A new study analyzing mixed hardwood forest plots in Maryland indicates that forests in the Eastern United States may be growing faster than they have in the past 225 years. On average, the forest is growing an additional 2 tons per acre annually--the equivalent of a tree with a diameter of 2 feet sprouting up over a year.

Forests and their soils store the majority of the Earth's terrestrial carbon stock. Small changes in their growth rate can have significant ramifications in weather patterns, nutrient cycles, climate change and biodiversity. Exactly how these systems will be affected remains to be studied.

The new paper focuses on the drivers of the accelerated tree growth. The chief culprit appears to be climate change, more specifically, the rising levels of atmospheric CO2, higher temperatures and longer growing seasons.

Forest ecologists know that the trees they study will most likely outlive them. One way they compensate for this is by creating a "chronosequence"—a series of forests plots of the same type that are at different developmental stages. At the Smithsonian Environmental Research Center (SERC), the authors meticulously tracked the growth of trees in stands that range from 5 to 225 years old, which allowed them 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.

The authors say it was not enough to document the faster growth rate; they 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 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 lengthened 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.

Ecosystem responses are one of the major uncertainties in predicting the effects of climate change. Parker thinks there is every reason to believe his study sites are representative of the Eastern deciduous 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.