| Čoahkkáigeassu: | Background and Aims: The Sierra Madre Oriental hosts a high floristic diversity, but its northernmost portion remains poorly explored. There is a lack of studies documenting the richness of the family Orchidaceae in this region. The aim of this work was to contribute to the knowledge of orchid diversity in the north of the Sierra Madre Oriental in Tamaulipas.
Methods: The orchid inventory was conducted through field work in the locations Conrado Castillo, Hidalgo municipality, and Los San Pedros, Güémez municipality. Each recorded genus was assigned its phytogeographic affinity. The orchid richness of Conrado Castillo and Los San Pedros was compared with 14 other localities in the country located along the Gulf of Mexico slope and southeastern Texas, using the Sørensen index, to construct an areas dendrogram with the UPGMA method.
Key results: In the study area, 35 species were recorded, with 29 in Conrado Castillo and 18 in Los San Pedros; 11 taxa were present in both localities. Bletia jucunda, Corallorhiza striata, Corallorhiza wisteriana, Deiregyne densiflora, Goodyera brachyceras, Goodyera oblongifolia, Malaxis abieticola, Malaxis brachyrrhyncha, Schiedeella transversalis, Sotoa confusa, and Tamayorkis hintonii are new records for the flora of Tamaulipas. Eighty-six percent of the recorded species are terrestrial, with the Pinus-Quercus forest showing the highest orchid richness (80%). The most represented floristic affinity was the southern (82% of genera), followed by Asian (14%) and boreal (4%) affinities. The orchid flora of the studied area is similar to that found in localities of Nuevo León and Texas.
Conclusions: Although the study area is located in the boreal region of Mexico, it houses a tropical orchid flora with tropical affinity. The 11 new records reported here increase the Orchidaceae richness for Tamaulipas to 102 species.
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