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“Tree Decay: An Expanded Concept”, Agriculture Information Bulletin Number 419, April 1979, Alex L. Shigo. Illustrations by David M. Carroll https://www.fs.usda.gov/treesearch/pubs/4425
“Compartmentalization of Decay in Trees” (CODIT) Agriculture Information Bulletin Number 405. Shigo, Alex L., Marx, Harold G. Drawings by David M. Carroll. 1977 http://www.treesearch.fs.fed.us/pubs/5292
“Tapholes in sugar maples: What happens in the tree”, General Technical Report NE-47. Walters, Russell S., Shigo Alex L. 1978 http://www.nrs.fs.fed.us/pubs/6205