Reproductive effort of Caragana microphylla under long-term grazing exclusion in a semi-arid grassland

Abstract

Grazing exclusion is a commonly used method to protect and recover herbaceous plants in grasslands, and woody expansion is a common phenomenon in grasslands. However, the effects of grazing exclusion on shrub reproduction remain poorly understood. We investigated the effects of long-term grazing exclusion (0, 8, 30 and 34 years) on reproductive effort of Caragana microphylla in the semi-arid Inner Mongolia Steppe. Our results showed that the 8-year grazing exclusion significantly increased the number of flowers, juvenile pods, mature pods and seed (seed number increased 33.5%) of C. microphylla, however, the 30- to 34-year grazing exclusion significantly decreased seed production (seed number decreased 77.9% and 88.1%) of C. microphylla. The 8-year grazing exclusion had no significant effect on the single flower weight, while the 30- to 34-year grazing exclusion significantly decreased (31.7% and 38.7%) the single flower weight. The grazing exclusion had no significant effect on the seed number per pod. The 8-year grazing exclusion significantly increased sexual allocation (31.6%), and it had no significant effect on sexual allocation efficiency; however, the 30- to 34-year grazing exclusion significantly decreased the sexual allocation (50.4% and 70.4%) and sexual allocation efficiency (55.5% and 60.1%). The 8-year grazing exclusion significantly increased seed weight and seed vigor (seed weight increased 18.7%, seed vigor index increased 28.7%), whereas the 30- to 34-year grazing exclusion decreased these indexes (seed weight decreased 14.2% and 21.0%, seed vigor index decreased 9.1% and 14.7%). These results indicated that the 8-year grazing exclusion increased reproductive effort of C. microphylla, while the 30- to 34-year grazing exclusion sharply decreased the reproductive effort of this shrub species. Our study suggests long-term grazing exclusion limited the expansion of shrubs in grassland by decreasing sexual reproduction and thus inhibiting population growth of shrubs.