Seeing the Forest and the Trees

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Each year, as winter fades and temperatures begin to rise, satellite sensors record the first appearance of spring foliage over the eastern United States. These images capture an expanding front of green spanning the region and advancing from the south. Mark Schwartz, professor of geography at the University of Wisconsin, Milwaukee, calls it the “green wave” and says these satellite images are just one perspective to study the timing of biological events in nature.

Though satellite technology is one of the newer tools available for use in the study of nature’s calendar, this science of “phenology” itself emerges from a long tradition. For example, in Japan the dates of the flowering of cherry trees have been tracked for close to a millennium, and in the United Kingdom some families have dedicated themselves to recording phenological data over generations. Most notably, Robert Marsham, known as the father of phenology, and his descendants kept unique records of spring’s arrival on the family’s country estate from 1736 until 1958.

While no similar tradition has emerged in the United States, in recent years casual and scientific observers seem to agree: Spring is coming earlier. According to Schwartz, “Over the period from 1955 until 2002, the start of the first leaf has come earlier at a rate of 1.2 days per decade.” Some monitoring stations in Vermont, from which he has collected data, “have shown advances at a rate of three days per decade.”

It’s in the Genes

According to Mark Losleben, assistant director of the USA National Phenology Network, plants and animals have developed phenological adaptations over evolutionary time scales to protect themselves from the harsh conditions of winter, and they ready themselves for optimal growing conditions in spring and summer.

Changes in the phases or behaviors of these species are “hard wired,” he said, to phenological triggers encoded in their DNA. For instance, Losleben said, “Seeds of some plant species might require a number of days below freezing in order to sprout.” Other species may require a different set of climatic, moisture and soil conditions to come into bloom. According to Losleben, each plant species has its own mechanism suited to its role in the ecosystem and place in the environment.

On the other hand, Losleben says pollinators, such as honeybees and other insects, may rely on an independent set of genetically encoded environmental triggers to cue the foraging behaviors that result in pollination. According to Losleben, a sudden change in one variable — in this case, temperature — could throw off the synchronization between species and have “a cascading effect within the ecosystem.

“We’re looking at potentially rapidly changing times,” he said, “and we don’t want to be caught up short. The best chance we have to avoid this is to learn as much as we can about the environment and how it is changing.”

Wall-to-Wall Coverage

Vast amounts of data are needed to obtain a full picture of the complex interactions between species, the environment and human activity, and this will require considerable effort, says Julio Betancourt, with the U.S. Geological Survey, Desert Laboratory, and one of the founders of the National Phenology Network. The goal is to obtain “wall to wall” coverage of the U.S. with phenological observations at every scale.

“In the United States, we spend $2 billion on satellites; part of the objective is to look at the growing season,” Betancourt said. It is, however, also “important to understand how organisms interact.

“There’s a lot going on in the green wave,” he said, much of which is beyond the capabilities of satellite sensors to detect. “It takes actual ground observers to predict the onset of changes in phenophases.”

While Betancourt notes that the U.K. and the EU as a whole have had phenological monitoring networks in place for many years, according to Schwartz, the United States has had no comparable program: “I like to joke we’re the Third World of phenology,” he said.

Bentancourt said he “got together with Mark Schwartz in 2005” to begin to address the “gap science and monitoring,” and the collaboration grew into a “consortium” of agencies and universities interested in the issue.

Lilacs in the Field

During the 1960s in the Northeast, a network of cloned lilacs had been implemented, Schwartz said, “as an aid to agricultural planning.” In the days before instruments, “They could use the cloned plants as phenological measuring devices.”

According to Schwartz, the idea then was to observe the lilacs over a number of years and generations of plants and to correlate their phases with optimal growing conditions for important farm crops. Participating farmers could be advised to use the cloned lilacs in their area as a guide, telling them, for instance, that “the best time to remove the mulch from the strawberry patches is two days after the lilacs bloom in your area.” However, Schwartz says, the system fell into disuse and was nearly forgotten.

During the 1980s while working on his dissertation, Schwartz “rescued” the remnants of the lilac network and “began using lilac data to calibrate climate models.” Later, he said, “As people were becoming more and more interested in climate issues, it made sense to integrate lilacs into the phenology network.”

In 2006, the grassroots consortium of agencies and universities officially established the National Phenology Network, and in August 2007 they opened offices in Tucson, Ariz. From there, the network seeks to coordinate ongoing phenological data collection activities while analyzing existing data and cultivating new data collection methodologies and sources. They have also made it a priority to enlist public support at every level, from university-trained specialists to citizen-scientists.

In addition to Schwartz’s lilacs, the network has selected about 60 additional indicator species for monitoring. Schwartz, meanwhile, continues to rebuild the lilac network. After successfully distributing 250 of the specialized plants to interested participants in 2006, he says it’s becoming a challenge for nurseries to keep up with demand.

To enlist public support, a field campaign called Project BudBurst has begun recruiting citizen-scientists to gather additional phenological observations. Members of the public can register to participate through the Project BudBurst Web site.

Sandra Henderson, national coordinator for the project, says the Web site will accept data on any number of the 60 targeted species. However, she said, “We’re not going to limit participation if individuals don’t have access to the targeted species.” For instance, she said, “If the only plant they had access to were the dandelion,” people could write in that option and the Web site could accept their report.

Henderson says Project BudBurst is off to a good start this season with more than 3,000 registered participants, and she expects to sign up many more. “We’ll still be hearing from people in Alaska into the month of June,” she said.

Betancourt welcomes the involvement of citizens who participate in existing science and ecological networks, such as the National Weather Service Cooperative Observer Program. “We’d like to get them to make phenological observations in addition to those they already make,” he said.

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