Seed dispersal in streams

How do plants in streams and rivers end up where they grow?

A simple answer is that many of them come floating with the water, a process called hydrochory. Hydrochory stands for the passive dispersal of organisms by water. It is an important means of propagule transport, especially for plants.  A substantial proportion of species growing in or near water have propagules – fruits, seeds or vegetative units – able to disperse by water, either floating, submerged in flowing water, or with the help of floating vessels.  For example, during floods, many propagules are dispersed on accumulations of floating debris.

Hydrochory has a number of important implications:

  1. It can enable plants to colonize sites out of reach with other dispersal vectors, but the timing of dispersal and mechanisms of establishment are important for successful establishment.
  2. Genetically, it may reduce spatial aggregation of genetically related individuals, lead to high gene flow among populations, and increase genetic diversity in populations receiving many propagules.
  3. At the population level, it may increase the effective size and longevity of populations, and control their spatial configuration.
  4. It is also an important source of species colonizing recruitment-limited riparian and wetland communities, contributing to maintenance of community species richness.
  5. It may even influence community composition in different landscape elements, resulting in landscape-level patterns. For example, many rivers have populations of alpine plants in the riparian zone, far away from the mountains.
Drift material in the riparian zone of the Vindel River in northern Sweden. Such debris may contain large numbers of plant propagules, like seeds of Angelica archangelica, one of which has germinated. The seeds of this species can float for more than one year.

Drift material in the riparian zone of the Vindel River in northern Sweden. Such debris may contain large numbers of plant propagules, like seeds of Angelica archangelica, one of which has germinated. The seeds of this species can float for more than one year.

How has hydrochory been impacted by people?

Humans have impacted hydrochory in many ways.  For example, dams affect hydrochory by reducing peak flows and hence dispersal capacity, altering the timing of dispersal, and by presenting physical barriers to dispersal, with consequences for riverine plant communities.  Hydrochory has been inferred to be an important vector for the spread of many invasive species, but there is also the potential for enhancing ecosystem restoration by improving or restoring water dispersal pathways.  Climate change may alter the role of hydrochory by modifying the hydrology of water-bodies as well as conditions for propagule release and plant colonization.

/Christer Nilsson