Extensive hydrochory uncouples spatiotemporal patterns of seedfall and seedling recruitment in a ‘bird‐dispersed’ riparian tree

Abstract

Summary Initial recruitment is by far the most variable and least predictable phase of the plant life cycle. Its dynamics remain little known, particularly in complex seed dispersal systems and over multiple reproductive cycles. Seeds of the tree Frangula alnus are usually dispersed by frugivorous birds. Mediterranean populations, however, are mostly restricted to riparian forests, and their seeds may be re‐distributed during the winter by elevated stream water flow. Strength and frequency of peak discharges vary greatly from year to year. I investigated spatiotemporal variation of initial recruitment by monitoring fruit production, seedfall and seedling emergence over 4 years along permanent transects in two F. alnus populations growing along small mountain streams in southern Spain. Seedling abundance varied greatly among years and independently of fruit or primary seedfall abundance. Spatial distributions of both seedfall and seedling emergence were highly consistent among years. Bird‐mediated seedfall density decreased exponentially with both increasing distance from the nearest fruiting tree and the decrease of its crop size. Seedling emergence was, however, uncoupled in space from the previous year's seedfall, both across single sample points and entire 60‐m transects. Dispersal experiments with seed mimics revealed that flooding caused secondary dispersal of much of the population seed pool over several scores of metres. Seed re‐distribution by water appears to override patterns of primary (bird‐generated) seedfall and to determine spatial patterns of seedling recruitment. Seedling emergence is, however, more stable across years than would be expected given the unpredictability of rainfall (and thus stream flow), suggesting that seed deposition may be primarily determined by relatively stable microsite characteristics. The dynamics of seed rain patterns are of critical importance for plant population demography and spatial structures. The finding that even complex, multistep dispersal systems may produce remarkably consistent year‐to‐year distributions of recruits has important implications for the interpretation of long‐term patterns of riparian plant regeneration at the landscape level and resulting conservation management.