Morgan Moeglein investigates the big bets trees make at the scale of the buds where leaves form for the season to come.

Every fall across the temperate latitudes, the leaves of woody plants complete development, reach their final form, and begin to senesce. Leaf senescence marks the end of a growth cycle that may seem to begin at bud break in the spring, but for many leaves actually began much earlier, during the previous growing season, hidden from view within a resting bud. Leaves that initiate and mature within a single growing season are known as neoformed leaves. Preformed leaves, by contrast, initiate during one summer, undergo one or more periods of winter dormancy, and complete their life cycle in the following summer. Botanists have long noted the presence of incipient leaves within dormant resting buds. Many unanswered questions about leaf preformation remain, however. When we look at a tree at the end of the summer, how many of those leaves were preformed? Do different species employ different strategies when they invest in preformed leaves for future seasons?

I have spent the last year trying to characterize how plants invest resources in their overwintering buds and the leaves within. This work led me to the Arnold Arboretum, where I set out to investigate how preformation contributes to leaf development across woody taxa. The first step in this work was to look very closely at dormant buds in the field.

Surveying buds across species in the depths of winter, without the obstruction of leaves, is the best time to appreciate how much bud morphology varies. Some overwintering buds are obvious, such as the long, shiny beech buds or the furry, gray magnolias. Then there are the minuscule and hard to find: the buttonbush bud barely raised from the surface of the twig, or the10 winterberry with its few, tiny, camouflaged bud scales. Walking through the collections sampling dormant buds for future microscopy, I easily snapped the bigger buds from their branches with my fingers, while smaller specimens had to be shaved from twigs with a razor blade back in the lab.

Even more remarkable than the external bud morphology observed in the collections is what you find inside after dissections under the microscope: the large, showy buds usually contain mature-looking leaves in miniature. The morphological intricacies of an oak or sweetgum leaf are already apparent, intact in these preformed leaves, months before they expand to their mature size. Even in the smallest buds, many leaves are already initiated, their shape recognizable in its relation to the mature form. A tiny, flattened catalpa bud may contain ten leaf primordia, similar to the number of mature leaves that will be present at the end of the growing season. It’s astonishing to realize, while looking at buds in the field, that the form and number of mature leaves could be determined so far ahead of time, within the tiny space of the smallest buds.

Large, showy buds usually contain mature-looking leaves in miniature.

By comparing preformed leaves dissected from within buds to mature, fully expanded leaves, I hope to deduce how much of the multi-seasonal cycle of leaf development occurs within buds relative to the end-of-season total. Looking at preformed leaf and bud measurements relative to mature leaf measurements allows us to measure relative investment in different leaf components, and begins to reveal patterns in preformed investment. These strategies can tell us something about how plants prepare for the future. Some species may preform more leaf tissue that is more fully developed in preparation for expected circumstances in the coming growing season, while others may wait and neoform the leaves they need in real time as a response to present growth conditions. There are also different ways to invest. One species might invest in many small leaves, while another might invest in fewer leaves that are larger and more completely developed. With so many different ways to invest in preformation, the range between preformation and neoformation begins to seem more like a spectrum than a dichotomy. Based on observations so far, I suspect the species surveyed will vary widely in the timing and extent of their investment in future leaves.

Surveying the contents of overwintering buds can help us learn how plants invest in leaves from season to season—but it may also better our understanding of the evolution of leaf form more broadly. Leaf shape varies massively across plants, but the variation within species or within individuals can be just as impressive. Leaf shape along a single growing tip can vary over the course of a season and this variation may correspond to preformed and neoformed trajectories arising from different developmental programs. Leaf-shape plasticity could provide standing variation for evolution to act upon, or it might open up different photosynthetic options as the season progresses. If the shape of preformed leaves is largely predetermined within the dormant bud, and if this shape is different from the shape of neoformed leaves later in the season, the confined nature of development within a bud may have some influence on leaf shape. The same may be true when comparing across species with different preformation strategies; a species that preforms more may be more likely to have certain leaf shapes than a species that produces more leaves through neoformation. There are many possibilities, but looking within dormant buds at the earliest stages of leaf development across species and environments may illuminate our understanding of the variation we see across mature leaves.

Morgan Moeglein is an assistant professor in biology at Norwich University, and was a Putnam Postdoctoral Fellow at the Arnold Arboretum in 2021–2022.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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