Loss of branches due to winter storms could favor deciduousness in oaks

Abstract

Premise Ecologists have an incomplete understanding of the factors that select for deciduous, evergreen, and marcescent leaf habits. Evergreens have more opportunities for photosynthesis but may experience costs when abiotic conditions are unfavorable such as during ice and windstorms. Methods We documented branch loss for species of oaks (Quercus spp.) in a common garden in California during an unusual windstorm. Results Branches of marcescent trees were more likely to break during the storm, and this pattern had a negligible phylogenetic signature. Branches of evergreen and marcescent species were mostly alive before breaking, which likely accrued a fitness cost, while those of deciduous species were mostly already dead. One explanation for the overrepresentation of broken branches from marcescent species is that their petioles are inflexible and have greater wind loading compared to the flexible petioles of evergreen leaves and the leafless condition of deciduous branches. Conclusions These results suggest that branch loss during unusual winter storms may be an important cost of a marcescent leaf habit.

Not all temperate deciduous trees are leafless in winter: The curious case of marcescence

“…A null hypothesis for any trait, including marcescence, is that it is an evolutionary byproduct with no current adaptive function. There is a strong phylogenetic pattern at our site for leaf marcescence, with nearly all species exhibiting the trait belonging to the plant order Fagales (oaks, beech, and chestnut; Appendix S1: Figure S4), a conclusion that has been noted elsewhere (Karban & Pearse, 2021). In these plant families, it is possible that the marcescent leaf habit may be a byproduct of incomplete evolution of deciduousness from evergreen ancestors.…”

“…By extension, delayed autumn senescence might increase marcescence, or early autumn frosts might lead to failure to form a proper abscission layer. A recent study of California oaks demonstrated that marcescent species are at higher risk to branch loss during winter storms than other species (Karban & Pearse, 2021). Longer term observational data are needed, alongside marcescent leaf removal experiments.…”