"Green tree" retention has been an ecological concept in even-aged, managed forests for several decades. In theory, aggregated retention (reserve patches) can provide old forest characteristics within the context of a regenerating forest stand, while allowing for free-to-grow status in the balance of the regenerating stand.
The Aitkin County Land Department (ACLD) has been retaining patches during clearcut harvests for a number of years and assumed that the ecological benefits were being provided. In the summer of 2014, we decided to do an informal study to measure some of the ecological benefits of reserve patches.
Pre-treatment stand description and condition
Stand establishment and management history
What actually happened during the treatment
We did not have baseline data, so we sampled recent and older reserved areas to look at old forest characteristics, specifically snag trees and coarse woody debris (down logs). Eleven sites were selected based on years since harvest, geographic distribution, and accessibility (see map 1). The average reserve patch size tended to be larger than typical (1.8-acres) as they had to be in our forest inventory as an inclusion or visible by aerial photo.
Basal area plots (10-factor) were taken at each patch center to determine live and dead (snags) standing trees. A 100th acre plot was used to determine CWD (down logs). Snags were defined as being >8' tall and >4" dbh and down logs were defined as being >4' long and >4" diameter.
Results (see Table 1) indicate that 16% of the standing trees in the patches (live and dead) were snags with an average diameter at 4.5' (dbh) of >12". The dominant live trees (by basal area) were red oak, paper birch, and sugar maple while paper birch, aspen, and red oak were the primary snag trees. CWD plots indicate over 200 down logs per acre. The data was divided into two categories >10-years and <10-years post harvest. The 5 patches > 10-years post harvest have around twice as many snags and down logs as the 6 patches < 10-years post harvest.
Table 1: Stand data results.
Summary / lessons learned / additional thoughts
Based on our informal study, old forest values are provided by aggregated retention in even-age regeneration harvests and not surprisingly these benefits increase as the stand ages; which can be important for a number of wildlife species that require old forest characteristics in our managed forest landscape.