Geographer Bill Turner and economist Jackie Geoghegan teamed up to examine the impact of changing land use in Mexico's Southern Yucatán. Melissa Floyd '01, working with Turner and several graduate students, developed a method to classify protected land based on levels of biodiversity.
Seeing the forest for the trees
Research conducted by Professors Bill Turner and Jackie Geoghegan
What can happen to a tropical rainforest when the logging industry moves in? Or to protected land when a nearby highway is built? Deforestation. Professors Bill
Turner (geography) and Jackie Geoghegan (economics), along with several graduate and undergraduate students,
have been studying changes in land cover
and land use, especially in relation to deforestation.
Turner, Geoghegan and graduate students
are focusing on Mexico's Southern Yucatán in a project
conducted in cooperation with scientists at Harvard
University and El Colegio de la Frontera Sur
(ECOSUR), Mexico. Undergraduate Melissa Floyd '01 examined protected
land in central Massachusetts.
What is land use and land cover?
Land use and land cover are variables used to create a model of a landscape. Land cover
describes what currently exists at a place on the earth's surface. For example, land
cover classes in Mexico's Southern Yucatán peninsula can include wetland forest, upland
forest, secondary growth, cropland, pasture, bracken fern (an invasive species) and
inundated/semi-inundated savannas. Land use refers to the way in
which society uses a particular area of land, whether for agriculture, recreation,
commercial activity or industry, to name a few examples. Land cover and land use are
interrelated and are often studied together. The same type of land cover can have
different land uses. An area of forest might be used for logging, for recreation, or for
conservation. Researchers are particularly interested in being able to predict how land
cover and land use might change over time.
The Southern Yucatán peninsula in Mexico was once the heartland of the ancient and
mysterious Mayan civilization, and descendents of the Maya still live there. Nowadays, in
addition to Mayan ruins, the Southern Yucatán is home to a large expanse of humid tropical
forest that has been increasingly vulnerable to logging and agriculture since the
construction of a highway in 1967. Concerns about deforestation led to part of the region
being converted to a protected area. Global deforestation is currently of great concern to
scientists around the world because it is thought to affect the earth's climate.
Turner and his colleagues are exploring the following questions in regard to a 10,000
square kilometer region in the Southern Yucatán :
- How much deforestation has occurred since the 1960s, and how fast has it occurred
- What types of land use and land cover are taking the place of the forest
- How plant and animal communities are being affected
- Who are the people making decisions about where deforestation occurs and how do they
make those decisions
- How an understanding of change in the Southern Yucatán region can be used to predict
land use and land cover change in other places
Sources of information
The researchers are using several different sources of information to study these
questions. Some sources provide information about the natural environment, while others
provide information about the people that change the environment.
One of the most important information sources consists of remotely sensed images taken at different time periods. Remote
sensing technology has made the study of land use and land cover much easier because it
can provide pictures of large areas of the earth's surface all at once. However,
interpreting the remotely sensed imagery is a complex process and an important problem for
study in itself. Different methods of interpretation
can result in slightly different
maps of land use and land cover. Geographers still need to travel to different parts of
the study area to confirm that they are interpreting the images
correctly-- a process called field checking
The images, along with their geographical locations, can be stored in a computer.
Remotely sensed images are divided into pixels (squares) of the same size, each of which
represents an area on the earth's surface. Each individual pixel can be linked to a
database describing its characteristics such land use, land cover, soil type, and
elevation. Turner and his colleagues are fortunate in having two types of Landsat imagery
available for study covering the whole time period of interest, from the 1960's to
Geographers need to go "into the field" for another reason. In order to
understand fully why a place is characterized by a particular land cover/land use, they
must talk directly with the people who manage the land. Decisions about how land is used
are often made by an individual, such as a farmer. But decisions affecting land use are
also made at many different levels of governmentlocal, regional and national. At
each level, geographers must understand how and why these decisions are made. They must
also study census records and other archival material to understand the characteristics of
the people who live there.
Turner and his colleagues are trying out three different statistical methods in order to
predict the probability that land use will change or remain the same in the future. Each
of these methods is based on the assumption that a given parcel of land can only support
one type of land use at a given time.
Markov chain analysis. This method assumes that a parcel's current land use is
the best predictor of how it will be used in the future. For example, assume that there
are two possible types of land use, forest or agriculture. Four types of transitions over
time would be possible, and, based on an examination of such transitions in the past, each
would be assigned a probability score that:
an agricultural parcel remains agricultural
- a forest parcel remains forest
- an agricultural parcel changes to forest
- a forest parcel changes to agricultural
Discrete choice probabilistic method. In this technique, each parcel of land
is given a "utility" ranking, based on how suitable it would be for each
possible kind of land use. Suitability can involve many factors, including distance to
the nearest road or market, soil type, elevation, etc. A given parcel of land might be
highly suitable for cultivation of chiles, but very unsuitable for logging, thus receiving
a high utility rank for chile farming, but a low one for logging. Researchers can then
look at an unknown parcel and its utility rankings and make an educated guess about which
land use is most likely occuring there.
Hazard or duration model. This technique blends aspects of the Markov chain
and discrete choice approaches. Like the Markov model, it assumes that current use is a
good predictor of future use. It extends the Markov model by looking a how long the
current land use has been active. Also considered in the equation are independent factors
that influence a parcel's suitability for a particular use.
Graduate student research in the Southern Yucatan
Rinku Chowdhury: How have
different organizations involved in forest conservation affected the way forests have
evolved over the past 15 years?
Eric Keys: How are forests
likely to be impacted by the widespread adoption of intensive chile production by small
How will biodiversity and the
release of carbon dioxide into the atmosphere be affected by different trends in
Undergraduate research in central Massachusetts
Melissa Floyd: Are protected lands providing an
environment favorable to biodiversity and wildlife habitats?
Figure 1: Black rectangle indicates area of Professor Turner's research.
Figure 2: Grad students Laura Schneider and Rinku Chowdhury standing among Mayan ruins.
Figure 3: The Maya still farm in their ancestral area, growing corn (maize), beans and squash.
Figure 4: A Mayan house.
Figure 5: Forest area.
Figure 6: Preparing land for Intensive chile production leads to deforestation.
Figure 7: 1987 land cover map created from satellite imagery. Enlarge.
Figure 8: 1997 land cover map created from satellite imagery. Enlarge.