From tree +‎ cookie.

tree cookie (plural tree cookies)

1. (botany, forestry) A preserved horizontal cross-section of a tree trunk or limb, often demonstrating the physical structure and growth rings of the source plant.
   • 2019, “Activity: Tree Cookies”, in Project Learning Tree‎^[1], Sustainable Forestry Initiative, archived from the original on 26 June 2024:

     Obtain tree cross-sections or “tree cookies” from a local tree-trimming service or other source, or make your own by sawing the trunk or limb of a fallen tree into sections 1–2 inch (4–5 cm) thick. To prevent them from splitting, dry the tree cookies either on a sunny driveway for five days or in a kitchen oven set to “warm” for five hours. Turn the cookies over periodically to dry both sides evenly.

   • 2020-06-16, Science World‎^[2], ASTC Science World Society, archived from the original on 22 April 2024:

     When a cross section of a tree is taken it is often referred to as a "tree cookie" due to its shape. The first year of growth is at the center of the tree cookie, while the last year’s growth is where the wood meets the bark.

   • 2021 May 17, “Tree Cookies”, in Idahoforests.org‎^[3], Idaho Forestry Products Commission, archived from the original on 29 July 2024:

     Tree cookies are cross sections of tree trunks that foresters and teachers use to illustrate how trees grow. Tree cookies reveal the many different layers that make up a tree.

2. A cookie shaped to resemble a tree.

• tree disc
• tree disk
• wood cookie