Bird-mediated seed dispersal of fleshy fruits of mediterranean shrubs in semiarid forest patches: the role of Pinus halepensis Miller trees as seed receptors

Abstract

In Mediterranean semiarid areas, limited water supply confers to woody non-arboreal formations (scrublands) a capital importance. Fleshy fruiting shrubs provide food resources for vertebrates giving rise to well-known mutualisms, poorly studied in marginal Mediterranean areas. We assessed seed dispersal in three shrub species (Rhamnus lycioides spp. lycioides Brot., Asparagus albus L., Pistacia lentiscus L.) considering birds as the main vector and pine trees (Pinus halepensis Miller.) as key structural elements in seed deposition and establishment. The study area is a small forest fragment with an artificially high standing density of pine trees. The initial hypothesis is that birds direct dispersal towards the tree stratum, thus conditioning shrub regeneration. Collector trays, placed under shrubs and pine trees (potential perches), were used to estimate fruit consumption and seed rain. Shrub density and regeneration were assessed through vegetation sampling differentiating three age classes (seedlings, saplings and adults). The highest estimated consumption by birds was recorded on A. albus (76 % of the fruits available). R. lycioides and P. lentiscus were less consumed (47 and 36 %) but achieved a relatively important dispersal towards perches (0.88 % of the total fruit crop, and 1.89 % of the fruits consumed in Rhamnus, and up to 2.94 and 8.18 % in Pistacia, compared to only 0.09 and 0.13 % in Asparagus). More than 80 % of A. albus seedlings grew under the tree canopy, also the microhabitat preferred by A. albus and R. lycioides saplings. Approximately half of the extant adult shrubs were under the tree layer (more than 80 % considering dead trees), although canopy cover was only 13.81 %. Only a small fraction of the seeds was deposited under trees, but their canopy seemed to provide a favourable regeneration niche, at least during the germination and establishment phases. The management of tree density seems thus crucial for fostering the transition of these reforestations towards healthier and more resilient woody formations.