Relationship between understory plant diversity and environmental factors in Eucalyptus robusta and Acacia mangium forests

In 2021， the Hainan Tropical Rainforest National Park was established， within which the operations of certain plantation forests were discontinued， the natural succession was happened， and the understory vegetation is exceptionally abundant. In the report， 60 sample squares （10 m × 10 m） were designated in the Acacia mangium forest， while 53 sample squares of the same size were designated in the Eucalyptus robusta forest. The findings are as follows： （1） The A. mangium forest harbors a total of 109 families， 312 genera， and 530 species of vascular plants in the understory， whereas the eucalyptus forest encompasses 96 families， 302 genera， and 445 species. （2） The canonical correspondence analysis （CCA） was performed， and the eigenvalues of Axis 1 and Axis 2 were determined to be 0.4954 and 0.3954， respectively， accounting for 58% of the total eigenvalues （1.5341）， which elucidate the relationship between the understory plant communities and environmental factors. When annual precipitation， mean annual temperature， elevation， mean daily temperature variation were used as individual explanatory variable， the depression exhibited significant effects （P<0.05）. （3） As the elevation increased， both the Shannon-Wiener and Simpson indices initially rose and then declined. Similarly， the Shannon-Wiener index， species richness， Simpson index， and species richness showed an initial increase followed by a decrease with increasing depression. With the increase of average annual temperature， the Shannon-Wiener index， Simpson index， and species richness all exhibited an initial increase followed by a decrease. The Shannon-Wiener index， Simpson index， and species richness decreased with increasing annual precipitation.