Ozarkian Atrichum

Back in early February, I collected a moss that had to be in the genus Atrichum (Polytrichaceae). It was acrocarpous with lanceolate leaves possessing lamellae (gill-like flaps) along the costa (midrib) and had serrate teeth on the margins. But it was a species that I didn’t recognize. There are two species commonly encountered in the Midwest, A. altecristatum and A. angustatum. Both can be readily identified by field characters. This one didn’t match either. Then last Sunday, I collected another Atrichum that looked different. Needing answers I turned to the Flora of North America volume on acrocarpous mosses. In it, I learned that there are only five species of Atrichum in Missouri and that one of these species is restricted to bald cypress swamps in the Bootheel. At this point I thought to myself “if my summation is correct, I have collections of all four Atrichum known from the Ozarks (of Missouri, at least)”.

Having been intimidated by bryophyte keys many times before, this time I stood steadfast, mustered my patience, gritted my teeth and dove in head first. Three hours later, I emerged victorious; blooded but unbowed. And as with any successful quarrel with a difficult plant group, at the end I asked myself “could it really be that easy?”. Here’s the breakdown:

From left to right: A. crispulum, A. crispum, A. altecristatum, A. angustatum. Note their relative size.

As luck would have it, A. angustatum is not only the most common species in the Ozarks, it is also the easiest to identify. Here is what it looks like in the field.

Identifying A. angustatum comes down to two characters. First, the leaves will be significantly less than 1.0 mm wide. Second, the costa and associated lamellae (basically what would appear to be the midrib of the leaf macroscopically) will account for one quarter or more of the leaf width. Terminology aside, pretty easy stuff. Here is a closer shot of a leaf where you can see the proportionally wide costa.

While all four seem to be acidophiles, you will commonly find A. angustatum in the driest of acidic habitats; cherty acid uplands, margins of igneous glades and sandstone outcrops. Like many mosses, it grows in large colonies on exposed soil such as tip-up mounds, ditch margins and that special little litter free micro-habitat that exists around the perimeter of large tree trunks. For the most part, A. angustatum is the shortest Atrichum in the Ozarks (see line-up photo above).

The second most common species in the Ozarks is A. altecristatum (formerly known as A. undulatum; in part). It is also readily identified by macroscopic field characters. The first thing to notice is that the leaves are strongly undulate (wavy like a lasagna noodle).

Other species have this character, but it is definitely more pronounced in A. altecristatum. The second character is height. As you can see in the line-up photo at the beginning of this post, there is a progression in height. It is taller than A. angustatum, but significantly shorter than A. crispulum and A. crispum. The rule of thumb is that A. altecristatum is one to three centimeters tall and the two taller species are two to six centimeters tall. I know what you’re thinking, “there is that damn overlap that makes this stuff so difficult”. Fear not, most specimens fall toward the extremes. A third character is habitat. Atrichum altecristatum is the weediest of the four species and occurs on areas of bare soil in mesic to dry-mesic forests. While its occurrence in intact native landscapes is well documented, it clearly increases in abundance under certain levels of disturbance; chiefly woods that have been heavily grazed. Perhaps it is the soil compaction coupled with the increase in exposed soil, but an abundance of this species indicates abuse. This is important because the remaining two species demonstrate a high fidelity to intact natural communities. They also occur in wetter habitats.

Atrichum crispulum also has undulate leaves. In fact, even under a microscope they are nearly identical to those of A. altecristatum. However, plant height and habitat easily differentiate the two.

Here is A. crispulum that has been rehydrated on my porch. This particular specimen is close to five centimeters tall; nearly twice the height of the average A. altecristatum. Atrichum crispulum is a species of undisturbed plant communities and is found in moist to wet protected slopes and wet sandy ledges in shady ravines. It has just recently been recognized as occurring in Missouri and is state listed as SU. Any new locations would be good info for the Natural Heritage folks.

The last species to round out the Ozarkian Atrichum experience is A. crispum. It is also found in wet habitat with intact native integrity.

It differs from the previous three species in lacking undulate leaves. In height it resembles A. crispulum, but it is more sparsely foliose.

That’s it! Height, undulations and habitat are all you need to distinguish the Atrichum of the Ozarks. Don’t ask me why the keys are based on the length of single cells and other equally tedious characters. It goes to show that with some field experience, some collecting and the process of elimination, moss identification is not only possible, its kind of fun.