Assessing the recovery of forest understory vegetation after clearcut logging across a 445-year chronosequence

The conversion of natural forested lands to managed forests has reduced the number of older, structurally diverse forests worldwide. In the conifer forests of the Pacific Northwest (USA), the long-term impacts of timber harvesting are not fully understood. We used a chronosequence of forests in southwestern Oregon that ranged from 25 to 445 years of age to compare changes in plant communities in logged stands with those in stands in late-succession conditions. The chronosequence consisted of 13 50m² permanent plots with similar elevation, aspect, slope, and forest type that were previously sampled in 2003. In 2021, we resurveyed the herbaceous understory in each plot to evaluate if the relationship between stand attributes and understory vegetation had changed over the 18-year period. Our results support the non-linear relationship between stand age and richness also observed in 2003, such that richness was highest in the youngest stands, reached a low point in mid-aged stands, and then reached high levels of richness in the oldest stands. However, changes in structural conditions and the response of understory vegetation varied among stands. We found that canopy cover increased (24%) over the 18-year period, with most of the increase occurring as young, recently clearcut sites entered a phase of canopy closure. These same stands lost an average of 11 understory species between 2003 and 2021, and the community composition of these younger stands became more similar to closed canopy stands. We observed a decline in evenness and diversity in the mid-age and older stands due to an increase in dominance of shade-tolerant plants. Overall, these results demonstrate a legacy effect of the historic phase of clearcut logging in the Pacific Northwest in which a large portion of forests on the landscape have now entered a period of high canopy cover and low richness and diversity.