RBP - Red Bark Phenomenon

RBP on Eastern White Pine (Pinus strobus)

 In the last couple decades or so, people have increasingly been noticing a peculiar rust-red color appearing on the trunks of trees here and there in New England and other regions. It has come to be known as "Red Bark Phenomenon" (RBP), a term coined by the Connecticut Agricultural Experiment Station. 

Affected trees typically show this coloration on just one side of the tree, usually the side facing north or west. Often it's found on trees near a water body- a pond, stream, or river. The condition does not affect just one tree species; on the contrary, it has been found on a wide range of tree species, both conifers and hardwoods, both native and introduced. But it seems to most often show up on eastern white pines (Pinus strobus), hemlocks (Tsuga canadensis), and oaks (Quercus spp), among the many others.

What causes this bark discoloration? Researchers have determined that it's a coating of algae in the Trentepohlia genus. It's a green alga, but contains large quantities of carotenoids and other compounds that hide its green chlorophyll from our view and make it appear an orange-red color. It does no known harm to the trees, being just a crusty coating on the bark's exterior. Trentepohlia is typically found in humid locations. The reason it appears on just the shadier side of trees is that a lot of sun exposure would quickly dry it out.

Sometimes it's seen on just one or two trees at a given site, as was the case with the white oak shown. 

RBP on White Oak (Q. alba)

In other locations, whole stands of trees might host the algae (see the photo of the white pine stand). 

 When I was first learning to identify scarlet oak (Q. coccinea), the particular trees I was looking at had rusty orange coloring in the bark furrows, which I assumed might be a characteristic of the species (similar to the red oak in the photo below). Then I found others that had no such color. That confused me a bit until I realized that I was seeing RBP.

  

RBP on White Pines

 

 

 

 

 

RBP in Red Oak (Q. rubra) bark furrows

 

 The next two photos are closeups of RBP on white pine and white oak bark.

Trentepohlia species can be found on other substrates besides bark, such as moist rocks, concrete, buildings, etc. They can also combine with certain fungi to form lichens. 

It may be easy to confuse other bark conditions with RBP. For instance, dead or dying hemlocks and other species may have their outermost bark layers chipped off by woodpeckers searching for insect larvae. Over time, woodpeckers may chip away at virtually the entire length of the trunk, so the entire tree looks orange. At a casual glance, this may look like RBP, particularly on hemlocks. With the current ash tree devastation being caused by the emerald ash borer, it's common to see bark chipped off white ashes (Fraxinus americana) too, exposing a somewhat orange color.

Not RBP: Dead White Ash (Fraxinus americana)

Not RBP: Dead Eastern Hemlock (Tsuga canadensis)