In the context of global climate change, Southeast Asian tropical rainforests face multiple stressors, including reduced precipitation, altered population density, and shifts in soil microbial communities. However, systematic experimental evidence on how these factors interact to affect tree regeneration is scarce.

In a new study published in Journal of Ecology on July 13, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators revealed the growth and functional traits of Parashorea chinensis, a keystone dipterocarp tree in Southeast Asian rainforests, are shaped by the interplay of drought, neighborhood density, and beneficial soil fungi.

To investigate these interactions, the researchers conducted a year-long greenhouse experiment in Yunnan, China. The researchers manipulated three variables: water availability (wet versus dry conditions), seedling density (one, two, or eight seedlings per pot), and soil fungal communities (with or without fungicide to disrupt ectomycorrhizal associations). They then monitored seedling growth and measured 11 functional traits, including leaf area, specific leaf area, root length, and stem density.

The results showed that seedling growth is collectively regulated by water availability, seedling density, and mycorrhizal fungi. Although water availability was the primary driver, its impact was substantially amplified by seedling density. The combined effect of low water and high density was multiplicative.

The researchers also found that an increase in drought frequency could disrupt the positive feedback loop between Parashorea chinensis and soil fungi, which could undermine the species' competitive dominance. Additionally, high density intensifies intraspecific resource competition. Future reductions in large vertebrates and pest-disease accumulation will increase the complexity of population dynamics.

The researchers further found that increasing drought frequency may disrupt positive feedback between Parashorea chinensis and soil fungi, potentially undermining the species' competitive dominance. Additionally, high density intensifies intraspecific resource competition, while future reductions in large vertebrates (altered herbivory patterns) and pest-disease accumulation will increase complexity in population dynamics.

The study emphasizes that drought, changes in neighborhood density, and alterations to soil fungal communities occur simultaneously in natural forests. This makes single-driver studies insufficient for understanding how forests respond to global change.

"Our results reveal that trait-growth relationships shift across water availability, neighborhood density, and soil biotas," said YANG Jie of XTBG. "Ignoring these context dependencies could lead to misleading predictions of how tropical forests will respond to future droughts and defaunation."