More than 50 species were studied in the humid part of the eastern Chaco with their special peculiarities and differences in dendrological aspects as well as in bark and wood structure so as to compare them with the other Chaco-forest types. Trees in the humid Chaco are higher as compared with those of the semi-arid Chaco. The percentage of trees belonging to the lowest height category is higher in the semi-arid Chaco than in the humid Chaco. While only one species of the semi-arid Chaco belongs to height category 2 (20 to 30 m), 12,5% of the trees in the humid forest reach even a height of over 30 m (cat 1). Species with the thickest stems and a DBH above 70 cm reach the highest percentage in the humid forest, while the DBH of the species in the semi-arid region is low and only Chorisia insignis reaches more than 70 cm. Macroscopic bark structure and microscopic bark and wood structures are described and compared in detail. A very large region in Argentinia, Brazil, Paraguay and Bolivia and several forest types were studied and compared. In a future publication the authors will deal with the mountains Chaco forests to make an exhaustive comparison between the three types of forests, their dendrological and structural peculiarities, their important species with their requirements and their useful products.