Variation in Morphology and Functional Traits of Dictyonema Glabratum From Contrasting Habitats in the Venezuelan Andes

Specimens of Dictyonema glabratum from various elevations in the Venezuelan Andes show clear differences in morphology. Optimal development of thalli is attained in the lower Paramo at 3500–3600 m; at the upper altitudinal limit of distribution at about 4300 m only poorly developed thalli are found; in the montane forest zone at about 1700 m the thalli attain about half the average surface area and two thirds of the thickness of those in the optimal habitat. The histological structure of the thalli also shows significant differences. The relative volumes of photobiont to mycobiont, as an indicator of the relationship autotrophism/heterotrophism, is 1.6:1 in specimens from the upper Paramo; 0.9:1 in open habitats on the lower Paramo; 0.5:1 in shaded habitats; and 1.4:1 in the montane forest. The three altitudinal morphotypes are not only distinguished by structure but also by carbon dioxide gaseous exchange patterns. Specimens from the optimal habitat exhibit the highest photosynthetic capacity. The respiration activity reflects a homeostatic adjustment to the prevailing temperature regime: the respiration of specimens from 4300 m at 1°C approximates to same intensity as that of D. glabratum from the lower Paramo zone at 6°C. Nevertheless, metabolic homeostasis is only possible over a narrow temperature amplitude. On account of the large heterotrophic component in lichens, at altitudes below 1500 m where the temperature average is 20°C or more and the nocturnal temperatures are constantly high, the dry matter consumption due to respiration would become extremely high. This suggests that D. glabratum may be limited in its downward distribution by an increasingly unfavourable carbon budget.