Frankia Nodules

Day 171: Here it is, the botanical curiosity which necessitated a second trip to Swofford Pond and Sulphur Creek Falls. My initial photos from a week ago were sufficient to obtain an expert's identification, but regrettably were rather poorly focused. It's not easy to get a clear photo when you're in dark woods with nowhere to stand a tripod because of the steepness of the slope! So what is it? Nothing particularly rare, just not often seen because the roots on which it grows are generally covered by soil and aren't exposed until the host tree falls over. It's a root nodule, birthed from a bacterial infection (beneficial, not harmful) by one of several generalized species of Frankia (of the Frankias, only F. alni is classified taxonomically, representing its host species of Alder). Frankia colonies form in the absence of nitrogen in the soil, i.e., they are nitrogen-fixers. The blebs of a mature colony are filled with bacterial filaments which convert atmospheric nitrogen into a form which can then be utilized by their hosts.

My first thought upon encountering it was that it was some type of gall, the same supposition which had been made in 1679 when it was first discovered (thank you, Google). I have seen some very unusual outgrowths as the result of infestation by gall-producing insects including one I mistook for a lichen growing on blue huckleberry. Then I wondered if it could have been produced by a nematode, but that too was wrong. I owe great thanks to our Park ecologist who referred my initial photos out to a second expert with whose aid the mystery was solved.