Science - June 5, 2020

Tropical forests can’t cope with heat

Roelof Kleis

Do tropical trees grow faster if there is more CO2 in the air? Not all trees, and especially not if its too hot, a WUR study shows.

© Shankar Raman

Plants convert CO2 and water to carbohydrates using sunlight. This process is called photosynthesis and forms the basis for growth. Extra CO2 acts as a fertiliser, promoting growth. This fertilisation is often seen as a bonus effect resulting from climate change. But is it? Research conducted by professor Pieter Zuidema shows this may not be entirely true.

Australian cedar
Zuidema meticulously studied 5318 growth rings on 129 Australian cedars (Toona Ciliata) on four separate locations in Australia and Southeast Asia. These growth rings represent a growth history from 1950-2014 and show an increase in the CO2 levels of more than a quarter (28 per cent) during this period. This increase affects how the trees react to the annual shifts in temperature and precipitation.

The Australian cedar is the tree of choice for researchers studying growth rings
Pieter Zuidema, professor of Forest Ecology and Management

‘The Australian cedar is the tree of choice for researchers studying growth rings’, says Zuidema. ‘The tree is fast-growing, light-loving, deciduous and has clearly visible growth rings.’ Zuidema and his colleagues were able to study the rings without having to travel: the trees had already been sampled for previous research. The effort lay in the statistical analysis of the ring width.

Climate sensitive
The readings clearly showed the trees to be climate-sensitive. The growth varies strongly per year, but all trees show the same pattern: in good years the trees grow fast, in poor years they don’t. Zuidema explains that this variation allows us to statistically analyse the effects of an increasing CO2 level separate from the precipitation and temperature. With surprising conclusions.

Toona ciliata2.jpg

The fertilisation effect of CO2 does exist, but only in cooler regions (with an average temperature of 200C ). Warm years lead to more effective photosynthesis and thus, increased growth. Trees are less sensitive to drought because they make efficient use of the available water. In warmer regions (25 0C on average) however, the CO2 bonus effect is lost. Worse still, in warm, dry years, growth slows down.

Tropical trees can’t handle heat very well
Pieter Zuidema

According to Zuidema, this is because leaves become so hot in warmer years, that photosynthesis is hampered. ‘Tropical trees can’t handle heat very well’, Zuidema states in a surprising conclusion. ‘We observe a change in the climate sensitivity of tropical forests due to increased CO2 levels.’ He quickly adds that this effect has so far been shown only in this species of tree.

‘This study is focused on a single species, that shows strong annual growth fluctuations, and may not represent the majority of tropical tree species’, Zuidema adds. ‘Still, we must adjust our expectations regarding the CO2 bonus. We are currently launching a study to research this effect ion the Himalayas, in areas that vary in height and thus in temperature.’