Plants' impact on landscape evolution made evident

November 2007

PROBLEM

Plants are more than stakeholders in Earth’s carbon, water and geologic cycles; they are major players. They regulate CO2 in the atmosphere by converting it into plant mass, and they release the oxygen we breathe. They impact the global water cycle by absorption and transpiration, and have an enormous impact on the evolution of Earth’s surface by altering soil properties and controlling water and wind erosion.

Scientists modeling Earth’s cycles sometimes include vegetation, but commonly treat plants as static in type and lifespan, rather than as biota that wax and wane in response to their environment. Yet the magnitude of vegetation’s short-term impact is evident in the large seasonal fluctuation of CO2 in the Earth’s atmosphere, which decreases drastically during the growing season. If plants have such an enormous impact on climate seasonally, might we be shortchanging their effect in long-term models of hydrology and climate?

Professor Rafael L. Bras of MIT’s Department of Civil and Environmental Engineering—who uses computer models to create precise descriptions of natural processes in hydrology and landscape evolution—believes that truly accurate models require that plants be represented as dynamic players.

APPROACH

In two recent papers, Bras and co-authors model the effects of vegetation and vegetation-erosion on landscape evolution and compare the models’ output with reality. One set of successive models used climate and soil data representative of the Oregon Coast Range, which runs more than 200 miles north-south, has an average elevation of 1,500 feet, a mild climate, and is home to a wide variety of plants and animals.

FINDINGS

In the first iteration, Erkan Istanbulluoglu and Bras formulated a model to represent the evolution of a featureless, flat landscape in response to climate. They limited the land-forming processes to geologic uplift; water-driven soil erosion and deposition; and the slow movement of soil from ridges triggered by raindrops, animals or other diffusive processes. They did not include vegetation. This model developed a stark landscape spiked with eroded slopes, similar to the bare, rugged badlands of Utah.

In the next iteration, the scientists included landslides and static vegetation. A very different landscape appeared, one that more closely resembled the actual range, but not yet accurately. Because the consistent vegetation cover protected the soil from run-off erosion, the topography developed into just a few highly elevated slopes; erosion was dominated by landslides.

Next, the researchers included dynamic vegetation in the model—vegetation that grows over time and dies in response to weather, fire and erosion. The topography that emerged was visibly very similar to that of the actual Oregon Coast Range.

IMPACT

Bras’ simulations provide a basic understanding of the co-evolution of vegetation and terrain, and underscore the importance of including dynamic vegetation in long-term models of hydrology and geomorphology. Vegetation does impact landscape evolution; the signature of vegetation is evident on terrain topography.

In a paper published in Water Resources Research in June, he and D.B.G. Collins describe plants’ rooting strategies—depth and distribution—in different ecosystems. They show that plants successful in a climatic and soil-type niche are those whose rooting strategies minimize the probability of moisture stress. Researchers can now incorporate that description of how climatic and soil factors contribute to changes in root configuration into ever more accurate models of landscape evolution.

This year, Bras is recipient of both the American Geophysical Union’s Robert E. Horton Medal, the highest award given to hydrologists by geophysicists, and the Simon W. Freese Award, the highest environmental recognition given by the American Society of Civil Engineers. For more information, contact OnBalance@mit.edu.

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