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Research shows clearing rain forests caused climate change

by Melanie Titanic-Schefft
photos by David Lentz

Droughts and climate change led to the eventual abandonment of Tikal, a major Mayan city of affluence that thrived for about 1,500 years in present-day Guatemala.

According to University of Cincinnati research just published in the prestigious Proceedings of the National Academies of Sciences (PNAS), effective and sophisticated land-use practices sustained the thriving Mayan city of Tikal for more than 1,000 years until drought and climate change led to the abandonment of the city, which was once home to tens of thousands of people at its peak in A.D. 700.

Those are the findings of David Lentz, UC professor of biological sciences in the McMicken College of Arts and Sciences, and his colleagues. They analyzed surveys, satellite imagery, archaeological information, water, food and forest resources and pollen data at Tikal to establish the timeline and causes of its rise and subsequent demise of the prominent Mayan city. Their paper, “Forests, Fields and the Edge of Sustainability at the Ancient Maya City of Tikal,” is in the December issue of PNAS.

Faculty and student members of that research team are from UC, the University of Texas, Carnegie Mellon University, NASA, Brigham Young University and the Instituto de Antropologia e Historia and include: David Lentz, Nick Dunning, Vern Scarborough, Ken Tankersley, Silvia Alvarado, Walter Burgos, Sheryl Carcuz, Chris Carr, Mauricio Diaz, Rob Griffin, Angela Hood, Brian Lane, Raquel Macario, Blanca Mijangos, Kim Thompson, Eric Weaver, Thomas Sever, Liwy Grazioso, Carmen Ramos and Richard Terry.

Flourishing city clears way to its own demise

Archaeologists divide Mesoamerican development into three major time periods: 

• PreClassic or Formative period extending from B.C. 1500 - A.D. 300

• Classic period extending from A.D. 300-950 

• PostClassic period extending from A.D. 950-1521

Says UC’s Lentz, “Our goal is to understand how the Maya lasted as a highly complex society for well over 1,500 years in a tropical environment. Their resource needs from forests were very great because they used trees to construct monumental buildings, for the creation of plaster (which was a surface cover for buildings and roadways), for cooking and many other needs. They had a sophisticated, long-lasting management system that apparently began to go awry during the Late Classic period” (A.D. 600-950).

Tikal’s population grew and prospered via their agricultural proficiency, including irrigation, terracing and slash-and-burn cultivation, along with controlled agroforestry and water conservation techniques. However, when Lentz and his team investigated mineral deposits in regional caves to try and understand the city’s demise, they found remarkable evidence for unusually persistent episodes of low rainfall during the ninth century that corresponded with the abandonment of Tikal.

Lentz credits this skillfully managed ecosystem for sustaining a large preindustrial community. However, the self-sustaining and effective land-use eventually resulted in the reduction of a consistent annual rainfall that was necessary to sustain their lives. In other words, human activity in the form of intensive agriculture and deforestation reached a tipping point, causing prolonged drought.

That’s because in spite of elaborate irrigation, the Maya eventually cleared about 60 percent of their forest, which had a dramatic impact on the moisture in the air. Because rain forest density creates conditions for high humidity and rain, the subsequent depletion of those forests caused a corresponding decrease in moisture and rainfall. This subsequently contributed to drought and thus, the depletion of vital water reservoirs in a climate where rain was a valuable resource, common only during the winter rainy season.

Food, fuel and land use

Lentz estimates the population of Tikal at about 45,000 by the amount of fuel and food they had.

Looking at pollen data and soil levels around the perimeter of the city, the UC team discovered that by the Late Classic Period, Tikal’s forest areas were reduced by 60 percent of the approximately 1,100 square kilometers of total land available. The crop fields used to grow maize (corn) and bajo was about one third of the available land and the rest was used as an urban area where they lived.

Closely examining soil and rock samples, Lentz and colleagues’ research revealed that, at first, the Maya managed their forests well, cutting down only a few trees at a time and replacing them as they went. For many years they had a viable conservation system, however, the UC scholars point out through their research that the Maya later made several mistakes that eventually caused problems with erosion.

“But then, as they moved into the classic period, they somehow figured out ways to control erosion because the erosion was a lot less during the classic period, which shows that they developed a lot of techniques for protecting their environment,” says Lentz. “And these behaviors kept that city going for over a thousand years.”


Using forest growth data, Lentz was able to determine how the Tikal forest supplied essential resources such as wood for fuel and construction material, habitat for game and wild plant food for pharmacopoeia from medicinal species.

Sectioning off six hectares (10 acres) of the rain forest, Lentz and his students marked and measured each tree to determine how much of the forest had to be managed on a sustainable basis. They found that the most commonly used trees were:

  • Inkwood - used for burning, later commercially extracted by the British in the 18th and 19th centuries to be used to make dye
  • Chicle tree - used for building
Example of wood carving found at Tikal temp, Guatemala

Examples of Maya culture wood use from ancient Tikal forest.

Examples of wood used at Tikal temple, Guatemala.

The heaviest demand on the forest trees was for:

  • Firewood fuel needed for cooking
  • Firing of ceramics
  • Construction timbers
  • Fuel to manufacture plaster was also a considerable need, since the ceremonial core of the city was covered in plaster

The UC team discovered Tikal wood use from a variety of sampling techniques:

  • They put cores down into the reservoirs and analyzed samples from the depths of the layers they retrieved
  • They found that the whole area around the temples was cleared and plastered over with a very high-quality plaster made from burned limestone, used as the binder for their plaster
  • This process took wood to burn the limestone to make this plaster, which adds to the need for cutting away forest trees

Agriculture and water resources

The Tikal fields were used to grow maize and a variety of other crops and, as Lentz points out, as in a lot of places, a city often grows on the best land, so the Maya also grew orchards and kitchen gardens around their houses.

Using the Late Classic village of Cerén as a comparative model, Lentz and colleagues showed that Tikal operated on a much larger scale, but used essentially the same array of crops. Tikal grew maize, three species of beans, two species of squash and several species of root crops, including sweet potato. The Maya at Tikal also used the margins around the swampy areas called bajos to grow what they could during the dry season.

Tikal grew as a city around a system of springs emanating from what is now the Temple Reservoir. The temple sits at the head of a long ravine that was blocked off in places to impound the water that flowed from the springs. As Tikal continued to grow, they built large plazas on either side of the ravine and canted the pitch of the pavements so that seasonal rainwater would flow into the reservoirs.

Temple 3 rising out of the tropical forest that now enshrouds Tikal.

Temple 3 rising out of the forest that now enshrouds Tikal.

So while they had a very effective and efficient water conservation and storage system, without the rainfall and with no way to import water into their city, they had no way to sustain the city. 

Therefore, when drought affected water supplies and agricultural capacity, the residents began to migrate to coastal areas, as Tikal had no draft animals to carry food or water from distant areas into the city.

Finally, the city was abandoned sometime around A.D. 870. Migratory patterns show that the Tikal Maya who survived the ninth century droughts, eventually moved to areas with more permanent water sources down around Lake Petén Itzá in Guatemala or to what is today referred to as Belize.  

Research support: This research was supported by the National Science Foundation, the Wenner-Gren Foundation and the Alphawood Foundation.

More information: Read Lentz' paper, Forests, fields, and the edge of sustainability at the ancient Maya city of Tikal.

Check out Lentz' book "Tikal" to be published in February 2015.