Vegetative regeneration of biennial Oenothera species after disturbance: Field observations and experiment

It has been recently shown that some annual and biennial species of man-made habitats cope with severe disturbance by resprouting (vegetative regeneration) from their bud bank and do not only rely on regeneration from seeds. Nevertheless, information on the ecology of this phenomenon is rare. In a field study, we answered the question how frequent is resprouting from root buds in populations of the ruderal biennial herb Oenothera biennis, and how it is affected by habitat conditions. In an experiment, we tested the hypothesis that higher severity of injury and later life-cycle phase of the injured plants suppress resprouting from both axillary and root buds in O. biennis and also in its closely related congeners O. fallax and O. glazioviana.In 25 out of 29 studied ruderal populations of O. biennis severely injured individuals were found; however, only half of these populations included injured individuals that resprouted from roots. Among these populations, the number of root-sprouting individuals varied highly (from 3% to 67% of injured individuals). The largest populations and the highest percentage of root-resprouting individuals were found in urban habitats with sandy/gravelly substrate, a low vegetation cover, and a high frequency of disturbance.In the experiment with three Oenothera congeners, removal of aboveground biomass with all axillary buds largely led to the death of plants of all three species. When a portion of the basal axillary buds remained intact, individuals of O. biennis mostly failed to regenerate, whereas individuals of O. fallax and O. glazioviana survived and formed seeds. A higher severity of injury suppressed resprouting in Oenothera congeners in this experiment. However, the relationship between life-cycle phase and the ability to resprout remains unclear in Oenothera species.This study showed that resprouting after severe injury is an important feature of Oenothera individuals occurring in man-made habitats and may represent an alternative strategy to regeneration from the seed bank under disturbance conditions.