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

Abstract

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 reference 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, eight 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.

Assessing restoration success by predicting time to recovery—But by which metric?

“…Piqueray et al, ). However, we acknowledge that species richness and other generic variables may be valuable metrics shedding light on the restoration process when restoring the species composition is not feasible (Halpern, Antos, Kothari, & Olson, ). For example when ecosystems are considerably degraded, appropriate reference systems do not exist and full recovery will neither be attainable or desirable (McDonald et al, ).…”

Rates, patterns, and drivers of tree reinvasion 15 years after large‐scale meadow‐restoration treatments

“…At BG, it took little more than a day for a crew of six using hand tools (loppers and saws) to cut and scatter the more than 800 trees sampled. Although prescribed fire can also be used to kill newly established tree seedlings, in this system it can have adverse effects-reducing survival or slowing recolonization of meadow specialists (Halpern et al 2016(Halpern et al , 2019. Given the pace of recruitment and slow growth of shade-tolerant A. grandis, a 20-year maintenance schedule for removal may be sufficient.…”

“…In 1995, it was targeted as a priority area for restoration as part of the Upper McKenzie Watershed Analysis, with the objectives of enhancing wildlife habitat (forage quantity and quality), restoring native grassland/meadow, and reducing fuel loadings. The current experiment, initiated in 2003, was designed to test the efficacy of tree removal-with or without fire-in achieving these objectives (Halpern et al 2012(Halpern et al , 2014(Halpern et al , 2016(Halpern et al , 2019.…”

“…Results after 8 years indicated that tree removal promoted widespread colonization by meadow species across a range of encroachment states (decades-to century-old forests). However, burning of woody residues had adverse effects-reducing survival of some target species and promoting expansion of a highly competitive meadow sedge, thus stalling reassembly (Halpern et al 2012(Halpern et al , 2016(Halpern et al , 2019. Over the same period, conifers reinvaded gradually at low density, irrespective of fire treatment, primarily along the forested edges of plots (Kremer et al 2014).…”

Patterns of conifer invasion following prescribed fire in grasslands and oak woodlands of Redwood National Park, California

“…In this context, perturbations to the fire regime can result in major changes to vegetation structure, analogous to responses that can occur following the removal of key components of the food web (Beschta & Ripple 2009; Ripple et al 2016). One example of such an effect is the invasion, or encroachment, of conifers into woodlands and grasslands following fire exclusion (Coop & Givnish 2007; Zald 2009; Sahara et al 2015; Halpern et al 2019).…”