The morphological basis of variation in net photosynthetic and respiratory responses of the mat-forming lichen species Stereocaulon virgatum and S. tomentosum

Abstract

Response patterns of net photosynthesis and dark respiration are examined in two species of Stereocaulon: S. tomentosum Fr., from early successional pine forest communities of the southern Canadian Rockies, and S. virgatum Ach., an early colonizer in tropical cloud-forest environments. These responses, measured in both intact and dissected mat segments, are described in the context of the influence of morphological variations on patterns of water vapor transport. Saturation response curves are fitted to data, allowing description of maximal rates of both net photosynthesis and dark respiration, water contents at which rates are half maximal, maximal water efficiency, and moisture compensation points. In S. tomentosum the closed-canopy nature of the lichen mat profile results in the development of a distinct shade ecotype in lower thallus segments. This canopy profile also impedes water vapor transport from within the mat profile, creating a more mesic microclimate for photobionts located at depth within the lichen mat. This stands in contrast to the open upper canopy profile of S. virgatum, which allows greater convective exchange at depth and appears to preclude the development of distinct sun–shade photobiont ecotypes. Net photosynthetic activity remains high in fully saturated thalli of S. tomentosum, yet in S. virgatum it is depressed by over 50% at full thallus saturation. This greater depression of photosynthetic uptake at full thallus saturation in the species of the more mesic environment contradicts a priori assumptions based on previous concepts of xeric–mesic response gradients in lichens. These responses are discussed in context of other selective pressures influencing lichen mat morphology.