Past tree influence and prescribed fire exert strong controls on reassembly of mountain grasslands after tree removal

Woody-plant encroachment represents a global threat to grasslands. Although the causes and consequences of this regime shift have received substantial attention, the processes that constrain reassembly of the grassland state remain poorly understood. We experimentally tested two potentially important controls on reassembly—the past influence of trees and the effects of fire—in conifer-invaded grasslands (mountain meadows) of western Oregon. Previously, we had reconstructed the history of tree invasion at fine spatial and temporal resolution. Using small subplots (10 × 10 m) nested within larger (1-ha) experimental plots, we characterized the fine-scale mosaic of encroachment states, ranging from remnant meadow openings (minimally altered by trees) to century-old forests (lacking meadow species). Subsequently, we removed trees from six plots, of which three were broadcast burned and three remained unburned (except for localized burn piles). Within each plot, subplots were sampled before ..., Species_cover_exp_subplotsSpecies cover in unburned (UB) and burned (B) experimental subplots before (2004) and after (2007, 2009, and 2013) treatment. Includes 30 meadow species present in at least 1% of samples (subplots or transects x sampling dates).
Species_cover_RMSpecies cover in the 117 reference-meadow transects sampled in 2013. Includes 30 meadow species present in at least 1% of samples (subplots or transects x sampling dates).
GF_cover-richness_exp_subplotsGrowth-form cover and richness in unburned (UB) and burned (B) experimental subplots sampled before (2004) and after (2007, 2009, and 2013) treatment.
GF_cover-richness_RMGrowth-form cover and richness in the 117 reference-meadow transects sampled in 2013.
Burn_severityPost-burn estimates of burn severity.
Distance_to_RMMean compositional (Bray-Curtis) dissimilarity and structural (Euclidean) distance of experimental subplots to reference meadows before (2004) and after (2007, 2009, and 2013) treatment.
Within-plot_heterog..., ,