Evolution of herbs: key to the conundrum might be tolerance not avoidance

Abstract

Abstract Aims Woody plants represent the ancestral growth form in angiosperms with herbs evolving repeatedly from them. While there are a number of hypotheses about drivers of the evolution of the herbaceous habit, the ability to avoid frost damage in winter by discarding their aboveground biomass has often been invoked as the main force in their evolution. We propose instead that any unpredictable disturbance might have been much more important than the seasonal frost, as herbs easily survive repeated disturbance. Methods We tested this hypothesis by comparing herbs and woody plants in their ability to deal with three types of simulated disturbances, more predictable winter freezing, less predictable spring freezing and herbivory. Comparison was made in an experimental common garden setup with 20 species differing in woodiness. We evaluated the effects of these disturbances on mortality and regrowth of plants. Important Findings Herbs did not have an advantage over woody plants in survival when exposed to winter freezing. In less predictable conditions of spring freezing herbs survived the treatment better than woody plants and this advantage was even larger in case of the simulated herbivory treatment. The advantage of herbs over woody plants in less predictable conditions suggests that herbaceous growth form might be an adaptation to unpredictable disturbance, which herbs are able to tolerate thanks to their ability to survive loss of aboveground biomass. Consequently, factors such as mammal herbivory or fire might have been the most likely factors in the transition from woody species to herbs.