Environmental stress and Umbilicaria lichens: The effect of subzero temperature pretreatments

Five species of Umbilicaria were used in experiments to determine the effect of moderately below zero temperature pretreatments. Both air-dry and fully-imbibed plants were exposed to-25° C for nine months. The experiments were conducted using samples collected in midsummer and mid-winter. The gas exchange activity of the plants was measured following the low temperature stress, and was compared to the response of unstressed plants. A four-light-level by three-temperature-assay of net CO₂ exchange was used throughout. The results showed that exposure of dry plants to-25° C has few effects, but that exposure when imbibed produces a range in response from serious damage to apparent stimulation. For all species the results are consistent with what would be expected knowing the extent and duration of the snow cover is the natural habitat.