EFFECTS OF DEFOLIATION ON RADIAL STEM GROWTH AND PHOTOSYNTHESIS IN THE MOUNTAIN BIRCH (BETULA-PUBESCENS SSP TORTUOSA)

1. Effects of herbivory on radial growth of mountain birch (Betula pubescens ssp. tortuosa) were investigated in defoliation experiments in a subarctic environment in northern Sweden. The effects of foliage loss on the rate of photosynthesis and the annual radial increment were assessed to test whether compensation had occurred. 2. Fifty per cent defoliation had no significant effect on ring width. In trees defoliated 100%, rings formed during the treatment year and each of the following 3 years were only 15-25% as wide as the corresponding rings in control trees. Remaining foliage on 50%-defoliated trees had a maximum area-based photosynthetic rate 35% higher than that of control trees, whereas the photosynthetic rate of regrowth foliage on 100%-defoliated trees was not significantly different from that of control trees. 3. Leaf nitrogen concentration and whole-tree leaf area showed normal values 2 years after defoliation. 4. The daily CO2 assimilation in mid-August of 50%-defoliated trees was estimated to be 67% that of undefoliated trees. The fact that ring widths of these trees were similar to those of control trees suggests that the reduction in photosynthesis was compensated for by stored carbohydrates or by a re-allocation of carbon, or both.