Our school textbooks teach us that plants make their own food using water from the soil, carbon dioxide from the atmosphere, and energy from sunlight. This process, known as photosynthesis, would thus be limited by the availability of these three resources. But it seems that plant growth is not as simple as this. For trees, growth can also be limited by the rate at which new cells can form in the trunks and branches, a process that is affected by temperature.
Understanding the factors that limit the growth of the world’s forests will become more critical as the effects of global warming take hold. Will increased atmospheric carbon dioxide help trees to grow? Or will extremes in temperature and precipitation restrict this growth? Until now, the question of whether plant growth is limited by the rate of photosynthesis or by the environmental conditions that affect cell growth was not adequately answered, despite its fundamental nature.
A new study led by University of Utah researchers, along with an international team of collaborators, has now attempted to answer this question. They have measured and compared the rates of photosynthesis and woody growth in forest trees at sites in North America, Europe, Japan and Australia. Their results, published today in the journal Science, suggest that we need to rethink the way we forecast forest growth in a changing climate, and that forests in the future may not be able to absorb as much carbon from the atmosphere as we hoped.
“A tree growing is like a horse and cart system moving forward down the road,” said Professor William Anderegg, who is the principal investigator of the study. “But we basically don’t know if photosynthesis is the horse most often, or if it’s cell expansion and division. This has been a longstanding and difficult question in the field. And it matters immensely for understanding how trees will respond to climate change.”
The scientists distinguish between a tree’s carbon source and its carbon sink. Carbon dioxide in the atmosphere is the carbon source – the tree uses this during photosynthesis to make food for its own use. Some of the carbon-based food is then used to grow new wood cells, and to enlarge existing ones. The tree’s wood is thus its carbon sink.
If the growth of trees is source-limited, then the amount of carbon dioxide available in the air is what would determine the rate of photosynthesis, and tree growth would be relatively easy to predict in a mathematical model. In this case, rising carbon dioxide levels in the atmosphere would be a good thing and forests would be expected to expand and proliferate.
However, if tree growth is sink-limited then they can only grow as fast as their cells can divide and mature. Many environmental factors can impact cell growth rate, including temperature, the supply of water and nutrients, and the rate of photosynthesis. In this case, modeling potential tree growth in future would have to take into account the impacts of these different factors.
In order to find out whether tree growth is limited by source or sink factors, the researchers compared the rates of photosynthesis and cell growth at forest sites on different continents. Measuring cell growth (sink) rates was done by sampling the growth rings of the trees.
“Extracting wood cores from tree stems and measuring the width of each ring on these cores essentially lets us reconstruct past tree growth,” said study lead author Antoine Cabon.
Measuring carbon sources was more difficult. Source data was measured with 78 eddy covariance towers, 30 feet tall or more, that record carbon dioxide concentrations and wind speeds in three dimensions at the top of forest canopies. “Based on these measurements and some other calculations, we can estimate the total forest photosynthesis of a forest stand,” explained Cabon.
The researchers expected to find that the processes of photosynthesis and cell growth were coupled – that when photosynthesis increased (or decreased), a corresponding increase (or decrease) would be seen in cell growth. They did not find this. Trees did not necessarily add more growth when the photosynthetic rate increased.
“Strong coupling between photosynthesis and tree growth would be expected in the case where tree growth is source-limited,” said Cabon. “The fact that we mostly observe a decoupling is our principal argument to conclude that tree growth is not source-limited.”
Although the decoupling of these two processes was recorded in environments across the globe, there were instances where it was weaker. The researchers were interested in what conditions led to stronger or weaker decoupling. Fruit-bearing and flowering trees, for example, exhibited different source-sink relationships to those of conifers. Greater diversity in a forest increased coupling, whereas dense leaf canopies decreased it.
Finally, coupling between photosynthesis and growth increased in warm and wet environments, while in cold, dry conditions the processes were decoupled and tree growth was more limited by the rate of cell growth.
According to Cabon, this last finding suggests that the source vs. sink issue depends on the tree’s environment and climate. “This means that climate change may reshape the distribution of source and sink limitations of the world’s forests,” he said.
The key finding from this study is that vegetation models, which use mathematical equations and plant characteristics to estimate future forest growth, may need to be updated. “Virtually all these models assume that tree growth is source-limited,” said Cabon.
For example, he explained, current vegetation models predict that forests will thrive with higher atmospheric carbon dioxide. “The fact that tree growth is often sink-limited means that for many forests this may not actually happen.”
That has additional implications: forests absorb and store about a quarter of our current carbon dioxide emissions. If the growth of forests slows down, their ability to take in carbon and mitigate the effects of climate change may be significantly reduced.