Declining potential nectar production of the herb layer in temperate forests under global change

Abstract Wild pollinators are crucial for ecosystem functioning and human food production and often rely on floral resources provided by different (semi‐) natural ecosystems for survival. Yet, the role of European forests, and especially the European forest herb layer, as a potential provider of floral resources for pollinators has scarcely been quantified. In this study, we measured the potential nectar production (PNP) of the forest herb layer using resurvey data across 3326 plots in temperate forests in Europe, with an average time interval of 41 years between both surveys in order to assess (i) the importance of the forest herb layer in providing nectar for wild pollinators, (ii) the intra‐annual variation of PNP, (iii) the overall change in PNP between survey periods and (iv) the change in intra‐annual variation of PNP between survey periods. The PNP estimates nectar availability based on the relative cover of different plant species in the forest herb layer. Although PNP overestimates actual nectar production, relative differences amongst plots provide a valid and informative way to analyse differences across time and space. Our results show that the forest herb layer has a large potential for providing nectar for wild pollinator communities, which is greatest in spring, with an average PNP of almost 16 g sugar/m 2 /year. However, this potential has drastically declined (mean plot‐level decline >24%). Change in light availability, associated with shifts in canopy structure and canopy composition, is the key driver of temporal PNP changes. Synthesis . Our study shows that if management activities are carefully planned to sustain nectar‐producing plant species for wild pollinators, European forest herb layers and European forests as a whole can play key roles in sustaining wild pollinator populations. Samenvatting Wilde bestuivers zijn cruciaal voor het functioneren van ecosystemen en de productie van voedsel voor mensen. Ze vertrouwen vaak op ecologische hulpbronnen van verschillende (semi‐)natuurlijke ecosystemen voor hun overleving, zoals nectar en stuifmeel. Toch is de rol van Europese bossen, en zeker de kruidlaag van Europese bossen, als potentiële leverancier van dergelijke hulpbronnen voor bestuivers nauwelijks gekwantificeerd. In deze studie hebben we de potentiële nectarproductie (PNP) van de kruidlaag van bossen gemeten met behulp van herhaalde vegetatieopnames in 3326 plots verspreid over verschillende gematigde bossen in Europa. Het gemidd