Ecology of desiccation tolerance in bryophytes: A conceptual framework and methodology

Bryophytes have long been understood to be desiccation tolerant (capable of reviving from an air-dry state, DT). While mechanistic studies focusing on the expression of this complex trait are numerous over the last 50 years, a firm conceptual framework for the ecology of desiccation tolerance in bryophytes has lagged behind. As a result of this, mechanistic studies have gone ahead without an ecological foundation, and this path can be scientifically risky. The motivating elements for this article include (1) the low visibility and lack of implementation of a conceptual framework for studying the ecology of DT in bryophytes, (2) the lack of a protocol for the step-by-step handling of field-collected or laboratory cultured specimens for study of desiccation tolerance with particular attention to the deacclimation of plants, (3) a recognition that most studies of desiccation tolerance tend to conflate two of the most important factors of desiccation, rate of desiccation and water content, and (4) recognizing that the ecological factors of rehydration, aside from recovery, are nearly always ignored in experiments on desiccation tolerance in bryophytes. Therefore this review postulates a conceptual framework for the ecology of desiccation tolerance, presenting the principal factors of desiccation (rate of drying, water content at equilibration, duration dry, and rate of rehydration) and how they may interact, presenting the principal physiological phases of rehydration (recovery, hardening, dehardening), and highlighting the importance of specimen handling in preparation for an experiment on DT. Useful metrics for assessment are derived that include the minimum rate of drying tolerated at a given water content, the minimum or maximum water content tolerated, the maximum duration dry tolerated, and the minimum dehardening time exhibited.