For many years, scientists have observed a curious phenomenon among plants – most species thicken their leaves when levels of carbon dioxide in the atmosphere rise. This leafy weight-gain, which can see leaves fatten-up by as much as a third, has been observed in many species of plants, from woody trees to crops such as wheat and rice.
Two scientists from the University of Washington assessed the impact of this response on the climate. They took existing climate change models that use the high atmospheric carbon dioxide levels expected later this century, and found that when thicker leaves were added to the equation the amount of carbon in the atmosphere increased further. Their new model predicted an extra 5.8 petagrams (or 6.39 billion tons) of carbon in the atmosphere per year when leaf thickness was taken into account, levels not far off the amount of carbon released due to human-generated fossil fuel emissions (8 petagrams, or 8.8 billion tons).
Abigail Swann, assistant professor of atmospheric sciences and biology, who conducted the research with doctoral student Marlies Kovenock, explains that the results are due to the fact that plants with thicker leaves photosynthesise less and are therefore less effective as a carbon sink. This isn’t due to the individual leaves acting differently but rather a cumulative effect. ‘What we found is that if plants make their leaves thicker, those leaves cost more to the plant to build and so we found that plants built less leaf area in total. That meant that the photosynthesis rate at a global rate was lower.’ In short, fewer leaves = less photosynthesis = less carbon dioxide converted into oxygen = more carbon in the atmosphere.
Swann explains that they also looked at the temperature effect of these leaf changes. Their simulations indicated that global temperatures could rise an extra 0.3 to 1.4ºC beyond what has already been projected to occur by scientists studying climate change. ‘About half of that temperature effect came from the physical effects of the fact that there’s less leaf area,’ says Swann. ‘In places such as the Amazon where it’s quite hot, a lot of cooling of the land surface occurs due to evaporation of water through leaves (transpiration), and so with less leaf area there’s less cooling of the surface through that process.’
Swann wants to understand this process further. Less total leaf area doesn’t seem like a good competitive strategy for plants that have to compete for light, and so it’s not yet clear why they do it. Though there are some theories as to why plants thicken their leaves, there’s no decisive answer to the question and unlocking the mystery could reveal more about the global effect of the phenomenon.
For Swann the key thing to note is that this new model illustrates how many factors are at play in our climate system. She wants to see physiological changes to plants investigated further and plant traits, such as changing leaf mass, considered in climate projections. ‘Biology does have a pretty big impact on the climate we experience in the future and it’s one of the things we don’t understand very well,’ she says. ‘We need to understand the responses that plants have, and we need to know more about how plants work because there are global-scale implications.’
For now it seems likely that plants will continue to thicken-up given the continual rise in carbon dioxide levels, mostly caused by human industrial activity. In 2013, carbon dioxide levels surpassed 400 parts per million (ppm) for the first time in recorded history and the concentration of carbon dioxide in the atmosphere today hovers around 410 parts per million. Scientists predict that within a century, it may rise as high as 900ppm. When it comes to the implications of this on plants we’re only just starting to pay proper attention.
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