By Matt Walker
Editor, Earth News
Fresh but fragrant air
Climate change will make the world more fragrant.
As CO2 levels increase and the world warms, land use, precipitation and the availability of water will also change.
In response to all these disruptions, plants will emit greater levels of fragrant chemicals called biogenic volatile organic compounds.
That will then alter how plants interact with one another and defend themselves against pests, according to a major scientific review.
According to the scientists leading the review, the world may already be becoming more fragrant, as plants have already begun emitting more smelly chemicals.
"The increase is exponential," says Professor Josep Penuelas, of the Global Ecology Unit at the Autonomous University of Barcelona, Spain.
"It may have increased already by 10% in the past 30 years and may increase 30 to 40% with the two to three degrees (Celsius) warming projected for the next decades."
Biogenic volatile organic compounds (BVOCs) are routinely emitted by plants into the atmosphere.
Such chemicals differ in size, properties and origin, and can range from isoprene, monoterpenes, sesquiterpenes, and so-called green leaf and herbivore-induced volatiles to oxygenated volatile organic compounds such as carbonyls, acids and alcohols.
All play vital roles in helping plants grow and metabolise, communicate with one another and reproduce, and protect or defend themselves from herbivores such as browsing mammals or insect pests.
But plants emit different levels of such compounds depending on environmental conditions.
While significant research has been done to assess the impact of global warming on further CO2 exchange in the atmosphere, little focus has been given to how changing temperatures will alter emissions of important compounds such as BVOCs.
So Prof Penuelas and Dr Michael Staudt of the Centre for Functional Ecology and Evolution in Montpellier, France conducted a major review of how climate change will alter the expression of these compounds.
"Based upon the work reviewed, we can be reasonably sure that climate and global change in general will have an impact on BVOC emissions," they write in the journal Trends in Plant Sciences.
"The most likely overall impact is an increase in BVOC emissions mostly driven by current warming, and that the altered emissions will affect their physiological and ecological functions and their environmental role."
In particular, they say higher temperatures will cause plants to produce more BVOCs, and also lengthen the growing season of many species, further adding to the BVOCs produced.
By enhancing the activity of BVOC synthesising enzymes, and making it easier for such compounds to diffuse into the air, rising temperatures will cause a sharp, exponential increase in BVOCs.
Global emissions may already have increased by 10% in the past 30 years and could increase by an additional 30 to 45%, they say.
For example, studies have shown that artificially increasing the air temperature by three to four degrees Celsius makes heath growing on a sub-Arctic island emit between 56 and 83% more isoprene.
On high alert?
Higher temperatures will also enable more high-emitting plants to colonise higher latitudes.
Temperature will not be the only factor making plants emit more fragrant chemicals.
Changes in land use could mean that rainforests are being replaced by plantation trees, such as palms and rubber, that emit many more volatile organic compounds.
The scientists also suspect that higher concentrations of CO2 in the atmosphere, greater levels of UV radiation reaching the poles, and increased amounts of ozone pollution, will all affect how plants produce BVOCs, though it is much less clear how.
Overall though, they feel the impact on plants around the world could be significant and underappreciated.
Communication between plants could be affected.
For example, some BVOCs such as terpenes, methyl jasmonate or methyl salicylate act as airborne signals between plants, warning them of an attack by herbivores.
Plants forced to produce more of such chemicals could therefore be in a constant state of high alert.
Or it could be that a more fragrant atmosphere confuses pollinators such as bees, altering plant reproduction, or insect pests.
"Temperature is a very powerful driver of emissions," says Prof Penuelas.
"The increased emissions will likely affect physiology and ecology, ie the functioning of life."
He and Dr Staudt say a number of more detailed long term studies need to be performed to better understand the impact of elevated BVOC emissions in different habitats.
For while they can be reasonably sure that BVOC levels will increase, and the world will become more fragrant as a result, the problem is too complex to yet gauge many of the consequences.