Does stream structure affect dispersal by water? A case study of the invasive tree Ailanthus altissima in Spain

摘要

Riparian zones are highly susceptible to invasion by alien plants. For both invasive and non-invasive plants, water has been proved to be an important vector for seed dispersal, connecting distant populations and, therefore, contributing to the downstream expansion of invaders. However, the effect of intrinsic factors of watercourses, such as habitat quality, on the invaders’ dispersal ability is largely unknown. We compared the dispersal ability of Ailanthus altissima fruits through a release experiment in two contrasting 100 m-long stretches of a near-natural and a degraded reach of the same river. One hundred fruits were released in the centre of the channel and allowed to float during 90 minutes. Results showed that fruit retention was five times lower in the degraded reach, suggesting that the loss of complexity in the fluvial habitat structure may increase dispersal distances of A. altissima fruits in headwater streams of similar characteristics. The lack of potential depositional zones, such as pools, meanders, or biological structures in the degraded reach increased the water dispersal success of A. altissima fruits. Among the studied retentive structures, macrophytes were found to be the main natural barrier to limit dispersal of A. altissima fruits. Velocities of dispersed fruits were highly variable within each reach but did not depend on the habitat quality. This study provides reasons to consider the heterogeneity and complexity of riparian and fluvial habitat to control the dispersion of fruits of the invasive tree A. altissima.