Responses of understory vascular plant communities up to 6 years after retention harvesting in planted Abies sachalinensis forests

Abstract

Planted forests managed to supply timber are usually harvested by clearcutting but are also expected to contribute to biodiversity conservation. The retention forestry approach that retains important forest structures, such as trees and decayed logs at harvest, can be applied in planted forests to mitigate the negative impacts of clearcutting. Planted forests are disturbed by harvesting and postharvest silvicultural management activities, such as site preparation, planting, and weeding. The responses of understory vascular plant communities to various retention harvesting and postharvest silvicultural management were studied by comparing the composition of understory plant species before and after harvesting at the Retention Experiment for plantation FoREstry in Sorachi, Hokkaido, Japan (REFRESH). Dispersed and aggregated retention sites were established in planted Abies sachalinensis forests. In the dispersed retention sites, broad-leaved trees established in conifer plantations were retained. Clearcut and unharvested stands were used as controls and patch-cutting sites (one third of the area was clearcut by patches of 1-ha) were also established. Harvesting took place the year after the preharvest survey, and postharvest surveys were conducted in the first, third, and sixth years thereafter. The effects on species richness, the persistence of species occurrence in the preharvest survey, and the species composition of understory plants were then examined. Colonization by early seral species resulted in changes to the understory plant communities after harvesting and site preparation, irrespective of overstory tree retention. The species composition within aggregated retention patches was not altered, even though most overstory trees were blown down and are expected to provide a refuge for forest species. The presence of retained trees and steep slopes enhanced preharvest species persistence, which was presumed to be due to hampered soil disturbance. Harvesting and site preparation methods that avoid soil disturbance may help to maintain habitats for more understory plants in planted forests. The effects of weeding on the persistence of understory species differed from that of harvesting and site preparation. The trajectories for each site in the Non-metric multidimensional scaling ordination space were almost parallel from the preharvest to postharvest surveys, suggesting that the preharvest vegetation left a legacy. The direction of movement was reversed in the survey six years after harvesting. It is necessary to determine the resistance and resilience of understory plants within the management cycle (planting through harvesting) to establish management practices that maintain the diversity of understory plants in planted forests.