Even plants can have too much of a good thing

No doubt this will horrify Pat, but this is one of my favourite YouTube videos. If you’ve ever expressed a doubt about something, only to be told it must be ok because it’s natural, this is the video for you.

There’s a cheerful inanity about the video. Water is natural, a daily need and essential for life, so who wouldn’t object to their home being flooded a create a new reservoir? There is an interesting question behind the video though. Everyone knows that plants breathe carbon dioxide. If that’s the case could climate change be an opportunity? Surely a more lush and verdant planet is good news? Sadly, like a lot of climate change science it’s a bit more complicated than that. A paper by Silva, Anand and Leithead in PLoS One yesterday shows that, despite increasing levels of CO₂, tree growth is slowing.

The paper, Recent Widespread Tree Growth Decline Despite Increasing Atmospheric CO₂ compares what we know from dendrochronology with a study of Water Use Efficiency. What researchers found is the water use efficiency increased over the twentieth century, but growth was more complex. It had a positive correlation with precipitation and/or a negative correlation with warmer climate. Because rainfall and temperature are both connected to the weather in general it’s hard to pull those two apart in the natural world.

As rallying cries go, it’s harder to come up with something worse than “It’s a bit more complicated than that!” but it looks like Silva, Anand and Leithead have made a useful contribution showing this is the case. It’s a similar finding to other researchers.

I’m looking at it from the outside so my difficulty is the simple story, more CO₂ is good for plants, makes sense. It seems to be backed up by the science. There’s a study by Rey and Jarvis from 1997, Growth Response of Young Birch Trees (Betula pendula Roth.) After Four and a Half Years of CO₂ Exposure. They found there’s an increase in biomass with more CO₂. They also found that there were changes below ground with finer roots. That doesn’t sound important because in pop science everyone equates respiration with breathing and everyone knows that plants breathe through their leaves.

Unfortunately this complexity is endemic in the academic papers. I read Plant Respiration and Elevated Atmospheric CO₂ Concentration: Cellular Responses and Global Significance by Gonzalez-Meler, Taneva and Trueman. They discuss the effect of CO₂ on enzymes on respiration and the problems of errors in measuring effects. Grace, Berninger and Nagy throw in some simple complexity, if that’s the right phrase, in their review Impacts of Climate Change on the Tree Line. An increase in saplings above the tree line sounds like it should lead to an increase in trees. Instead it could lead more to an increase in animals that graze on saplings. It’s obvious when you think about it, but from the outside ecology is incredibly complicated.

That’s one of the reasons why I like Silva, Anand and Leithead’s work. To a non-botanist like me it has reasonably simple message. The part that says increased CO₂ increases water use efficiency appeals to me as the part of me that thinks plants breathe in CO₂. The part that then says, but this also shows signs of water stress also appeals to the part of me that gets thirsty on a hot day.

The research shows that plants aren’t going to choke on the increased concentrations of CO₂ that are currently predicted. But the emphasis on water stress in this paper shows that they’ll need a drink. If a drink is out of reach then what better term is there for what is suffered than water stress? I think for people who understand that plants depend on CO₂ it’s a helpful way to show that an increase isn’t automatically a good thing and that it’s a bit more complicated than that.

Image credit: Thirst by Todd Baker at Flickr. (cc)