Significance of bryophytes and lichens in arctic coastal plain

Introduction

In the whole Alaskan arctic region, bryophytes (commonly known as "mosses") and lichens are shown to be important in the tundra communities in terms of their cover, production and phytomass. They are generally not freely consumed by herbivores, though they are the main producers in the tundra region; many mosses are slow to decompose, resulting in high phytomass to production ratios indicative of slow turnover. The accumulating bryophyte phytomass has high thermal insulating, water-holding and cation exchange capacities. Thus, it exerts a powerful influence on soil temperature and water regimes and on nutrient cycling, and it forms a carbon sink of significance in terms of global warming. Main species of mosses are Aulacomnium, Sphagnum,Tomenthypnum spp., and main species of lichens are Cladonia, Peltigera, Cetraria spp.. (Sarah J. Woodin & Mick Marquiss, 1997)

Bryophytes and lichens constitute a large part of the total biomass of present-day tundras, and in Rangifer distribution areas the latter are an important part of the food web.  They are important in the structure and function of the ecosystems because of their effects as insulators and filters.  Their insulating properties is partly from increased reflectance and partly from the numerous air pore space when dry.”  The soil under bryophyte and lichen mats tends to be cold and moist.  Lichens and mosses affect nutrient cycling of the ecosystem by intercepting aerial deposition and leaching from dripping aboveground vascular plant parts. (Cowles, 1984; Rosswall and Granhall, 1980) Lichens with cyanobacterial symbionts and bryophytes with cyanobacterial associations provide the main input of nitrogen into the ecosystem. (Alexander et al., 1978; Baselier et al., 1978,; Kallio, 1975)

Mosses contribute significantly to above-ground production and phytomass in the region. In the extensive, relatively dry tussock tundra in Alaska, the production of mosses is around 70gm-2 but in the wetter areas, as near the arctic coastal plain, the production can be as high as 164gm-2, because moss production generally increases with water availability and marine shore is usually more plentiful in nutrients. Lichens are generally less productive than mosses, but they contribute substantial phytomass in lichen heaths and woodlands. (Sarah J. Woodin & Mick Marquiss, 1997)


Lichens on the arctic coastal plain
Photo: Subhankar Banerjee

Factors for their success - an analysis on their physiology

There are several factors responsibles for the success of mosses and lichens in Alaskan region. The two groups are remarkably similar in attributes beneficial in severe environments. Both tend to occupy and create relatively favourable microenvironments. Many species exhibit a broad response of net assimilation rate to temperature, with maxima at 10-15oC but with positive net assimilation and dark respiration continuing at or below 0oC. Light compensation and saturation intensities are typically lower in mosses and lichens than in vascular plants, and compensation levels decrease with temperature permitting positive net assimilation under cool, low-light conditions. (Sarah J. Woodin & Mick Marquiss, 1997)

Moreover, most species have little access to soil moisture and lack an effective cuticle; this enables them to absorb water through much of their surface but results in rapid water loss under drying conditions. Yet, though the plants become inactive when dry, they resume normal metabolism rapidly on remoistening. Thus, mosses and lichens are thus adapted to switching rapidly between periods of metabolic activity and rest, utilizing favourable conditions whenever they occur. This may be facilitated by micromorphological features thought to facilitate simultaneous uptake by moss leaves of both water and carbon dioxide. They also resist frost very well by conferring tolerance of cytoplasmic dehydration resulting from extracellullar ice formation. (Sarah J. Woodin & Mick Marquiss, 1997)

Reference:
1.    Sarah J. Woodin & Mick Marquiss. (1997). Ecology of Arctic Environment, under the article: Longton, Royce E. (1997). The role of bryophytes and lichens in polar ecosystems