Nutrient access affects trees' carbon storage abilities

A groundbreaking study led by Western Sydney University has found that common Australian trees are unable to store as much carbon dioxide as previously thought.

Conducted at the university’s EucFACE facility, the world’s only free air CO₂ experiment in native woodland, the study saw scientists expose large tracts of remnant native eucalypt forest to treatments of elevated CO₂ at 550 ppm — the amount estimated to be in air levels in 30–50 years’ time.

Writing in the journal Nature Climate Change, the researchers found that photosynthesis levels increased consistently by 19% under elevated CO₂; however, this did not translate into increases in wood, stems and leaves over the three-year measurement.

“We expected that if they took up more carbon dioxide in a higher CO₂ atmosphere, they would put more of it in their wood,” said lead scientist Professor David Ellsworth. “And not only didn’t they put it in their wood, but we didn’t find it going into the leaves or anything above ground.”

So why haven’t scientists noticed this before? It turns out that previous experiments in the US and Europe saw researchers add extra CO₂ to plots in temperate forests, which saw trees increase their growth by around 23%. The problem, said Professor Ellsworth, is that these forests are located in areas with adequate supplies of nutrients — unlike in Australia, where conditions for plants are not quite so ideal.

“Australia’s soils are very old and weathered by millions of years of sun and rain, meaning soils are very low in nutrients, and most of the available nutrients are tied up inside wood, leaves and roots,” he said.

“It means that our soils simply lack the available nutrients that would let trees take advantage of the extra CO₂ they find in the air,” Professor Ellsworth continued — a theory that was proven when his research team added phosphorus to their tested trees and found a consistent increase in tree growth of 35%.

The study’s results unfortunately do not bode well for climate modelling, with many international climate agencies having taken their data from research in temperate forests. As noted by Professor Ellsworth, “Since many of the world’s subtropical and tropical forested regions exist on low-nutrient soils, our results indicate that global estimates of carbon storage in forests could be too high.”

With up to 30% of the world’s forests potentially affected by these findings, Professor Ellsworth suggests that current climate models should be revisited, taking into account the role of soil nutrients such as nitrogen and phosphorus when estimating future carbon sinks.

Image credit: ©Tommaso Lizzul/Dollar Photo Club