Here in New England, springtime can feel like an emotional rollercoaster. A bout of 60-degree days, t-shirts and sun can suddenly become a week-long cold snap and a half-foot of wet snow. This year, between the 3rd and 6th of March, Boston experienced a temperature fluctuation of 56 degrees! Less than two weeks later, the temperature had plunged into the 20s, and my flip-flops had been thrown back into the closet in disgust.
Amidst these erratic changes, plants must time their vegetative and reproductive cycles in response to a number of constantly changing variables. Unfettered by emotion, they possess sophisticated mechanisms for determining the proper timing of phenological events such as flowering and leaf-out. Temperature and photoperiod are the two major factors that control these events in temperate woody plants, although exact timing depends heavily on the microhabitats of individual specimens. The timing of these critical events begins a chain of plant-animal interactions that control most ecosystem processes.
Temperate plants must undergo a period of dormancy and prolonged cold exposure called a chilling requirement, which differs in length among species. After this requirement has been met, rising air temperatures signal to plants to break dormancy and begin moving energy from roots towards buds. Increasing photoperiod, or exposure to sunlight, plays an important role in signaling flowering and leaf-out, but only in some species. Birches, hazelnuts and poplars, for instance, have no photoperiodic requirement, and often break dormancy during early warm spells.
The timing of phenological events is crucial to a plant’s life cycle. Flowering must be synchronized with insect activity, or occur during the pre-leafout period when pollen can travel freely on air currents. Many tree species at the Arboretum, such as elms, alders and willows, have already flowered and begun their reproductive cycles. For other species, early leaf-out means valuable exposure to sunlight, and the ability to create and store energy, before forest canopies fill out. Honeysuckles, lilacs and willows have already begun to leaf-out, along with many other shrubs.
This early action is often risky. Opportunistic behavior, such as a quick leaf-out in response to warm temperatures, means greater exposure to harmful frosts. If you visited the Arboretum this week, you might have spotted damage caused by Monday’s snow and freezing temperatures. The rapid growth of ice crystals through unprotected plant tissues can destroy cell walls, inducing dehydration and tissue death. Such damage can be costly for plants and results in wasted flowers and foliage.
In the long run, a finely-tuned response to environmental change can mean the difference between proliferation and mere survival. Mechanisms such as photoperiodicity are thought to protect against early warm spells that can coax plants into exposing vulnerable tissues. Variations in leaf-out timing may also offset the costs of herbivory, anatomical differences, and microhabitat conditions.
As you visit the Arboretum, take note of the species you see flowering and leafing-out in early spring. Studying shifts in phenological events is a fantastic way of observing our changing climate. In fact, long-term phenological data sets, some ranging back centuries or even millennia, illustrate the stark impact of rising temperatures on flowering and leaf-out timing. The implications of changing phenology are enormously significant–ecologically, agriculturally and horticulturally (as in the case of the magnolias to the left).
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 Thursday, April 14 from 5:00pm–7:30pm at the Hunnewell Building. Click here to register. Read more about Tree Spotters in Silva [pdf].