Many have assumed that warmer winters as a result of climate change would increase the growth of trees and shrubs1（灌木） because the growing season would be longer. But shrubs achieve less yearly growth when cold winter temperatures are interrupted by temperatures warm enough to trigger growth. When winter temperatures fluctuate（波动） between being cold and warm enough for growth, plants deplete2 their resources trying to photosynthesize and end the winter with fewer reserves than they initially3 had. In the summer they have to play catch up, said Melanie Harsch, a University of Washington postdoctoral researcher in biology and applied4 mathematics. She is lead author of a paper on the subject recently published in PLOS One.The roots are especially sensitive to temperature fluctuations5, Harsch said. Warming winters result in higher root respiration6, which uses up carbon reserves as plants make and release oxygen, leading to less carbon available during the regular growing season.Harsch and her colleagues studied two species of shrubs on Campbell Island, an uninhabited UNESCO World Heritage site in the southwest Pacific Ocean about 375 miles south of New Zealands mainland. They studied two large shrubs, Dracophyllum longifolium and Dracophyllum scoparium, which are evergreen7 broadleaf species that can grow up to about 15 feet tall and live up to 240 years.Researchers found that while warmer, drier winters helped seedlings8 get established, it adversely9 affected10 growth of older plants.For growth to occur you need sufficient precipitation（降水，沉淀） and temperature and nutrients11. Growth should only happen during the summer on Campbell Island when temperatures are above 5 degrees Celsius12, Harsch said. Five degrees C is about 40 F. On Campbell Island most winters are cool and below this 5 degrees Celsius, so the plants are not active. The plants we studied are evergreen and there is little snow cover, so they are sensitive to changes in temperature.In this study, researchers cut out discs, called cookies, from just above the shrubs root collar, and measured the width between each ring to determine growth. They found that plant growth decreased as winter temperatures went up.On Campbell Island the snow is ephemeral, so the plants usually are not covered, Harsch said. If were going to see an effect in changing winter conditions, were going to see it at Campbell Island decades before we see it at, say, Mt. Rainier, where there is a lot of snow and winters are colder.Harsch said plants in areas like Campbell Island may eventually adjust to warmer winters, but the transition period will be tough as temperatures bounce above and below what plants need to stay dormant13（休眠的，静止的）, causing the plants to draw down their resources.It may eventually be warm enough in the winters so that plants can photosynthesize and grow year round, like they do in the tropics, she said. Its this transition part that plants are not adapted for.
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1 shrubs 灌木 参考例句： The gardener spent a complete morning in trimming those two shrubs. 园丁花了整个上午的时间修剪那两处灌木林。 These shrubs will need more light to produce flowering shoots. 这类灌木需要更多的光照才能抽出开花的新枝。
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