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

Premise of the study. 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.

Key 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 prior to 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.

Methods
We recorded the number of branches that had fallen during a windstorm in a common garden of oak species. We characterized trees as being either evergreen, marcescent, or deciduous and compared the likelihood that downed branches were associated with these leaf retension habits. We also recorded whether downed branches had leaves and whether they were still green and alive when they broke. We evaluated whether observed patterns had a phylogenetic signature by mapping the likelihood of branch loss on a published oak phylogeny. We evaluated one possible mechanism for the high rate of branch loss among marcescent oaks by experimentally subjecting green and brown leaves of Quercus lobata to a standardized gust of wind and observed how far the wind moved the leaves.