Epiphytic orchids in tropical dry forests of Yucatan,Mexico – Species occurrence,abundance and correlations with host tree characteristics and environmental conditions

Tropical dry forests have been less studied in terms of their resident epiphyte flora compared to wet forests. We studied five species of epiphytic orchids in two dry forest fragments differing in tree composition, stature and rainfall regime. We compared the vertical distribution within the host tree, epiphyte-host associations and seasonal variation in microclimatic conditions in a tropical dry deciduous (Celestún) and a semi-deciduous forest (Kaxil-Kiuic) of the Yucatan Peninsula, Mexico, during the wet, early dry and dry seasons. Light, vapor pressure deficit, air temperature, and dew were measured on two heights (1.5 and 3.5 m) of the host with the highest abundance of orchids. Surprisingly, orchid abundance was higher in the Celestún deciduous forest, the site with low precipitation. High epiphyte abundance in the middle canopy stratum of the hosts in both forests was arguably related to a favorable combination of micro-environmental factors. In both forests, about 90% of all orchids grew on a single host tree species. Although bark roughness and the area of the substrate were the most important host characteristics that influenced the abundance of orchids in the Celestún deciduous forest, this did not explain this preference. Climatic variation was greater among seasons than between microenvironments in the host trees. The most abundant species, Encyclia nematocaulon, had a great capacity to occupy different strata in both forests, even in tree tops with very large micro-environmental fluctuations during the year.