As humanity continues to burn fossil fuels, the delicate balance of life on Earth is changing. This is true of trees, many of which are growing faster as a result of increasing concentrations of carbon dioxide (CO2) in our atmosphere.
But not all trees are reacting this way. In particular, eucalypts – Australia’s iconic forest trees – have not benefited from the increase in CO₂ as expected.
Why not? Our new research, published today in Nature, shows that it’s an underground battle for phosphorus, a mineral nutrient in the soil that’s essential for tree growth. The results suggest that in some parts of the world, rising CO₂ means that tiny insects in the soil “hold onto” their phosphorus, making less available to trees.
This is alarming news, because according to current projections, global forest growth is intended to limit the damage from global warming.
What our study includes
Our study used data from a Western Sydney University experiment known as Airless CO₂ Enrichment with Eucalyptus, or EucFACE. The experiment is located in a century-old Cumberland forest in the Hawkesbury district of Sydney.
CO₂ is released into the forest through a computer-controlled system. Scientists then monitor the effects on trees, soils and the wider ecosystem. Over six years, CO₂ rose to levels expected around 2050 (under the current trajectory of business-as-usual emissions).
Our previous studies found that forest trees did not show any additional growth at high CO₂ levels. We suspected the cause was low soil phosphorus availability and set out to test this.
Sally Tsoutas
Phosphorus is essential for the photosynthesis process that makes trees grow. Phosphorus in the soil is provided by insects known as microbes. These microorganisms break down dead and decaying matter, and in the process change the phosphorus into a form that plants can take up with their roots.
Most Australian soils are naturally low in phosphorus because they are derived from ancient, nutrient-depleted rocks. The same is true for most soils in tropical and subtropical regions. This makes the phosphorus service provided by microbes even more important.
We took phosphorus into all parts of the ecosystem, tracing its journey from the soil to the trees. We found that under high CO₂ conditions microbes retain more of the phosphorus they produce to aid their metabolism. This left less available for trees to capture.
This happened despite the trees trying to “bargain” for phosphorus by releasing extra carbon into the soil to feed the microbes.
In addition, trees are great phosphorus “recyclers” – they remove half of the phosphorus from each leaf before it falls. But this was still not enough to support additional tree growth.
Why this matters
Our study is the first to show how the phosphorus cycle is affected by high CO₂ – and in particular, the role of soil microbes.
The results are important for predicting soil phosphorus availability and plant productivity in forests and woodlands as CO2 levels increase in the atmosphere.
Current climate projections assume that increased CO₂ will lead to more forest growth globally. Forests are a vital carbon “sink” – that is, they draw carbon from the atmosphere. Thus, increased forest growth was predicted to go some way towards limiting the effects of climate change.
If our results are taken into account, future warming would be higher than current predictions. However, it is important to verify our results in other locations, with other tree species. New experiments are being set up by foreign teams, including in the Amazon rainforest, to test the findings.
Importantly, our results do not mean that forests are not a crucial carbon sink. Forests hold a large amount of carbon. Avoiding deforestation and planting new forests are valuable tools for preserving and increasing carbon stocks.
Our research demonstrates the importance of considering soil when growing trees. We also hope that our research will stimulate further efforts to find phosphorus in ecosystems, especially in tropical rainforests where phosphorus is often very limited.