Tree Line
Oregon's Mount Washington also sports a distinct tree line. USGS photo by Lyn Topinka - image courtesy USGS/Cascades Volcano Observatory.
DB: This is Earth and Sky. A listener writes, “My family hiked up Mt. Washington in New Hampshire . . . and noticed that the tree line was very distinct . . .Why?”
JB: A tree line – the elevation above which trees don’t grow – can be higher or lower depending on factors that scientists don’t always agree on. Factors include precipitation and the length of the growing season in a given area.
DB: Mount Washington is the highest peak in New Hampshire’s White Mountain range. Its tree line coincides with the height of its wintertime cloud base – and some researchers believe that’s why the tree line there is so distinct. In the White Mountains, in winter, moist air is forced up the mountain slope. The moisture forms supercooled water droplets at about 1,500 meters – about 4,500 feet. When the droplets contact a tree, they freeze into what’s called “rime” ice. The very heavy rime ice coats tree branches and weighs them down. New growth dies back – and you’ve got the mechanism for a tree line.
JB: Wind limits tree growth, too. In winter on Mount Washington, the winds average 80 kilometers or 50 miles an hour – with gusts up to hurricane intensity. Meanwhile, on slopes less exposed to wind, the tree line might be higher. Special thanks today to the U.S. Forest Service and to the National Fish and Wildlife Foundation – supporting the conservation of native fish, wildlife, plants, and their habitats. We’re Block and Byrd for Earth and Sky.
The following individuals were interviewed for today’s show. Our thanks to:
Peter Crane
Director of Programs
Mount Washington Observatory
North Conway, NH
Daniel J. Zarin, Ph.D.
Assistant Professor of Forest Ecology
Department of Natural Resources
University of New Hampshire
Durham, NH
Author’s Notes:
The highest wind ever measured on Earth was clocked at 370 kilometers – about 230 miles – per hour. It was recorded on Mount Washington in 1934.
Soil often gets thin at high elevation.
Some bare mountain tops were burned off by fire. The thin organic soil at the top of the mountain takes a long time to rebuild.
You may have heard the tree line called the timberline, but they are not exactly the same thing. Timberline really refers to the upper limit for commercial timber while tree line refers to the elevation above which no trees exist. In between the two lies what ecologists call an ecotone, a zone of transition from closed forest to alpine vegetation.
Under some circumstances, high summertime temperatures at high elevation may kill tree seedlings. Other studies suggest that if summers are neither warm nor long enough, trees can’t sufficiently prepare themselves for the cold winters they face at high elevation.
In general, the tree line is several thousand feet lower in eastern mountains of the US than out west. That’s due to the lower cloud base that results from moister conditions.
Additional Teacher Resources
NASA, Earth Observatory, Media Alert Archives: Global Warming and the Onward March of the Pine
Climate change could dramatically increase the forest cover of the Earth’s mountains, ecologists are predicting. This article explains that using data from the Austrian Alps, ecologists have developed a model that predicts the area covered by the local pine will increase from 10% today to 60% by the turn of the next millennium. The article then covers the natural forces that may potentially cause the change.
The Oxford Journal, Annals of Botany: Impacts of Climate Change on the Tree Line
Trees are excluded from the coldest parts of the world. At high latitudes and high elevations they always give way to dwarf shrubs. The boundary of the forest is known as the tree line, although it is usually not a distinct line. Commonly, one observes both immature trees and old dwarf trees scattered above a ragged “line.” This boundary is observed in all parts of the world and exhibits common features. This article discusses how global climate change could potentially change this line around the world.
