A Quick Introduction to Forests and Carbon
You've probably never thought about them this way, but trees are, essentially, not much more than long tubes of carbon stacked on top of each other. A large percentage of woody biomass is just carbon (about 47% on average). This is because trees store carbon as a result of their natural physiological processes, when they pull in CO2 from the air which reacts with water in the chloroplasts to create glucose and oxygen. The glucose is used by the tree, and the oxygen is released back to the atmosphere. This process (photosynthesis) is key to why corporations and governments have been turning to forests as "nature-based" solutions to mitigating global climate change.
On the surface it makes a lot of sense. If trees store carbon naturally as they grow, then planting more trees should remove more CO2 from the atmosphere. That's technically true, but like all good science it's more complicated than that, for several reasons: 1) not all trees and forests store carbon equally; 2) trees and forests don't grow naturally everywhere; 2) forests are sources of wood which is a product that society needs; 3) there are economic incentives for harvesting wood and/or clearing forest land for other uses; and 4) forests face a growing list of threats.
Breaking down some of these reasons more specifically helps reveal why these are issues with both ecological and economic implications. Each of the reasons on their own are more complex than can be covered in a blog post, but it's worth spending some time with the broader themes, so I've split this discussion into two parts. Part One (this post) takes a deeper look into what we do and don't know about the science of managing forests for storing carbon, and Part Two will cover carbon offset programs in terms of how they operate, what they aim to do, and the economics of why they fail.
What We Know
As I mentioned above, absorbing CO2 from the atmosphere via photosynthesis is one of the primary services that forest ecosystems provide to society. Globally, only oceans cycle more carbon from the atmosphere than forest environments do. Forest ecosystems account for more than 80% of aboveground carbon in terrestrial systems, and estimates put the total absorption of CO2 by the world's forest at around 4.4 trillion pounds per year, which equates to roughly 30% of all human-caused CO2 emissions.12
Again, this is all very important for regulating atmospheric CO2 and O2, but one important thing forest ecologists have learned is that the rate at which forests do this is not distributed equally around the planet. Different forests cycle and store carbon differently depending on numerous factors, and those differences have important implications for the economics, ecology, and geography of forest conservation.
The age of the forest is one important characteristic that influences carbon uptake and storage. Younger trees tend to have higher uptake rates due their overall faster growth rate, but trees don't stop growing or storing carbon as they age, even if they slow down. Older forests also hold more total carbon in both their trees and soil than do younger forests. This has lead to the development of "ecological forestry", of which one idea is to selectively harvest at low intensities to open up canopy gaps that allow younger, faster-growing trees to take advantage and store more carbon in the process, while also preserving the overall aging forest structure so that stored carbon isn't released. This is something our lab is currently working on at Penn State.
Another key factor is not just the age of the forest, but how that age is distributed across space. Forests that are uniform in age are often referred to as "even-aged", while forests that have a mix of younger and older trees dispersed throughout are "uneven-aged". Even-aged forests can, technically, sequester more carbon as tree biomass. But when looking at total ecosystem storage, uneven-aged forests have higher amounts due to greater storage in the soil. This tips even more toward uneven-aged forests when combined with selective harvesting which can increase the total carbon stored in the system.3
The diversity of the tree species in a forest also plays a role in how much carbon is stored. In general, forests that are more diverse are also more productive. A number of studies have found that higher levels of tree species diversity promotes greater ecosystem productivity and carbon storage.4567 This holds true across many regions and biomes, but does vary some depending on things like forest age, tree density, and local site conditions.
Lastly, managing forests for carbon storage depends is the extent to which we can increase the amount of land covered by forests globally. There's two ways this can be accomplished: reforestation, and afforestation. Reforestation refers to planting trees in areas that have historically been forested, but forests have been reduced in size or lost. Afforestation, on the other hand, is when trees are planted in areas not previously forests (creating new forests). Globally there has been an increase of both in recent decades, leading to a 65% increase in planted forest area since 1990.8 This seems like a positive development, but one issue is that plantings often have low species diversity and the species that are chosen are fast-growing but vulnerable to pest outbreaks. There's also some uncertainty about the effect of planting trees in areas that were not historically forested, with some studies actually showing that this can decrease soil carbon levels. Most research suggests that reforestation, combined with the conservation of old forests, is the best approach.
What We Don't Know (yet)
We know a lot about managing forests and how to prioritize carbon storage, but we don't know everything. Most of the current gaps in the research require a "zooming in" to fill, or in other words, we need to narrow our research in order to address more specific kinds of questions about forest ecology at finer scales. Paradoxically, though, this creates the need for broader research that synthesizes the more specific findings. So, in short, we need more research.
One important area that needs more attention is the evaluation of the models used to predict tree and forest growth. Many of the equations that use tree diameter to estimate biomass (and carbon) are based on broad taxonomic groups that don't reflect the relationships between tree growth and local site conditions. This means that estimates of carbon storage that are based on these models may be under- or over-estimating the actual amount.
Another area that relates to this is the fact that the relationships between tree growth and local site conditions are only now being examined thoroughly. Much more information about how the soil, topography, aspect, and climatology of different regions influence tree growth is needed. We also need a better understanding of how different forest types respond to climatic variation.
If these issues weren't complex enough already, the added complication of ongoing climate changes requires more knowledge of how shifting temperature and precipitation patterns are influencing tree and forest growth. We need to be able to predict what forests will do in the future in order to manage for carbon storage long term. This requires all the previously mentioned types of information.
So, for as much as we do know, there's also that we don't. Which means there's a lot of work for scientists to do.
A quick preview of Part Two
Despite the uncertainties, we already know enough to start managing forests in ways that prioritize carbon storage. In fact, this is already underway and as I mentioned above, many governments, corporations, and even individuals can already participate in forest carbon offset programs by buying carbon credits to offset their emissions from travel, manufacturing, development, computing, and other activities. There is one glaring issue though: these offset programs don't work. Study after study is finding that carbon credit offset programs don't sequester nearly as much carbon as they are selling credits for, and this includes forest carbon credits.9 Yes, forests store carbon, but they only offset emissions if more carbon is being stored that what was emitted. As simple as that sounds, it's a complex issue, which is why I'm splitting it from this post. So, in Growing Carbon Part 2, I'll take a dive into the economics of forest carbon credits and why these programs fail to mitigate global climate change.
To cite this article, use the following:
Lockwood, B. (2024). Growing Carbon Part One: The science of managing forests to store carbon. Brief Ecology Newsletter, 5. Retrieved from: https://benlockwood.substack.com/p/growing-carbon-part-one
Köhl M, Lasco R, Cifuentes M, et al. Changes in forest production, biomass and carbon: results from the 2015 UN FAO Global Forest Resource Assessment. For Ecol Manag. 2015;352:21–34. https://doi.org/10.1016/j.foreco.2015.05.036.
Ameray, A., Bergeron, Y., Valeria, O., Montoro Girona, M., & Cavard, X. (2021). Forest carbon management: A review of silvicultural practices and management strategies across boreal, temperate and tropical forests. _Current Forestry Reports_, 1-22.
Taylor AR, Wang JR, Kurz WA. Effects of harvesting intensity on carbon stocks in eastern Canadian red spruce (Picea rubens) forests: an exploratory analysis using the CBM-CFS3 simulation model. For Ecol Manag. 2008;2
Pretzsch H, del Río M, Ammer C, et al. Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient through Europe. Eur J For Res. 2015;134(5):927–47. https://doi.org/10.1007/s10342-015-0900-4
Naeem S, Thompson LJ, Lawler SP, Lawton JH, Woodfin RM. Declining biodiversity can alter the performance of ecosystems. Nature. 1994;368(6473):734.
Tilman D, Knops J, Wedin D, Reich P, Ritchie M, Siemann E. The influence of functional diversity and composition on ecosystem processes. Science. 1997;277(5330):1300–2.
Liu, Y., Hogan, J. A., Lichstein, J. W., Guralnick, R. P., Soltis, D. E., Soltis, P. S., & Scheiner, S. M. (2024). Biodiversity and productivity in eastern US forests. _Proceedings of the National Academy of Sciences_, _121_(14), e2314231121.
FAO. The state of the world’s forests 2020. FAO and UNEP. 2020. https://www.fao.org/3/ca8642en/ca8642en.pdf.
Haya, B. K., Evans, S., Brown, L., Bukoski, J., Butsic, V., Cabiyo, B., ... & Sanchez, D. L. (2023). Comprehensive review of carbon quantification by improved forest management offset protocols. _Frontiers in Forests and Global Change_, _6_, 958879.
Great article! Looking forward to pt 2