Productivity‐dependent effects of adapted mowing regimes on arthropod communities in roadside grasslands

Abstract Arthropod diversity is rapidly declining in semi‐natural grasslands. Among the driving factors are nitrogen deposition and climate change, which typically increase vegetation productivity and extend the growing season. These changes can negatively affect plant diversity and simplify vegetation structure and thereby indirectly reduce arthropod diversity. Adjusting the mowing frequency, height and/or timing may counteract these effects and address shifts in seasonal activity of arthropods. This study evaluated the effects of these adjustments on arthropod diversity under current environmental conditions. We performed a 3‐year (2021–2023) field experiment in 10 Dutch roadside grasslands across a productivity gradient. The standard late August mowing (control) was compared to early November mowing and to June mowing at high (15 cm) or low (6 cm) height followed by either late August or early November mowing. In 2023, we assessed the effects of these mowing treatments on ground‐dwelling arthropods (sampled using pitfall traps), emerging arthropods (soil‐ and vegetation‐dwelling arthropods sampled using emergence traps) and flower availability (used as a proxy for food availability for pollinators). Our results show that delaying the final cut to early November was advantageous for late‐season emerging arthropods and flower availability regardless of site productivity. Emerging arthropod biomass in early autumn was approximately equal to that in spring and summer, underscoring the importance of delaying the final cut. In low‐productive sites, increasing the mowing frequency from one to two cuts per year did not benefit any species group. Conversely, in high‐productive sites, increasing the mowing frequency generally increased the diversity of ground‐dwelling arthropods, particularly in summer, and increased flower availability in spring. These benefits were most pronounced with a low cut in June. Yet, the June cut caused a relatively long (>1 month) reduction in flower availability and a lower summer biomass of emerging arthropods, regardless of cutting height. This suggests that regimes with two cuts per year could be more beneficial if applied in a phased or rotational manner. Synthesis and applications . Adjusted mowing dates and, in productive grasslands, an increased mowing frequency may benefit grassland arthropod communities under current environmental conditions.