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There’s more to tree growth than hot air

One of the common claims about climate change is that it will be good for plants. Plants eat CO2 so more carbon dioxide must mean more food for plants. There are some problems with this idea. One is that plants also need water to process carbon dioxide. Climate change could mean less rainfall, so plants might not take advantage of the increase food. It turns out even in places where rainfall increases, it still might not be good news for trees.

Location of Flakaliden, Sweden, near Umeå.
Flakaliden, Sweden. Image: Google Maps.

Tree Physiology has a special issue on experiments at Flakaliden. This is a research station in the boreal forest of northern Sweden. There’s now over thirty years of research there into tree growth, ecosystem function and nutrition. It might look out of the way, but it’s important work. The boreal forest is about a third of the world’s forested area. What happens in the far north matters, and a paper with free access by Sigurdsson et al. gives a glimpse into the future. The title Growth of mature boreal Norway spruce was not affected by elevated [CO2] and/or air temperature unless nutrient availability was improved does take some of the suspense out the findings, but it’s still important work.

The reason why the research needs to be done is that results from earlier experiments seem to contradict each other. Some studies have found elevated CO2 increases tree growth. Other have found it doesn’t. Sigurdsson et al. have looked to see if it’s not just CO2 that is needed but something else. But you know that from the title.

The idea that CO2 increases growth is obvious enough, but why temperature? The answer is that boreal forests are highly seasonal. Temperature isn’t simply a linear effect, it’s also a cue for the plant that winter is over and it’s time to start growing. So any good experiment will also look at the effect of temperature because more growing season could also mean there’s more growing.

Spruce under snow
The Spruce looks like an unlikely victim of global warming. Photo: Ole Husby.

Flakaliden is important here because there are so many long-term experiments. The site is well-known, so the papers authors are able to cite earlier research sauing that Nitrogen deposition averages 3kg per hectare per year. The more you know about what is going into a system, the better you know your experiment. The team were able to add more control by growing the trees in chambers. The soil was isolated in one chamber, the above-ground tree was in another. They could then heat and humidify the air under controlled conditions so they knew what was going in to the tree. They kept an eye on the rain and snow outside, and made sure the test trees got the same amount of water with sprinklers.

Once you have your run your experiment for three years, you can harvest the trees and measure them.

The experiments showed as the title says the the growth of mature boreal Norway spruce was not affected by elevated CO2 and/or air temperature unless nutrient availability was improved. This finding has consequences for climate modelling. Currently the more CO2 = more trees model is widely accepted. What Sigurdsson et al. suggest is that this doesn’t take into account the nutrition of the trees.

The present study, together with the results from other manipulation experiments at Flakaliden (fertilization, irrigation, soil warming), convincingly show that the effect of air temperature on tree growth is not through aboveground processes, but rather through an indirect effect on soil temperature that is likely to increase nutrient availability.

If this is the case, they say that this will not have the same effect on the nutrient-ppot Boreal forest that it will have elsewhere on the planet. It’s no wonder the title of the paper is so clear; it’s a very important message.

Image

Spruce. Photo by Ole Husby. This image licensed under a Creative Commons by-sa licence.

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