Recovery of understorey vegetation after stem-only and whole-tree harvesting in drained peatland forests

Abstract

The demand for small-sized trees, logging residues, stumps, and lateral roots for energy production has increased during recent decades and therefore whole-tree harvesting (WTH) has become a more common harvesting method in forests. However, this may cause a more pronounced delay in the recovery of forest vegetation than conventional stem-only harvesting (SOH), especially in sensitive peatlands, and thus increase soil erosion. The effects of WTH have not been investigated on peatlands before this study. Recovery of understorey vegetation of drained peatland forests after two different tree harvesting methods, stem-only harvesting and whole-tree harvesting, was investigated in Eastern Finland at eight silviculturally managed peatland forests largely comprising Scots pine, Pinus sylvestris, and Norway spruce, Picea abies. In SOH, trunks only were removed from the sites whereas in WTH, tree trunks, branches, and stumps were removed. In each site, understorey vegetation (tree seedlings, dwarf shrubs, graminoids, herbs, and bryophytes) was inventoried on both mounded and unprepared soil (surfaces) on 220 permanent sample plots one and five years after harvesting. WTH had more pronounced effects on vegetation than SOH. Five to six years after the treatments, the occurrence of dwarf shrubs was lower in WTH than in SOH, whereas the cover of graminoids increased from both SOH and WTH. Soil preparation affected negatively on the recovery of peat and forest bryophytes, but positively on the recovery of some graminoid and herb species. Peat properties, e.g., pH and water table level, were found to regulate the recovery. WTH caused a longer delay on the recovery of understorey vegetation than SOH, especially if soil had been prepared. Thus, WTH cannot be recommended in drained peatland forests.