In the 1500s, the ponderosa pine forests of Jemez province in New Mexico were home to between 5000 and 8000 people. But after Europeans arrived in the area, the native population plummeted by more than 80%, probably because of a series of devastating epidemics. A new study suggests the crash took place 100 years after the first contact with Europeans. It also suggests that the sudden drop in the local population had dramatic ecological effects, including an increase in forest fires.

The authors of the paper used a "terrific combination" of dendro-ecology—which uses the rings of trees to determine their ages and reconstruct past environments—fire ecology, and LiDAR, a remote sensing technique based on laser light, says Steve Lekson, a Southwestern archaeologist at the University of Colorado, Boulder, who wasn't involved in the study. “It’s such an amazing approach,” agrees Richard Nevle, an environmental scientist at Stanford University in Palo Alto, California, who has studied the ecological effects of the American Indian population crash. “No one has really pulled all these different pieces together so well before. It raises the bar.”

American Indian populations plummeted after the arrival of Europeans in the New World, largely because of the spread of smallpox, typhus, measles, and other infectious diseases. But archaeologists and historians have debated the exact timing and severity of the decline. Did diseases race out ahead of colonial settlement, decimating communities that hadn’t even met Europeans? Or did it take more sustained contact between the two populations to spark epidemics?

The new study suggests the latter, at least in the Jemez province in northern New Mexico. At one time, the region was full of villages and fieldhouses, says Matthew Liebmann, an archaeologist at Harvard University and lead author of the paper. “You can’t walk around on top of these mesas for more than a couple hundred yards without coming across an archaeological site.” Although few of these sites have been excavated, they have been well preserved because they lie within a national forest.

So Liebmann and his team just needed a way to map the ruins without the trees getting in the way. That’s exactly what LiDAR allowed them to do: By sending pulses of laser light down from an airplane, the team soon had a map of the Jemez province, sans trees. Then, they converted the volume of rubble visible today into an approximate number of rooms that must have existed in each of the large villages. From this, they estimated how many people lived there just before the first Spanish explorers arrived in the area in 1541: between 5000 and 8000. That made the province “probably one of the more densely occupied areas of the American Southwest on the eve of European contact,” Liebmann says.

To figure out when the Jemez villages were abandoned, Liebmann and his colleagues counted the rings of trees now growing on the landscape. When a tree is growing, say, inside what was once a house, “the age of that tree would give you at least a minimum time since the structure was last occupied,” says Tom Swetnam, a dendrochronologist at the University of Arizona in Tucson who worked with Liebmann. Many trees now growing in and around the Jemez villages sprouted in the 1630s and 1640s, suggesting that the communities were abandoned around that time, the team reports today in the Proceedings of the National Academy of Sciences.

That’s about 100 years after the first contact with Europeans, but it coincides with the establishment of Catholic missions, the first permanent European settlements in the region. That suggests the area's colonization triggered the depopulation, says Nevle, not something that happened before, such as occasional contact with Spanish explorers. The exact details remain unclear, but perhaps “it was something about bringing so many people together in the mission system that provided the critical mass for diseases to move across the population,” Nevle says.

Colonial documents mention plagues sweeping through Jemez in the 17th century, which supports the idea that a large part of the depopulation was due to disease. The few survivors eventually retrenched in one village near the main mission, which may have also contributed to the abandonment of other settlements. Four decades later, by the early 1680s, there were fewer than 850 Pueblo people left in Jemez province, LiDAR data from the remaining village and Spanish colonial documents indicate. That’s a loss of 87% of the original population. The native peoples’ descendants, members of the Pueblo of Jemez, still live there today.

The tree samples also revealed scars from forest fires, which allowed Swetnam to see how the fire ecology of the Jemez province changed after the population crash. When the villages were still occupied, “people were using every stick of wood for firewood,” Swetnam says, so forest fires didn’t have much fuel to work with. Before 1620, extensive fires burned through Jemez about once every 17 years, he calculated; after 1620, that increased to every 11 years, presumably because people were no longer clearing the underbrush and chopping down trees for construction.

What’s more, “there’s no evidence that any of these villages that we’re studying had any catastrophic fires go through them,” Swetnam says. That is in stark contrast to today, as forest fires are becoming both more frequent and more intense in the Jemez region. “The role of ancient Pueblo people in culling or thinning understory provides a really intriguing model for how we might decrease the number of catastrophic fires,” Lekson says. Liebmann says he is already working with the Pueblo of Jemez to help them apply his study’s lessons about sustainably controlling fire.