Facilitation of bilberry by mountain birch in habitat severely disturbed by pollution: Importance of sheltering

Earlier we demonstrated that in heavily polluted industrial barren dwarf shrubs grew and reproduced better under top-canopy plants than in gaps, while in undisturbed forests they performed better in gaps than under tree canopies. During 2002-2004 we used bilberry, Vaccinium myrtillus to experimentally test the hypothesis that positive effects observed in industrial barrens were due to mechanical sheltering of dwarf shrubs by canopies of birch, Betula pubescens subsp. czerepanovii. Sheltering by wooden fence during 2 years improved performance and alleviated stress (as indicated by decrease in FA) in bilberry plants growing in open microsites, compared to both the same plants in pre-treatment year and non-sheltered (control) plants. Performance indices of artificially sheltered V. myrtillus reached the level observed in plants naturally sheltered by birch canopies. Removal of natural shelter, i.e. cutting of neighbouring birch, caused steady decline in vegetative biomass production, while changes in shoot mass, leaf mass and leaf FA of bilberry were not significant, and performance of these plants during the two post-treatment years remained higher than performance of bilberry in open microsites. Our data indicate that shelter effects to the great extent explain facilitation of dwarf shrubs by top-canopy plants in industrial barrens. Amelioration of unfavourable environment by both natural and artificial shelters was mostly due to protection from wind and extreme soil temperatures. We suggest that in harsh environmental conditions positive plant-plant interactions are primarily mediated by changes in biophysical environment.