Effects of conifer basal area on understory herb presence, abundance, and flowering in a second-growth Douglas-fir forest

Abstract

Although overstory trees exert competitive effects on understory plants, it is not clear how this competition affects the distribution and performance of herb species. This study seeks to clarify the relationship between understory herb performance and overstory basal area in second-growth Pseudotsuga menziesii (Mirb.) Franco stands. Data on 11 understory herb species were collected in a 100-ha watershed. Statistical models were constructed to control for the effects of slope, aspect, soil type, and distance from the central stream or peripheral ridge lines. Presence of old-growth associated and forest generalist herbs was positively associated with conifer basal area, as well as with north-facing aspects and proximity to the stream channel. Presence of release herbs, subordinate forest species that respond positively to canopy disturbance, was largely independent of measured variables. Abundance of individual species showed weak and inconsistent relationships with conifer basal area. In contrast, flowering of almost all species was negatively related to conifer basal area. Regression tree models suggested that conifer basal area may have stronger negative effects farther from the moist environments along stream channels. I conclude that patterns of presence of slow-growing forest species may be determined primarily by past events, while flowering better reflects current stand conditions.