Root Rivals, Shoot Allies: Soil Moisture Rewrites the Rules of Plant Neighborhoods

In the tropical forests of Xishuangbanna in southwestern China, where more than 80 percent of annual rainfall arrives in a single wet season and the dry months bring little more than fog, the identity of the plant standing next to you can mean the difference between thriving and barely surviving. A new controlled glasshouse experiment has revealed that four native tropical legume species respond to their neighbors in strikingly different ways depending on how wet or dry the soil is, shifting their entire resource-acquisition strategy between roots and shoots in a pattern that challenges simple assumptions about competition in plant communities.

The study, conducted at the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences, grew four legume species—Crotalaria pallida var. obovata, Crotalaria assamica, Flemingia macrophylla, and Ototropis multiflora—under three soil moisture levels representing the natural range found in the region’s forests between dry and wet seasons. Each species was grown alone, with four individuals of the same species, or in mixed-species pots containing two individuals each of two different species. After 30 weeks of growth, the researchers measured an extensive suite of functional traits, from specific leaf area and stem length to specific root length, average root diameter, root-to-shoot ratio, and nodule production, alongside antioxidant enzyme activities and oxidative stress indicators.

The results reveal a fundamental asymmetry in how plants respond to neighbors of their own kind versus neighbors of different species, and this asymmetry flips depending on water availability. Under low soil moisture, plants grown with conspecific neighbors developed thicker roots with larger average root diameters and higher specific root length than plants grown with heterospecific neighbors or alone. Root-to-shoot ratios were also elevated under intraspecific competition in dry conditions, indicating that plants facing members of their own species invested disproportionately in belowground structures. This pattern is consistent with theoretical predictions from game theory, which suggest that when individuals share identical resource requirements, they may overproliferate roots in a scramble to preempt shared soil water—a phenomenon sometimes described as a belowground tragedy of the commons.

In contrast, interspecific interactions told a very different story. Plants grown with heterospecific neighbors showed greater leaf area, higher average leaf mass, and increased aboveground and total biomass across all soil moisture levels. Under medium and high moisture conditions, the biomass advantages of growing alongside a different species were particularly pronounced. Flemingia macrophylla, for instance, showed biomass increases of 330.6 percent under interspecific competition at medium moisture and 286.5 percent under high moisture, compared with much smaller or even negative responses under intraspecific competition. Crotalaria pallida, by contrast, showed a 50.4 percent biomass reduction under intraspecific competition at medium moisture, highlighting how species-specific growth strategies modulate the outcome of neighbor interactions.

The researchers quantified these interaction outcomes using the relative interaction index, a metric ranging from negative one to positive one, where negative values indicate competition and positive values indicate facilitation. Belowground biomass RII values were more negative under intraspecific interactions, particularly for Flemingia macrophylla under low soil moisture, confirming that conspecific neighbors imposed stronger competitive effects on root systems. Aboveground biomass RII values, however, were generally positive under interspecific competition across species and moisture levels, with the strongest positive responses observed in Crotalaria assamica, Flemingia macrophylla, and Ototropis multiflora. This dissociation between belowground and aboveground interaction outcomes suggests that the same pair of plants can simultaneously compete belowground and facilitate each other aboveground, depending on their relative trait differences and the prevailing moisture regime.

Physiological measurements added another layer to the story. Under low soil moisture, catalase activity and malondialdehyde content—a marker of lipid damage from oxidative stress—were significantly higher under intraspecific competition than under interspecific or no-competition treatments, indicating that conspecific neighbors in dry conditions generated greater cellular stress. Conversely, peroxidase and superoxide dismutase activities were higher under interspecific competition, with the greatest differences observed under low moisture. These divergent antioxidant responses suggest that the two types of neighbor interactions impose qualitatively different physiological burdens, with conspecific competition under drought producing more oxidative damage while heterospecific associations may trigger a different protective enzyme profile.

Principal component analysis of fifteen functional traits revealed that the first two principal components together explained 39.0 percent of total trait variation. The first axis was dominated by biomass and growth-related traits, including root-to-shoot ratio, aboveground biomass, leaf area, nodule dry mass, and specific stem length, while the second axis was dominated by leaf and physiological traits, with specific leaf area contributing nearly 24 percent of the variation. Plants under high moisture clustered toward the positive side of the first axis, while those under low moisture occupied the negative side, confirming that soil moisture was a primary driver of multivariate trait coordination. The loading patterns also showed that growth-related traits loaded positively on the first axis while root diameter and root-to-shoot ratio loaded negatively, reflecting the fundamental trade-off between rapid growth and conservative resource retention.

Nodulation patterns provided further insight into how legumes adjust their symbiotic nitrogen acquisition under changing conditions. Nodule number increased with soil moisture, with plants under intraspecific competition producing more nodules under high moisture. Nodule dry mass was primarily regulated by moisture availability, with greater investment under wet conditions and reduced investment under drought. These plastic adjustments in legume-rhizobia symbiosis demonstrate that even the belowground mutualisms that define legume ecology are sensitive to the combination of water availability and competitive context, adding a further dimension to the resource-allocation decisions plants must make.

The findings support the functional equilibrium hypothesis, which predicts that plants preferentially allocate biomass toward the organs responsible for acquiring the most limiting resources. Under water limitation and conspecific competition, the limiting resource is soil water, and plants responded by investing in roots. Under favorable moisture and heterospecific neighbors, competition for belowground resources is relaxed, and plants shift carbon allocation toward shoots and leaves to capture light. However, the study’s authors caution that these results come from a controlled glasshouse experiment using pots, young plants, simplified sandy soil, and a relatively short observation period, which may limit direct extrapolation to mature tropical forest communities where soil microbial communities, root architecture, and multi-species interactions are far more complex.

Nevertheless, the implications for understanding tropical ecosystem dynamics under climate change are significant. As precipitation patterns become increasingly variable, the balance between competition and facilitation in plant communities may shift in ways that depend not only on how dry or wet the soil becomes but also on which species happen to be growing side by side. The study demonstrates that plant-plant interactions are not fixed as either competitive or facilitative but are contingent outcomes shaped by neighbor identity, organ-specific resource overlap, and environmental context. For restoration practitioners designing mixed-species plantings in seasonally dry tropical regions, the message is clear: the choice of which species to combine, and expectations about how those combinations will perform as rainfall patterns shift, should account for the divergent root and shoot strategies that each species brings to the neighborhood.

Subject of Research: How soil moisture and neighbor identity shape root and shoot trait strategies and competition-facilitation outcomes in tropical legume species

Article Title: Divergent Root and Shoot Trait Strategies Shape Competition–Facilitation Shifts Between Conspecific and Heterospecific Neighbors Under Contrasting Soil Moisture Conditions

Article References: Sher, J., Li, Y., Zhang, Y.-B., Han, M., Shah, G. M., & Zhang, J.-L. (2026). Divergent Root and Shoot Trait Strategies Shape Competition–Facilitation Shifts Between Conspecific and Heterospecific Neighbors Under Contrasting Soil Moisture Conditions. Ecology and Evolution, 16(10), Article e74586. https://doi.org/10.1002/ece3.74586

Image Credits: AI Generated

DOI: 10.1002/ece3.74586

Keywords: plant competition, soil moisture, functional traits, tropical legumes, root economics, facilitation, biomass allocation, Xishuangbanna, intraspecific competition, interspecific competition, stress gradient hypothesis, trait plasticity