Forsythias, magnolias, and cherries are blooming in every corner of the Arboretum. Tall white tufts of serviceberry (Amelanchier spp.) reach out from conifer stands, and redbud branches burst with pink blossoms. Dogwoods, azaleas, and lilacs prepare for their annual show. With the world awash in floral hues, it’s easy to forget the most important of spring’s colors: green.
To the casual observer, the emergence of leaves can seem almost instantaneous. In reality, the greening of trees and shrubs in Massachusetts occurs during a 4 to 6 week interval, beginning in early spring. During this period of leaf-out, plants respond to rising temperatures and prolonged daylight by moving energy towards dormant buds, which hold preformed reproductive and vegetative tissues waiting eagerly for the advent of spring. Plants must meet required thresholds of growing degree days (a measurement of heat accumulation used to understand tissue development rates) before these tissues begin to enlarge and eventually emerge. Within weeks, billions of juvenile leaves–tiny solar panels loaded with chlorophyll–are ready to begin generating energy for trees and shrubs.
The timing of leaf-out can vary widely between different species. For instance, honeysuckles (and many other shrubs) leaf out well before beech trees. Of our native New England tree species, the birches, alders and willows tend to leaf out much earlier than others such as oaks and hickories. Stem anatomy may play a key role in these timing strategies, as the smaller vascular structures of early-leafing trees allow them to withstand the freezing temperatures that sometimes occur during early spring. Leaf-out timing can also vary within each species. A beech tree on a warm, sunny hillside may leaf out before a beech in a cool, shady ravine. Furthermore, the timing of leaf-out within each species is weather dependent and can vary dramatically from season to season. A short winter and warm spring may coax greenery into appearing long before its typical leaf-out date.
What’s so important about leaf-out, and why do scientists study its timing? While healthy foliage allows plants to undergo the majority of their photosynthesis throughout the growing season, the appearance of juvenile leaves also triggers a cascade of ecological processes. New greenery is a primary energy source for many animals – most importantly, insects – which hatch, develop, and become food for other organisms. The advent of blossoms and new leaves is often marked by a flurry of avian activity, as birds capitalize on this insect bounty.
Some species, such as the notorious winter moth (Operophtera brumata), synchronize their life cycles to hatch and begin feeding while juvenile leaves are still tender and free from noxious compounds, such as tannins. The tiny caterpillar to the left, along with millions of others, are already busy feeding on trees throughout the Arboretum. Luckily, healthy trees can often withstand partial defoliation and later produce a new flush of leaves. Biological treatment, such as the introduction of a parasitic fly species, Cyzenis albicans, may effectively combat the rise of this invasive pest at the Arboretum. Intimate knowledge of a plant’s life cycle is crucial to the successful control of pathogens, as treatments must be timed to coincide with specific events.
Flowers allow plants to reproduce and maintain genetic diversity among populations. They provide food and habitats for many animals. They are often visually striking. These are undoubtedly important qualities. But leaf-out marks the true beginning of spring activity, as solar energy captured through photosynthesis becomes available to the food chain. On the surface of each leaf, photons end a 92 million mile journey through the vacuum of space to convert water and atmospheric carbon dioxide into the sugars that support a plant’s entire life cycle. The leaf is more than a humble organ – it is the site of a miraculous process that sustains all life on earth! By studying the timing of leaf-out cycles, we enhance our understanding of the ecological and chemical processes that are crucial to our survival on this planet.
If you’re interested in learning more about phenology, botany, or ecology, join the Arnold Aboretum Tree Spotters, a group of citizen scientists responsible for collecting data from 11 species throughout the Arboretum. Our next training session will take place on Sunday, May 15th from 2:00pm–4:30pm at the Hunnewell Building. Click here to register. Read more about Tree Spotters in Silva [pdf].