Better Measurement of Forests’ Contribution to Soil Carbon Sequestration - a Q&A with YCNCC Researchers Mark Bradford and Alex Polussa

In late April, Yale School of the Environment (YSE) Professor Mark Bradford and Yale Applied Science Synthesis Program (YASSP) Research Project Manager Alex Polussa, co-authored a peer-reviewed article in Global Change Biology on “Designing National Forest Inventories for Accurate Estimation of Soil Carbon Change.”

YCNCC News recently sat down with Bradford and Polussa to discuss the research and its implications for national greenhouse gas inventories, as well as potentially carbon markets.

YCNCC: YASSP is a major YCNCC research and impact initiative. For readers who are unfamiliar with the program, can you provide a quick overview?

MB:  Sure! YASSP was founded in 2022 as a joint initiative between The Forest School (at YSE) and the YCNCC, with a mission to generate science that supports decision making for stewardship of lands for biodiversity, climate and co-benefits. We’re a space for open collaboration among practitioners, academics and policymakers to develop applied science that generates robust evidence about the efficacy of management decisions. In particular, we’re developing the necessary science for accurate and robust quantification of outcomes, such as carbon storage,  allowing users to determine best management choices. I like to say that we’re doing decision science to de-risk and optimize land management.

YCNCC: What was the specific motivation of this research?

AP:  National monitoring networks are really valuable for understanding how ecosystems are changing over time because they help answer questions like: are forests acting as a carbon sink or source? These measurements are telling us not just how forests are changing, but also estimating things like national greenhouse gas emissions which can shape policy around land management. Rob Buchkowski (YSE PhD 2019), the lead author on this paper, brought the Canadian National Forest Inventory (NFI) network to our attention because, after its establishment in the early 2000’s, it is now reaching its second and third round of remeasurements. No one has really had the opportunity to look at these large scale networks with more than a single measurement. So, the empirical data from the NFI provided a real opportunity and motivated us to ask questions about how reliably we can estimate changes in soils and where are the areas we can improve.

YCNCC: At YCNCC, we’ve been thinking a lot about “co-drivers” for the implementation and scaling of natural climate mitigation solutions. Can we consider increased soil carbon sequestration a “co-driver” for forest management projects?

MB:  I’d suggest that we have to think about soil carbon both for the quantification of climate mitigation and as a co-driver. For example, in the mature forests that are immediately in our “backyard” here at Yale (i.e. eastern U.S. temperate forests), there is as much carbon stored in the soil as there is in the wood of the trees. Together they account for more than 80% of the carbon in those ecosystems, so if we’re doing reforestation or other forest management we have to pay attention to the carbon beneath our feet along with the carbon we can see, to ensure we’re accounting accurately. From a co-driver perspective, we need to view that carbon in the soil – and the need to protect it and/or restore it – as a way to “climate-proof” our forests. The history of land conversion has depleted the soil carbon in what are now re-establishing secondary forests, and that carbon is a critical determinant of soil fertility. Higher carbon amounts in surface soils allow for more efficient drainage and better retention of plant-available water. In a world that is increasingly experiencing extreme climate events, such as flooding and droughts, maintenance of forest biodiversity, tree growth and non-timber forest products (e.g. water supply and purification, maple syrup) relies on having healthy, fertile soils to support the trees.

YCNCC: Does this research have potential implications for carbon markets – including current voluntary, Paris Agreement, and future compliance markets?

MB: There are major implications but they are often “out of sight”. Let me explain: Monitoring data from national infrastructure, such as the Canadian NFI, generate data that inform country-level greenhouse gas budgets. These budgets are then used to assess whether countries are on track to meet their Nationally Determined Contributions (NDCs), which are the self-defined, climate action plans that are the cornerstone of the 2015 Paris Agreement. With Articles 6.2. And 6.4 of the Paris Agreement, countries can trade carbon credits to help them achieve their NDCs but naturally accumulating carbon – such as most carbon in the soils of Canada’s managed forests – cannot be traded. However, you need well-designed monitoring networks to ensure that you accurately account for naturally accumulating carbon. If you incorrectly claim more carbon is accumulating than it is, then a country has to buy fewer credits to meet its NDCs, reducing demand on carbon markets and hence investment in stewarding forests. Alternatively, if a country overestimates its naturally accumulating carbon and has met its NDC, it risks claiming there is a surplus of carbon and can sell credits from other activities that qualify as additional, leading to trading of “phantom” credits, which can undermine confidence in markets.

YCNCC: What does this work tell us about designing monitoring networks?

AP: Well for one, these long-term monitoring networks are incredibly valuable, but there have historically been limited data to show what they can realistically estimate. Soil carbon is highly variable across space, so people believe that detecting relatively small changes over time is not possible. And one thing we found was that variability within sites was often as large as variability between sites. We also saw that changes can emerge simply from background variability and sampling effects. However, looking at multiple remeasurements allowed us to separate real trends from sampling noise. Also, increasing the number of plots improved our ability to detect change more than increasing the number of microplots within a site. I think it is encouraging that as these networks continue to sample over time, we can continue to refine and expand these networks depending on what kinds of change we want to detect, whether that is broad regional trends, or more specific responses to land management practices. 

YCNCC: What next steps do you have planned to advance this work?

AP: With this work, we are showing how we can glean insights from long-term remeasurement datasets that can improve current designs. These datasets don’t just strengthen confidence for an individual project, but provide insights to how we can more broadly estimate changes in ecosystem properties. There are a lot of ongoing projects with data that are capable of improving empirical approaches, but they’re often difficult to obtain or are fragmented. So one of the things our team has been working on is to expand access to more long-term datasets and encourage organizations to make these kinds of data available. Identifying design levers for improving change detection is valuable for science in general, but also for evaluating the efficacy of nature based solutions and carbon accounting. By bringing these datasets together, we can innovate and better understand empirical uncertainty and measurement. 

YCNCC: Any last words?

MB: I’d be remiss not to highlight Rob Buchkowski, a YSE alum (masters and PhD) and assistant professor at Western University in Canada, who brought us in as collaborators, and the Canadian National Forest Inventory for making such an incredible, long-term dataset available for evaluation to not only further improve their own monitoring network but to serve as the illustrative example for other countries. The design of such networks is taking on even greater importance given how interconnected their data are to NDCs and the trading of carbon credits to help us, as a public good for the planet and all peoples, limit climate change. It’s for this reason that in YASSP we’re working with about 50 project partners to advance project designs to produce accurate accounting, looking to fill this “yawning” data gap for natural climate solutions. Such work is only possible because we’re a central YCNCC research and impact initiative.