in extent and may therefore seem inconsequential. However, in addition to providing photosynthates to the whole plant, individual leaves may preferentially feed local sinks, including pri- mordial meristems that develop later. For leaf damage, this may have local consequences and, in the case of preformed meristems, may result in time lags. To investigate such consequences and to determine the degree of independence among shoot modules, we studied shoot traits after damage to individual leaves in long shoots and vegetative and generative short shoots of field-growing mountain birch, Betula pubescens subsp. czere- panovii (Orlova) Hamet-Ahti. For short shoots, the results suggest that, due to the catkin, generative shoots have a higher priority in resource use than vegetative shoots. The overwinter survival of vegetative short shoots was decreased after their young leaves had been damaged early in the previous season. Otherwise, there were no significant effects of leaf damage on vegetative shoots. In generative shoots, leaf damage affected size and reproduction instead of survival. Because leaf damage significantly decreased leaf size-in the generative shoots in the posttreatment year, leaves presumably competed with the catkin for resources within the shoot. Damage also reduced the catkin mass and the mean seed mass in the treatment year. In the post- treatment year, catkin mass and total seed mass were reduced by the treatments, but dif- ferences in the number of seeds and mean seed mass did not quite reach significance. The treatments had no effects on leaf mass of neighbors of vegetative or generative shoots in the treatment year or in the posttreatment year. In the treatment year, removal of long shoot leaves from a growing long shoot did not affect growth of the long shoot, indicating that such growth was supported by resources external to the shoot. However, local effects were evident in the posttreatment year: leaf mass decreased in shoots that developed from axillary buds in long shoots whose supporting leaves had been clipped in the previous year. Consequently, removal of individual leaves may have local effects on shoot survivorship, reproduction, and growth. Timing of damage and shoot type modified the consequences of local leaf damage, and there were no consistent responses for all situations. Due to the limited extent of damage, the effects were not likely to result from nutrient loss due to damage. Instead, consequences of local leaf damage could be explained largely by effects on local sinks and meristem primordia.