Contrasting impacts of climbing plants on host tree reproduction in a drought-stressed forest

Climbing plants, or climbers, are known to negatively affect the survival and reproduction of tropical and temperate humid forest trees through competition and structural parasitism. These impacts are attributed to their growth strategy, which relies on other plants for mechanical support and allows them to divert resources away from structural investment toward vegetative and reproductive functions. Such negative interactions may ultimately influence the composition and dynamics of plant and animal communities around them. Effects of climbers on hosts may differ in drought-stressed systems because investments in foliage and reproductive structures could favor facilitative interactions, such as abiotic stress amelioration through shading or pollinator attraction through synchronous flowering. Knowledge of climber–host interactions in Mediterranean and drought-stressed forests in general is limited. To test the hypothesis that climbers can facilitate host reproduction via synchronous flowering in a drought-stressed Mediterranean forest, we assessed the effects of climber cover on the probability of flowering, fruiting, and final fruit set of Crataegus aronia, a common insect-pollinated tree. Overall, climber cover was negatively related to flowering probability but positively related to fruiting probability and final fruit set. These effects differed between climbers with distinct flowering phenology relative to the host. Non-co-flowering climber effects on flowering probability were negative. However, co-flowering climber effects on fruiting probability and final fruit set were positive. These positive effects on C. aronia are in stark contrast with pervasive negative effects of lianas in tropical moist forests. The differing effects of climbers with distinct phenology suggest involvement of pollinator attraction. Although limited to the studied tree species in a Mediterranean forest, the documented positive effects of climbers in this drought-stressed forest and their relation with synchronous flowering highlight the need for trait-based analyses of climber–host interactions across systems, particularly to expand our understanding of their ecological roles under varied climate conditions.