1 Stress may affect compensatory plant growth and plant responses to herbiv
ory 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 Finl
2 In unpolluted sites, the defoliation of both willow species adversely aff
ected sexual reproduction and increased the formation of epicormic shoots,
but did not change either shoot growth or leaf size. Timing and extent of c
lipping 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 f
ruit production and an increase in leaf fluctuating asymmetry, a non-specif
ic indicator of stress.
4 Activation of dormant buds was a common compensatory response to herbivor
y in unpolluted sites. The lack of an increase in epicormic shoots in pollu
ted sites may be due to already weakened apical dominance.
5 Compensatory abilities may be affected both by direct damage caused by po
llutants and pollution-induced increases in environmental stress. Although
S. borealis shows no visible sign of stress in polluted sites, regrowth fro
m dormant buds and enhanced shoot growth were seen under these conditions.
Resources were not therefore available for further compensation of damage c
aused by defoliation. Our results fit the hypothesis that fast growing plan
ts have low compensative abilities.
6 Salix caprea shows more pollution-induced stress than S. borealis, expres
sed as increased developmental instability near the weaker and shorter shoo
ts near the stronger pollution source. This may explain the differences in
their compensatory responses in polluted sites.