Human-induced disturbances may change vegetation and carbon (C) and nitrogen (N) processes in the forest floor and the soil beneath it. The aim of this dissertation was to study the effects of physical and chemical disturbance on boreal forest soil and vegetation. The specific aims were to evaluate the rate and direction of the forest ecosystem recovery from the disturbance and to assess how C and N processes are affected by different disturbances regimes. Two contrasting soil-affecting treatments – stump harvesting and sprinkling infiltration – were studied as case studies representing a disturbance. Sprinkling infiltration alters the chemical composition of forest soil, whereas stump harvesting results in changes especially in the physical structure of the forest soil. Furthermore, in contrast to stump harvesting where C and nutrients are removed from the soil with the removed biomass, sprinkling infiltration adds large quantities of C and nutrient-rich surface water into the forest soil. As stump harvesting and sprinkling infiltration are relatively newly introduced land use practices, very little is known of their long-term effects on boreal forest soil and vegetation.
The effects of stump harvesting on forest soil surface disturbance, C and N pools and mineralization rates, understory vegetation, seedling growth and coarse woody debris (CWD) were studied in Norway spruce (Picea abies (L.) Karst.) stands located in Central and Southern Finland. The results of this study indicate that stump harvesting causes soil mixing and relocation of organic matter in the soil profile, which in turn is reflected to the soil C and N dynamics as soil C and N pools tended to be lower following stump harvesting. Stump harvesting combined with site preparation tends to cause more extensive soil surface disturbance than site preparation alone, and the mixing effect of stump harvesting persists on soil surface after a decade since harvest. Furthermore, this study underlines that stumps, coarse roots and fine coarse roots represent a significant portion of the CWD, belowground biomass and nutrients in a forested stand, and thus their extraction results in substantial and direct removal of biomass, C and nutrients from the stand.
The effects of sprinkling infiltration on forest soil, tree growth and understory vegetation and their respective recovery were studied in an experimental stand that had been infiltrated with surface water in order to produce artificial groundwater. The study revealed that the previously observed changes soil chemistry had persisted in the experimental stand; soil pH and base cation concentration as well as the rate of net N mineralization were still significantly higher at the infiltrated plots after a 12–15-year recovery period. These results lead to the conclusion that sprinkling infiltration results in the long-term neutralization of the forest soil. In contrast to tree growth, theunderstory vegetation had not benefited from the added nutrients and organic matter, instead the large amounts of added water had created conditions unfavorable to certain plant species. In conclusion, sprinkling infiltration is an environment altering treatment which, based on the findings of this study, can have short-term effects on tree growth and long-term effects on soil processes and understory vegetation and ultimately, ecosystem recovery.
The results of this study demonstrate that disturbances affect the function and structure of forest soil and these changes can persist for at least a decade on the surface of the soil in the organic layer and deeper in the mineral soil. Furthermore, this dissertation highlights the need for long-term perspectives in ecosystem management and planning.