Here are some ideas
Facial Recognition -- probably not a solution for your Project
how about taking a look into the extensions directory?
https://puravidaapps.com/extensions.php
- Computer Vision Extension by Makeblock: for color detection, face detection and feature analysis (using online API).
- Face Detection Extension by Andres Daniel
Measure by Augmented Reality ( AR ) - might work
You might be able to use AR with App Inventor by using the ActivityStarter https://gadgetstouse.com/blog/2020/05/13/measure-anything-with-your-phone-camera-without-touching-it/ Not tested. GoogleMeasure It might work with your device.
Existing non App Inventor AR measuring apps : https://play.google.com/store/apps/details?id=com.grymala.photoruler&hl=en_US&gl=US
or this App Inventor article about using AR http://ai2.appinventor.mit.edu/reference/other/vr.html
Measure using HTML
If all else fails
Use a ruler, measure the width of the tree trunk; write it down. Enter the value in your app database.