The impact of ten years at -20°C on gas exchange in five lichen species.

Rates of net CO2 exchange in five sympatric species of Umbilicaria were measured after 10 years at-20°C. During that time, the lichens had been at either a high (saturated) or a low (air-dry) water content. The results showed an immediate, return to normal rates of gas exchange for air-dried then frozen U.vellea. Rates returned to normal for air-dried U. deusta within four days. The three other species studied, U. mammulata, U. papulosa and U. muhlenbergii showed intermediate responses. Water saturated then frozen thalli of U. vellea were irreparably damaged after 10 years; even fungal respiration was severely impaired. In U. mammulata, U. papulosa and U. muhlenbergii, photosynthesis was eliminated but fungal respiration rates were not influenced. In contrast to this, water saturated then frozen thalli of U. deusta showed a nearly equal photosynthetic capacity to the original rates following 10 years at -20°C. In all cases the magnitude of the effect of the long-term subzero treatment correlated well with the known distribution patterns of the lichens in the field.