Effects of pollution-induced habitat disturbance on the response of willows to simulated herbivory

Abstract

1 Stress may affect compensatory plant growth and plant responses to herbivory may therefore be modified by pollution. We compared the effects of leaf clipping on growth, reproduction and developmental stability of two willow species (Salix borealis and S. caprea) growing at different distances from two copper-nickel smelters (Monchegorsk, NW Russia and Harjavalta, SE Finland). 2 In unpolluted sites, the defoliation of both willow species adversely affected sexual reproduction and increased the formation of epicormic shoots, but did not change either shoot growth or leaf size. Timing and extent of clipping had little effect on the responses. 3 Compensatory responses to leaf clipping in polluted habitats were reduced, especially in S. borealis, as reflected by decrease in shoot growth and fruit production and an increase in leaf fluctuating asymmetry, a non-specific indicator of stress. 4 Activation of dormant buds was a common compensatory response to herbivory in unpolluted sites. The lack of an increase in epicormic shoots in polluted sites may be due to already weakened apical dominance. 5 Compensatory abilities may be affected both by direct damage caused by pollutants and pollution-induced increases in environmental stress. Although S. borealis shows no visible sign of stress in polluted sites, regrowth from dormant buds and enhanced shoot growth were seen under these conditions. Resources were not therefore available for further compensation of damage caused by defoliation. Our results fit the hypothesis that fast growing plants have low compensative abilities. 6 Salix caprea shows more pollution-induced stress than S. borealis, expressed as increased developmental instability near the weaker and shorter shoots near the stronger pollution source. This may explain the differences in their compensatory responses in polluted sites.