From tree rings to Carbon sequestration
- Quantify standing carbon stock of a tree and its development over time.
- Quantify annual carbon sequestration of a tree and its changes over time.
- Expand estimates of carbon stock and annual sequestration from trees to forest
Trees intercept light, sequester carbon and store that carbon in wood (stem, branches, and roots) and foliage. Approximately 50% of the dry weight of a tree is carbon, and to fix one ton of carbon, a tree needs to fix 3.67 tons of CO2. Since we can’t weight the standing trees while growing in a forest to estimate its carbon content, foresters and scientist have developed relationships between tree size measurements of standing trees, typically diameter at breast height (DBH), and the dry weight of the tree. Those relationships are called allometric relationships and are developed by measuring the DBH of a tree, then cutting it down and weighing its components (stem, branch, foliage, and roots). These allometric equations allow us to use measurements of the diameter of standing trees to estimate their dry weight and carbon content.
In this lab, you will learn how to quantify the standing carbon stock of a tree and a forest, using allometric equations and tree ring measurements at breast height.
Stem analysis is a technique that can be used to quantify the diameter growth of a tree by measuring tree ring widths either in a section of a tree that has been cut, or in increment cores taken from standing trees.
Measurements of the size (radius) of each tree ring in a section of a tree at breast height allow us to calculate diameter growth at breast height. If we know the diameter of the tree at each age, we can then use allometric equations to calculate tree biomass. In addition to diameter growth, the radius of each tree ring can be used to calculate the basal area of a tree, defined as the cross-sectional area at a given height, and calculated with the following formula:
Tree basal area = π * radius 2
In this lab, we will use tree ring data for a loblolly pine tree sampled as part of a Silviculture class at NC State University, with data provided by Dr. H. Lee Allen. To estimate tree biomass, we will use the following allometric equation for loblolly pine, developed by Gonzalez-Beneke, et al. (2014):
Tree biomass = 0.037403 * DBH 2.676835
Where tree biomass is above ground biomass in kg/tree, and DBH is diameter at breast height in cm.
Given that approximately 50% of the dry weight of the tree is carbon, the following equation allow us to calculate carbon content in the above ground biomass of a tree:
Tree carbon = Tree biomass * 0.5
Furthermore, to fix a kg of carbon a tree needs to fix 3.67 kg of CO2, then:
Tree CO2 eq = Tree carbon * 3.67
With this equation we can calculate standing stock of carbon, expressed as CO2 eq, at each age, as well as annual carbon sequestration, estimated by the annual increment in standing stock of carbon.
Those calculations are for individual trees, if we want to get an estimate of the standing stock of carbon and annual carbon sequestration of a forest, we need to multiply the values at the tree level by the number of trees in a forest. A reasonable number for loblolly pine would be 1200 trees per ha.
- Graph the relationships for diameter and basal area growth versus age.
- Graph the relationships for standing stock of carbon and carbon sequestration versus age.
- Calculate annual increments in all variables and graph them versus age.
- At what age did annual diameter and basal area increments peak?
- At what age did annual carbon sequestration peak?
- How many trees or hectares of forest would compensate your annual household emissions?
Gonzalez-Benecke, Carlos A., Gezan, Salvador A., Albaugh, Timothy J., Allen, H. Lee, Burkhart, Harold E., Fox, Thomas R., Jokela, Eric J., Maier, Chris A., Martin, Timothy A., Rubilar, Rafael A., Samuelson,Lisa J. 2014. Local and general above-stump biomass functions for loblolly pine and slash pine trees. Forest Ecology and Management, 334:254-276 23 p