Increasing the number of forest trees around the world is becoming increasingly limited by water as the climate warms up, according to new research by an international team of scientists from the University of Arizona.
The effect is most evident in the Nordic climates and high altitudes, where the primary limitation of tree growth was low temperatures, reports the team this week in the online journal Scientific advances.
Our study shows that in most of the land, trees are becoming more and more limited to water, "said Flurin Babst, the first author to research at the Tree-Ring Research Laboratory and the Swiss Federal Research Institute WSL Zurich.
"This is the first time that someone has engineered the responses of growing trees to the climate at a near-global scale," Babst said.
The researchers compared the annual trees growth rings in two time periods, 1930-1960 and 1960-1990. Growth rings are wider when conditions are better, narrower when conditions are worse. Ring width measurements were taken from trees at approximately 2,700 locations stretching across every continent except Antarctica.
For the two time periods, the team also represented the average temperature, precipitation and drought stress measures on plants on a network covering the temperate and boreal regions of the world.
Adding tree ring data to the map allowed scientists to see if climate change in the 20th century corresponded to changes in the growth of world trees.
Co-author of co-author David Frank said, "We have seen areas where temperature increase is limited in the early 20th century, but we are now seeing changes to limiting drought-humidity."
Comparing the 1930s to 1960s with the 1960-1990 average temperature increased by 0.9 degrees F (0.5 degrees C) and the land area where the growth of trees was primarily limited by the temperature fell by 3.8 million square kilometers, the size of Brazil.
Babst was surprised that such a small change in temperature would change such a large area of trees from being limited to temperature to limiting the water.
"It's much more than I expected," said Babst, who is now a researcher at the Federal Research Institute of WSL.
The findings have implications for the future growth of forests and trees and for forest management, said Frank, director of the Trees Tree Research Laboratory and dendrochronology professor at the AU.
"Low growth indicates increasing stress on plants – which may be related to mortality," he said.
The paper "Redistribution of the Twentieth Century in Global Climate Growth Factors" is scheduled for online publishing in Romania Scientific advances on January 16th. More information about co-authors and funding agencies is at the bottom of this press release.
Since the introduction of systematic satellite observations in the late 1970s, scientists can study changes in vegetation growth on large surfaces by comparing satellite images from the same place in time and by measuring "summer" in images. Greenness is a measure of how leaf plants are and how fast they grow.
However, satellite observations were not available for most of the 20th century. Moreover, measuring "summer" in a satellite image can not tell how many individual plants have increased from year to year.
"Satellites only see the leaves – they do not see the wood where the carbon is stored," Babst said. "We wanted to offer a perspective of wood."
He and his colleagues wanted field measurements of tree growth to compare with satellite data. By using several databases of arboric ring measures around the world for the 1930-1960 and 1960-1990 periods, researchers could see if the average tree growth has changed.
Additionally, as team research combines a substantial data base of growth data from tree rings around the world with global climate data over the same period of time, new results will help test and improve computerized models of how climate affects vegetation.
Co-authors of Babst and Frank are Valerie Trouet from the UA; Olivier Bouriaud from Strada Universitatii in Suceava, Romania; Benjamin Poulter of NASA Goddard Space Flight Center in Greenbelt, Maryland; and Martin P. Girardin of Quebec's Natural Resources Canada and the Université du Québec à Montréal, Canada.
The "European Union-Horizon 2020" program and the Swiss National Science Foundation funded research.