生态学家发现森林增长速度加快

Speed is not a word typically associated with trees; they can take centuries to grow. However, a new study to be published the week of Feb. 1 in the Proceedings¹ of the National Academy of Sciences has found evidence that forests in the Eastern United States are growing faster than they have in the past 225 years. The study offers a rare look at how an ecosystem生态系统 is responding to climate change. For more than 20 years forest ecologist Geoffrey Parker has tracked the growth of 55 stands of mixed hardwood硬木，阔叶树 forest plots in Maryland. The plots range in size, and some are as large as 2 acres. Parker's research is based at the Smithsonian Environmental Research Center, 26 miles east of the nation's capital.

Parker's tree censuses人口普查 have revealed that the forest is packing on weight at a much faster rate than expected. He and Smithsonian Tropical Research Institute postdoctoral fellow Sean McMahon discovered that, on average, the forest is growing an additional 2 tons per acre annually⁵. That is the equivalent等价的，想等的 of a tree with a diameter直径 of 2 feet sprouting⁶发芽 up over a year.

Forests and their soils store the majority of the Earth's terrestrial陆地的 carbon stock. Small changes in their growth rate can have significant ramifications⁷分枝，分杈 in weather patterns, nutrient⁸ cycles养分循环, climate change and biodiversity生物多样性. Exactly how these systems will be affected⁹ remains¹⁰ to be studied.

Parker and McMahon's paper focuses on the drivers of the accelerated tree growth. The chief culprit犯人，罪犯 appears to be climate change, more specifically, the rising levels of atmospheric¹¹ CO2, higher temperatures and longer growing seasons.

Assessing how a forest is changing is no easy task. Forest ecologists know that the trees they study will most likely outlive比……活得长 them. One way they compensate¹² for this is by creating a "chronosequence年代序列"—a series of forests plots of the same type that are at different developmental stages. At SERC, Parker meticulously细致地，拘泥地 tracks the growth of trees in stands that range from 5 to 225 years old. This allowed Parker and McMahon to verify that there was accelerated growth in forest stands young and old. More than 90% of the stands grew two to four times faster than predicted from the baseline chronosequence.

By grouping the forest stands by age, McMahon and Parker were also able to determine that the faster growth is a recent phenomenon. If the forest stands had been growing this quickly their entire lives, they would be much larger than they are.

Parker estimates that among himself, his colleague Dawn Miller¹⁴ and a cadre基础结构 of citizen scientists, they have taken a quarter of a million measurements over the years. Parker began his tree census⁴ work Sept. 8, 1987—his first day on the job. He measures all trees that are 2 centimeters or more in diameter. He also identifies the species, marks the tree's coordinates¹⁵ and notes if it is dead or alive.

By knowing the species and diameter, McMahon is able to calculate the biomass生物量 of a tree. He specializes in the data-analysis side of forest ecology. "Walking in the woods helps, but so does looking at the numbers," said McMahon. He analyzed¹⁶ Parker's tree censuses³ but was hungry for渴望 more data.

It was not enough to document the faster growth rate; Parker and McMahon wanted to know why it might be happening. "We made a list of reasons these forests could be growing faster and then ruled half of them out," said Parker. The ones that remained included increased temperature, a longer growing season and increased levels of atmospheric大气的 CO2.

During the past 22 years CO2 levels at SERC have risen 12%, the mean temperature has increased by nearly three-tenths of a degree and the growing season has lengthened¹⁷ by 7.8 days. The trees now have more CO2 and an extra week to put on weight. Parker and McMahon suggest that a combination of these three factors has caused the forest's accelerated biomass gain.

Ecosystem² responses are one of the major uncertainties¹⁸ in predicting the effects of climate change. Parker thinks there is every reason to believe his study sites are representative of the Eastern deciduous¹⁹ forest, the regional ecosystem that surrounds many of the population centers on the East Coast. He and McMahon hope other forest ecologists will examine data from their own tree censuses to help determine how widespread the phenomenon is.