In the spring of 2020, when cities like Boston have ground to a halt due to the novel coronavirus, T. S. Eliot’s nomination of April as the “cruellest month, breeding / Lilacs out of the dead land” certainly feels apt. The arrival of flowers seems incongruous amidst a pandemic, and the season, for unrelated factors, can be uncertain and unpredictable for plants as well.

As part of my study of the climate-change vulnerability of the Arnold Arboretum’s diverse maple (Acer) collection, I have been visiting trees of 37 maple species at the Arboretum once or twice a week and recording whether their buds have opened and how development of their leaves and flowers has progressed.

maple collection late april
The maple collection (Acer) in late April. Photo by Jon Hetman

This process of budbreak, one step in the annual cycle of plant phenology, represents a double-edged sword for maples and other temperate trees. A quick and robust start in spring gives a tree more opportunities to capitalize on April’s lengthening days and abundant rains. Yet my ongoing experiments show that those trees that are further along, phenologically speaking, are more vulnerable to the dangers of a late winter freeze. This tension creates a trade-off between liberal growth and conservative bet-hedging and makes some species both better at responding to a changing climate while also more vulnerable to its dangers.

Early Bloomers

There are relatively few native North American maples, just nine species compared to over 100 indigenous to East Asia. But among them is a homegrown duo, a pair of “sister species,” meaning that they are more closely related to each other evolutionarily than to any other species. I’m speaking here of those neighborhood friends, red (Acer rubrum) and silver (A. saccharinum) maple. Fascinatingly, these trees behave differently, by far, from other maples. They produce flowers in late winter, far before other maples have begun breaking bud. This year—a relatively warm, and thus phenologically early one—the Arnold’s red and silver maples were sending out flowers by the first week of March. Regardless of the timing of this event, these flowers will be fertilized and begin ripening into seeds, which are often quite obvious by the time leaves begin to emerge (around April 15, this year). In the meantime, red and silver maples are distinguishable by the haze of red to yellow flowers surrounding their otherwise bare branches.

Red maple (Acer rubrum) with bright red spring flowers.
The bright red flowers of red maple (Acer rubrum) began appearing at the Arboretum in early March.

Though other maples flower before producing leaves, none, to my knowledge, allows five weeks to pass between the two events. And at least when planted in North America, few other species begin waking up for spring as early as the red and silver maples flower. At the Arnold, three East Asian species vie with red and silver maple for the title of Earliest Bloomer in early to mid March. Two are another pair of sister species the Kawakami maple (Acer caudatifolium) and Manchurian striped maple (A. tegmentosum). Closely related and relatively similar looking, these relatives of the native striped maple (A. pensylvanicum) have beautiful green bark and luxurious, soft leaves. They each begin breaking bud around the same time that red and silver maples flower, but it takes some time for their leaves and flowers—which generally emerge together—to fully open up. Interestingly, these visually similar sisters could not be more distinct in their ranges of origin. The Kawakami maple, native to Taiwan and growing in areas that rarely experience a hard freeze, is a subtropical outlier at the Arboretum, while the Manchurian striped maple, as its name suggests, occupies some of the coldest places colonized by the genus. A third species, bearded maple (A. barbinerve), also hails from far northeastern China, eastern Russia, and Korea and breaks bud early, though it is not closely related to the other early bloomers.

Red and silver maples abound near the Hunnewell Building, but all three East Asian early bloomers can be compared most easily on single loop around Peters Hill. I recommend starting at the Poplar Gate, where the Arnold’s only three Kawakami maples (accessions 435-84*A, B, and C) are planted in a lonely trio near the aspen (Populus) collection. A short walk west on Peters Hill Road leads to a pair of Manchurian striped maples (accessions 1400-77*D and E) under the pine planting along Weld Street. Finally, heading south toward Peters Hills’ historic gravestones will lead visitors to two bearded maples (318-97*A and B) just off the main road. As so often happens at the Arnold, a fifteen-minute stroll can bridge vast geographic (from Manchuria to Taiwan) and evolutionary (59 million years of evolution; Li et al. 2019) divides, all while showcasing three species of gorgeous, early-bird maples.

Buds of Kawakami maple (Acer caudatifolium)
Opening buds of Kawakami maple (Acer caudatifolium). Photo by William (Ned) Friedman

Better Late than Never

As of April 20, this year, several of the Arboretum’s maples show very little bud development, giving the external appearance of a tree stuck in the dead of winter. Why do these holdouts sacrifice weeks of potential photosynthesis and access to pollinators, biding their time while other trees leaf and flower? One reason may be the conservativism I mentioned above: these species could have evolved under conditions in which false springs—warm weather followed by a cold spell, are a greater risk. In fact, there is some emerging evidence in my data of a trend of earlier bud-break from maples originating in higher latitudes. Perhaps species growing at intermediate latitudes, commensurate with Boston or further south, have indeed experienced selection in favor of more conservative, later leaf-out dates to avoid the potentially disastrous damage caused by a false spring? Of course, the Kawakami maple, a southerly species with an early leaf-out date, demonstrates the limits to this theory, which might just apply to species from colder climates.

Who among the maples are dragging their feet this year at the Arnold? Noticeably, a set of three close cousins (not exactly sister species, but all members of the same group within the maple genus) have all waited through the middle of April for even a sign of bud swelling. These include the North American dominant sugar maple (Acer saccharum) as well as its European relatives, the Montpellier (A. monspsessulanum) and Greek (A. heldreichii) maples. A walk down the Willow Path provides ample evidence of these species biding their time while other maples throw caution—and pollen—to the wind. Also notable among the late bloomers are two close relatives among the exclusively East Asian trifoliate maple group: the Nikko (A. maximowiczianum) and paperbark (A. griseum) maples. These Arboretum favorites will ultimately produce sets of petite, compound leaves, but presently show only the very earliest signs of breaking bud. This is not to say, though, that all late bloomers are related: the Balkan maple (A. hyrcanum), a close relative to the sugar and Greek maple, and the trifoliate trident maple (A. buergerianum) have both put out their leaves already.

Emerging leaves of paperbark maple (Acer griseum)
Emerging leaves of paperbark maple (Acer griseum). Photo by Suzanne Mrozak

Yet even as the last holdout species welcome spring, the maple genus’s diversity will continue to delight those with a discerning eye. Among and even within species, one can find variety in sex, with some plants only producing male or female flowers and others generating flushes of them, switching from male to female, then to male again. Some individuals will not flower in a given year, whereas others will end up loaded with seeds. This is all to say that spring marks only the beginning of the excitement in the maple calendar. The reds and oranges of fall foliage may steal the show, but the the genus’s thrilling springtime overture should not be missed.