Vascular plant species diversity in low elevation coniferous forests of the western Olympic Peninsula: a legacy of land use.
Timber management practices affect spatial patterns of seral stages and understory plant species diversity. We quantified diversity patterns in understory vascular plant species, including exotic plant species, across four broad age classes of low-elevation coniferous forests of the western Olympic Peninsula, Washington, USA - regeneration (age 0-19 years), young (20-79 years), mature (80-199 years) and late-seral (≥200 years). Species richness varied significantly with age class: highest in newly regenerating stands, lowest in the young class, and increasing to the late-seral stage. We observed an age-class effect for total percent cover by growth form and for distribution of exotic plant species. Shrubs had highest cover in the regeneration category; forbs and subshrubs had highest cover in late-seral forest. Ferns and graminoids did not vary across age classes. Twenty percent of the species found in regeneration plots were exotic, while all species encountered in late-seral plots were native. The proportion of exotic species was intermediate in young (7%) and mature (6%) forests. On the western Olympic Peninsula the late-seral age class is the most limited in spatial extent and has the least connectivity across ownerships. Management efforts aimed at conservation of late-seral plant species at broad spatial scales will need to consider the lack of contiguity of late-seral forest and the role of ownership in its distribution.