Grassland restoration with and without fire: evidence from a tree‐removal experiment

Abstract

Forest encroachment threatens the biological diversity of grasslands globally. Positive feedbacks can reinforce the process, affecting soils and ground vegetation, ultimately leading to replacement of grassland by forest species. We tested whether restoration treatments (tree removal, with or without fire) reversed effects of nearly two centuries of encroachment by Abies grandis and Pinus contorta into dry, montane meadows in the Cascade Range, Oregon, USA. In nine, 1-ha plots containing a patchy mosaic of meadow openings and forests of varying age (20 to > 140 yr), we compared three treatments affecting the ground vegetation: control (no trees removed), unburned (trees removed, slash burned in piles leaving 90% of the area unburned), and burned (trees removed, slash broadcast burned). We quantified changes over 3-4 years in soils, abundance and richness of species with differing habitat associations (meadow, forest, and ruderal), and recruitment of conifers. Except for a transient increase in available N (especially in burn scars), effects of burning on soils were minimal due, in part, to mixing by gophers. Tree removal greatly benefited meadow species at the expense of forest herbs. Cover and richness of meadow species increased by 47% and 38% of initial values in unburned plots, but changed minimally in burned plots. In contrast, cover and richness of forest herbs declined by 44% and 26% in unburned plots and by 79% and 58% in burned plots. Ruderal species and conifer seedlings were uncommon in both treatments. Although vegetation was consumed beneath burn piles, meadow species recovered significantly after three years. Long-term tree presence did not preclude recovery of meadow species; in fact, colonization was greater in older than in younger forests. In sum, temporal trends were positive for most indicators, suggesting strong potential for restoration. Contrary to conventional wisdom, tree removal without fire may be sufficient to shift the balance from forest to meadow species. In meadows characterized by historically infrequent fire, small-scale disturbances and competitive interactions may be more critical to ecosystem maintenance and restoration. Managers facing the worldwide phenomenon of tree invasion should critically evaluate the ecological vs. operational need for fire in ecosystem restoration.

Contrasting effects of land use legacies on grassland restoration in burnt pine plantations

“…Prescribed burning is a commonly used method to eliminate conifer litter following tree removal (Halpern et al, 2012). The effects of burning depend on fire intensity, species tolerance to fire, and regeneration ability after fire (Funk and McDaniel, 2010).…”

“…The increase in available nutrients following burning may also promote target and non-target species differently (Sturgess and Atkinson, 1993). Fire is unnecessary when altered light conditions by tree removal alone effectively assist re generation (Halpern et al, 2012). Although fire is not part of the natural disturbance regime, low intensity fire does not impair open sand grasslands in the region (Ónodi et al, 2014).…”

“…Pine plantations alter vegetation composition in former grasslands (Szitár et al, 2014). Disturbance associated with the establishment of plantations promotes early-seral annuals (Maestre and Cortina, 2004), while altered light conditions favour forest herb establishment (Bremer and Farley, 2010;Halpern et al, 2012). Pine plantations have lower grassland species richness and abundance than former grasslands both in the aboveground vegetation and in the seed bank (Piqueray et al, 2011).…”

Plant functional groups: scientometric analysis focused on removal experimentsPlant functional groups: scientometric analysis focused on removal experiments

“…By knowing the role of a certain species group, it is possible to predict the effect of its introduction or removal from a community, such as the alteration on soil nutrient availability or a decrease in decomposition rates (Kong et al 2011;McLaren & Turkington 2010a). In this context, experimental studies are important because they can shed light on several ecological aspects: competition between distinct functional groups (Klanderud 2005;Liancourt & Tielboerger 2009); plant succession (Baldwin & Mendelssohn 1998;Cater & Chapin 2000); the effect of introduced plants (Halpern et al 2012); and the effect of changes in soil nutrients due to the removal or introduction of functional groups (McLaren & Turkington 2010a).…”

Hydrologic Response to Conifer Removal from an Encroached Mountain Meadow

“…In some regions, fire in addition to removal of encroaching vegetation may be the most effective management tool to promote the restoration of degraded meadow habitat because it was the primary mechanism associated with removal of encroaching vegetation before changes in fire regime (Caprio and Graber, 1999). However, meadow restorations performed by removing encroaching vegetation without the subsequent implementation of selective slash or broadcast burning have demonstrated that conifer removal alone is sufficient for short-term restoration of some encroached meadows, especially meadows with a history of infrequent wildfire (Swanson et al, 2007 andHalpern et al, 2012). Furthermore, broadcast burning and slash burning may be counterproductive to meadow restoration because of the attendant increases in soil nitrogen content and exposure of mineral soil, which induces conditions favorable to the establishment of ruderal species.…”