干旱区生态环境知识资源中心

Exclosures are widely used for rangeland restoration in semiarid woodlands. However, grass recovery could be hampered if degradation exceeded certain thresholds. In this study we assessed four years effects of seasonally grazed exclosures vs. open rangelands on understory cover (grasses, low shrubs and litter) and peak standing biomass in three increasing degradation stages mature forests, secondary forests and shrublands in semiarid Chaco woodlands. We found that grass cover and biomass increased tenfold in four years in mature and secondary forests but remained virtually null in shrublands. In rested forests, the grass cover increments remained relatively constant regardless the annual rainfall amount, both in the driest year 2013 (531 mm) and the wettest year 2015 (924 mm). Only in an extraordinarily wet year (2015) did grass biomass increased in rested shrub lands. In that wet year, low shrubs cover higher at higher overstory degradation decreased in all woodlands. Our results suggest that shrublands may constitute a new steady state unable to recover by grazing exclusion itself, but probably by its combination with wet periods.