Influence of harvesting intensity of logging residues on ground vegetation in coniferous forests.
Ground-vegetation succession was analysed at 4 Swedish coniferous sites (dominated by Picea abies, Pinus sylvestris, or both species) 8-16 years after clear felling. At each site, 3 levels of logging residue harvesting were applied in a randomized block design: (a) above-stump whole tree harvesting (no residues remaining); (b) harvesting all tree parts except needles; and (c) conventional harvest (residues left on site). Vegetational changes at each site were analysed by principal component analyses. Successional changes were related to the first principal components, whereas residue treatments were related to components of the second or lower order. Treatment related differences in species cover, as analysed by a 2-way ANOVA, were noted at all sites, 8 as well as 16 years after harvest. The covers of most vascular plant species, notably herbs and graminoids, were generally lower after whole tree harvesting, while the covers of epigeic lichens and Vaccinium myrtillus were higher. Bryophytes were mostly indifferent to logging residue treatment, except for Pleurozium schreberi which had a lower cover in plots without logging residues. The overall differences were small and resembled those found along natural soil fertility gradients. The presence of woody debris appeared to have a weak, suppressive effect on the cover of the graminoids Deschampsia flexuosa and Luzula pilosa. This effect was oly evident on the first study occasion, about 8 years after felling. The results suggest that the major influence of logging residues left on site was related to their nutrient contribution. The impact tended to decrease over time and, in a long-term perspective it is suggested that treatment differences in species cover will be small, especially in forests developing dense canopies. However, the possibility cannot be excluded that repeated whole-tree harvesting will produce more drastic effects by reducing soil nutrient availability.