Effectiveness and limitations of fencing on the conservation of the grassland specialists in a semi-natural grassland degraded by increased large herbivores

Abstract Rare grassland herbs are increasingly threatened by increase in large herbivores. Exclosures that exclude large herbivores can be effective tools for the protection and conservation of grazed plant communities. However, installation of the exclosures in abandoned semi-natural grasslands has two aspects: protection of vegetation and elimination of disturbance by herbivores. We aimed to clarify the exclosures promote or hinder the conservation of grassland herbs in a semi-natural grassland. The field survey was conducted in a semi-natural grassland under grazing pressure by sika deer in central Japan. Fences were erected in 2010 and 2011. Plant species composition and species in flower were recorded from 2011 to 2022 inside and outside the fences. We compared the change of species composition with species composition data recorded in the same grassland patch in 1980s, before deer numbers increased, as reference. Within the fence erected in 2010, species composition was getting similar to the reference from 2011 to 2017, but regressed by 2022. Correspondingly, the cover of grassland herbs and the number of grassland herbs in flower first increased and then decreased. Tree species conversely increased from 2017 to 2022. Within the fence erected in 2011, the species composition was hardly getting similar to the reference, and grassland herb cover remained lower. Long-term fencing can lead to a decline in grassland herbs with an increase in competing shrubs though the grassland herbs temporarily increase within the fence erected earlier. Anthropogenic disturbance may be necessary for the conservation of grassland herbs in long-term fenced grassland communities.