Photosynthesis in a widespread and important sub-Arctic moss and lichens species in pine ecosystems of the ZOTTO tower footprint area

In boreal forests, bryophytes and lichens usually dominate the ground floor layer and contribute up to 50% of ecosystem gross CO₂ exchange (Bisbee et al. 2001; Goulden & Crill 1997). Sphagnum spp. are the most important contributors in wetland C uptake, and feathermosses and lichens play a significant role in well-drained sites (Nilsson & Wardle 2005; O’Connell et al. 2003; Jarle W. Bjerke et al. 2013). Given their important ecological roles in such a widespread biome, it is surprising that still a few studies have attempted to understand the intrinsic factors that control moss-lichen cover carbon dynamics specifically under ongoing climate change in high latitudes.

The aim of this work was to determine the stocks of moss-lichen stratum and photoassimilation activity of its dominant species during the growing season. The study has been conducted in Central Siberia near Zotino tall tower observatory (ZOTTO, 60 ° N, 89 ° E) in lichen- and feathermoss-dominated pine forests. First, to assess the phyto (bio) mass stocks the grass-shrub and moss-lichen layers were sampled in 100 replicates in each type of forest from 20x25 cm subplots (S = 50 cm²). The intensity of CO₂ photoassimilation was determined in situ by Walz GFS-3000 (Heinz Walz GmbH, Effeltrich, Germany) infrared gas analyzer. Photosynthetic activity of lichens and feathermosses was measured during the growing season of 2018 in June, July, August and September around the mid-day time. For every time point we also analyzed CO₂ exchange dependence from temperature, photosynthetically active radiation (PAR) and CO₂ concentration.

The dominants of ground vegetation for the moss-lichen layer were Cladonia stellaris, Cladonia rangiferina, Cetraria islandica, Pleurozium schreberi, Hylocomium splendens, Aulacomnium palustre. The moss-lichen layer accounted for 78-96% of the total phytomass of ground floor in studied pine forests and comparable (486 g/m²) to the photosynthetic phytomass of the tree canopy (pine needles). During the growing season, carbon assimilation by the moss-lichen layer varied in a relatively narrow range: from 38 ± 4 to 42 ± 5 mgCO₂ / m² / hour for lichen C. stellaris and from 93 ± 11 to 99 ± 13 mgCO₂ / m² / hour for moss P. schreberi. Thus, moss-lichen layer dominants maintained high photoassimilation activity throughout the growing season. Temperature increased the intensity of CO₂ assimilation and no inhibition was observed at maximum T used in our study (+40 ° C). There were no differences in the temperature dependence of CO₂ photoassimilation between feathermosses and lichens. However, they differed in dependence from PAR. Mosses showed 2-fold larger response of CO₂ assimilation intensity to increase of PAR comparatively to lichens. The rate of photosynthesis of both moss and lichen showed log growth with increasing CO₂ levels up to 2000 ppm. Compensation poit was varying from 170 to 284  ppm.

This study was supported by the Russian Foundation for Basic Research project  № 18-05-60203 "Landscape and hydrobiological controls on the transport of terrigenic carbon to the Arctic Ocean".