This is a busy time of year for Richard B. Primack, a biologist at Boston University. He and his colleagues survey the plants growing around Concord, Mass., recording the first day they send up flowers and leaves.
Compared to the last five springs, things are pretty slow right now around Concord, in large part because of the relatively cold winter and chilly March.
But Dr. Primack wouldn’t call this a late spring. “It’s just much later compared to our recent memories of spring,” he said.
Dr. Primack knows this thanks to Henry David Thoreau. During the 1850s, Thoreau carefully recorded the arrival of spring at Walden Pond, one of Concord’s most famous sites. Dr. Primack has combined Thoreau’s data with his own and those of other naturalists to create a record of the seasons stretching across 160 years.
As Dr. Primack writes in his new book “Walden Warming,” spring has started earlier and earlier over the decades. It now arrives about three weeks sooner than in Thoreau’s time.
The pattern Dr. Primack sees at Walden Pond is part of a grand, planet-wide march. Many studies — based both on observations in the field and on satellite images taken from space — indicate that spring is shifting earlier.
The changing spring is one of the most striking impacts attributed to global warming. But in both hopeful and troubling ways, new studies are showing that warming’s effects are broader, affecting plants from spring to fall.
Amy M. Iler, a postdoctoral researcher at the University of Maryland, and her colleagues recently exposed some of that complexity in a mountain valley in Colorado, where they and other scientists have been monitoring flowers since 1974.
Recently, they analyzed records for 121 species to look for significant trends in their growth. They published their study last month in The Proceedings of the National Academy of Sciences.
Over all, the flowers have followed the same trend seen around the world. The first flowering in the valley now comes 25 days earlier than in 1974.
But on species by species, the pattern isn’t so simple. “There’s a complexity that we can see,” said Dr. Iler.
It turns out that only about half of the species are starting to flower earlier in the spring. Many other species have responded to the warming in other seasons. Some are reaching their flowering peak earlier in the year, for example. Other species are extending their flowering into later in the fall.
There are probably many factors at work behind these complex changes. And a new study published this week in Nature sheds some light on them. It suggests that the rising temperature isn’t the only thing that’s shifting the seasons. The carbon dioxide that causes global warming may be directly affecting the plants, too.
The study was based on the results from a long-term experiment set up in 2006 by Jack A. Morgan of the federal Agricultural Research Service and his colleagues. They divided up a patch of Wyoming prairie into plots of grass.
On some plots, they installed heaters to keep the air a few degrees warmer than normal. On other plots they placed carbon-dioxide pumps. And on still other plots, they provided both heat and extra carbon dioxide. Since then, the scientists have been watching how the prairie grows under the different conditions.
The scientists found that in the warmer plots, some species flowered sooner in spring. And they also found that on plots with extra carbon dioxide, some plants kept growing later into autumn.
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“It’s a very clear message,” said Dr. Iler, who was not involved in the research service’s study.
Dr. Iler and other researchers are investigating how these factors change the seasons. Warmer temperatures make spring come earlier, it seems, because many plants have evolved to use heat as a signal to start growing.
Dr. Iler and her colleagues have also found evidence that the melting of mountain snow can also spur plants to awaken in the spring. Unusually early melts often lead to early blooms.
In the fall, Dr. Morgan and his colleagues argue, carbon dioxide allows plants to keep growing by letting them hold onto more water.
Plants have to carry out a tricky balancing act between taking in carbon dioxide and losing water in order to grow. That’s because they absorb carbon dioxide from the air by opening tiny holes called stomata.
“As part of the process of letting in carbon dioxide, water moves out,” said Heidi Steltzer of Fort Lewis College in Colorado, an author of the Nature study.
As water evaporates from their stomata, the plants have to replace it with water from the soil. By autumn, they pump the soil dry. The lack of water helps to bring their growing season to a halt.
In their study, Dr. Steltzer and her colleagues found that extra carbon dioxide left prairie plots with more water in the soil. They suspect the additional gas allowed the plants to grow without opening their stomata as wide. They lost less water, and so their water supply lasted them longer into the fall.
Since the mid-1800s, carbon dioxide levels have risen more than 40 percent. Dr. Morgan suspects that it may be extending the growing season in many ecosystems where water can end up in short supply.
“It’s a fairly large fraction of the earth,” he said.
In the coming decades, carbon dioxide levels are expected to rise still more, driving up the planet’s average temperature. Spring will probably continue to advance, and some plants will grow longer into autumn.
The changing seasons may have both positive and negative effects. Dr. Morgan and his colleagues have found that the Wyoming prairie become more productive with an extra supply of carbon dioxide, for example.
On the other hand, Dr. Iler worries that the changing flowering times of plants may disrupt their pollination. Some species may end up competing with each other for visits from pollinating bees. “It’s reshuffling the community,” she said.
Eventually, the beneficial impacts of the changing seasons may screech to a halt. That’s because global warming is expected to bring drought to many parts of the world. Extra carbon dioxide may no longer save plants enough water to continue growing.
“If things really get warm and toasty, then I can see the benefit that carbon dioxide brings may be overcome,” said Dr. Morgan.
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