DOI: 10.5061/dryad.v5f76v4
title: Data from: Past tree influence and prescribed fire exert strong controls on reassembly of mountain grasslands after tree removal

description: 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 and
 periodically after tree removal to quantify the individual and interactive
 effects of past tree influence and fire on grassland community reassembly.
 Adjacent, uninvaded meadows served as references sites. ‘Past tree
 influence’ was defined as the multivariate (structural or compositional)
 distance of subplots to reference meadows prior to tree removal.
 ‘Reassembly’ was defined as the distance, or change in distance, to
 reference meadows at final sampling. Consistent with theory, we observed
 greater reassembly of plant community structure than of composition, as
 loss of meadow specialists was offset by establishment of
 disturbance-adapted meadow generalists of similar growth form.
 Nevertheless, 8 years after tree removal, most subplots remained
 structurally and compositionally distinct from reference meadows.
 Furthermore, fire had both destabilizing and inhibitory effects: it
 reduced survival of meadow specialists across the range of encroachment
 states and, where past tree influence was greater, it stalled reassembly
 by promoting expansion of a highly competitive native meadow sedge. The
 slow pace of reassembly, despite abundant open space, suggests strong seed
 limitation—a condition exacerbated by burning. We present a novel test of
 the importance of past tree influence and fire for restoration of
 tree-invaded grasslands, offering insights into how constraints on
 community reassembly vary along a continuum of tree-altered states.
description: <strong>Species_cover_exp_subplots</strong><br>Species 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). <strong>Species_cover_RM</strong><br>Species 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). <strong>GF_cover-richness_exp_subplots</strong><br>Growth-form cover and richness in unburned (UB) and burned (B) experimental subplots sampled before (2004) and after (2007, 2009, and 2013) treatment. <strong>GF_cover-richness_RM</strong><br>Growth-form cover and richness in the 117 reference-meadow transects sampled in 2013. <strong>Burn_severity</strong><br>Post-burn estimates of burn severity. <strong>Distance_to_RM</strong><br>Mean compositional (Bray-Curtis) dissimilarity and structural (Euclidean) distance of experimental subplots to reference meadows before (2004) and after (2007, 2009, and 2013) treatment. <strong>Within-plot_heterogeneity</strong><br>Within-plot heterogeneity of species composition (Bray-Curtis dissimilarity) and community structure (Euclidean distance) before (2004) and after (2007, 2009, and 2013) treatment.