Layer structure and species composition of primeval habitat forest communities that include the tallest tree in karst areas of China

Parashorea chinensis is a symbolic tree species in tropical seasonal rain forests in China. Its primeval habitat forest communities show clear adaptations to a tropical seasonal environment, and have the greatest diversity of tree species and the highest proportion of tropical flora, in China. Recently, a P. chinensis with a height of 72.4 m was discovered in the primeval forest of tropical karst peak cluster depression in Nonggang, Guangxi. This tree broke the record for the tallest tree in China's karst areas. Here we examine the properties of the forests that can support such remarkable trees, in order to update our understanding of the vegetation characteristics of tropical karst forests. We draw on four primeval forest community survey plots of P. chinensis in Nonggang, in order to summarize the characteristics of layer structure, species composition, plant areal types, leaf features, ecological habits, and dominant genera of these forest communities. The characteristics of the region around the tall tree were compared with those of other communities of P. chinensis in China. The results were as follows: (1) Giant trees exceeding 50 m in height are scattered across the canopy of the tallest tree's community. The continuous arbor and shrub layer can be divided into four layers, approximately 36.9 m, 25.6 m, 9.5 m, and 2.7 m high. There are 64 tree species, 24 shrub species, 31 vine species, 26 grass species, and 8 epiphyte species within the 2400 square meter plots. (2) The arbor layer is dominated by tropical flora plants such as P. chinensis and Dracontomelon duperreanum, karst endemic plants such as Excentrodendron tonkinense and Cephalomappa sinensis, and deciduous trees such as Garuga forrestii and Allospondias lakonensis. (3) Species of tropical flora account for 93.7% of the trees in the arbor layer, with species from tropical Asia (Indo-Malay) accounting for 65.6%. Mega-phanerophytes account for 15.6%; evergreen plants account for 85.9%; compound leaf plants account for 39.1%; leathery leaf plants account for 60.9%; and typical karst plants account for 28.1%. (4) The greatest similarity to the dominant tree forest is shown by Napo County in Guangxi. Maguan and Mengla Counties in Yunnan, Bama, Tianyang, and Dahua Counties in Guangxi are progressively less similar. These relationships indicate that the influence of tropical climate on community forest structure is greater than that of their geology. In summary, the primeval forest communities of P. chinensis in Nonggang thrive in the closed habitat environment of the tropical karst peaks cluster depressions, benefiting from the relatively humid and warm local climate. Characterized by the presence of iconic tropical giant trees such as P. chinensis and Dracontomelon duperreanum, these forests exhibit the intricate forest layers of a tropical seasonal rain forest. Due to the combined influence of the tropical northern margin's geographical location and karst landform, these communities' forest canopies contain a certain proportion of deciduous tree species, with relatively low proportions of mega-phanerophytes and epiphytic plants. The communities are rich in drought-resistant plants with features such as leathery leaves, microphyll leaves, compound leaves, and typical karst plants.