[st26]

It's known in the ham community that pine needles diminish the range of UHF radios, so it fits with pre-existing knowledge.

Reportedly needles are about the same length as a UHF antenna, and so they tend to absorb the radiation.

5G is, I believe, somewhere in the UHF spectrum.

[Junk_Collector]

Trees are quiet good at scattering RF mostly do to the water in them, their density of leaves/needles and the random orientation that they grow in. This leads to significant attenuation of the signal even if it isn't absorbed by anything. It's also worth noting that dried wood is almost RF transparent.