Propolis is a vital element of a healthy hive, and is attached by bees to the rough surfaces of the nest cavity. Propolis creates an anti-microbial atmosphere that protects the bees from mould and pathogens. This atmosphere can only be created in a warm nest as is found inside a tree with good insulation and high thermal inertia; Condensate from the metabolism of winter honey beads onto the propolis walls. This dissolves some of the propolis that eventually evaporates and circulates into the nest atmosphere.
In the video link above, Alasdair Mackenzie shows the incredible effort bees will put into creating a healthy hive atmosphere. The effort reflects the importance of propolis. A side benefit for the tree is the stabilisation of the tree cavity.
Emmanuel's beautiful picture above of a propolis encrusted nest was a winning entry in the 2018 Natural Beekeeping Trust's photographic competition.
Related Papers and articles:
Nest Scent & Warmth - Johann Thür,
Propolis - a very special substance - Torben Schiffer, in cooperation with Jürgen Tautz
Textured Hive Interiors Increase Honey Bee (Hymenoptera: Apidae) Propolis-Hoarding Behavior - Cynthia R L Hodges Keith S Delaplane Berry J Brosi
Propolis envelope in Apis mellifera colonies supports honey bees against the pathogen, Paenibacillus larvae - Renata S. Borba, Marla Spivak
In Vitro Studies to Evaluate the Antistaphylococal Aureus Activities of Propolis: A Wonder Honey Bee Product in Comparison with Antibiotics: Ampicillin and Amoxycillin - Anita Devi, Neelima R Kumar and Jaspreet Kaur