Abstract
Leaf habit, whether a plant is deciduous or evergreen, is one of the most conspicuous traits in plant species, ecosystems, and biomes. In this review, we discuss theories and empirical patterns of deciduous and evergreen habits with a focus on the optimal strategies. Historically, optimal leaf habits and leaf lifespans (LL) have been explained from the viewpoints of carbon economy and nutrient use. In the leaf economics spectrum, deciduous and evergreen species are positioned at the resource-acquisitive and resource-conservative sides of the spectrum, respectively. Seasonal variations in leaf traits of deciduous species could be understood simply as optimization of carbon economy in relation to the progress of the growth season. Evergreen species, however, exhibit more complex patterns, which could be understood as acclimation to seasonal environmental changes or resource allocation to leaf replacement and reproduction. Optimal timing of leaf shedding in evergreen species varies depending on the limiting factor for leaf retention. The return on investment (RI) through the lifetime of a leaf should be greater in evergreen species when there is no environmental suppression. However, RI of evergreen species is often suppressed by seasonal environment or species interaction. Deciduous species can have comparable RIs with evergreen species when they can occupy better environment such as gaps in forests. In conclusion, which leaf habit is advantageous can be understood from trait optimization perspectives. Such knowledge should help forecast how forest ecosystems will respond to changing climate and other anthropogenic environmental impacts.