Logging residues and ground vegetation in nutrient dynamics of a clear-cut boreal forest

Abstract

In this thesis are studied the role of decomposing logging residues and developing ground vegetation in nutrient dynamics of a clear-cut area. The main aims were to study how much nutrients are released from logging residues during the first three years after clear-cutting and what is the role of ground vegetation in the retention of nutrients on site after clear-cutting. The study was conducted in eastern Finland in a Norway spruce dominated mixed forest, part of which was clear-cut and part left uncut. The decomposition of Norway spruce (Picea abies (L.) H. Karst.), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth) logging residues i.e. foliage, fine roots (diameter 2 mm) and branches (diameter 1 cm), were studied using the litterbag method. Aboveand below-ground biomass of the ground vegetation was sampled on one uncut plot and two clear-cut plots one year before and five years after clear-cutting. In total, 33 % of the dry mass and C, 49 % of P, 90 % of K and 8 % of Ca were released from logging residues in three years, but there was no net release of N because more N accumulated in the roots and branches than was released from the foliage. The loss of mass, C, P and K was greatest during the first year, whereas there was no net release of Ca until the third year. Most of the released nutrients originated from the foliage. Total ground vegetation biomass and nutrient pools decreased after clear-cutting to one half or even lower, but returned to precutting levels within 4-5 years, and the pools of P and K became even larger. In the first year after clear-cutting more N and Ca accumulated in the logging residues than were stored in the ground vegetation. The results indicate that logging residues are a potential source of the elevated dissolved C and P, K and Ca observed in surface waters soon after clear-cutting, but are not a net source of N during the first three years. The ground vegetation is capable of taking up only a small fraction of the nutrients that are released from logging residues during the first two years after clear-cutting and the decomposing dead ground vegetation is a potential source of leached nutrients. The results suggest that nutrients released from logging residues are initially retained on site primarily through soil processes and microbial immobilization. Solely microbial immobilization in logging residues can initially play a more important role in the retention of N and Ca than the ground vegetation. Ground vegetation, however, recovers rapidly from clear-cutting and it becomes thereafter a significant nutrient sink.