The shelf is a fruiting body — the reproductive structure. The organism has been living inside the wood digesting it long before anything appeared on the outside. So the shelf is not the problem, it is the announcement. The practical translation is that there is a volume of decayed wood inside the trunk or the root collar, and the real job is establishing how much sound wood remains around it and whether that remaining shell is enough to hold the tree up. In Danville — Danville Main Street Historic District and Danville Courthouse Square Historic District included, that's normally a hazardous tree removal job at $800–$5,000. Within weeks.
Why this comes up in Danville
Duke Energy Indiana serves the town and the area south and east, per the IURC's service-territory GIS; just north of Danville the territory becomes Hendricks Power Cooperative, an REMC. Danville runs municipal water, wastewater and stormwater — roughly 3,500 water customers in Center Township — but not electric. Worth knowing: 170 IAC 4-9's two-week-notice and 25-percent-canopy-consent rules are written for investor-owned utilities and name Duke, AES/IPL, I&M, NIPSCO and CenterPoint — not the REMCs.
Danville's canopy
The Main Street Historic District developed circa 1844–1920 across just 9 acres and 37 contributing buildings, and the Courthouse Square district circa 1865–1960 over 70 acres — those blocks hold the town's oldest and largest trees on narrow lots with sidewalks on both sides. Danville does actively manage its right-of-way canopy through a standing Tree Advisory Committee, though the specific inventory and replanting-ratio requirements attributed to Chapter 97 could not be verified.
Hendricks County (county seat) County rules
No permit is required to remove a tree on private property in Danville. Beyond that, the town's Chapter 97 tree code could not be verified from a retrievable source: what danvillein.gov does confirm is that a three-member Tree Advisory Committee exists to encourage the planting, care and preservation of trees in the public right-of-way. Chapter 97 is reported to require written permission from that committee and the Staff Project Coordinator before removing, or trimming more than one-fifth of, any right-of-way tree, on a free 30-day permit — confirm that requirement with Town Hall before any work in the tree lawn.
The tree lawn in Danville
Chapter 97 is reported to define TREE LAWN as public property lying between the edge of pavement or curb and the property line, excluding any sidewalk, while § 97.22 puts the burden on the resident: the adjoining owner is responsible for planting, removing and maintaining that vegetation so it does not become a hazard or obstruct passage. In Danville the tree lawn would then be town land, your responsibility, with the town's free permit required first — but neither provision could be verified against a retrievable copy of the code, so confirm with Town Hall.