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.

The Trend of Grassland Restoration and Its Driving Forces in Ningxia Hui Autonomous Region of China from 1988 to 2018

“…Grazing reduces community resistance by preventing perennial herbs from competing with exotic plants [21], while grazing prohibition reduces the increase in exotic plant species [22]. At the same time, grazing prohibition is beneficial to the recovery of beetle communities [23], promotes the growth of nematodes [24], arbuscular mycorrhizae [25] and cryptosporidium [26], and is conducive to the expansion of woody plants [27]. Vegetation changes and bare land reduction [26] caused by the short-term grazing prohibition are beneficial for grassland birds to build nests and improve wildlife habitat quality [28,29].…”

“…(2) From the perspective of community productivity and diversity of grassland ecosystem caused by grazing prohibition, moss [61], grass layer [62], shrub layer, and woody vegetation will recover rapidly after grazing prohibition [27]. After 5 years of grazing prohibition, bryophytes in Austrian Tyrol have increased significantly, and the tall species in the subalpine steppe increased [61].…”

“…After 5 years of grazing prohibition, bryophytes in Austrian Tyrol have increased significantly, and the tall species in the subalpine steppe increased [61]. After 8 years of grazing prohibition, the number of flowers, young pods, mature pods and seeds, seed weight and vigor of Caragana microphylla increased significantly [27].…”

“…Long-term grazing prohibition will reduce the community diversity index and affect the ecological function of grassland [27]. Long-term grazing prohibition will also reduce the grassland community diversity index and affect grassland ecological function [36].…”

“…The effects of long-term grazing prohibition on species richness come from multiple sources, including reducing vegetation diversity by reducing soil seed banks [66], limiting prairie shrub expansion by reducing sexual reproduction and thus inhibiting shrub population growth [27], reducing willow species diversity by reducing the new stem height growth and supplementation forms a closed canopy [67]. After 34 years of grazing prohibition, the seed yield, single flower weight, sex distribution, seed weight, and seed vigor of C. microphylla were severely reduced [27]. Long-term fencing led to a significant decrease in plant density and abundance in meadow communities, which was not conducive to the growth and renewal of new seedlings, and affected the succession and development of alpine meadow communities [68].…”