I've been puzzling over this little mystery for years now. In many areas entire groves of pines are coated with
this orange stuff. It is as if a truck loaded with orange spray paint exploded nearby, and a dense
orange mist wafted through the grove, settling
on tree trunks as it passed. I guessed that this was a crustose lichen ("crustose" means "in a thin crust"), but
close examination of the coatings reveals no sign of the structure that would be typical in lichens.
It turns out to be a subaerial terrestrial microalgae. Before today, I didn't even know that there are species of algae
that live on land. But this and its many relatives do just fine on land. Somehow it gets into the
air, where it is dispersed by rain through wide areas, which explains its presence on large expanses of trees. This has been found to be the cause of occasional sightings of orange-colored rain. And although it lands on everything, it manages to make a permanent home on tree bark. It seems especially common on red and white pine. This microalgae is an epiphyte: it does
no harm to its host.
So ... if this is algae, why isn't it green? Because it is filled with carotene, the same stuff that makes carrots orange. This is probably what protects it from too much sunlight.
7/29/2023 · Groton Place and Sabine Woods, Groton, Massachusetts ID is uncertain
7/5/2012 · Groton Place and Sabine Woods, Groton, Massachusetts ID is uncertain
7/20/2012 · J. Harry Rich Dirt Road, Groton, Massachusetts · ≈ 2½ × 3½″ (6.6 × 9.8 cm) ID is uncertain
The underside of the bark is rust-colored as you can see here, but the coating on the outside is visible at the edges. · 8/31/2023 · Groton Place and Sabine Woods, Groton, Massachusetts ID is uncertain
7/25/2013 · Beaver Brook Assn Conservation Lands, Rte. 130, Hollis, New Hampshire · ≈ 7 × 4½″ (16 × 11 cm) ID is uncertain
Trentepohlia odorata var. umbrina (Kützing)Hariot
Trentepohlia umbrina description by Thomas H. Kent, last updated 25 May 2020.