Carbon dioxide levels have been increasing since the industrial revolution and have been increasing really really fast since the 1950s. You’ve all seen the hockey stick graph.

I’ve been looking at pinyon pine tree rings for the past year and think there might be something going on with increasing CO2 and what the rings are doing. So I’ve got a stack of papers to read through and thought I’d share some of them with all of you.

Carbon dioxide is pretty good for plants because plants need carbon for photosynthesis. We’re interested in what higher levels of CO2 will do to plants because if plants are growing more and eating more carbon dioxide it could help slow down global warming. But giving most plants most places extra carbon dioxide doesn’t seem to do much in the long run because plants need lots of other things to grow, like nitrogen and water, and carbon isn’t usually the most limiting. It’s the same for you: it doesn’t matter how many vegetables you eat if there’s no water.

But what about plants where carbon might be a limiting factor for growth? Carbon can be limiting in hot, dry places because to get carbon, plants have to lose water. Plants have little tiny pores in their leaves called stomata. They have to open these to let in CO2, but water escapes whenever they’re open. A lot of plants that live in hot, dry places have evolved a different kind of photosynthesis to deal with this.

Another place carbon might be limiting is at very high elevations. Atmospheric CO2 concentrations decrease with increasing elevation. That means that in the same amount of space there’s less CO2. This happens to all gases. Flagstaff is above 2000 meters. When people move here, they have a hard time breathing for awhile because of the lower oxygen concentration.

Lamarche et al. looked at tree rings in the 1980s of bristlecone pine growing at 3100 meters. They found that the trees had increasing growth since about 1840. Initially they thought that this was due to warmer temperatures, but then when it cooled down in the 60s, the trend kept going and even accelerated. So, the faster growth wasn’t due to hotter temperatures.

But was it caused by higher CO2 levels? Plants can only use so much CO2, no matter how much is available. Like at Thanksgiving dinner, there’s lots of food available, but you can only eat so much. In the 1960s, CO2 was between 223 and 230 ppm at 3500 meters. For spruce, that concentration is well below what it considers CO2 saturation and so is probably well below what bristlecone considers saturation.

While the authors didn’t really have enough data from enough places to say for sure CO2 makes trees growing at high altitudes grow more, what they do have certainly suggests it.

This paper is over 20 years old, so we’ll see what more recent papers have found…

LAMARCHE, V.C., GRAYBILL, D.A., FRITTS, H.C., ROSE, M.R. (1984). Increasing Atmospheric Carbon Dioxide: Tree Ring Evidence for Growth Enhancement in Natural Vegetation. Science, 225(4666), 1019-1021. DOI: 10.1126/science.225.4666.1019