Roots, Litter, and Seasonal Drought Together Inhibit Plant Growth in the Herbaceous Layer in a Subtropical Moist Forest of Southwestern China

The mechanisms of the maintenance of plant diversity in forests have been extensively studied because of their ecological importance. The study of the regeneration and growth dynamics of herbaceous understory communities in forests is relatively more common than that of woody plant overstory and understory communities. To investigate which environmental factors (plant roots, forest litter, or both) control seedling survival, growth, and production in the herbaceous layer in the context of increasingly severe seasonal drought caused by global climate change, we performed a seedling growth experiment of the herbaceous layer influenced by the interaction of plant roots and forest litter through a manipulative complete block experiment, crossed with an irrigation experiment, in a montane subtropical moist evergreen broad-leaved forest of southwestern China. Within both the control and watered plots, we established four experimental subplots with plant roots and forest litter included (R+L+); plant roots included, but forest litter excluded (R+L-); plant roots excluded, but forest litter included (R-L+); and both plant roots and forest litter excluded (R-L-). After one year, the R+L+ treatment in the control plot had statistically less species richness and plant individuals, shorter mean and maximum seedling heights, and less dry biomass of plant seedlings than those in the other seven experimental treatments. Across all the experimental replicates, the pooled data showed that plant roots, forest litter, and seasonal drought, together, inhibited all the dependent herbaceous growth variables. Our study demonstrates how plant roots, forest litter, and seasonal drought synergistically regulate seedling establishment and the growth of the herbaceous layer in tropical and subtropical forest understory. This synergistic regulation changes plant physiological responses and forest evolution through controlling plant diversity and the individual richness of the herbaceous layer in the context of sustained global climate change.