Just getting there takes hours of hot, sweaty hiking through lowland Papua New Guinea forests: three hours from the road to the base camp, then another seven to the site. That’s when the real work begins: tagging, measuring, and identifying 250,000 trees scattered over 50 hectares.

Stuart Davies estimates there’s between 600 and 700 different tree species among those quarter-million trunks, between 5 and 10 percent of which will be new to science.

But the effort isn’t being expended solely to discover new trees. Davies and colleagues at the Center for Tropical Forest Science (CTFS), which he directs, are building an observatory — pointed at planet Earth — out of trees.

The center is an unusual partnership between Harvard’s Arnold Arboretum and the Smithsonian Tropical Research Institute. Headed by Davies, who is the Arboretum’s director of Asia Programs and who holds a joint appointment at the Smithsonian Tropical Research Institute, the center was established in the early 1990s to provide a stable, long-term scientific framework for tropical forest study through a series of forest plots — exhaustively documented and regularly censused — across the tropics.

Trees are natural climate-monitoring stations. They are stationary and long-lived, allowing monitoring of a single spot over time. Their growth is sensitive to changes in sunlight, rain, temperature, and other environmental factors. They’re also sensitive to the global warming gas carbon dioxide, which they use in photosynthesis.

Some scientists have speculated that as carbon dioxide levels in the atmosphere rise, as is expected in most climate change scenarios, tree growth will increase, locking up at least some of the carbon humans release and helping to dampen the coming changes.

But they don’t know for sure.

Climate change isn’t just about carbon dioxide, however. Scientists predict not just an earth that is warming; on average, they expect localized changes in temperature that far outstrip global averages — changing rainfall patterns and insect and animal distributions.

But the CTFS’s forest network isn’t just about climate change, either. Scientists studying many aspects of forest biology have used the plots, which are censused every five years. Davies said the plots, which themselves are undisturbed, are selected to represent forest types that are used by humans, so that the basic forest biology can be understood and put to use in designing sustainable-use plans and, if needed, restoration projects.

At Harvard, faculty, graduate students, and undergraduates have used forest plots in Thailand, Borneo, Malaysia, and elsewhere for studies involving everything from carbon flux to insect vocalizations.

Charles Bullard Professor of Forestry Noel Michele Holbrook said the plots’ longevity, their geographical breadth, the consistency with which data on the trees are collected, and the associated data, such as forest soil types, make the network a unique resource.

“The plots are important for a lot of reasons. Most important is their longevity, the fact that they’re accumulating data over time. They become more valuable with each census,” Holbrook said. “There’s nothing like them anywhere.”

The availability of data from previous censuses makes research on the plots far easier than it would be if a researcher had to start from scratch in a different forest, said Holbrook. Holbrook pointed out that she is overseeing a senior honors thesis focused on Borneo’s forests that wouldn’t have been possible if the student had to lay all the scientific groundwork herself.

The latest plot, in Papua New Guinea’s northern province of Madang, is part of an expansion of the forest network over the past two years, driven by interest in climate change, which has seen what was largely a tropical network of 21 forest plots grow to 32, either established or under way and encompassing both tropical and temperate forests.

The new plot has been on the drawing board for almost two years, Davies said, and has been helped along by grants from the National Science Foundation and from John Swire & Sons Ltd., a British company that runs shipping operations in Papua New Guinea. The forest tract is biologically significant, Davies said, because it will be the first one east of Wallace’s Line — an invisible line running off the island of New Guinea’s western coast that separates the flora and fauna with roots in Asia from those with roots in Australia.

“We have identified certain areas that are unique in biodiversity, that are biogeographically important, and that are underrepresented in our global network. New Guinea was an obvious one,” Davies said.

Davies, working with a colleague from the University of Minnesota, George Weiblen, began to identify the plot location after the local people — who belong to the Wanang clan group — approached Weiblen, who runs an entomological research station in the area.

The Wanang, who, like other indigenous clans own their traditional lands, had watched neighbors sell logging rights to foreign companies who had come in and devastated the forests, leaving the local people with a small royalty but no forest, which had supported their livelihood for centuries.

“The northern slopes of New Guinea from 200 meters to 800 meters are all one forest type. That’s good for logging, because they can basically go in and just mow it down, take out all the important species,” Davies said. “It’s really the story of tropical forestry in other parts of the world coming to New Guinea now.”

The Wanang asked Weiblen if there would be a way to have their forest studied, as an alternative to logging, and Weiblen contacted Davies. The clan agreed to let the CTFS use the 50-hectare plot and agreed to limit their use of a 1,000-hectare buffer zone around it, leaving the study site as undisturbed as possible. In exchange, Davies said, the CTFS will provide a royalty in the form of community development, such as education and health care, the details of which are still being worked out. In addition, he said, the project will provide jobs as porters, cooks, and other camp staff to the local people, who are mainly living as subsistence farmers — raising pigs and growing crops like cassava and corn — and who have very little cash income.

The plot’s scientific staff will also be mainly made up of Papuans, Davies said. Team leaders will be trained at the CTFS facility in Panama. In March, Davies and Weiblen will return to conduct on-site training in the tagging and identification of trees.

As with all CTFS plots, field-workers will tag, locate, and identify every tree in the plot larger than a centimeter in diameter at breast height. They’ll also collect 40,000 to 50,000 voucher specimens. Davies estimated the early work will take two years to complete.

“It’s just a hard slog,” Davies said. “It’s a big job.”

The bulk of the work will be done by four to six teams of three people each, rotating into the forest and out again.

“Just like any job that’s in the middle of the forest with few options for recreation, we have to change those teams over. You can’t let them stay too long, they’ll go stir-crazy,” Davies said.

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