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Springtime under global warming: the forest trees

With maximal three-day error, Henry Thoreau reportedly could tell the exact date, in springtime, given information on some wildflowers in bloom and trees in leaf. Years of land surveying and ceaseless “sauntering” through the wood lots, meadows, and swamps around Concord had given him the extensive knowledge of phenological events documented in his published Journal. In his time, as well, conditions and events in the natural world were held to be unalterable and dependable, the seasonal alterations of temperatures and of greening and flowering of plants determined by forces beyond human control.
The comforting sense of permanence of the natural world such as prevailed at that time has been changed through the phenomenon of global climate change. By several weeks, as being documented by researchers throughout the world, spring can arrive earlier now for many plants. Per degree (Celsius) of warming, botanical journals report, their spring leafing and flowering has advanced an average of 4 to 5 days. And plants are moving, too, northward to higher latitudes or upslope to higher terrain as warming of the earth progresses. Having finely attuned themselves to land and climate conditions in a particular area, though, over millennia, the speed of ongoing warming puts many at risk of extinction. Firmly rooted to the ground and slow of growth, trees may be especially vulnerable. Bill McKibben for instance cites researchers’ estimate that forests can move at most half a mile in a year, through natural growth along their edges. To keep up with rapid shifting of climate zones, as now appears to be occurring, their northward march would have to be more than thirty times faster. As he phrases the seriousness of adaptive failure for them, trees may “die looking out at the sights they were born to.”
Fortunately, accordingly to an article in the journal Science in March, all tree species are not affected in the same manner. Some forest trees “know better” than to leaf out in response to warming weather, alone. An internal clock in the species that dominate our northern temperate forests, the oaks and hickories, hackberry and beech, for example, tracks length of day time as the most important signal for dormancy break. In a series of studies, when transplanted to subtropical parks where temperatures were exceptionally high for them, these trees held back bud burst until the critical day length had passed. Genetic tuning to this tracking mechanism prevents their sprouting at the “wrong” time, such as in an abnormally warm spell in winter.
Certain other tree species, the birch and poplar and hornbeam among them, require a sufficiently long chill period in winter before responding to warmer temperatures. In contrast, the researchers find, many ornamental, non-native plants, such as lilac, seem ready to “adopt a more risky life strategy” in this regard, through spring-time response to warming temperatures, alone.
Perhaps the standard for climate tracking they evolved over eons of time can give our mature-forest native trees a little edge, one might hope, in harder times ahead.