Black alder may enhance riparian buffer mitigation of pine-plantation effects on macroinvertebrate food webs in headwater streams

• High-quality litter from riparian alder reduces the nutritional imbalance between benthic consumers and basal resources. • Autochthonous resources become more important in streams lacking riparian alder. • Riparian alder may lead to longer instream food chains, particularly during winter. • Stream food-web structure persists across riparian types due to benthic macroinvertebrates feeding plasticity. Over the past century, drylands have undergone significant landscape transformations. Abandonment of traditional crops and pastures led to development of extensive afforestation programs with conifers, which often lacked an ecologically sound orientation, raising concerns on their potential consequences on recipient ecosystems. Forest streams heavily rely on inputs of terrestrial organic carbon and thus are particularly vulnerable to human-driven changes in catchment and riparian forests. One point of uncertainty is whether existing stands of deciduous trees in the riparian zone may buffer headwater stream food webs from the impacts of afforestation on the surrounding landscape. We used stable isotopes of carbon and nitrogen to investigate whether the presence of the nitrogen-fixing black alder in the riparian zone alters the impacts of pine plantations on macroinvertebrate food webs of headwater streams. We observed a consistent consumption of leaf litter by shredders, but a higher importance of autochthonous support to all macroinvertebrate functional feeding groups than initially expected, especially in absence of alder. In addition, we discerned a potential trend toward a food chain lengthening at streams holding riparian alder in winter. Overall, our results indicate that riparian alder can enhance the buffer effect exerted by other broadleaf species through a reduction of the usual wide nutritional imbalance existing between benthic consumers and resources, which may translate into longer food chains. These findings highlight the critical role of riparian vegetation, particularly deciduous species like black alder, in maintaining headwater stream ecosystem integrity within afforested landscapes. Incorporating riparian vegetation management into afforestation planning can enhance stream food-web stability and support more balanced aquatic ecosystems.