Climate, and not fire, drives the phylogenetic clustering of species with hard-coated seeds in Mediterranean Basin communities

Abstract

Hardseededness is a common trait in Mediterranean plant communities, although the origin of its selection is controversial. It may be a mechanism of persistence to fire temperatures, but could also form part of a gap-detecting mechanism to provide germination cues under arid conditions. To disentangle this, we studied the phylogenetic structure of plant communities against fire frequency and aridity gradients. The phylogenetic structure in Mediterranean Basin ecosystems was analysed for the hardseededness trait as a whole and was separated by the families composing this trait (Fabaceae and Cistaceae). This study focused on woody perennial species. The phylogenetic structure was also contrasted against soil classes. Hardseededness on the whole, and for the Fabaceae family alone, showed phylogenetic clustering as aridity increased. Cistaceae displayed the opposite pattern with phylogenetic clustering in most humid areas, together with a significant soil effect. Surprisingly, fire frequency had no influence in any case. This climate-driven phylogenetic clustering indicates that the hardseededness trait could confer some fitness advantage under dry conditions. For this reason, coexisting species were more closely related in the community with increasing aridity. This effect was especially evident for the Fabaceae family. These results shed some light on the evolutionary selection of this adaptive trait under Mediterranean conditions. Our results question the role of fire in the selection of the hardseededness trait in Mediterranean Basin ecosystems and indicates that climate is the most important factor. Therefore, we should be cautious in assigning to fire a preponderant role in the selection of some plant traits.