The stability of moss populations on stream banks along two first-order, temperate-forest, headwater streams

Abstract

The occurrence of terrestrial mosses in temperate deciduous forests is limited by abundant leaf litter that covers almost all of the soil, except along stream banks where mineral substrates are exposed by wind. However, stream banks are also subject to erosion that is a major disturbance for moss populations. Utilizing transplants and observations over a three period (2015–2017), we examined maintenance of current populations and establishment of new ones along the banks of two headwater streams, one in Virginia (Meadowood) and one in Illinois (Vishnu). Of the 224 plots, some with and some without mosses, that we monitored at the two stream study areas, 78 (35%) were lost to erosion or stochastic events. Areas with established moss populations had similar losses than areas without mosses (43% lost vs. 39% lost); however, losses at the two sites were very different—in Illinois, 41% of the original plots were lost, compared to 29% in Virginia. These differences in losses from erosion correspond to 32% greater number of high (>2 cm) and extreme (>4 cm) precipitation events at the Illinois site. In general, areas with mosses were lost at the same rate as those with leaf-covered bare soil. Leaf cover of moss plots, measured at height of the autumn leaf fall, averaged 12% at the Illinois site and 14% in Virginia, compared to plots without mosses that averaged 56–57% leaf cover, indicating that established moss populations are associated with exposed microsites, whereas microsites without mosses have high leaf cover. We examined the potential for colonization of new sites using transplants. Mosses transplanted onto bare soil successfully established 52% of the time, with unsuccessful ones mostly lost due to high leaf cover. Bare areas that did not receive transplants were colonized only 14% of the time, while bare areas within established moss populations were recolonized 48% of the time. We conclude that: 1) stream bank bryophyte populations are limited by leaf litter on the forest-side and erosion on the stream-side—the latter influenced by high precipitation events; 2) new potential habitats are often covered by leaf litter prohibiting establishment; 3) establishment of new populations in the local area appear to also be limited by availability (or dispersal) of diaspores; and 4) despite these limitations, losses from erosion appear to be counter-balanced by persistence and slow expansion of existing populations that remain largely litter free owing to slightly raised canopy surfaces and rapid recolonization of internally disturbed microsites.