Laboratory for
Climatology and Remote Sensing

Abstract:

Recent work on bryophyte diversity in lowland forests of northern South America has suggested the existence of a new type of cloud forest, the “tropical lowland cloud forest” (LCF). LCF occurs in river valleys with high air humidity and radiation fog, and is rich in epiphytes. We explored the lichenological characteristics of putative LCF in a lowland area (200–400 m a.s.l.) near Saül, central French Guiana, using macrolichens (including large crustose species) as indicator taxa. We analyzed macrolichen diversity on 16 trees in two 1 ha plots, in LCF and in lowland rain forest without fog (LRF). Sampling efficiency was ca. 80% in both forest types. Canopies of both LRF and LCF were richer in lichen species than understory trunks. Species richness of macrolichens was rather similar in the two forest types but species composition was significantly different. Cyanolichen richness in LCF was ca. 2.5 times higher than in LRF; in contrast, LRF had 4 times more species of green-algal Parmeliaceae. Our study suggests that cyanolichens except for Coccocarpiaceae serve as indicators of LCF. We explain the detected diversity patterns by differences in water availability due to fog precipitation and higher humidity. This is indicated by the higher relative air humidity in the lowland cloud forest, which was >6% higher than in the rain forest.

Abstract:

Recent work on bryophyte diversity in lowland forests of northern South America has suggested the existence of a new type of cloud forest, the ‘tropical lowland cloud forest’ (LCF). LCF occurs in river valleys in hilly areas with high air humidity and morning fog, and is rich in epiphytes. We explored epiphyte abundance and canopy microclimate of LCF in a lowland area (200–400 m asl) near Saül, central French Guiana. We analyzed the vertical distribution of epiphytic cover and biomass on 48 trees, in LCF and in lowland rain forest (LRF) without fog. Trees in LCF had significantly more epiphytic biomass than in LRF; mean total epiphytic biomass in LCF was about 59 g/m2, and 35 g/m2 in LRF. In all height zones on the trees, total epiphyte cover in LCF exceeded that in LRF, with ca 70 percent mean cover in LCF and ca 15 percent in LRF. During both wet and dry seasons, mean diurnal relative air humidity (RH) was higher in LCF than in LRF, and persistence of high RH after sunrise significantly longer in LCF. We suggest that the prolonged availability of high air humidity in LCF and the additional input of liquid water through fog, enhance epiphyte growth in LCF by shortening the desiccation period and lengthening the period of photosynthetic activity of the plants.

Abstract:

• Additional water supplied by dew formation is an important resource for microbes, plants and animals in precipitation-limited habitats, but has received little attention in tropical forests until now. • We evaluated the micro-environmental conditions of tree stem surfaces and their epiphytic organisms in a neotropical forest, and present evidence for a novel mechanism of diurnal dew formation on these surfaces until midday that has physiological implications for corticolous epiphytes such as lichens. • In the understorey of a lowland forest in French Guiana, heat storage of stems during the day and delayed radiative loss during the night decreased stem surface temperatures by 6?C in comparison to the dew-point temperature of ambient air. This measured phenomenon induced modelled totals of diurnal dew formation between 0.29 and 0.69 mm d )1 on the surface of the bark and the lichens until early afternoon. • Crustose lichens substantially benefit from this dew formation, because it prolongs photo- synthetic activity. This previously unrecognized mechanism of midday dew formation contrib- utes to the water supply of most corticolous organisms, and may be a general feature in forest habitats world-wide