Mowing increases the vulnerability of sand grassland restoration to secondary invasion despite the sensitivity of invasive species to drought

Increased frequency and severity of droughts as a result of climate change coupled with invasions by alien species may have a significant impact on vegetation development and thus on ecological restoration efforts. We studied the impact of restorative mowing, time and drought severity on sand grassland restoration in Hungary. Using data spanning 1995–2019, we explored the dynamics of target, invasive and C4 species. Restoration interventions involved the removal of invasive Robinia pseudoacacia (1994/1995) and regular mowing (1995–2001) at three sites. Rising temperatures and more frequent summer droughts were measured at all sites based on long-term weather data (1995–2020) from the FORESEE-HUN v1.0 database. Target species were not significantly affected by drought, C4 species had higher proportions at higher drought severity, while invasive species generally had higher proportions at lower drought severity. Mowing was neutral to target species and had a positive impact on invasive and C4 species. Mowing amplified drought responses only for C4 species in the short term in August. Our results confirm that sand grassland vegetation is resistant to drought during restoration. Mowing was found to control targeted invasive R. pseudoacacia but is insufficient for accelerating the regeneration of sand grassland and can promote secondary invasion by herbaceous species, with long-lasting effects, despite the negative impact of droughts on invasive species. Our results also highlight the need to consider not only the length of the monitoring period, but also the frequency of data collection and its relationship to environmental changes, such as the expected frequency of droughts.