Tree rings show water loss limits global forest growth

New strategies aimed at conservation and sustainability play a crucial role in stemming the loss of biodiversity recorded around Europe and the rest of the world. Analysing the responses of animals and plants could help improve the management of ecosystem functioning and the provision of its services.

An international team of researchers have shown that the growth of trees is becoming more limited by water rather than temperature particularly in northern climates and at higher altitudes. Partially supported by the EU-funded BACI project the team published its findings in the journal ‘Science Advances’.

“In this study, we aim to provide the missing empirical baseline of how tree growth responds to climate variability across temperate and boreal forest biomes. To reach this unprecedented spatial scale, we leveraged a global network of cross-dated tree-ring width data from 2710 sites,” the researchers said. “At each site, we quantified the response of interannual variability in tree growth to four climate parameters (subsequently called “climate response”),” they added.

The variables were assessed during two 30-year periods, from 1930-1960 and 1960-1990. “These parameters were used as metrics of energy [temperature (T)] and water [precipitation (P)] availability, atmospheric water demand [vapor pressure deficit (VPD)], and drought [standardized precipitation evapotranspiration index (SPEI)].”

The researchers noted that “trees became more limited by atmospheric water demand almost worldwide. These changes occurred under mild warming, and we expect that continued climate change will trigger a major redistribution in growth responses to climate.”

Changes in climate response

As summarised in a news item by the University of Arizona for the two time periods involved the team mapped these parameters “onto a grid covering the world’s temperate and boreal regions. Adding tree-ring data to the map allowed scientists to see whether the changes in climate during the 20th century corresponded to the changes in growth of the world’s trees.” In the same news item co-author David Frank said: “We saw areas where in the earlier part of the 20th century temperature limited growth. But now we are seeing shifts towards moisture-drought limitation.”

When the two 30-year periods were compared, “the average temperature increased 0.9 degrees F (0.5 degrees C) and the land area where tree growth was primarily limited by temperature shrank by 3.3 million square miles (8.7 million square kilometers), an area about the size of Brazil,” the news item noted.

According to the news item, first author Flurin Babst “was surprised that such a small change in temperature would shift such a large area of trees from being temperature-limited to being water-limited.” He said: “That’s much more than I expected.” His colleague Frank added: “Reduced growth is indicative of increased stress on plants–which can be linked to mortality.”

The BACI (Detecting changes in essential ecosystem and biodiversity properties – towards a Biosphere Atmosphere Change Index: BACI) project was launched in 2015 “to tap into the yet-to-be realized potential of existing and scheduled space-borne Earth observations” the project website states. “In conjunction with ground data it will allow us to derive new essential ecosystem variables and to detect changes in ecosystem functioning.”