LBL Scientist Studies How Plants Respond to Greenhouse Effect

Summer 1989

By Jeffery Kahn (

Global warming implies a world of changes. Melting icecaps, rising oceans, and inundated coastal areas are the apparent initial consequences, but the reverberations of climate change are boundless. Overheated cities could trigger heart attacks. Farming belts could turn to dust bowls. Island nations could disappear.

The repercussions of greenhouse warming range throughout the Earth's natural resources. Plants and animals would be stressed, reducing biodiversity, and increasingly the likely number of extinctions. Likewise, air and water quality could suffer.

Walt Westman, an ecologist in Lawrence Berkeley Laboratory's Applied Science division, is researching how plant communities will respond to the greenhouse effect. Research in this area is relatively new. In examining California vegetation, Westman has created a groundbreaking methodology which can be used to analyze how altered climate could affect other plant communities elsewhere.

Scientists believe the burning of fossil fuels and deforestation will cause atmospheric carbon dioxide levels to double some time in the next century, causing a gradual climate change. Computer climate models of the Earth are still crude, but they agree in forecasting slight increases in temperature and precipitation in California.

For the California flora, says Westman, "That means that generally, vegetation that prefers warmer and wetter conditions will start invading habitat now occupied by vegetation preferring cooler and drier conditions."

Westman's study focuses on the coastal sage scrub community. Ranging from San Francisco into northern Mexico, coastal sage scrub is one of the two most common shrub-land plant communities in Southern California. Coastal sage scrub occupies the driest area along the coast, and is of particular significance to urban residents. It is the main source of ignition for the wildfires that periodically rage in Southern California.

Before exploring the effects of a climate change, Westman examined the antecedent conditions that govern life within this community of 500 plant species. He assessed over 40 possible factors including a complex of current climactic variables, substrate, fire variables, grazing, and air pollution. Based on these analyses, he and a former student at UCLA, George Malauson, built a computer model to simulate the changes that occur within the community over time, before and after fire.

"Both the model and our field data show that summer conditions are more important in predicting what grows where," says Westman. "This is a Mediterranean climate with a low rainfall that is heaviest in winter; the main stress on the plant is the lengthy summer dry season and its intense temperatures. Plants that can survive summer evapotranspirative stress will survive. It's the intensity of summer temperature and the extent of annual rainfall that determines species distribution."

Using his working ecological model as a foundation, Westman created a new model that simulates the influence of the forecasted climate changes in the 21st century.

Plants in the model are subjected to increased precipitation and temperatures, increased ozone levels, and varied fire frequencies. Westman and Malauson are the first scientists to examine the combined effect of climate change, heightened air pollution, and altered fire frequency on vegetation.

"Altered fire frequency results from the altered rate of vegetation growth, and hence fuel," explains Westman. "Altered air pollution would result from the change in the incidence of inversions, plus increased summer temperatures which would lead to more rapid photo-catalytic conversion of hydrocarbons and nitrogen oxide into ozone. Ozone is the main component of smog, and is strongly damaging to coastal sage."

With this complex of variables in its digital memory, the model is able to simulate how the plant community changes under various scenarios over a two century span. Different climate conditions and different fire frequencies interact and can produce radical changes in the coastal sage scrub community. Many scientists believe that climate change would result in extinctions, but the model makes a substantial case for this prognosis.

"We run a fire every 10, 20, 30, or 40 years over our 200-year cycle and see how the plant community recovers, how many species return after the fire. Interestingly," says Westman, "under doubled CO2 levels with a 10-year fire frequency and increased ozone levels, we lose all the species."

Currently, controlled burns of scrub lands are done about every 12 years in the Los Angeles area. "Fire hazard reduction practices may come into conflict with efforts to preserve these species under altered climate," concludes Westman.

Following wildfire on coastal sage scrub, mudslides, which envelop and crush homes, cars, and anything in their path, often occur. Climate change could alter the frequency of mudslides, and the rate of erosion of mountainous terrain. Further, erosion itself has collateral consequences. Erosion rates effect sediment buildup in reservoirs, the clarity of estuarine waters, and the productivity of fisheries.

Westman says the next step in his continuing research is to include chaparral, the other dominant nonforest ecosystem in Southern California, in his climate model, and analyze the competitive relations between these two shrub types.

Ultimately, he says, models of how plant communities respond to climate change could be linked to other models simulating erosion, sedimentation, and flooding. Coupled, they would provide a powerful warning of how the incidence of fire and flood might change life in the 21st century.