Asko Lõhmus, University of Tartu
Riinu Rannap, University of Tartu
professor Anne Tolvanen, University of Oulu
This thesis integrates the present knowledge on the impacts of forest drainage on biodiversity, assess the long-term impact of the drainage on habitat quality and species assemblages, and gives implications, how to harmonize ditching practices with sustainable management. Forest drainage manipulates the key components of the ecosystem – hydrology and dominating organisms (trees, Sphagnum mosses) – and consequently the whole biota and ecosystem functioning. Forest drainage is practised in vast areas to increase timber production. Its environmental impacts, yet, have gained little attention, compared to e.g. effects of harvesting.
The harpoon heuristic depicts the post-drainage transformation of the ecosystem. After the initial stick – i.e. the ditching – the harpoon penetrates deeper pressing its barbs ( i.e. irreversible changes) one after another into the ecosystem. The irreversibilities are caused by several ecological and socio-economical feedback mechanisms. During the long-term draining, populations of hundreds of species reorganize. For example, drainage is detrimental for at least one fifth of the species in swamp forests, though about the same number of new species colonizes the area after drainage.
A large part of drainage impacts are delayed and indirect, functioning via slowly changing forest structure. Lichens and bryophytes were especially sensitive to the latter. Thus the impact of forest drainage could be mitigated by increasing the diversity of overstory and abundance of logs.
Drained forests were not unambiguously impoverished, but contained valuable habitat components and therefore also threatened species. For example drainage did not seem to decrease the overall amount of small water bodies, though it did change their features – natural water bodies were replaced with ditches. Widespread anthropogenic water bodies, such as ditches and wheel rut pools provide varied habitats, thus could complement local restoration and conservation attempts. The latter are effective if targeted to multiple species with similar habitat requirements, and to key sites in the landscape.
In the thesis a systematic approach for harmonizing forestry ditching practices with the aims of biodiversity protection is described. According to this, representative ‘focal species’ from sets of sensitive species should be selected, depending on landscape context and the stands allocated into four main groups of management approaches. Hundreds of species should be examined to identify a few representative biodiversity targets for drainage mitigation programmes. Some bryophytes and lichens growing on live and dead trees were provided as representatives of the biota of natural swamp forest.
The general implications based on this thesis suggest paying more attention to the impacts of forest drainage on biodiversity and applying appropriate mitigation measures according to landscape context. The further research to support biodiversity conservation practises includes assessing less studied but potentially sensitive species groups, details of the impact mechanisms, searching for and testing of focal-species and habitat management approaches.