Abstract: Pollination is essential for biodiversity and ecosystem function, yet artificial light at night (ALAN) threatens these interactions, particularly in understudied high-altitude ecosystems. We conducted a field experiment in an alpine meadow on the Tibetan Plateau to evaluate the relative contributions of diurnal and nocturnal pollination and their sensitivity to ALAN in two species with contrasting mating systems: the self-incompatible Pedicularis kansuensis and the self- compatible Euphrasia regelii. By manipulating light conditions (ALAN vs. natural dark) and pollinator access (full exclusion, nocturnal only, diurnal only, and open pollination), we quantified pollinator visits, conspecific pollen deposition, and seed set. The self-incompatible P. kansuensis relied substantially on nocturnal Lepidoptera (mainly Noctuidae), which accounted for 72.3% of night-time visit. This nocturnal pollination contributed an additional 26.71% of conspecific pollen number per stigma and accounted for 35.71% of total seed set. However, ALAN severely disrupted this interaction, reducing visitation frequency of Lepidoptera by 49.1%, nocturnal pollen receipt by 53.8%, and seed set by 56.0%. In contrast, the reproductive success of the self-compatible E. regelii was unaffected by ALAN, as it lacked nocturnal pollinators and relied on diurnal visitors and autonomous selfing (accounting for 65.0% of seeds). Our findings demonstrate that ALAN selectively impairs plant reproduction by altering pollinator composition, with effects critically dependent on a species' mating system and pollinator dependence. The results underscore the urgent need to integrate the conservation of nocturnal pollination into light-pollution mitigation strategies, particularly in sensitive alpine ecosystems.

Key words: Alpine meadow, Nocturnal pollination, Artificial light at night, autonomous selfing, pollen deposition