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As the earth warms up, many plants and animals are moving uphill to keep their cool. Conservationists are anticipating much more of this, as they make plans to help natural systems adapt to a warming planet. But a new study in Science magazine finds that plants in Northern California are bucking this uphill trend. NPR's Richard Harris explains.

RICHARD HARRIS: Coping with climate change is, quite literally, an uphill struggle for ecosystems looking to remain in a temperature zone where they do best.

Mr. SOLOMON DOBROWSKI (University of Montana): We see it, consistently, for mobile species such as insects and animals. A lot of the real foundation studies for this have come out of studies looking at butterflies, for example.

HARRIS: So Solomon Dobrowski, at the University of Montana, expected he'd see the same trend when he looked into historical movements of plants in a vast area of Northern California. He dug through a remarkable record of the region's vegetation collected back in the 1930s, thanks to a federal project started during the Great Depression. He then compared that with modern vegetation surveys.

Mr. DOBROWSKI: And what we found was counter to our expectations. We found that, in fact, the preponderance of plants that in our study area - which is probably the northern half of the state, would be the simplest way to describe it - had actually moved downhill, roughly 80 meters - so about 240 feet.

HARRIS: Individual plants don't move, of course, but the optimal range of many different species has been creeping downhill. So, more new seeds sprouted downhill, and more new plants took root. And this was true not just for annual plants, but for bushes and even trees. Why would that be, he wondered, considering that the area has warmed up? He and his colleagues say the answer lies, not in the temperature, but in the amount of life-giving rain and snow. It turns out this region has been getting wetter.

Mr. DOBROWSKI: These plants are tracking water availability more so than temperature.

HARRIS: Not that big a surprise, when you come to think of it. But until now, ecologists doing this kind of study had mostly noticed a trend linked with temperature. Dobrowski says that still holds, in many cases, but...

Mr. DOBROWSKI: The simple message that things are going to move uphill and towards the poles may not be the answer in all cases.

HARRIS: This adds some pretty big wrinkles to conservation plans. For example, it's not always a good assumption that protecting areas upslope from plants will help protect their future habitat as the climate changes. Plus, Dobrowski says, if ecosystems see their optimal temperature moving uphill and their water supply moving downhill, that could be quite awkward at times.

Mr. DOBROWSKI: If a number of plants are moving down slope and organisms that are adapted to rely on those plants such as butterflies, for example, which are more constrained by temperature and moving upslope, you could have situations in which plants and animal communities are even disrupted further.

HARRIS: This is quite a sobering finding for ecologists trying to anticipate what will happen to natural systems in the coming century. Scott Loarie, a researcher at the Carnegie Institution of Science on the Stanford campus, says most of the research has been focused on the warming part of global warming.

Mr. SCOTT LOARIE (Researcher, Carnegie Institution of Science): One of the reasons that so many of our results on what we expect will happen with climate change have centered on temperatures, because we know the most about temperature. Pretty much all of our evidence shows that the earth is going to get hotter, and there's very little dispute about that. We don't know very much about precipitation, but that doesn't mean to say that we dont think precipitation is important.

HARRIS: Loarie says the new study underlines just how important precipitation can be. Unfortunately, in many parts of the world, scientists simply can't say whether climate change in the long run will bring more moisture or more drought. Loarie says California is a case in point. The various climate forecasts disagree.

Mr. LOARIE: So it's really a crap shoot in California, whether we're facing a drier or wetter future.

HARRIS: And that means it's entirely possible that the plant communities that have been marching downhill for the past 80 years, will eventually reverse course and head back up the slopes.

Richard Harris, NPR News.

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