Understory floristic composition of planted western white pine stands in the northern Rocky Mountains

Understory plants are an integral part of forests, serving a variety of functions that help maintain healthy ecosystems. The structure and composition of the understory are influenced by numerous biotic and abiotic factors, with light being critical. The introduction of the pathogen Cronartium ribicola, which causes white pine blister rust, into North America in the early 20th century led to the near total loss of western white pine (Pinus monticola) from moist forests of the Northern Rockies. Management is reintroducing blister rust-resistant western white pine across the landscape, but the effects on the understory are unknown. We examined the effects of stand structure and proportion of western white pine in the overstory on understory diversity of vascular plants in closed canopy stands dominated by blister rust-resistant western white pine across northern Idaho. Habitat series explained the greatest amount of variation (34%) in species presence-absence, while canopy cover accounted for 25%, basal area of all trees for 18%, and the proportion of western white pine composition by 14%. Our analysis revealed positive relationships between the proportion of western white pine in the overstory and both the presence of understory plants and the cover of several understory species. For both the presence and cover, separate sets of thirteen species were found to have a positive relationship with the proportion of western white pine in the overstory, with eight species in common. This research fills a knowledge gap by using data from a range of stands across northern Idaho with varying abundance of western white pine in the overstory to evaluate the relationship between the understory and overstory composition. As land managers plant more western white pine trees, we are likely to see the concomitant increase in understory plant diversity across the landscape, in addition to numerous other benefits, including disturbance resistance and resilience.