Tree rings tell of water and climate change

By unlocking the secrets of tree rings, scientists have created a global database of trees’ water-use efficiency, which they hope will help improve rainfall predictions and management of the world’s water resources.

The study, published in the journal Nature Climate Change, reveals that trees are not acclimatising well to increasing carbon dioxide concentrations – a discovery that lead researcher Mark Adams, from the Swinburne University of Technology, Australia, says is “truly surprising”.

Numerous high-profile papers have reported that water-use efficiency increases with rising carbon dioxide levels, and this had become widely accepted as fact.

But the new study, the first to collate global data on the stable carbon isotope composition of tree rings, turns that notion on its head.

Tree-ring carbon isotopes are directly related to trees’ water-use efficiency and are therefore vital for understanding past climates and predicting future climates across the globe, Adams explains.

Tropical latitudes, which span sections of North and South America, Africa, Asia and Australia, are important regulators of climate, especially rainfall, from evaporation or transpiration of plants.

Plants’ ability to control water loss to the atmosphere also impacts how much remains in the soil and is available for run-off – although their water-use efficiency varies. 

In a previous study on leaf respiration, Adams’ team discovered that leguminous trees such as acacias (Acacia spp.), which produce seeds in pods, use water more efficiently than non-legumes. 

In subsequent research with tree-rings, they found that rainfall played a major role in driving water use efficiency in tropical forests – drier forests were acclimatising to climbing carbon dioxide concentrations much faster than wetter forests, with important implications for climate modelling. 

“This striking result really required us to look further as the modellers who predict climate and weather, and those who predict run-off and streamflow, now routinely include water-use efficiency as part of their models,” Adams says.

“In fact, their models are much less accurate without knowledge of plant-controlled transpiration flux of water to the atmosphere.”

By analysing 422 records of tree-ring isotopes stretching back more than 150 years, his team has now found that water-use efficiency increased most in the years following the Great Depression and World War II – periods with much slower increases in carbon dioxide levels. 

This has dropped considerably since then, Adams says.

“Since WWII, the rate of increase in water-use efficiency has been slowing, virtually without pause, so that by the first decade of the present century, [it] was hardly increasing at all.”

They also found that trees respond differently to increased carbon dioxide concentrations: flowering trees (angiosperms) have slowed their response much more than non-flowering trees (gymnosperms), coming to a virtual halt.

This might change with the current coronavirus pandemic.

“If there is a pause in carbon dioxide emissions as a result of reduced economic activity generally,” says Adams, “then we may see a rebound in the rate of change in water-use efficiency.”