Rising atmospheric carbon dioxide (CO2) is the most certain driver of global change. We put out more and more CO2 into the atmosphere in the year, yet our terrestrial biosphere seems to be doing a pretty good job sequestering part of that carbon, according to some studies. More efficient plant carbon uptake via photosynthesis, increased water use efficiency and improved plant nutrient access provided by an efficient symbiotic fungal network seem to underlie this apparent boost in the strength of the terrestrial carbon sink. Most of these findings emerge from research studies on relatively young forests and grasslands in temperate regions and mostly limited by nitrogen availability. Old forests have much slower turnover and growth rates and patterns observed in young forests and plantations are not necessary mimicked by their older relatives. Besides, beyond the temperate regions, nitrogen limitation to growth is overridden by scarce available phosphorus, particularly in geologically old continents and tropical regions.
In our paper published in Nature Climate Change, we show that, contrary to the predictions of most dynamic vegetation models, above ground productivity in an old-growth native eucalyptus forest does not increase in response to rising atmospheric CO2. We demonstrate that it is not that old trees are not capable of growing more, but instead, it is phosphorous availability in the soil that constraints biomass production and accumulation under elevated CO2. These results are surprising, given that we found that elevated CO2 had a consistent stimulation effect on photosynthesis at the leaf level, which was not translated into greater wood or leaf production.
Eucalypt forests are the dominant vegetation type in Australia, but they are also planted all around the world. Besides, phosphorous limitations are not endemic to this island-continent, but are wide-spread in tropical and subtropical regions. Therefore, our results are not only relevant for this specific ecosystem type but have important implications for all phosphorous limited forests (for example in tropical and sub-tropical regions), eucalyptus plantation around the world and old-growth forests.