http://www.galk.de/index.php/arbeitskreise/stadtbaeume/theme...
Durch viele solcher Wurzelverwachsungen bilden die Bäume eines solchen Reinbestandes eine Art 'Superorganismus' (GRAHAM & BORMANN 1966, EIS 1972, FINK 1999). Dies erklärt nun auch das beobachtete Phänomen der Stocküberwallung (Abb. 4-6), da nach dem Absägen eines Baumes weiterhin Nährstoffe von den benachbarten, noch lebenden Bäumen über diese Wurzelverwachsungen zugeführt werden können und ein solcher Stock überwallen kann (GÖPPERT 1842, WICHMANN 1925). In gewisser Weise "parasitiert" damit der abgesägte Stock auf dem Wurzelsystem der Nachbarbäume. Andererseits können die Nachbarbäume das Wurzelsystem des gefällten Baumes mitbenutzen.

https://ideastations.org/science-matters/question-your-world...
Trees share vital nutrients, minerals, sugars, and water via underground root exchange. This can happen in two ways, both are equally amazing. One method is by means of fungal hyphae. Long thin strands of this fungus grow around and between tree roots. Some have even been known to stretch for miles! The fungus will drill into the roots of trees and thus become a means by which substances can be exchanged between trees, on a highway of fungal growth. Another way trees can share is by root grafting. This is a process in which roots of different trees grow in the same area, overlap, and physically fuse together. Trees of different species are even capable of fusing their roots together and turning the combined root structures into one massive underground circulatory system.

http://archive.kuow.org/post/scientists-peek-hidden-world-tr...
A Douglas fir stump kept alive by a neighbor

https://www.tandfonline.com/doi/ref/10.1080/0021213X.1987.10...
LIVING STUMPS—CIRCUMSTANTIAL EVIDENCE FOR ROOT GRAFTING IN PINUS HALEPENSIS AND P. BRUTIA PLANTATIONS IN ISRAEL

--------------------------------------------------
Note added at 1 day 17 hrs (2018-09-05 10:20:11 GMT)
--------------------------------------------------

Similar context: http://www.nhm-wien.ac.at/hallstatt/wirtschaftsraum/waldwirt...

Page 362 has an illustration, and the explanation matches the Museum's explanation of Stocküberwallung.
https://books.google.de/books?id=9Qqe0Vcxon4C&pg=PA319&dq=St...
“Der” Wald: Den Freunden und Pflegern des Waldes geschildert
von Emil Adolph Rossmässler

https://www.tandfonline.com/doi/abs/10.1080/0021213X.1987.10...
The exposed cambium of the stumps forms a callus, referred to by Wichmann (1925) as a "turban" growing over the cut surface.

"Bark caps from living stumps" fits your description.
https://vancouverislandbigtrees.blogspot.com/2013/04/douglas...
One of the curiosities of Vancouver Island's Douglas-fir forests are stumps that have healed over completely such as the one shown above. These capped stumps make one wonder how a dead tree can continue to grow in this way. If you could peer into the darkness underground you would find answers, because it is all about the roots.
Several species of trees produce root grafts including Douglas-fir and the true firs. Grafting happens when the root systems of two trees come into contact and grow together. Trees joined in such a way become in effect one organism.
This scenario is common in Douglas-fir forests, and stumps adjacent to live trees may grow over and heal completely in a strange mushroom-like bark cap.