The Spatio-Temporal Distribution of Fog in French Guiana as a Precondition for the Appearance of a new Vegetation Type - the Tropical Lowland Cloud Forest

Tropical cloud forests are characterized by high epiphyte diversity and abundance with a crucial driver for this being the frequent occurrence of fog that constitutes an additional water source for epiphytic vegetation. The ecological value of fog in these ecosystems is widely recognized and although tropical cloud forests in montane regions are known to be biodiverse and abundant in epiphytes, recent work indicates similar levels of both diversity and abundance in tropical lowland systems. An important question is whether fog plays a similar role in lowland systems as it does in montane areas, particularly since the physical mechanisms for fog formation in the tropical lowland are not well understood. The major aim of the present study was to provide an in-depth investigation of the fog phenomenon in French Guiana, including the spatio-temporal fog dynamics, and with this, to enhance the understanding of the impact of fog on epiphytic vegetation in tropical lowland forests. The main hypotheses suggest the frequent occurrence of radiation fog in valley forests throughout French Guiana, supported by nocturnal katabatic flows, and the formation of the epiphyte-rich “tropical lowland cloud forest” (LCF) as a new vegetation unit, which is restricted to areas frequently affected by fog. Testing these hypotheses required a specific experimental setup, an interdisciplinary approach and the development of a novel fog detection scheme. The first hypothesis concerning the frequent spatio-temporal occurrence of valley fog could be verified by the statistical assessments of the meteorological data derived in the field and the results of the satellite-based fog-detection scheme. The identified meteorological processes confirm the hypothesised mechanism of radiation fog formation and demonstrated that katabatic flows play a role. Therefore, nocturnal cold air drainage, typical in complex terrains of the midlatitudes, should also be regarded as a trigger of fog formation in the tropical lowland. The integrative assessment of canopy microclimate and epiphyte parameters verifies the second hypothesis. It was demonstrated that epiphyte diversity, abundance and biomass are significantly higher in valley forest as a consequence of a more favourable canopy microclimate due to frequent fog episodes. These findings provide strong evidence for the existence of the hitherto neglected tropical lowland cloud forest as a novel forest type. Thus, the results indicate that LCF and LRF should no longer be viewed as a single formation. Epiphyte diversity and distribution in LCF and LRF may represent useful characteristics for discriminating these forests. Further, the results indicate that the region of frequent occurrence of radiation fog in French Guiana can be regarded as optimum habitat for tropical lowland cloud forests. Overall, both hypotheses of this study can be confirmed. The investigations of the presented work have shown that fog formation is a regularly occurring phenomenon in space and time in French Guiana with significant impacts for epiphytic vegetation