Aspects of heterogeneity : effects of clear-cutting and post-harvest extraction of bioenergy on plants in boreal forests

Abstract: Abstract. The objectives of this thesis are to evaluate (1) the influence of slope aspect on boreal plant responses to clear-cutting and (2) the effects of post-harvest extraction of bioenergy (logging residues or slash) on plant composition, richness and performance in clear-cuts. Such insight is essential for understanding changes in species composition and richness in response to clear-cutting and application of intensified harvesting systems. The focus is on productive and managed spruce dominated forests and focal organisms are mosses, liverworts (i.e. bryophytes) and vascular plants. Space-for-time substitution studies were performed in south- and north-facing slopes located in 10 forests and 10 adjacent clear-cut stands in central Sweden. Differences between forests and clear-cuts were interpreted as effects of clear-cutting. The results show that the response of all three focal groups differed between aspects. More species were lost in south-facing slopes and clear-cutting reduced species richness of liverworts as well as of bryophytes and vascular plants associated with sheltered habitats. By contrast, clear-cutting caused no reduction in any group and more species were added in north-facing slopes. As a result north-facing clear-cuts generally had higher species richness than their forest counterparts. The disparate patterns in species’ response between aspects were most likely caused by initial microclimatic differences and a greater microclimatic change in south-facing slopes, in response to clear-cutting. A paired comparative study of conventionally harvested (i.e. slash left) and slash-harvested clear-cut stands was performed 5-10 years after clear-cutting in south-central Sweden. Both the species composition and the richness of mosses and liverworts were affected by slash harvest, whereas the composition of vascular plants was not. Slash harvest also reduced richness of mosses and liverworts associated with forests and organic substrates (e.g. dead wood and litter). Species richness of vascular plants and bryophytes associated with inorganic substrates (i.e. mineral soil) was unchanged. Differences between conventionally harvested stands and slash-harvested stands were most likely a result of reduced cover of organic material reducing substrate availability and shelter in the latter. Increased mechanical disturbance in slash-harvested stands that destroys remnant forest vegetation and favours pioneers may also play a role. A bryophyte transplant experiment was performed in seven clear-cuts in central Sweden and monitored over one vegetation period. The results show that logging residues (or slash) and forest edges may shelter ground-dwelling bryophytes by buffering the clear-cut microclimate. In conclusion, both slope aspect and extraction of forest bioenergy affect plant survival in clear-cut boreal forests. As surviving plant populations facilitate re-colonisation, north-facing slopes and conventionally harvested clear-cuts (i.e. slash left) may potentially recover faster than south-facing slopes and slash-harvested clear-cuts.