Consequences of reduced light for flower production in conifer-invaded meadows of the Pacific Northwest, U.S.A

Woody-plant encroachment threatens the biodiversity and ecosystem functioning of grasslands and meadows worldwide. An important but rarely described consequence of the transition to woody-plant dominance is the reduction in flowering of herbaceous species. We modeled community-wide relationships between flowering and light in two tree-invaded meadows (BG and M1) in the western Cascade Range, Oregon (USA). At BG, trees established over two centuries, forming a gradient of encroachment states (open meadow to old forest) and declining levels of light (91% to 8%), with most (85%) of the decline occurring within 2–4 decades of tree establishment. At M1, where trees formed distinct edges, we also tested whether distance from edge can serve as proxy for light in modeling flowering response. Flowering declined significantly with reductions in light at both sites: for a 10-percentage-point reduction in light, probability of flowering decreased by 35% (BG) and 21% (M1), median flowering density (flowers per m2) by 15% and 8%, and median flowering effort (density per unit cover) by 10% and 9%, respectively. At M1, distance to edge was a poorer predictor of flowering due to its nonlinear relationship with light: > 80% of the reduction in light occurred within 4.5 m of the edge. Our results reveal strong, nonlinear relationships of flowering in time (most rapid early in the invasion process) and space (steepest at the forest edge). In a landscape dominated by forests, conifer invasion of mountain meadows can reduce the local and larger-scale diversity of plants and their insect pollinators.