Woody plants are known to exert strong competitive and facilitative effects on herbaceous plants.  However, few studies have considered how the strengths of these effects change over time or vary with traits of the interacting species. We studied the temporal dynamics of tree-herb interactions in a montane meadow in western Oregon where two conifers, Pinus contorta and Abies grandis, have invaded continuously for more than 80 yr.  We used a chronosequence approach, selecting individuals of both species to represent a broad range of tree ages.  Effects of trees were measured by the differences in cover and richness of resident meadow and colonizing forest understory species in paired transects (equal numbers of 0.2 x 0.5 m quadrats) beneath and adjacent to the conifer canopy.  Transects were oriented to the northeast and/or southwest from the bases of individual Pinus (n = 31 NE and 36 SW) and Abies (n = 38 NE and 36 SW).  General linear models were used to test for the hypothesized effects of tree age (increasing with time), tree species (greater in Abies than Pinus), directional shading (stronger to the NE), and their potential interactions (e.g., stronger temporal trends in Abies than in Pinus).

Results/Conclusions

In general, both tree species exerted negative effects on meadow species and positive effects on forest herbs (significant directional change in seven of eight one-sample t-tests).  However, we did not detect a negative effect of Pinus on cover of meadow species.  In addition, in 37% of transects, cover of meadow species was greater beneath than adjacent to trees. This occurred more frequently under Pinus than Abies (p<0.001), more frequently under younger (<30 yr) than older (>60 yr) trees (p<0.0001), and more frequently to the SW vs. NE (p<0.01).  Tree age had a significant effect on the magnitude of decline in cover (but not richness) of meadow species.  Declines under Pinus, however, were an indirect effect of increasing density of Abies beneath the canopy.  For forest understory species, increases in cover and richness with tree age were significant under Abies, but not under Pinus (significant age-by-species interaction).  Our results highlight the importance of time and species identity in understanding the dynamic and often unpredictable effects of tree establishment in these herbaceous communities.