By Deborah Zabarenko, Environment Correspondent

WASHINGTON (Reuters) - Fast-growing weeds have evolved over a few generations to adapt to climate change, which could signal the start of an "evolution explosion" in response to global warming, scientists reported on Monday.

This means that the weeds will likely keep up with any attempts to develop crops that can adapt to global warming, said Arthur Weis, a professor of ecology and evolutionary biology at the University of California, Irvine.

But some long-lived species -- like the venerated California redwood tree, with a life-span of hundreds of years -- will not have the capacity to adapt so quickly, because their life cycles are so long, Weis said in a telephone interview.

The quick-growing weedy plant known as field mustard showed the ability to change reproductive patterns over a period of just seven years, Weis said.

"If you take a climate shift, such as we've had here in southern California, in a very few number of generations you can get a change in ecologically important traits that can allow these fast-growing weedy species to hang on and actually do well despite the change in environments," he said.

Weis and his colleagues cultivated two sets of mustard seeds in a greenhouse: one set collected in 1997, just before a five-year drought, and a second set collected in 2004, after the drought ended.

The plants were divided into three groups, with each getting different amounts of water, ranging from drought-dry to soggy. In every case, the post-drought generation of plants flowered earlier, meaning the plants could produce seeds before the soil dried out. Late-bloomers would wither before any seeds were produced in a drought year.

How fast a change is this, on the evolutionary timetable? Weis calculated that this represents a 16 percent acceleration of the mustard plants' life-cycle over seven generations.

"That's a pretty big change in age of maturation," he said.

Asked to hypothetically compare this to evolutionary changes in people, Weis offered what he termed a very crude analogy: if humans evolved at the same rate as the mustard plants in the experiment, the average onset of the age of reproduction in humans would slip from 16 years to 13 1/2 in seven generations.

Weis is spearheading a project to collect, dry and freeze seeds from around North America so they can be studied 50 years from now. He figures that global warming will prompt lots of evolutionary changes and scientists will want to have evidence of plants before the changes occurred. The effort is called Project Baseline.

"If global climate change is coming, and it is, we have this huge unplanned experiment in evolutionary biology facing us," Weis said. "Climate change could lead to an evolution explosion. ... This gives scientists an unprecedented opportunity to look at the actual nuts and bolts of evolutionary change."

The idea is for scientists in the mid-21st century to go back to the same locations where plants are being collected and note the differences between the plants from the different time periods.

Research by Weis and his team was published in the journal Proceedings of the National Academy of Sciences.