At least here in southern Sweden, the autumn colours have been fantastic this year! As an evolutionary ecologist one starts wonder: why does trees differ in level of coloration? Is it only a benefit to the tree? Or are there costs associated with it as well? And why are some leaves red early in spring? In the new Early View paper, “Red young leaves have less mechanical defence than green young leaves”, Ying-Zhuo Chen and Shuang-Quan Huang have found answers to some of these questions. Here is a short version by Shuang-Quan Huang:
People in North Temperate Zone often enjoy seeing colorful leaves in autumn, an obvious phenomenon in deciduous forests. Evolutionary biologists Hamilton and Brown (2001) considered that autumn leaf coloration is expensive because it involves the costs of pigment synthesis, resource loss and loss of primary production (photosynthesis). A functional hypothesis proposed by Hamilton & Brown (2001) and Archetti (2000) suggest that leaf redness could be an adaptive strategy as a warning signal reducing insect attack (anti-herbivory hypothesis), but is still largely controversial.
Many species have red young leaves in spring. For example, an investigation of tropical plants in a Nature Reserve, Singapore showed that 60% species with young leaves were red (see Dominy et al. 2002). The anti-herbivory hypothesis has also been adopted to explain color change during leaf development. We are interested in why so many species produce constant green leaves in its life cycle if leaf-color change involves an obvious benefit (less herbivory). A similar question was asked by David Lee (2002) who argued that if anthocyanins in leaves confer some physiological/selective advantage, how do the species lacking anthocyanins compensate?
To test whether green leaves reduce herbivory by physical defense as an alternative to the supposed warning signal of red leaves, we conducted comparative analyses of leaf color and protective tissues of 76 woody species around our campus Wuhan University, a subtropical area in central China. We found that the species with green young leaves showed a significantly higher incidence of enhanced cuticle, multiple epidermis, and trichomes compared to species with red young leaves. This analysis suggests that green leaves may compensate for the lack of anthocyanins by adopting enhanced physical defense. Our finding of relatively poor mechanical protection in red young leaves may provide new evidence for the adaptive explanation of leaf color change.