Two decades of forest understory vegetation development in a 445-year chronosequence in the Siskiyou mountains of southwestern Oregon, USA

Conversion of natural forests to managed forests has reduced the amount of older, structurally diverse forests worldwide. In coniferous forests of the Pacific Northwest (USA), where understory plants represent 90 % of the species but only 1 % of forest biomass, the long-term impacts of timber harvesting remain unclear. We investigated these impacts by remeasuring a chronosequence of forests in the Siskiyou Mountains, Oregon, initially sampled in 2003. In 2021, we resurveyed 13 plots (2500 m² each), ranging from 25 to 445 years in age, to assess changes in the relationship between age, canopy cover, environmental attributes, and multiple measures of understory diversity in early-seral logged stands compared to mature and old-growth conditions. Over the 18-yr period, canopy cover increased in early-seral stands (20 %), which transitioned into canopy closure, leading to an average loss of 11 species per plot, declines in early-seral indicator species, and increased compositional similarity to mature stands. Beta diversity was highest in early-seral stands and declined with stand age, a pattern that remained consistent across sampling periods. In old-growth stands, increases in a few shade-tolerant species drove declines in evenness and diversity, a pattern that partially aligns with expectations of increased plant cover under rising temperatures and reduced precipitation. These findings offer insight into legacy effects of clearcut logging and potentially climate change on a critical component of forest biodiversity: plantations have entered a phase of canopy closure, low species richness, and reduced diversity, while species diversity and evenness have also declined in old-growth stands.