Soil seed bank evaluation and seedling establishment along a degradation gradient in a semi-arid rangeland

Abstract

The effects of rangeland degradation on the size and species composition of the seed bank and seedling establishment in the field were quantified over a two year period (2000–2001 and 2001–2002 growing seasons). Soil seed bank sampling was carried out at three-monthly intervals from 0.25m2 blocks 50mm deep. Sampling was from rangeland artificially maintained in three different rangeland conditions viz. good, moderate and poor. The total seedling establishment in the field for every species was obtained at the end of the growing season. Soil temperatures at 100mm depth and soil-water content at 50mm depth intervals were recorded once a week. Due to the lower basal cover with rangeland degradation, soil temperatures increased significantly both daily and seasonally, while soil-water content also decreased. Rangeland in poor condition was characterised by a significantly higher seed density in the seed bank and more seedling establishment than grassland in good condition. No climax seedlings were present in rangeland in poor condition, while very few survived in rangeland in good condition at the end of the season. A mean seasonal seed bank density of rangeland in good, moderate and poor condition was respectively: 48, 74 and 98 seedlings m–2 for October; 28, 32 and 40 seedlings m–2 for January; and 58, 172 and 128 seedlings m–2 for April. Rangeland degradation showed a decrease in species richness in both the seed bank and seedling establishment in the field. A mean number of 1.67, 2.27 and 2.83 seedlings m–2 established in rangeland in good, moderate and poor conditions respectively at the end of the growing season.