Temperature and Timing

The scientific process is a long and complex trail that sometimes leads to dead-ends and conflicting results before clear conclusions can be drawn. Throw in a dose of climate change to muddy the waters, and environmental research can be downright complex. That appears to be the case with recent studies of how the changing climate is affecting the timing of leaf emergence in the spring.

Anecdotal evidence has suggested for some time that warming temperatures are causing trees and other plants to bud and leaf out earlier. An 18-year study by the Vermont Department of Forests, Parks, and Recreation at the Proctor Maple Research Center has found that to be true of sugar maples. While the researchers observed considerable variability from year to year, they found that, on average, budding occurred three days earlier in the 2000s than in the 1990s, and average leaf out was five days earlier.

Those observations seem to agree with experiments conducted at Duke Forest in North Carolina and at Harvard Forest in Petersham, Massachusetts. Scientists there examined a mix of native trees growing in open-topped, temperature-controlled chambers, some of which were heated three to five degrees above ambient temperature. Not only did the warmer temperatures produce earlier budding in the spring, “There is a certain time of the year when warming has the most impact,” said James Clark of Duke University.

Clark’s team found that unseasonal warming during the late winter and early spring – just a few weeks before the buds would be expected to open – has more effect on budbreak than at any other time of the year. They noted that some species are more sensitive to warming temperatures than others, and some advance their budding to match the earlier season, while others do not.

A team of German researchers has come to nearly the opposite conclusion, however. In a study of twigs from 36 different tree and shrub species that were exposed to different temperature and light conditions in climate chambers for six weeks, they found that plants tend to delay their spring growth following a warm winter. “An ample ‘cold sleep’ is what plants need in order to wake up on time in the spring,” said Julia Laube of the Technical University of Munich.

As a result of the warming climate, the German scientists predict that the spring growth of many forest trees may start later and later, which may provide an advantage to shrubs and invasive trees that are not dependent on the cold. Like the Duke study, the German results varied by species, with beeches, sugar maples, and hornbeams affected the most and birches, lilacs, and hazels the least.

More studies are obviously needed before scientists can say with certainty how climate change will affect the timing of spring growth.


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