Ecological Tolerances of Riparian Plants
The location of riparian plants on the floodplain depends largely on individual plant adaptations to variations in depth to the water table and soil texture. Many of the plants’ ecological tolerances can be explained by examining their location on the floodplain relative to the river channel.
Close to the river channel, plant roots have easier access to water, but the plants have to contend with increased flooding and physical battering from hydraulic forces during high flows. The root systems of plants that are frequently flooded are adapted to survive the low oxygen conditions created when roots are completely saturated. Typically, low floodplain species have flexible stems that can withstand the physical stress, and when branches do break off they can sprout into new trees if they become lodged into the river bank.
To view riparian plants, roll the cursor over any of the plant images below. You can also see the plant list here.
Soil particle sizes tend to be larger closer to the channel as well, because as the floods distribute sediments, larger particles (sand) fall out of suspension earlier, while finer sediments (silt) get carried farther out on the floodplain. This means that while plants closer to the channel may get flooded more often, the large soil particles allow water to drain through the soils quickly. Therefore, the plants in well drained soils must be able to either tolerate long periods of drought during dry seasons, or send their roots into the water table.
Farther away and higher than the main channel, there are many layers of soils that the river has deposited over time. These are fine textured, nutrient rich sediments that allow productive riparian forests to develop. The fine sediments can hold moisture longer, enabling many upper floodplain species to endure droughts. These fine soils on the upper floodplains are generally higher in organic matter and nutrient content, however they compact more easily, which causes them to be less aerated, and have lower amounts of oxygen. Plants located on the higher floodplains are typically not tolerant of long duration flood events. Due to the stratification of the many layers of sediment deposited over time, there may also be thin lenses of gravel, sand, silt or clay, and each of these can affect plant growth. These lenses in many cases explain why plants that should grow well based on the soil type at the surface are stunted or unhealthy. Plants at higher elevations to the water table must reach farther to access ground water or be able to tolerate periods of drought.
Along with the physical conditions (soils and hydrology) that affect individual plant locations on the floodplain, biological interactions among plants and with other wildlife can also determine the survival and growth of individual plants. For example, certain plants require open canopies where there is little competition for resources while others are competitive under a closed canopy. Additionally, wildlife can influence plant location through herbivory of seedlings and foliage. Both the physical and biological factors that affect plants are considered as the planting design is built. Once the suite of species that can be planted on a site are decided, River Partners biologists think about how different combinations of species will grow together over time.