Forest degradation and short-term seasonal exclosure effects on biotic and abiotic factors linked to rangeland rehabilitation in semiarid Chaco, Argentina

Abstract

In semiarid Chaco, wet season grazing exclosures allow a rapid grass recovery in mature and secondary forest patches, but not in the most degraded patches dominated by shrublands. In order to understand the causes of their dissimilar rehabilitation dynamics, we analyzed the effects of the forest-patch type (mature forest = MF, secondary forest = SF, and shrubland = S) and 4-year seasonal exclosures (vs continuous grazing) on biotic and abiotic factors (vegetation structure, incident radiation, grass seed bank, litter and soil traits) that could affect grass recovery. We found that vegetation structure, through determining litter quantity, controlled soil chemical traits. As forest degradation level increased, litter mass (MF = 6260, SF = 5410, S = 1220 kg DM/ha), total soil organic matter (MF = 5.1%, SF = 4.1%, S = 3.5%) and nitrogen (MF = 0.25%, SF = 0.22%, S = 0.18%) decreased, and the incident radiation at soil level rose (MF = 28%, SF = 41%, S = 52%). After 4 years of seasonal grazing rest, we found reduced soil bulk density (from 1.13 to 1.07 and 1.07 to 1 g/cm3) and increased total porosity (from 57 to 60 and 60 to 62%) at 0–10 and 10–30 cm depth, respectively, in all forest patch types. Exclosure also increased soil seed bank density (from 28 to 251 grass seeds/m2) both in the forests (with grass) and in the shrublands (even before grass recovery), which would receive seeds from the adjacent forest patches, which coexist in the same landscape matrix. Our results suggest that the low tree cover, litter, and soil chemical traits related to lower availability of water and nutrient in shrublands, could hamper grass recovery in recently established exclosures. However, improvements in the grass seed bank and soil physical traits related to higher water infiltration could favor its late recovery, which is triggered by an extremely wet cycle after several years of the seasonal exclosure. Exclosure is a promising management strategy for degraded areas involving intermingled patches of forests and shrublands.