Impacts of fire in active and passive restorations on boreal forest soils and vegetation

Abstract

ISSN 1795-7389 (online) ISBN 978-951-651-608-3 (pdf) ISSN 2323-9220 (print) ISBN 978-951-651-609-0 (paperback) Publishers:ABSTRACT Passive recovery or active restoration approaches may be used in the repair of degraded ecosystems. The effects of such measures on ecosystem patterns and processes, including boreal forest soils and vegetation, are poorly understood. This thesis examines the impacts of both active and passive restoration approaches on soil organic matter (SOM) and vegetation in the boreal forests of eastern Finland.The study sites were located in managed and protected boreal forests in the same region in Finnish North Karelia. In the study sites, I measured soil and vegetation patterns, and the environmental controls on SOM decomposition in relation to the proximity of decaying logs.In actively restored sites, the burned, partly harvested site had lower humus SOM stocks and displayed vegetation biomass and cover patterns that suggested stronger disturbance than the other sites. Burning decreased and homogenized vegetation diversity through spatiallyuniform extinctions and limited colonization 10 years after fire. Green tree retention partially alleviated the impacts of disturbance on vegetation biodiversity. Proximity of dead wood (but only of non-charred logs) enhanced conditions for SOM decomposition. Charred logs did not exhibit this effect, which suggests a previously unknown linking of forest fires to soil processes via charred wood.In the passive recovery sites, legacies of slash-and-burn regimes have persisted in the forests for more than a century. The disturbed forests had a higher volume of large birch trees and lower SOM stocks. In boreal conditions, passive restoration may take more than a century before ecosystem properties return to their pre-disturbance state. Soil properties may be more challenging to restore than above-ground tree structures.My results indicate that active and passive restoration approaches may produce quite different pathways and outcomes. In general, the active restoration approach with low severity fires that is currently applied appeared to not harm forest soils; in particular it left the deeper mineral layers intact, and may provide a more rapid way to restore ecosystem properties. However, there is an urgent need to cover a longer successional time series to reveal the exact differences between active and passive restoration trajectories. The inherent differences between the focus of the passive restoration approach (to recover ecosystem naturalness in a more holistic sense) and the active restoration approach (targeting specific species, habitats, structures and processes in the ecosystem) should be duly acknowledged.