Moisture availability is obviously of overriding importance to poikilohydric epiphytes, and lichens and bryophytes are differentially distributed along moisture gradients. Chlorolichens tend to be more tolerant of desiccation than cyanolichens, which tend to be more tolerant than bryophytes. Nearly all lichens require alternating wet and dry periods to maintain symbiosis (Honegger 1998). Prolonged hydration severely limits CO2 diffusion to the photobiont and, therefore, photosynthesis of lichens (Lange et al. 1993, 2000; Zotz et al. 1998). In contrast, many bryophytes can thrive under continuously wet conditions (Proctor 2000). Thus, bryophyte biomass is generally highest in rainforests, cyanolichens reach their greatest abundance in moist forests with a summer dry season, and chlorolichens are often the only epiphytes in dry forests. There are, of course, exceptions to these general patterns. For example, some epiphytic chlorolichens (e.g., Hypogymnia duplicata) are restricted to coastal rainforests ( McCune 1997), and some epiphytic bryophytes (e.g., Porella platyphylla) are very tolerant of desiccation ( Marschall and Proctor 1999). However, the typical abundance sequence (bryophytes—cyanolichens—chlorolichens) recurs along gradients of decreasing moisture availability at multiple spatial scales.