A Lovely Latitudinal Leaf Gradient
In wet tropical forests, woody plants with toothed or lobed leaves are few and far between, while most leaves in temperate regions (think maple, oak, birch) do display these characteristics. There is a very strong latitudinal gradient of leaf traits that has been so thoroughly confirmed that paleobotanists can actually use the degree of toothy-ness of a fossilized leaf to determine the mean annual temperature for the region and time period the fossil came from.
Why do tropical plants have smooth edged leaves and temperate plants have toothy, lobed leaves? That’s a question we have yet to answer. There are many ideas floating around out there. It may be because a few plant lineages with lobed and toothed leaves just happened to be successful in northern latitudes and have stuck around. But that doesn’t really hold up too well because there are numerous plant lineages with members that inhabit both temperate and tropical regions. This divergence of character in individual lineages likely occurred because of large scale environmental changes over the course of geological time.
Most studies that attempt to address the why of the leaf form latitudinal gradient focus on leaf function in late stages of leaf development or in mature leaves. They search for reasons why teeth and lobes would be beneficial to plants that live in areas that experience high degrees of seasonality. Some say that teeth and lobes allow for increased surface area and subsequently increased gas exchange, allowing the plants to perform photosynthesis more efficiently with the limited time they have to sequester energy. Teeth may serve as hydathodes, or modified water expulsion organs that keep leaf tissue from flooding in the spring time. It’s also been said that toothed leaves protect plants against herbivores.
Whatever the hypothesis, all have yet to be substantially supported. The idea that natural selection could act on leaf traits at a time in the plant’s life cycle when the leaves are not fully mature seems to have been ignored in all of the aforementioned hypotheses. Entertain, for a moment, the idea that selection is acting on leaf primordia in overwintering buds, as opposed to mature leaves.
All the way back in 1899, a botanist by the name of Sir John Lubbock posited that folds and venation patterns in leaves within unbroken buds were responsible for differences in form of mature leaves. About a century later, researchers used the principles of kirigami, or fold-and-cut origami, to predict the shape of maple leaves by relating the folds and cuts to the angles, depths, and boundaries of the maple leaf primordia within the buds. And it worked. Support for the bud packing hypothesis; check.
In further support of this hypothesis we can look to seasonal heteroblasty. Heteroblasty is the difference in leaf shape between leaves that are partially developed within overwintering buds (preformed leaves) and leaves that are not formed within the buds at all (neoformed leaves). Trees that have to go into dormancy during winter drop all their leaves and get their buds ready for the next year so as to jump on spring as soon as it arrives. The leaves that are the first to jump out into springtime are consistently rounder, more lobed, and toothier, than the neoformed leaves that come after them.
Seems like we have an answer here, but to go along fully with this hypothesis, one must assume that the leaves of temperate plants undergo much more in-bud development than their tropical relatives. Phenology of tropical plants remains poorly documented so we can’t be 100% sure in making this assumption. Nonetheless, the bud packing hypothesis shifts the focus away from mature leaves to a more holistic, whole plant perspective that has the potential to connect leaf shape with not only bud packing, but also bud packing, branching architecture, and even evolutionary biome shifts.
Edwards, Erika J., et al. “Unpacking a century-old mystery: Winter buds and the latitudinal gradient in leaf form.” American Journal of Botany (2016).