Mary C. Sollows, Katherine A. Frego, Christopher Norfolk
The Bryologist 104 (3), 421-429, (1 September 2001) https://doi.org/10.1639/0007-2745(2001)104[0421:ROBTFD]2.0.CO;2
Colonies of Bazzania trilobata (L.) S. Gray, a common and abundant species on the floor of closed canopy mixed and coniferous forests in New Brunswick, were observed to be pale and dry within a 2-yr-old clear-cut area. This multi-part study used a combination of approaches to assess the species' tolerance of various drying regimes. 1) To determine viability of field-dried shoots, bundles of shoots from the clear-cut and from an adjacent closed canopy forest (controls) were compared in terms of color and net photosynthetic rate (using Infra Red Gas Analysis, IRGA) before and after two months of laboratory “recovery” conditions (light, temperature, and humidity similar to those documented in closed canopy forest). Shoot elongation during recovery was also quantified. Field-dried shoots were viable: following recovery period, they had increased in length, shifted in hue, and displayed positive net photosynthesis, however none of these measures of metabolic activity equaled those of the controls. 2) The moisture content at saturation, determined for reference, was 1296.4% by weight. 3) The drying effect of the IRGA process was evaluated by subjecting moist pre-weighed bundles of shoots to three successive sequences of IRGA, re-weighing, and 24 hr of recovery. IRGA resulted in slight (approximately 40%) moisture loss from shoots initially averaging 368.9% moisture (S.E. = 9.96%), but did not cause a significant decrease in net photosynthesis within this range. 4) Tolerance of drying was tested on freshly collected, forest-acclimated moist shoots. Initial net photosynthesis (by IRGA) and moisture content were determined for replicate bundles of shoots. These were then subjected to drying for 1, 2, 4, 7, and 12 d, while control groups were stored at recovery conditions. Net photosynthesis and moisture content were re-measured after drying, and again after 24 hr of recovery. Moisture content of controls declined by approximately 40% during IRGA, and gradually thereafter in recovery conditions; net carbon gain declined over the course of the study. Moisture content of dried shoots declined by an order of magnitude in all treatments, but returned to near control levels after recovery. However, net photosynthesis dropped to zero with drying, and did not resume after rehydration. In view of the sensitivity of B. trilobata to laboratory drying, and its viability on a 2-yr-old clear-cut, we suggest that a) field conditions are neither as severe nor as prolonged as those tested in the laboratory and b) the colonial form prevents inner shoots from uniform exposure to severe conditions.