Stress responses of Salix borealis to pollution and defoliation.
Morphological characteristics of Salix borealis [S. myrsinifolia] were monitored during 1994-96 at 10 sites along a transect crossing the heavily polluted surroundings of the Severonikel smelter in NW Russia, and analysed with respect to pollution load and density of the principal herbivore, the leaf beetle Melasoma lapponica [Chrysomela lapponica]. Leaf size and shoot growth increased and leaf pubescence decreased with an increase in pollution, whereas variation in other characteristics was not related to pollution load. Leaf fluctuating asymmetry (FA) and leaf pubescence increased with an increase in herbivory, whereas variation in other characters was not related to foliar damage. A field experiment demonstrated that leaf FA and pubescence were high in the year after defoliation. The data on leaf FA, a non-specific stress indicator, suggested that for the extant individuals of S. myrsinifolia, defoliation resulted in detectable stress, whereas long-lasting severe pollution by sulfur dioxide and heavy metals caused no measurable stress response and may even stimulate growth of leaves and shoots. Leaf FA of woody plants may serve as an early indicator of the biotic stress caused by herbivory, which expands the use of FA as a tool in helping to compare the relative strengths of different stressing agents.