Mortality of retention trees due to blowdown damage in a retention forestry experiment in Hokkaido, northern Japan

Retention forestry is an approach that preserves features of the forest structure which are important for maintaining biodiversity at harvest to mitigate the negative impacts of clearcutting by retaining trees. Since these trees are exposed to rapid environmental changes upon the harvesting of the trees around them, understanding the mortality patterns is crucial so that the application of retention forestry may meet its aims. In this study, the mortality of dispersed retention trees was assessed at the retention forestry experiment sites in Hokkaido, Japan, from pre-harvest to the 6th year post-harvest. Naturally regenerated broad-leaved trees were retained at three density levels (ca. 10, 50, and 100 trees/ha) at the final cutting of a planted Abies sachalinensis forest, and all retention trees were monitored. Most dead trees were uprooted or stem-broken, suggesting that blowdown caused by typhoons and extratropical cyclones was the major cause of mortality. After harvesting, the mortality of retention trees was higher at lower retention levels, and annual mortality rates at each site were higher in the first 3 years (1.1 %-7.6 %) than from the 3rd to 6th year (0.0 %-2.6 %). Trees with smaller diameters had higher mortality rates for all species except Betula platyphylla. The form of mortality (uprooted, stem-broken, standing dead) varied among species: species with low wood density were prone to stem-breakage, whereas shade-tolerant species had a low probability of uprooting and standing dead. Species with longer longevity were less prone to uprooting and stem breakage. Stem-broken Tilia spp. trees often survived and their crowns recovered, potentially providing a tree-related microhabitat of partially dead trunks and branches. Preventing blowdown damage is important both for the management of planted forests and to ensure the presence of large old trees long after harvesting. Severe wind damage is increasing due to climate change; hence, management measures that include wind damage will become crucial in northern temperate regions including Hokkaido. In planted conifer forests, mixing of wind-resistant broad-leaved trees may increase resistance to wind damage. We recommend retaining species that are resistant to blowdown and trees with large diameters and low height-diameter ratios. Additionally, some trees may be retained in small patches to avoid individual retention trees being directly exposed to the wind.