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.

A common concern to planting for reforestation is seedling failure that is directly measurable by seedling early field performance of growth and survival. Root growth potential (RGP) is a commonly used metric of seedling quality and has been considered indicative of seedling field performance. The effect of RGP is thought to be dependent on planting site and underlining environmental conditions. Moisture stress often is considered the primary cause of seedling failure in addition to other environmental factors such as soil physicochemical properties in regions such as the Inland Northwest of the United States that is prone to growing season drought. In addition, it is interesting to test whether seedling early field performance is related to their morphological attributes and whether the morphological attributes are related to RGP. A comprehensive evaluation on early field performance of three planted conifer species of interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco), grand fir (Abies grandis (Douglas ex D. Don) Lindl.), and western larch (Larix occidentalis Nutt.) was conducted in this study. It was found that RGP did not show clear correlation with early field performance across species. RGP also was not significantly correlated with seedling morphological measures such as below- and above-ground biomass and root-to-shoot ratio (R:S, by mass). Early field performance of growth and survival varied greatly across individuals of seedlings. The most influential predictors of early seedling growth and survival were their initial size (indicative of energy reserve) and soil temperature that likely interacted with soil moisture. Our findings suggest that seed stock selection for reforestation probably should favor species and genotypes with greatest heat tolerance that may be better adapted to future conditions in the region.