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The world's climate is warmer on average than it was a hundred years ago. Plants in some places are emerging earlier in the spring and insects that like warm weather are on the move. But scientists are finding out that the culprit isn't just warmth. As NPR's Christopher Joyce reports, it's also the absence of cold snaps.
CHRISTOPHER JOYCE, BYLINE: The idea that a warmer planet could mean avocados in Scotland or bananas in Montana may sound silly. But in fact, tropical plants are moving north. For instance, mangroves in Maryland.
DANIEL GRUNER: We're walking into what we call our dirt lab. Plants and living insects...
JOYCE: Daniel Gruner opens a big steel door at a lab at the University of Maryland. Inside, shelves of mangrove plants in pots. Mangroves are those lush, salt-water-loving forests along coastlines that seem to hover on a bed of twisted roots. In the U.S., you mostly find them in Florida, not in Maryland, except in labs like this one grown for research.
But biologists have found that mangrove plants like these are headed north. They're cropping up in places they've never been before. Scientists thought it was because it's getting warmer up north - mangroves like it hot - but they were wrong. What mangroves are actually responding to are cold snaps or the lack of them.
GRUNER: They're responding to thresholds of temperature and not just to long-term warming trends.
JOYCE: The threshold that controls where mangroves can grow is exactly 25 degrees. Florida mangroves are creeping up the Florida coast because cold snaps that low just aren't happening the way they used to there.
GRUNER: But it's just the areas where they experience fewer of these cold snaps that really increased in mangrove area.
JOYCE: This surprised biologists. They've seen bark beetles and invasive weeds move into new places that have grown warmer. But with mangroves, it's the growing lack of really cold weather that has opened up the north to them. Now, maybe that's a good thing. Mangroves are nurseries for fish and crabs. They buffer the coastline from storms. But Kyle Cavanaugh, of the Smithsonian Institution, who has mapped their movement north, says mangroves are moving into areas that are already occupied.
KYLE CAVANAUGH: Areas where historically have been 100 percent salt marsh, now we're seeing mangroves move in and really flourish.
JOYCE: What's happening is a kind of battle between northern salt marshes and southern mangroves on the march.
CAVANAUGH: It seems like this mangrove salt marsh system is on a - is near a tipping point in certain areas where very small changes in temperature - in this case, the frequency of these extreme events - can lead to dramatic landscape scale changes.
JOYCE: Cavanaugh and Gruner discovered this phenomenon by studying satellite photos of the Florida Atlantic coast. They describe it in the Proceedings of the National Academy of Sciences. They point out that more mangroves might be a good thing but they fear that other plants that have been held back by cold weather - invasive weeds like the all-consuming kudzu - may also be following right behind the mangroves. Christopher Joyce, NPR News.
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