(Tree) Ring in the New Year: Looking back on old forest growth to predict our future

Article: Hararuk, O. , Campbell, E. M., Antos, J. A. and Parish, R. (2018), Tree rings provide no evidence of a CO₂ fertilization effect in old‐growth subalpine forests of western Canada. Glob Change Biol. doi:10.1111/gcb.14561

New climate, old trees

The start of a new year is a great time to reflect on the past and look forward to the future. Scientists also do the same thing as part of their work.  But instead of compiling “Top 10” lists for the year and making a list of resolutions, scientists instead study things that happened in the past as a way to give us a better idea of what might happen in the future. This can be a really useful way to answer questions about things where we can’t set up an experiment in the lab, which is often the case when studying the environment.

In this study, a team of scientists wanted to find out what will happen to forest growth as carbon dioxide levels (CO2) increase in the atmosphere as we continue to burn gasoline, coal, and other fossil fuels. We already know many of the serious impacts of climate change that will come with these rising carbon dioxide levels. One area where there is still some uncertainty, however, is how forests will react. Particularly, there are a lot of questions about what will happen to mature forests, where there are lots of old trees and there has not been any major disturbances (e.g. fires, infestations, or logging) for a long time.

More carbon dioxide? If trees like it they’ll put bigger tree rings on it

Understanding what will happen to these forests is especially important since they also act as carbon sinks, taking up carbon dioxide as they grow. This means that they can work to counteract climate change, but the degree to which they do this depends on how much they are growing. Many have predicted that the increasing atmospheric carbon dioxide could actually promote forest growth, and therefore carbon dioxide uptake. Since carbon dioxide is used in photosynthesis as the plant forms sugars, it is thought that rising carbon dioxide levels could result in faster growth since it would provide trees with more “food” under this hypothesis of CO2 fertilization.

Tree rings from a bald Cyprus that is approximately 700 years old. This tree grew in a nutrient-poor area, so the rings are very thin. Source: James St. John (CC BY 2.0)

In order to determine what might happen in the future, Oleksandra Hararuk and colleagues turned to the past. They collected samples of wood from an old forest in Canada in order to see how the growth of the trees responded to rising carbon dioxide levels in the atmosphere over the course of the 20th century. They assessed the growth by looking at the tree rings in the wood. Trees form a new ring each year, and the thickness of the ring is related to how fast the tree was growing (More on tree ring research!). Because of this, they could collect samples of these old trees and have a record of their growth going back centuries from measuring the tree rings.

To grow or not to grow?

When they analyzed the tree ring records, they found that there was actually a decrease in the growth rate at all their study sites during the 20th century. This was a different result from what was expected under the CO2 fertilization hypothesis and what was predicted by computer models. One possible reason for this is that the trees could already be at their maximum carbon dioxide uptake, and thus further increases in atmospheric carbon dioxide do not have any effect on the growth. It could also be a result of some other limiting factor such as not enough water or nutrients.

A tree ring record being examined underneath a microscope Source: Marc Dominianni (CC BY-NC-ND 2.0)

Regardless of the reason behind the decreasing growth rates, this study shows that even if there are increases in forest growth with increasing carbon dioxide in some places, the increases will not occur everywhere and not all to the same degree. Further, this means that forests might not take up as much carbon dioxide in the future as we thought they would. This will be an important consideration as we work to mitigate climate change. With this information from the past, we can resolve to work towards a better a future.

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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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