The role of competition, light and soil microbes on tropical tree seedling growth

Forests are dynamic ecosystems where light availability governs what and how species grow in the understorey. The light intensity can be easily manipulated in greenhouses or plant growth chambers but the role of soil microbes in seedling establishment and competition with other plants are much less understood. 

Dr Nianxun Xi from Sun Yat-sen University (China) and colleagues from China and France investigated the effect and interactions between abiotic (e.g. light availability) and biotic (e.g. competition, soil microbe composition) conditions on two species of Bauhinia tree species seedling growth. The scientists found that high light availability increases the influence of conspecific soil microbes on plant-plant interactions which helps to understand fundamentally how tree seedlings might establish under forest gaps. Intraspecific competition and “no competition” positively correlated with Gram-positive:Gram-negative bacteria ratio under low light but the ratio negatively correlated under high light conditions.

Hundreds of tree, shrub, herb and liana species belong to the genus Bauhinia. The two tree species,  B. brachycarpa and B. variegata, are good model species to compare fast-growing, shade-intolerant to slower-growing, shade-tolerant seedling growth. 

Xi and colleagues collected seeds and the topsoil around Bauhinia brachycarpa and B. variegata trees in a woodland in the Yuanjiang Valley in 2016. The researchers germinated the seeds in the greenhouses of the Xishuangbanna Tropical Botanical Garden. The research project consisted of a “conditioning” and a “response phase”. In the “conditioning phase”, 14 seedlings grew in individual pots for 9 months which allowed the soil microbes to build up specific to the two tree species. 

The “response phase” of the experiments consisted of a multifactorial block design of 144 pots for 12 weeks. Seedlings grew under low and high light conditions, on sterilised or “live doil” form the same species or from the other. Some seedlings experienced no competition (e.g. single plants grew in individual pots), or intra- and inter-species competition (e.g.one tree seedling grew along with another seedling in an individual pot). The scientists harvested all plants in August 2017 and based on plant biomass measurements, plant-soil feedback (PSF) index, competition intensity (e.g. competition irrespective of environment) and competition importance (e.g. impact of competition relative to environmental factors) were calculated for each species under different conditions. Phospholipid fatty acid analysis estimated fungal, Gram-positive and Gram-negative bacterial biomass in the soil from each pot. 

“Here we show that light availability mediates the balance between conspecific soil feedbacks and plant-plant interactions on seedling biomass during tree seedling establishment, and demonstrate the importance of both soil community structure and plant neighbour identity for PSF outcomes,” Xi and colleagues said. 

The seedling growth, biomass of microbial groups and plant-soil feedback (PSF) mainly differed due to light availability. The presence of competitors only led to different PSF values under high light availability suggesting that competition is less important in low-productivity (e.g. low light) environments. Intraspecific competition and “no competition” positively correlated with Gram-positive:Gram-negative bacteria ratio under low light but the ratio negatively correlated under high light conditions. Competition importance was stronger in B. variegata under intraspecific competition whilst under interspecific competition, competition intensity was stronger in B. brachycarpa. The presence of conspecific (e.g. soil originated from the same tree species from the “conditioning phase”) soil microbes decreased plant competition intensity in low light but increased plant competition intensity in high light. 

“We provide evidence that interspecific competition can alter the direction of plant-soil feedbacks under high light, with potential implications for seedling establishment in forest gaps and disturbed environments,” Xi and colleagues said. 

In an accompanying publication in the Environmental and Experimental Botany journal, the researchers investigated the plant morphology and nutrient levels from the same experiments. They found that the leaf nitrogen:phosphorus content decreased under high light conditions on the “live soil” and the root and lower leaf phosphorus content greatly relied on soil microbes. Xi and colleagues have begun to detangle the dynamic processes of tropical tree seedling growth and impact on nutrient cycling within forests. Understanding these patterns help scientists and foresters better understand how forest gaps and disturbances might impact forest communities.