Calculating forest carbon with tree rings

New measurement tool could help meet carbon sequestration goals

Traditionally, scientists have estimated how much carbon forests and other vegetation remove from the atmosphere with calculations relying on other measurements, such as emissions produced by humans, CO2 concentration in the atmosphere, and carbon stored in the ocean. But these calculations are imprecise. For a better understanding of above-ground carbon in forests, a group of scientists are looking at tree ring cores as a way to collect data directly from forests.

That data would help scientists gain insight into current and future forest dynamics, which would help landowners make effective decisions as to which trees should be harvested based on various important ecological factors, such as climate sensitivity and competition, that can increase the forest’s strength as a carbon sink. They believe being able to quantify and report the amount of carbon being removed from the atmosphere and stored in the forest would also be a big step in building a more robust and verifiable carbon credit system.

Most people see tree rings on a stump after the tree has been chopped down, but crews can use an increment borer to collect the ring data while the trees are still standing. They can drill the borer into a tree and remove a sample of wood that is about 4 millimeters wide (smaller than the width of a pencil) from the tree. The tree grows to seal the hole which prevents diseases, and the core can be taken to a laboratory for study.

The increment borer is a classic instrument and over the past 130 years it has been used to provide insight about everything from pollution to wood density and decay, but according to Dr. Margaret Evans, an assistant professor at the University of Arizona’s Laboratory of Tree-Ring Research, the value of tree ring data in measuring forest carbon has largely gone unrecognized until now.

A core can display a record of the tree’s life from sapling to present-day. Each year a tree adds a new layer of growth to its trunk to create the rings of a tree, with the oldest, smallest rings near the center and the newest rings just beneath the bark. Light-colored wood forms in the spring and early summer while the tree is growing rapidly, and a darker band forms in the late summer and fall when growth is slowing before the tree becomes dormant for the winter. The rings can provide information about a tree’s carbon storage. It records events like destructive floods or insect attacks that can affect carbon uptake.

But taking a core from a single tree is hardly enough to measure the carbon in a whole forest, let alone all the forests in the world. The process would require scale to be meaningful. The group doing the initial work on this hopes the tree-ring data collection method will spread across North America and beyond in forests with temperate trees that record annual rings. The tree-ring method is less likely to be useful in tropical forests, where the warm temperatures allow trees to grow year-round. Trees in these forests go dormant during periods of drought, but the rings that form may not be reliable enough to date the tree.

The easiest and most cost-effective way to ramp up tree-ring data collection in the U.S. is to use national forest inventories, like the Forest Inventory and Analysis (FIA) Program of the U.S. Forest Service. According to Evans, the largest time and financial cost for collecting data is sending a crew out to these sites, but programs like FIA already have return visits included in the budget.

“Many of the field crews are already taking cores from the tree to collect age information of the stand, but they are not necessarily saving it or bringing it back to the lab for someone to measure and look at it under a microscope,” said Dr. Kelly Heilman, Postdoctoral Research Associate at the Laboratory of Tree-Ring Research.

Turning the physical cores into data that can be used to estimate forest carbon is time-intensive and requires some special training, but Evans and Heilman pose this challenge as an opportunity for forestland programs and universities to collaborate. The cores can be collected and sent to university researchers who can analyze them to determine carbon storage and flux (such as how much carbon is being stored in the tree each year).

There is still a great deal of uncertainty when it comes to converting tree measurements to above-ground carbon estimations for a whole forest stand. “Tree ring data are great at telling you how that individual tree is changing over time, but that’s not the forest carbon – that’s just at the tree level, and really just the measurement of the stem of the tree,” explained Evans.

To measure carbon storage, researchers need to know the above-ground biomass of the tree, but they can’t simply weigh it as one might do with smaller plants. Instead, researchers rely on a set of equations based on data previously collected when people chopped down trees and weighed them to determine how a tree’s biomass is related to its diameter. Heilman explains these equations allow scientists to calculate the carbon storage, but there is always inherent uncertainty in this scaling. No two trees grow exactly the same, so even if they have the same diameter, they may have different amounts of biomass.

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