Every winter, floodwater covers this riverbank by about thirty metres. When the water retreats, the plants are not simply recovering: their leaves, stems, and roots are carrying different chemical priorities. Along China’s Yangtze River, a recently established riverbank ecosystem is covered by about thirty metres of water each winter, then exposed during the growing season.

That makes it a natural setting for asking how flooding changes plants. This matters beyond one river. River dams are becoming more common, and unpredictable rain is expected to increase, so riverbank plants may face changing depths and durations of flooding.

The central question is how three basic building materials of plants—carbon, nitrogen, and phosphorus—are divided among leaves, stems, and roots, and how that division differs between short-lived and longer-lived plants. It is like managing a household budget: the same household has money, food, and time, but spends them differently in the kitchen, bedroom, and workplace.

Plants likewise distribute these materials among different parts for different jobs. The study examined short-lived and longer-lived plants, looking separately at leaves, stems, and roots in the Three Gorges Reservoir riverbank.

It also examined flooding, plant communities, and soil conditions as possible influences. Compared with grasses and forests, these riverbank plants had higher nitrogen and phosphorus concentrations but lower carbon concentrations, indicating nitrogen-limited growth.

The plants also showed weaker carbon sequestration and lower nutrient-use efficiency, revealing the ecological consequences of that nitrogen-limited growth in the riverbank habitat. The plant parts did not share the same chemical balance. Leaves had higher amounts of nitrogen and phosphorus than stems and roots, and short-lived plants had higher amounts than longer-lived plants.

The plant’s life form and the job of each organ mattered: annual plants and leaves had higher nitrogen and phosphorus concentrations than perennials, stems, and roots. Flooding did not produce one uniform response. As elevation increased and flooding became less intense, carbon rose in many parts of short-lived plants and in the leaves of longer-lived plants, while the stems and roots of longer-lived plants changed little.

Nitrogen stayed fairly steady in most parts, except that it rose in the leaves of longer-lived plants. Phosphorus, meanwhile, decreased in many parts of short-lived plants. Leaves concentrate more nitrogen and phosphorus than stems and roots, supporting faster growth and photosynthesis.

This nutrient priority changes with plant lifespan: short-lived plants contain more of these nutrients, while long-lived plants retain more carbon in their stems. The strongest influences came from soil rather than surrounding plant communities, which were less influential in shaping variation in plant chemical balance.

Different plant parts responded to different soil features, including water content, soil density, acidity, and the amounts of carbon, nitrogen, and phosphorus in the soil. For longer-lived plants, soil water, soil density, elevation, and surrounding plant cover were important influences on leaf chemistry and nutrient balance.

For stems and roots, the important influences differed: water content, elevation, and community height mattered for stems, while acidity, plant cover, elevation, and water content mattered for roots. The results point to an indirect chain: flooding changes the soil and the plant community, and those changes then alter the chemical balance inside plant parts.

Flooding does not simply act directly on every plant in the same way. Hydrological changes can affect more than visible plant cover: they can alter soil nutrient dynamics, plant growth, and the stoichiometric properties of plant parts. The key lesson is that soil, more than the surrounding plant community, helps set these chemical priorities.

That means changing water levels can reshape riverbank growth by changing the ground beneath the plants.