Management methods for conserving a semi-natural grassland degraded by heavy grazing by large wild herbivores

• Mowing inside the fence promoted grassland-herb conservation. • Mulching inside the fence had limited restoration effects. • Non-grassland herbs increased outside the fence. Rare grassland herbs face increasing threats from large herbivores in semi-natural grasslands. Conservation efforts are needed to maintain plant species composition affected by grazing pressure. This study examined the impact of vegetation management with herbivore exclusion via fencing on declining grassland species affected by grazing. We conducted a field survey in a semi-natural grassland in central Japan, where sika deer populations have been increasing. In 2010, a fence was installed, and three treatments were applied within it: annual mowing, annual mulching, and no management. Changes in species composition were compared with baseline data from the 1980s, prior to the increase in the deer population. Until 2015, the species composition across all treatment areas closely resembled that of the reference community. By 2022, the mown area inside the fence showed the highest similarity to the reference and the greatest dominance of grassland herbs. In contrast, the unmanaged area showed a decline in grassland herb species after 2017, with an increase in woody species. The mulched area exhibited higher cover of non-grassland herbs relative to the other fenced plots, with recovery patterns similar to those observed in the abandoned area. These findings imply that fencing combined with mowing is the most effective strategy for conserving grassland herbs under grazing pressure.