Moss leaf water content and solar radiation at the moss surface in a mature black spruce forest in central Alaska

Abstract

In a mature black spruce (Piceamariana (Mill.) B.S.P.) forest near the University of Alaska, Fairbanks, AK, samples of four common moss species, Polytrichumcommune Hedw., Hylocomiumsplendens (Hedw.) B.S.G., Pleuroziumschreberi (Brid.) Mitt., and Sphagnumsubsecundum Nees. were collected at intervals during the 1976 season to determine the diurnal variation in leaf water content and daily water loss as functions of temperature, moisture, radiation, and wind speed. The field measurements were followed by laboratory experiments on intact cores in an open system and on excised shoots in closed cuvettes. In these experiments, water loss rates varied by species and were affected by vapor pressure deficits and wind speed; where vapor pressure deficit is more constant than these other factors, variations in light intensities had less effect on water loss rates. Polytrichumcommune, which translocates water from the soil, avoided moisture stress to a greater extent than the other moss species which were more dependent on water absorption through the leaves. Hylocomiumsplendens was below the water content for compensation for almost 50% of the July measurement period. Observed patterns of the rates of water loss and of the moisture required to reach field capacity are correlated with the moisture status of the mosses in the field. Field measurements of photosynthetically active radiation (PAR) at the moss surface and above the forest canopy showed that the transmission of diffuse radiation through the forest canopy increased with increasing cloudiness. PAR at the moss surface was above the compensation level for photosynthesis with sunny and cloudy conditions. Sunflecks (short periods of direct sunlight), which had an intensity of about 76% of the radiation incident on the forest canopy, occurred on up to 35% of the ground surface and provided a major source of the radiation received. Species varied in their pattern of attenuation of light through the moss canopy. A mixed stand of P. commune and P. schreberi and stands of H. splendens transmit light deeply. Sphagnumsubsecundum, on the other hand, shows rapid attenuation of light high in the canopy. The observed pattern of light attenuation helps explain the vertical position of the green–brown interface and the death of green material in the moss canopy.