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Nature's clock ringing in earlier springs
Published September 20, 2005
Nature’s biological clock measures time with the arrival of birds, the greening of leaves, and the budding of flowers. Now it seems the clock has sped up when it comes to spring’s arrival, a leap that many view as a response to global warming.
“I have heard that the mayflower should now be called ‘aprilflower’ because it’s switching its bloom time,” said Elisabeth Beaubien, national coordinator of the Canadian program PlantWatch. Similarly, her research has shown quaking aspen trees, admired for their white bark and yellow fall leaves, are blooming about eight days earlier on average in Edmonton since the mid-1930s (International Journal of Biometeorology, August 2000). Aspen thrives in many U.S. states as well, although its bloom times in this country generally go unrecorded.
Concern over these changes in time, and the lack of a U.S. network to systematically monitor them, drew about three dozen scientists to Tucson for a late August workshop sponsored by the Institute for the Study of Planet Earth at The University of Arizona (UA) with funding support from five federal agencies. The group hopes to launch a nationwide monitoring project soon, explained workshop organizer Julio Betancourt of the U.S. Geological Survey and UA. “This is probably something we can get going by the next growing season,” Betancourt said.
Co-organizer Mark Schwartz of the University of Wisconsin-Milwaukee has already set up a prototype website where anyone can sign up to submit their own national and regional observations (See sidebar on page 4).
During the workshop, Beaubien and other program managers shared advice about setting up citizen networks to observe phenology, the timing of annual life cycle events. Although the word itself sounds unfamiliar to many people, the practice of phenology stretches back to the last ice age, Beaubien reported. She noted that ancestors of the Blackfoot tribe used the blooming of buffalo bean as a sign to hunt bison bulls.
In the Southwest, the monsoon’s arrival has long been hailed as an opportunity for growth, by Hohokam farmers who planted beans and squash on floodplains thousands of years ago through modernday gardeners who collect rain in barrels. But while climatological records pinpointing the monsoon’s arrival show up in scientific literature and on the web, biological records documenting nature’s response to seasonal climate events remain rare for this country.
One of these rare bloom records involves a lilac network launched in 1957 by Joseph Caprio of Montana State University, who also shared insights at the workshop. At its peak, more than 2,500 volunteers from 12 western states, were posting information on the location and blooming times for lilac shrubs. Caprio also distributed cloned honeysuckle bushes to a smaller group of volunteers. Dan Cayan of Scripps’ Climate Research Division and colleagues Nature’s clock ringing in earlier springs Plant and animal cycles show earlier and longer warm-weather seasons continued on page 3 including Caprio recently reported that the observations show lilac and honeysuckle flowers in the West were blooming 5 to 10 days earlier on average in the second half of the 1957–1994 record compared to the first half (Bulletin of the American Meteorological Society, March 2001). Unfortunately, the number of volunteers dropped precipitously since Caprio retired in the early 1990s.
In a more comprehensive study, Terry Root and her colleagues at Stanford University compared phenological records for 145 species including trees, insects, flowers, and birds from sites in Europe, Asia, and North America. They found that four out of five species examined had shifted their seasonal timing in a way that appeared related to temperature increases from human-influenced global warming (Proceedings of the National Academy of Sciences, May 2005). During the assessed time frame, which varied by species and record but averaged 28 years, the measured seasonal events for birds and trees shifted ahead about five days, while herbaceous plants like grasses and wildflowers were starting about two days earlier.
Although the earlier springs may sound good to snowbound northerners who see robins as a sign of pending release from winter’s grip, these phenological shifts can signal problems ranging from earlier allergy attacks to potential missing links in the food chain.
An earlier start to the pollen season seems to be one consequence of warming temperatures, based on research in the Netherlands by Arnold van Vliet of Wageningen University and colleagues (International Journal of Climatology, November 2002). This should concern the roughly 15 percent of thepopulation who suffer from hay fever, and could be problematic for those with asthma as well, noted van Vliet, who attended the workshop.
The Netherlands study also found trees were responding more rapidly than grasses, with juniper and oak releasing pollen nearly three weeks earlier in the 1990s than they had in the 1970s. In this case, that’s good news for allergy sufferers, as grass pollens tend to aggravate allergies the most.
However, ecologists worry that disjointed shifts by different species could break food chain links and create other problems. Plants overall are responding more quickly to temperature than birds, and insects are reacting fastest of all, explained Jill Attenborough, program director of a project that collects phenology data from more than 21,000 regular observers in the United Kingdom.
“It helps people understand that the synchrony of the natural world is being threatened,” Attenborough suggested. The U.K. network is supported by the efforts of The Woodland Trust, a nonprofit group that in 2000 joined efforts to recruit volunteer observers after identifying climate change as the single biggest threat to ancient forests.
Scientists who studied insect outbreaks in southwestern forests have been working to document how warm temperatures contributed to recent attacks.
“The probability of outbreaks will increase as temperature increases because insects are cold-blooded,” as Neil Cobb of Northern Arizona University explained during a water summit in Flagstaff last month. Freezing winter temperatures help keep insects in check.
Although drought clearly contributed to a recent bark beetle outbreak, entomologists suspect warmer-than-usual winter Nature’s clock, continued continued on page 4 temperatures may have influenced its scale. Beetles devastated about 2.7 million acres of southwestern pine forests from 2001 through 2004, according to estimates from the U.S. Forest Service’s Southwestern Region.
Workshop participants were focused on the collection of phenology data to improve society’s understanding of climate variablity as well as change. For many, the ultimate goal would be to produce more skillful climate forecasts and better predict response to climate swings.
“Basically we see phenology as an indicator of climate impacts, whether it’s from natural variability or human-induced,” van Vliet said. He noted that the latter includes the urban heat island effect that occurs as paved areas expand from city centers.
Temperature vs. rainfall
In the Southwest, plants often respond to rainfall variability more dramatically than to temperature, as the greening of the desert this past year dramatically demonstrated.
“Many of the showiest species require large amounts of rainfall when temperatures are cool but not cold,” said Janice Bowers, a USGS researcher. Her research found the best wildflower years in the Sonoran Desert resulted when rainfall rates were about 50 percent higher than usual from September through March. What’s more, these higher rainfall years tended to occur when the July–December index signaled an El Niño pattern was active in the tropical Pacific Ociean. (Journal of the Torrey Botanical Society, January 2005).
The El Niño this past year ushered in red brome, cheatgrass, and other invasive grasses that helped spark a record fire year in Arizona. As of September 15, most of the state’s 725,903 acres impacted by fire this year burned in grasslands and grass-covered desert rather than forests, indicated Chuck Maxwell of the Southwest Coordination Center. (Meanwhile, only 23,097 acres burned in New Mexico.)
Rainfall certainly encouraged the extensive grass cover, but warmer spring temperature may help give invasive grasses a foothold over native species, some researchers suspect (Southwest Climate Outlook, February 2005). Unlike the more sparse cover of native wildflowers, a continuous grass cover can spread fire throughout desert ecosystems. Brushes with fire can be fatal to the Southwest’s poster cactus, the saguaro.
The phenology network actually could help battle the problem of invasive grasses leading fires into the saguaro’s realm, noted Betancourt. For instance, a citizen network reporting the growing presence of buffelgrass could alert officials to take action, such as spraying herbicides before the grass consumed the landscape. Word of starthistle reaching the flowering stage could inspire officials to release a natural enemy like the yellow starthistle seedfly, he added. These actions could become more important as rising temperatures dry out grasses earlier in the season.
The longer warm weather seasons showing up in records could translate into longer and more severe fire seasons as soils once covered by snow face the harsh light of day for extended periods. The litany of problems this would bring includes the release of more carbon dioxide back into the atmosphere.
The dry mass of plants and trees amounts to about half carbon—all of it drawn at some point from carbon dioxide in the air. Because carbon dioxide is the main greenhouse gas implicated in the acceleration of global warming, its ups and downs affect the amount of heat the atmosphere retains at the Earth’s surface. Northern hemisphere carbon dioxide levels rise and fall with the seasons as plants draw down this greenhouse gas in summer, then release much of it via decay when they die or drop their leaves in winter (Figure 1).
Like much plant growth itself, the seasonal timing of the carbon dioxide “drawdown” had shifted forward by up to a week as of the mid-1990s compared to the early 1970s, as reported by a longtime carbon dioxide recordkeeper, the late Charles Keeling, and colleagues (Nature, July 11, 1996). In a follow-up paper, Keeling, lead author Ranga Myneni of Boston University, and others found satellite evidence tosupport their argument that this carbon dioxide shift reflected increased plant growth during a longer growing season (Nature, April 17, 1997). They estimated that the northern growing season increased by roughly 12 days between 1982 and 1990, with two-thirds of the change attributed to an early spring and the remainder from a delayed autumn.
The opportunity to calibrate satellite “green-up” imagery with documented onthe- ground leafing out was touted as another important reason for a continentalscale phenology network in this country. More of these comparisons will help scientists understand how organisms that form the planet’s biosphere are responding to climate variability and change.
With more than 150 countries and a growing number of U.S. states vowing to reduce the input of carbon dioxide into the atmosphere, the helpful role of plants is destined to receive much attention by those trying to monitor the year-to-year changes in the carbon cycle. The implications for global warming go beyond carbon dioxide dynamics, as vegetation cover can also increase the Earth’s retention of direct solar radiation, especially when compared to snow cover.
In addition, farmers, ranchers, tourists, doctors, teachers, biologists, journalists, gardeners, land managers, and many other members of society also stand to benefit from continental-scale information linking climate fluctuations to the seasonal cycles of plants and animals, participants pointed out. With so many potential beneficiaries, the message from the workshop rang out loud and clear: It’s time for U.S. residents to synchronize their watches and clock nature’s biological cycles.