Trentepohlia umbrina (Kützing) Bornet

About plant names...

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
≈ 28 × 42" (70 × 105 cm) 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 umbrina description by Thomas H. Kent, last updated 25 May 2020.

© FloraFinder.org. All rights reserved.

7/5/2012 · Groton Place and Sabine Woods, Groton, Mass­a­chu­setts
≈ 7 × 4½" (17 × 11 cm) ID is uncertain

1/10/2012 · Tom Paul Trail, Westford, Mass­a­chu­setts
≈ 4½ × 7" (11 × 17 cm) ID is uncertain

1/10/2012 · Tom Paul Trail, Westford, Mass­a­chu­setts
≈ 7 × 4½" (17 × 11 cm) ID is uncertain

7/5/2012 · Groton Place and Sabine Woods, Groton, Mass­a­chu­setts
≈ 7 × 4½" (17 × 11 cm) ID is uncertain

9/28/2011 · Groton Place and Sabine Woods, Groton, Mass­a­chu­setts
≈ 3½ × 5" (9.2 × 13 cm) ID is uncertain

A side view: only one side of each tree is orange. · 7/5/2012 · Groton Place and Sabine Woods, Groton, Mass­a­chu­setts
≈ 16 × 9" (41 × 22 cm) ID is uncertain

Boundary where the coating starts, about a foot above the ground. · 7/5/2012 · Groton Place and Sabine Woods, Groton, Mass­a­chu­setts
≈ 8 × 12" (20 × 31 cm) ID is uncertain

1/10/2012 · Tom Paul Trail, Westford, Mass­a­chu­setts
≈ 4½ × 7" (11 × 17 cm) ID is uncertain