Hygrochasy, the opening of a fruit in response to moisture and subsequent dispersal of seeds via raindrops, is predominantly known from plants in arid regions. The evolution of this unusual dispersal mechanism is mostly associated with temporal and spatial dispersal restriction, the spreading of germination risks, safe site strategies or protection against seed predators. In recent years, however, hygrochasy was also shown in a number of plant species from different, non-arid habitats around the world.
The aim of this study was to test whether hypotheses developed for hygrochasy in plants of arid regions apply to unrelated hygrochastic species in different habitats globally.
Hygrochastic species of the Aizoaceae (Southern Africa) and Veronica (Plantaginaceae) (New Zealand) were compared with hygrochastic Oenothera (Onagraceae) (Northern America) in dispersal experiments and cluster analyses of morphological data, environmental and distribution data was carried out. Character evolution of dispersal mechanisms was also traced in all study groups.
For Aizoaceae and Veronica, strong support for some hypotheses was shown, whereas most present day hygrochastic Oenothera do not seem to conform to them. However, Oenothera evolved as part of the Madro-Tertiary flora, when the temporal restriction of dispersal might have played an important role in response to dry, highly seasonal climate with unpredictable rainfall and hygrochastic capsules would have been highly advantageous. Therefore, hygrochasy seems to be an ancient relict, which is still present in most species of one Oenothera subclade.
In conclusion, hygrochasy is a very successful strategy for various plant taxa if the restriction of dispersal in space and time is advantageous, for example in regions with restricted rainfall or a heterogeneous environment with small suitable habitat patches.