Retention tree characteristics have major influence on the post-harvest tree mortality and availability of coarse woody debris in clear-cut areas

• 55% of retention trees die and 80% of these fall down in 10 years after harvest.
• Dispersed trees have higher mortality than grouped trees.
• Effect of diameter on mortality varies between tree species.
• Pines have a lower risk to fall down than spruces or deciduous trees.
• Tree characteristics can be used to retain trees that best meet the aims of retention.

Retention forestry is widely used for reducing the negative biodiversity impacts of forest management. Its effectiveness depends, however, on the post-harvest dynamics of the retention trees, which in turn are likely affected by both tree- and stand-level factors. Understanding the effects of these factors on tree dynamics is thus essential for evaluating the biodiversity effects of retention forestry. We studied the impact of tree-level factors on the mortality and falling patterns of retention trees in pine-dominated boreal forests of eastern Finland. In total, 2738 retention trees were followed individually for 10 years after harvest on 12 study sites with an initial retention level of either 10 or 50 m3 ha−1. Prescribed burning was applied to half of the sites after harvest. Effects of tree species (pine, spruce or deciduous), diameter and location (grouped or scattered trees) on tree mortality and tree fall after death were assessed using survival analysis. During the 10-year survey period, 55% of the trees died and 81% of the dead trees fell down. The risk of mortality decreased with increasing diameter in all tree species on burned sites. On unburned sites, this applied only to pines and deciduous trees whereas for spruce the largest trees had the highest mortality. On all sites, dispersed trees had higher mortality than grouped trees. Pines had a lower risk to fall down after dying than spruces or deciduous trees. We conclude that tree species, diameter and location can be used to predict the post-harvest mortality and falling of retention trees and to select trees that best meet the aims of retention. However, considering the biodiversity effects, the predicted tree dynamics should not be the only basis for tree selection, but the habitat value of the retained trees must be accounted for as well.