Earth and Sky科普英語：Sierra Treeline 山脊樹林

Could global warming allow forests to expand into higher elevations than they have reached in more than a thousand years? Learn more about studying alpine treelines -- today on Earth and Sky.

JB: This is Earth and Sky. The highest point that trees grow on a mountain -- called the treeline -- is based in part on temperature.

DB: Andy Bunn is a PhD candidate at Montana State University. He suspects that treelines around the world will expand up mountain slopes in response to global warming. He's studying treelines in Sequoia and Yellowstone National Parks.

Andy Bunn: So we use alpine treeline as a bellweather -- as as an early warning system -- of global climate and try to understand if treeline is responding -- is treeline ebbing and flowing right now in response to climate warming?

JB: The difficult task of monitoring a mountain range's entire treeline is made easier by using computer models and satellite data to identify trees that are most likely to react to climate shifts.

Andy Bunn: Once we find those areas that fit our requirements of being climate sensitive we can actually go in there and start looking at the growth rates -- the growth rings of trees. We can start looking for germination of seedlings and we can start looking for these areas where there's dead wood rooted above the current treeline ...

DB: Bunn hopes these studies will help determine how forests may change in the coming century. For more, come to today's show on the web at earthsky.org. Special thanks to Andy Bunn in Montana and to NASA's Earth Science Enterprise. And Deborah Byrd, with Joel Block, for Earth and Sky.