Subalpine Spruce Forest Carbon Flux: The Role of Hylocomium Splendens

Abstract

Pulse release and uptake of carbon by forest floor Hylocomium splendens moss mats was examined in a subalpine white spruce/subalpine fir forest. Optimal conditions for net photosynthesis in Hylocomium splendens were 14 °C, 240-480 μmol m⁻² s⁻¹ and between 200-400% water content by weight. These conditions were rarely met under field conditions. However, release of CO₂ from organic soils frequently elevated CO₂ concentrations within moss mats by up to 200 ppm CO₂ above ambient, suggesting that elevated CO₂ concentrations within forest floor moss mats may counteract the effect of suboptimal microclimate conditions experienced by Hylocomium splendens, allowing it to achieve positive carbon balance under a much wider range of conditions. Observation in the summer of 1992 and 1993 show that pulse release of organics from Hylocomium splendens contributes between 10-110 mg ⁻¹ m⁻² total organic carbon (TOC) to groundwater leachate during the rehydration phase of every rain event. I hypothesize that mineralization of this TOC release and resaturation respiration from the moss mat contributes significantly to carbon cycling within mat environments. The pulse release of carbon may also have major influence on ecosystem function stimulating microbial decomposition, increasing net mineralization of nitrogen, and stimulating asymbiotic nitrogen fixation. Thus, Hylocomium splendens functions as both a source and sink for ecosystem carbon. It's most important contribution is that of an ecosystem capacitor. H. splendens accumulates gaseous carbon from the atmosphere through photosynthesis. Then, during rehydration, the bryophyte releases carbon in a water soluble form at concentrations higher than those found in the atmosphere.