Aphanorrhegma serratum (Hook. f. & Wils. in Drumm.) Sull. in Gray
Lidded Earth Moss
Basis for Listing
Aphanorrhegma serratum (lidded earth moss) is endemic to eastern North America within the temperate bioclimatic zone. It has been recorded regionally from Ontario, Nebraska, Iowa, and Wisconsin. In Minnesota, only two small populations in Chisago and Washington counties have been found (Mille Lacs Uplands and St. Paul-Baldwin Plains and Moraines subsections). With so few populations documented, there is not enough information available to detect a statewide population trend. Further survey work is needed to clarify the species’ abundance as well as the potential threats to its survival (Janssens 2005). Given its apparent rarity, Aphanorrhegma serratum was designated a species of special concern in 2013.
Aphanorrhegma serratum is a small light-green plant, 2.0-4.0 mm (0.08-0.16 in.) high, occurring either as scattered individuals or growing close together (not as tufts or mats). Stems are often forked. Upper leaves are 1.5-3.0 mm (0.06-0.12 in.) long, oblong to obovate, acute to short-acuminate, and bluntly serrulate in the upper ½ or less; costa (distinct mid-vein of leaf) ends near the apex, often disappearing in the slender tip (Crum and Anderson 1981; McIntosh 2007). Leaves are often twisted when dry, exposing the capsule that sits down within the leaves (McIntosh 2007). Setae (the stalk that supports the capsule) are stout and approximately 0.2 mm (0.008 in.) long, thus the capsules appear sessile. A calyptra, which is mitrate in shape (like a miter or bonnet) with 4-6 smooth lobes, sits atop the capsule. Capsules are light-brown when mature in August and are 0.4-1.0 mm (0.02-0.04 in.) wide and smooth. The species is often confused with Physcomitrium immersum (immersed physcomitrium moss) or Physcomitrella patens (spreading earthmoss), both of which are also uncommon in Minnesota and share similar ranges and habitat. Aphanorrhegma serratum, however, is readily distinguished by its capsule, which splits exactly in the middle at maturity, whereas the line of dehiscence in both P. immersum and P. patens is situated above the middle of the capsule (Nichols 1910; McIntosh 2007).
This species occurs on poorly drained soils that are subject to inundation, such as stream and river banks and floodplains or other low areas, such as within old fields. Across its range, many specimens have also been found on disturbed or open sites on bare soil, rarely among other mosses (McIntosh 2007).
Biology / Life History
This species is autoicous (having both male and female reproductive organs on the same plant). The genus Aphanorrhegma refers to an “unapparent” rupture of the capsule, when in fact, this is what happens: at maturity, the capsule’s distinct conical short-apiculate operculum (lid) opens and falls off, rather than rupturing along the line of dehiscence along rows of small delicate equatorial cells (Crum and Anderson 1981; McIntosh 2007). The species name (serratum) refers to its toothed leaf margins.
Conservation / Management
Although not much is known about this species or its preferred habitat in Minnesota, collections here and in other states suggest that it is typically found in floodplains along stream banks. Based on these reports, this species may be repeatedly moved or swept away by swift currents typical of these communities, while smaller disturbances may create habitat.
With climate change, however, extreme single-day precipitation events have increased nationally, especially over the last three to five decades, with the largest increases occurring in the Midwest and Northeast. With an increase in heavier or longer-duration rainfall events, soil erosion and damage to the surrounding plant communities can also be expected to increase, particularly if these areas have had the tree canopy recently opened or removed as this would limit or reduce infiltration and increase surface run-off and the removal of ground vegetation. As a result, habitat for A. serratum could be negatively impacted with a rise in heavy rainfall and erosion events.
Best Time to Search
The best time to search for A. serratum is from May through September or essentially anytime the ground is not covered by snow.
Erika R. Rowe (MNDNR), 2018
(Note: all content ©MNDNR)