Browsing by Author "Obregon, Andre"

Fog frequency and the meteorological processes leading to fog formation have never been studied in depth in tropical lowland forest areas. This study provides detailed evidence of frequent fog occurrence in lowland valleys of central French Guiana. Fog frequency showed a clear diurnal course, with a maximum before sunrise; average fog duration was 4.6 h. The diurnal course of visibility was positively correlated with the diurnal course of humidity in the above-canopy air. Fog persistence correlated significantly with atmospheric parameters during the dry season, but not during the rainy season. The main trigger of fog development in the lowland forest seemed to be precipitation, leading to higher soil moisture, greater evapotranspiration and, thus, higher water content of air. An increasing temperature difference between valley and hill sites after sunset, together with more frequent down-slope winds during nights with long fog periods, points at some influence of katabatic flows. The frequent occurrence of fog in the valleys correlated with significantly higher epiphyte diversities in valley forests as compared to hill forests, and supported the occurrence of the hitherto undescribed, epiphyte-rich "tropical lowland cloud forest" (LCF) in the valleys. The higher epiphyte diversity in LCF coincided with significantly higher relative air humidity in LCF than in hill forest. The ecological benefits of fog for the epiphytes in LCF are surplus of moisture and delayed onset of the stress period, particularly in the dry season. (C) 2010 Elsevier B.V. All rights reserved.

Recent evidence is emerging that tropical cloud forests are not restricted to mountains, but may also be found in tropical lowland areas, the so-called 'tropical lowland cloud forest' (LCF). LCF occurs in river valleys with high air humidity and morning fog, and is rich in epiphytes. We explored the diversity of bark-inhabiting liverworts in LCF, a group of organisms known to be sensitive indicators of humidity. To test the hypothesis that LCF differs in species richness and species composition from lowland rain forest without fog (LRF), we sampled liverwort diversity in LCF and LRF on 48 whole trees in two sites in French Guiana. Sampling efficiency (=no. of species found as percentage of estimated total number of species) was about 90% in both forest types. Species richness in LCF was significantly higher than in LRF and species composition differed in all height zones; moreover, LCF had three times more indicator species. Indicators of LCF included shade epiphytes and generalists that occurred also in montane forests, those of LRF were sun epiphytes characteristic of rather dry, open sites. The detected differences in liverwort diversity of LCF and LRF are explained by the more humid conditions in LCF as compared with LRF. A comparison of liverwort richness along elevational gradients in the Neotropics showed that species richness differs more strongly among forest type (rain forest, cloud forest) than among elevation and that lowland cloud forests may be richer in species than montane rain forests. The data indicate that elevational comparisons of bryophyte species diversity in the Tropics should make a distinction between rain forests and cloud forests.

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" (La:). LCF occurs in river valleys with high air humidity and radiation fog, and is rich in epiphytes. We explored the lichenological characteristics of putative [CF in a lowland area (200-400 m a.s.I.) near Said, 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 [CF and in lowland rain forest without fog ([RE). 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 LEE 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. (C) 2010 Elsevier Ltd. All rights reserved.

Recent studies on bryophyte and macrolichen diversity in lowland forests of northern South America have shown the existence of a novel forest type, the "tropical lowland cloud forest" (LCF). LCF is very rich in epiphytes and occurs in lowland river valleys where radiation fog in the morning provides an additional input of liquid water. Because of their dependence on frequent precipitation and low evaporation, Hymenophyllaceae (filmy ferns) are a suitable group for studying moisture availability. We sampled epiphytic Hymenophyllaceae on 32 trees in French Guiana, 16 in LCF and 16 in adjacent rain forest (LRF). Abundance of Hymenophyllaceae was significantly higher in LCF than in LRF. Only 10% of trees in LRF were inhabited by filmy ferns, in contrast to 70% in LCF. Moreover, the number of species recorded in LCF (9) was more than twice as high than in LRF (4), and the mean number per tree 8 times higher. Species restricted to the understory of LRF occurred in the canopy of LCF. We attribute the detected differences in diversity and vertical distribution of Hymenophyllaceae in the two forest types to the occurrence of fog in LCF, enhancing the availability of liquid water and thus Facilitating the establishment and growth of the filmy ferns. Also, radiation protection against evaporative loss seems to play a crucial role in the vertical distribution of filmy fern diversity. The observed differences in filmy fern diversity and distribution in LCF and LRF represent novel traits separating the two forest types, and indicate that Hymenophyllaceae species are sensitive indicators of lowland cloud forest.

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 Saul, 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/m(2), and 35 g/m(2) 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.

A land-cover classification is needed to deduce surface boundary conditions for a soil-vegetation-atmosphere transfer (SVAT) scheme that is operated by a geoecological research unit working in the Andes of southern Ecuador. Landsat Enhanced Thematic Mapper Plus (ETM+) data are used to classify distinct vegetation types in the tropical mountain forest. Besides a hard classification, a soft classification technique is applied. Dempster-Shafer evidence theory is used to analyse the quality of the spectral training sites and a modified linear spectral unmixing technique is selected to produce abundancies of the spectral endmembers. The hard classification provides very good results, with a Kappa value of 0.86. The Dempster-Shafer ambiguity underlines the good quality of the training sites and the probability guided spectral unmixing is chosen for the determination of plant functional types for the land model. A similar model run with a spatial distribution of land cover from both the hard and the soft classification processes clearly points to more realistic model results by using the land surface based on the probability guided spectral unmixing technique.

Fog is a crucial driver of epiphyte richness in tropical montane cloud forests but its spatial occurrence and role in tropical lowland areas is poorly understood. Recent studies in French Guiana have reported high epiphyte richness in previously undescribed "tropical lowland cloud forest" (LCF) due to radiation fog. Here, we analyze the spatial extent of fog/low stratus (FLS) in lowland forests of French Guiana using the frequency distribution by means of night-time MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data for the years 2007-2010. The analysis is based on a newly developed dynamic threshold-test method relying on brightness temperature differences between thermal and mid-infrared bands. Individual thresholds for the discrimination between fog/low stratus and cloud-free pixels were retrieved by radiative transfer calculations and validated using discriminant analysis. The thresholds dynamically depend on total precipitable water (TPW) and the terrain-induced maximum possible sub-pixel fog coverage. The results of the new retrieval were validated using in-situ data and compared to results from existing fog detection algorithms, showing an improvement of the new detection scheme regarding the capability to detect sub-pixel fog coverage under varying TPW. FLS frequency maps derived from the novel fog classification scheme indicate a widespread distribution of night-time fog in river valleys, marking a multitude of potential areas for LCF throughout French Guiana. LCF is probably not only a local phenomenon but also may be widely distributed in river valleys in the lowland tropics, with significant consequences for biodiversity mapping in tropical lowland areas. (C) 2014 Elsevier Inc. All rights reserved.