Global warming over the past decades has intensified tree growth in most northern (and upper) treelines in most areas around the world. An international team of scientists predicts that by the beginning of the next century, provided the temperature continues to rise, the relationship between temperature change and the rate of annual growth in such conditions will almost disappear. This means that the ability of trees to absorb CO2 will not increase in proportion to the increase in temperature. The research results are published in the journal Global Change Biology.

Over the past 100 years, global air temperatures have increased by 0.8 degrees Celsius. Global warming is projected to continue throughout the 21st century. This warming, coupled with an increase in atmospheric carbon dioxide, has accelerated tree growth in conditions where, until recently, it was limited by temperature variability. However, there is no certainty that the growth rate of trees will continue to increase.

An international team of scientists, including a researcher from the Krasnoyarsk Science Center of SB RAS, analyzed the trends in tree growth in the 20th century and made a forecast of the impact of climate change on forest development up to the beginning of the 22nd century. The developed models show that in the future, the dependence of the tree growth on temperature changes will become weaker, and low temperatures will no longer limit the tree growth in northern latitudes.

"Currently, the growth of trees in the north, first of all, depends on the variability of summer temperatures. Under these conditions, the slightest positive changes in temperature, especially in early summer, lead to the acceleration in the tree growth. Within certain limits, there is an almost direct relationship between summer temperature and growth. However, at temperatures above a certain threshold, a tree enters the comfort zone ", - explained to RIA Novosti one of the authors of the article, Doctor of Biological Sciences, leading researcher at the V.N.Sukachev Institute of Forest SB RAS and Siberian Federal University, Alexander Kirdyanov.

To solve the large-scale task, researchers collected data on the northern and upper forest boundaries for thirteen main forest regions of the world, including ecosystems in the tropical, temperate continental, Mediterranean, boreal and subarctic zones. The main information was obtained from wood cores, cylindrical wood samples with a diameter of about five millimeters which were used to estimate the age of trees and radial growth rate. To estimate climatic changes and compare them with the tree growth, scientists used statistical data on the average monthly temperature and amount of precipitation in 1901-2004.

Based on the data obtained, the scientists tested how the relationship between temperature and tree growth will have changed under moderate (0.9–2.6 ° C) and intense (1.4–4.8 ° C) warming scenarios by the end of the 21st century. Simulation of the radial growth of trees predicted a decrease in the temperature dependence of the growth rate of trees. As soon as the summer temperatures become comfortable for trees, a further increase in temperature ceases to affect the formation of tree rings to the same extent. This means that the ability of trees to absorb CO2 from the atmosphere will not grow in proportion to the increase in temperature.

“Having analyzed the data for half a century (from 1950 to 2004), we showed that the trees in the northern and upper treeline on four continents had increased their growth rate. A particular acceleration has been noted since the 1980s. These changes coincide with the start of rapid warming. However, the forecast for the same areas at the end of the 21st century showed a weakening of the relationship between changes in climatic parameters and tree growth. We interpreted this process as a loss of sensitivity to changes in summer temperature, which should be especially noticeable in boreal forests. As the relationship between temperature and growth disappears, local conditions or competition between plants will become decisive factors. How trees will adapt to further warming is of great importance for the fate of forests in northern ecosystems, ” said Alexander Kirdyanov.

Scientists believe that the lack of temperature dependence of tree growth can affect ecosystem processes in mountainous and polar regions. In particular, this will lead to the expansion of the distribution areas of trees and their colonization of  the mountain areas and tundra. These changes will also affect carbon stocks, nutrient and water cycling in the mountains and in the north.