Clearing vs. burning for restoring Pyrenean grasslands after shrub encroachment

Abstract

Anthropogenic activities have modified vegetation on subalpine belts for a long time, lowering the treeline ecotone and influencing the landscape mainly through grazing and fire. The abandonment of these traditional land use practices during the last decades and global warming are contributing largely to the colonization of woody species in subalpine grasslands, causing irreversible changes in ecosystem functioning. A variety of management strategies requiring the use of fire and/or clear-cutting are carried out to prevent the expansion of highly encroaching shrubs and improve the conservation status of subhumid high-productive grasslands ecosystems. However, it is still poorly understood how different management strategies affect the recovery of subalpine grasslands, which is influenced concurrently by the vegetation community involved. In this study we used field experiments to test the impact of management treatments on soil properties and vegetation responses in subalpine Bromion erecti grassland communities colonized by the pyrophyte shrub Echinospartum horridum (Vahl) Rothm. on the southern Pyrenees. Vegetation was monitored for 5 years in E. horridum plots (two plots per treatment) where the vegetation was removed by prescribed fire (Burnt treatment) or by mechanical removal (Clear-cut treatment). Undisturbed E. horridum plots were used as a control (C-Erizón) for shrub removal treatments and a Bromion erecti grassland community regularly grazed (C-Grass) was used as a control for subalpine grassland. Clear-cut treatment of E. horridum community was more effective to control E. horridum colonization than Burnt treatment and contributed to a higher extent to recover original grassland vegetation, because E. horridum seedling performed worse (lower germination rates) and plant species were more similar to the original grassland (legumes, non-legume forbs, and grasses). Shrubs and sub-shrubs cover in Burnt areas increased faster than in Clear-cut areas during the 5 years following the treatment, although it did not reach the level of C-Erizón. Species richness and diversity increased in comparison with C-Erizón in both treatments, but they were significantly lower than those in the C-Grass. Network connectivity was larger in well preserved grasslands, i.e, C-Grass, than in any other treatments, mainly dominated by negative plant-plant association. Soils nutrients declined in Burnt sites 4 years after the fire treatment and no difference was observed between Clear-cut and C-Erizón sites, although they did not reach the soil fertility values of C-Grass in terms of organic matter and C/N ratio. This study showed that grazing favors plant diversity and community complexity in subalpine grasslands. It also demonstrated that clearing is a better strategy than burning to restore grasslands after shrub encroachment because burning entails deeper soil degradation and faster recovering of the pyrophyte shrub, E. horridum.

Clearing shrubland as a strategy to encourage extensive livestock farming in the Mediterranean mountains

“…The average size of professional farms increased sharply between 1972 (27.6 LU) and2007 (121.7 LU), but showed a slightly decrease in the last decade. 1972, 20 more from 1993-2007and 12 more from 2007. In addition, between 1972…”

De la despoblación a la repoblación rural de las montañas

“…Se propone así una gestión mixta, que permita mantener un paisaje en mosaico dinámico que abarque los distintos estadios sucesionales, desde el mantenimiento de zonas forestales con muy baja intervención humana y zonas de matorral hasta zonas de aclareo para recuperación de pastos, cultivos y bosques de galería que permitan incrementar la conectividad del paisaje mediante el desarrollo de infraestructuras verdes (MITECO 2020). Para eliminar el matorral en las zonas seleccionadas para aclareo, se recomienda el uso del desbroce frente al fuego, evitando así la pérdida de suelo tras las quemas (Badía et al 2017;Nuche et al 2018;Alados et al 2019). Este modelo de gestión ha sido aplicado durante los últimos 35 años en los valles riojanos del Sistema Ibérico, con el desbroce progresivo de casi un 30% de la superficie ocupada por matorral en 1986 (Lasanta et al 2019).…”

Gestión activa frente a matorralización: buscando el equilibrio entre conservación y explotación en montaña

“…In a more recent article, Gómez et al (2019) recommend frequent clearance (every 1-2 years) and heavy grazing after clearing, in order to prevent the spread of unsuitable grasses for pasture, which are precursors of shrub recolonization. In turn, Alados et al (2019) studied the effects of prescribed burning and shrub clearance in sub-alpine pastures in the central Pyrenees on the recovery of E. horridum and on Bromion erecti pasture, finding that clearing was better at controlling colonisation of E. horridum than burning, as well as degrading the soil loss. They concluded by saying that clearing is a better strategy than burning to restore sub-alpine pasture land after invasion by shrubs.…”