Invasive weeds grow with global warming

The yellow star thistle, an invasive range land plant from Turkey, is spreading at the Jasper Ridge Biological Preserve at Stanford in Woodside. Photo by Jeff Chiu / the Chronicle. The yellow star thistle, an invasive range land plant from Turkey, is spreading at the Jasper Ridge Biological Preserve at Stanford in Woodside. Photo by Jeff Chiu / the Chronicle. Photo: Jeff Chiu, The Chronicle Photo: Jeff Chiu, The Chronicle Image 1 of / 1 Caption Close Invasive weeds grow with global warming 1 / 1 Back to Gallery

For educational purposes, the California Invasive Plants Council will sell you bouquets of plastic weeds, including yellow star thistle, tamarisk, leafy spurge and knapweed. Some recent studies suggest that many gardeners need not invest in these because they can expect more of the real thing to arrive as climate change advances.

A warmer world will have more atmospheric carbon dioxide and nitrous oxide. UC Irvine's Diane Pataki says nitrous oxide has 300 times the global warming potential of carbon dioxide per molecule. It's released when soil bacteria covert ammonium to nitrite.

Pataki planted fescue test lawns, varying their temperatures and fertilizer. She discovered that warmer plots with high soil moisture pumped out more nitrous oxide. These plots also had the most weeds, with warm-season species like crabgrass and Bermuda grass dominating the fescue.

Fast-growing weedy plants will seize the resource bonanza created by climate change. Dana Blumenthal, agricultural research service ecologist with the Department of Agriculture, has found that rapid growers adapted to nitrogen-rich soils tend to carry a heavy load of fungal and viral diseases in their native habitats. In new areas, they leave their pathogens behind, giving them an advantage over even fast-growing native species.

They meet new predators and diseases but may be less susceptible to them than native plants are. Dutch scientists looked at exotic plants from warmer climates that had become established in a nature preserve, comparing the newcomers with their closest native relatives. They reported that the invasive species were less vulnerable to soilborne microbes than the natives. Local aphids that fed on the native plants ignored the exotic ones, as did North African locusts.

Weeds from warm climates are poised to claim new turf as temperatures increase. But other invasives may lose ground. Princeton researchers Bethany Bradley, Michael Oppenheimer and David Wilcove used computer models of global climate change to predict the future ranges of weeds that are widespread in the West.

"Just as native species are expected to shift in range and relative competitiveness with climate change," they wrote in a study published in the journal Global Change Biology, "the same should be expected of invasive species." Using each weed's preferred habitat characteristics and a scenario in which fossil fuel emissions are not reduced, Bradley and her colleagues created an invasion risk map for each weed.

Their results were mixed. The bad news for California: yellow star thistle will keep its current range and probably spread farther here and in Nevada. Tamarisk, an exotic tree that sucks wildland creeks dry, will neither gain nor lose in a warmer West. The largest effects the Princeton group predicted were for cheatgrass and leafy spurge, which will shift their ranges north, and spotted knapweed, which will move to higher elevations.

Gardeners may also have to cope with a new cast of characters. Some researchers are trying to identify exotic species with weed potential before they jump the garden fence - plants with windblown or bird-dispersed seeds, species that reproduce vegetatively through runners, generalists that thrive in a variety of habitats - and working with growers and nursery folk to make sure new invasives don't get into the horticultural trade.

Climate change is a moving target, and some effects could turn out to be stronger than the models account for. There's also no foolproof way to identify which plants will become invasives. But with weeds moving both in from wildlands and out into them, it's safe to say that weed control will still be on every thoughtful gardener's agenda.