Adapted Plant Traits in Riparian Zones

• Riparian plants have anatomical and morphological adaptations that help them persist in the dynamic and variable conditions of the ecotone.
• Plant adaptations in the riparian ecotones occur in response to periodic flooding, anoxia, low luminosity, and unstable substrate.
• The adaptations can be in the shoot, leaves, and root systems.
• Adaptations vary based on life forms, species, location, and environment.

Riparian vegetation is an ecotone that occurs as a transition zone between aquatic and upland ecosystems. The vegetation has many plant traits that have been adapted to help it survive the dynamic conditions in these zones. Learning how these adaptations work can help rehabilitate and manage riparian systems degraded by anthropogenic water and land use practices.

What are Riparian Ecotones?

Figure 1: “Functions of a Riparian Ecotone, Purohit  & Khan (2022). (Image credits: https://wotr.org/blog/importance-of-and-threats-to-riparian-ecotones/)

Riparian zones are ecotones transitioning between aquatic ecosystems and uplands with a steep environmental and vegetational gradient and have unique biophysical and biotic characteristics. Riparian zones are crucial buffers between aquatic and terrestrial ecosystems where materials and energy are exchanged.

The unique ecotone features make them productive, and riparian ecosystems have distinct ecological functions, see Figure 1. Some essential functions of riparian ecotones are:

• Stabilizing shores
• Filtering sediments, nutrients, and pollutants from uplands
• Improving water quality
• Regulating water temperature
• Providing habitat and food for wildlife
• Increasing groundwater recharge

These functions are possible due to the high diversity of plant forms and species in riparian ecotones- grasses, herbs, shrubs, and trees.

Riparian Vegetation

The unique environment in riparian zones also produces a steep gradient in vegetation, as seen in Figure 2. The plant communities are complex, variable, and distinct from those in adjacent uplands and aquatic ecosystems.

Differences in plant type, species, and abundance exist between upstream and downstream of the same river. In Germany, perennial reeds and forest and swamp species are common upstream due to the year-round presence of water. Downstream, meadows, grasslands, and reeds of flowing waters are present. In North America, common species can be herbaceous Kentucky bluegrass (Poa pratensis) and sedges (Carex spp.), shrubs like currant (Ribes spp.), and trees like willow (Salix spp.), aspen (Populus tremuloides), etc. Red alder and western red cedar were more common upstream than downstream reservoirs.

More significant differences can occur between riparian zones of different rivers in different eco-climates. For example, trees (aspen) were common in Neveda’s Deer Creek, shrubs (willows) in Wet Creek, and herbaceous plants in Summit Creek in Idaho.

Plant communities’ differences are due to variable environmental conditions in time and space.

Figure 2: Illustration of the typical Western U.S. “sandwich” of sagebrush upland-riparian zone-aquatic systems, Clary & Martin (1999). (Image credits: https://www.fs.usda.gov/rm/pubs/rmrs_p011/rmrs_p011_049_055.pdf)

Factors Shaping Riparian Vegetation

The vegetation is affected by many factors that create unique and dynamic conditions in riparian ecotones, the major ones being the fluvial processes, climate change, and anthropogenic activity.

Fluvial processes: Many processes, like flooding, soil deposition, erosion, and transportation, create growing conditions unique to these ecosystems. These processes are also responsible for environmental variability in time and space.

Climate change: Extreme events like an increase in the number and intensity of flash floods and a rise in water can be stressors for riparian vegetation. Several native species could be at risk of extinction.

Anthropogenic activity: Several human activities impact riparian vegetation. These are converting natural upland terrestrial areas for pastures and agriculture, deforestation, and introducing new species. Farming can increase sediment, nutrients, pesticides, and pollutant load. Pumping of sewage and industrial waste into water will also affect riparian zones. People also change fluvial processes; building dams or using flood control measures can change water flow and soil deposition,

All vegetation growing in riparian zones have anatomical, morphological, reproductive, and physiological adaptations to natural processes that help them persist in dynamic and variable conditions, see Figure 3. Knowing the mechanisms plants use to adapt to riparian conditions can advise species’ choice in rehabilitating degraded riparian areas due to the artificial processes.

Morphological and anatomical adaptations in riparian plants occur in the roots, shoots, and leaves. The major adaptations, which are a result of plant phenotypic plasticity, are discussed below.

Figure 3: “Main morphoanatomical adaptations in riparian vegetation,” Garcia & Jáuregui (2020). (Image credits: https://www.intechopen.com/chapters/74207)

Root Adaptations

In flooded and waterlogged soils, roots have less oxygen because of reduced gas exchange with the atmosphere. Hypoxia or inadequate oxygen supply in roots induces anaerobic respiration and higher use of stored carbohydrates. So, the root biomass is reduced and is reflected in decreased root length and diameter. Lack of oxygen also leads to root blackening and mortality. The root strategies of riparian plants are responses to these conditions.

Adventitious roots

Riparian plants form adventitious roots for quick recolonization to replace affected or dead ones, as in Polygonum spp., Tabebuia rosea, etc. These roots may or may not be nodal and help in water and nutrient absorption, transport, and gas exchange. The timing of adventitious root growth is a response to waterlogging and flooding and also depends on species, stage of plant development, duration and depth of flooding, and water temperature.

Buttress roots and stilts

Stilts and buttress roots are common in riparian plants growing on banks of flowing water like rivers and streams and in trees without deep roots. These structures improve the anchorage of plants on substrates with a thin soil layer and provide stability. Stilts and buttress roots are more common in sloping areas and depend on flood duration. Drypetes spp. and Aquilaria malaccensis have stilts and buttresses.

Pneumatophores

Pneumatophores are specialized roots that grow upwards due to negative geotropism to improve plant oxygen intake. These roots have spongy tissue and lenticels on the surface. Pneumatophores are common in Euterpe and Mauritia, palms are found in swampy areas and headwaters of rivers.

Cortex aerenchyma

The cortex is an outer tissue between the epidermis and endodermis, where gas and water are transported. More aerenchyma tissue, which has an even distribution of intercellular spaces, in the cortex increases porosity.  It helps with air movement, improves oxygen flow in roots, and prevents anaerobiosis. Porosity is an adaptation to long-term floods. In mesic areas, plants have a porosity of only 2-7% of their volume, whereas in riparian plants, it is up to 60%. Rumex palustris, a riparian plant, has more aerenchyma than R. acetosa, which is intolerant to flooding.

Cracks, fissures, and lenticels on root surfaces also increase oxygen diffusion.

Wider diameter

Hypoxic conditions increase root diameter because plants grow a thicker cortex. However, a reduction in the stela tissue can reduce root width, as seen in Genipa americana adapted to flooding.

Root adaptations can depend on growth stages and life forms. So, the adaptations seen in herbaceous species and tree seedlings may not be found in adult trees.

Shoot Adaptations

Life forms play different roles in riparian ecotones. Trees and shrubs stabilize the banks and block wind. In contrast, herbaceous plants protect and stabilize the soil and can be useful in rehabilitating degraded areas since they are fast-growing. Shoot adaptations occur in response to the threat of hypoxia and muddiness that screens light.

Stems

Amphibious plant species at the water edge would have modified stems, such as above-ground stolons and rhizomes, where the cortex has more aerenchyma. For example, common riparian grasses Paspalum distinum has rhizomes, and Cynodon dactylon has stolons. Among shrubs, Ficus squamosa in riparian Thailand has stolons. The aerenchyma helps in oxygen flow and to keep the plants afloat on water. Moreover, they help in vegetative propagation when broken into fragments.

Stem Nodulation

Stem nodulation in riparian legumes helps to fix nitrogen instead of the usual root nodules in mesic plants. Along with the nodules, the stem can have more parenchymal cells to aid in sufficient oxygen uptake. Stem nodules are common in the tropics and sub-tropics in river or lake margins and wetlands. Species with stem nodulation are Sesbania, Discolobium genera, etc.

Epicormic growth

Species avoid anoxia stress through epicormic growth to spread above water. Seedling shoots grow out of water for better internal aeration with longer internodes.

Lenticels, fissures, and cracks

Flood-tolerant riparian plant species have hypertrophic lenticels, which are enlarged openings on stems above the water level for oxygen entry and release of carbon dioxide, as in the swampy plant Nyssa biflora.

Fissures and cracks on young stems of trees and grasses appear due to the expansion of aerenchymatous tissue, exposing internal tissues to the atmosphere for better aeration in flooded conditions. For example, cracks are seen in seedlings of Myracrodruon urundeuva in flooded soils.

Wood characteristics

Some riparian species have wood characteristics that help them survive in the ecotones.

• They can be tension wood, allowing stems to bend or develop crooks. The tension wood has little or no lignification and an internal gelatinous layer made of so-called G-fibers that allows stem movement, as in Sebastiana commersoniana.
• Macules made of parenchyma tissue with thicker walls allow anaerobic survival, as in Eugenia inundata.

Besides stems, shoot adaptations include those in leaves.

Foliar Adaptations

Leaves are the most plastic and most significant organ in plants. They are exposed to the environment, and many leaf size, mass, and anatomy adaptations help in riparian regions.

Elongation of the petioles

Submerged petioles are elongated, so leaves occur above water with low luminosity to the surface with better light availability. For example, Rumex palustris grows long internodes to keep leaves out of water.

Leaf shedding

Many tree species shed their leaves in the initial days and months of flooding and submergence. The tree grows new leaves towards the end of flooding. This phenomenon is seen in Amazonian floodplains.

Xeromorphic leaves

The leaves are xeromorphic if trees are evergreen in riparian zones and occur completely submerged during waterlogging. The leaves have a thick cuticle, more sclerenchyma, and bifacial mesophyll. In submerged conditions, paradoxically, trees experience water deficit due to less water uptake because the stomata close or roots die in waterlogged conditions. Xeromorphic leaves help the submerged plants overcome water deficiency.

Specific leaf area

Specific leaf area (SLA) is the leaf area ratio to leaf dry mass. An essential functional trait influences a plant’s investment in leaf growth and photosynthetic rate. It is also related to leaf nitrogen content, longevity, and architecture. SLA is usually higher for wetland plants than for drier areas, as it promotes gas exchange. Gradients in SLA upstream and downstream can occur depending on longitudinal trends and disturbances. Higher SLA is found in disturbed areas and upstream.

Stomata density

Riparian species can have higher stomata density and indicate low water use efficiency as they are ecological pioneers and grow in areas with low fertility due to flooding, as seen in poplar species. However, many species with more stomata can have a thick cuticle to reduce transpiration.

Histological characteristics

Riparian species can have contrasting histological properties. Guazuma ulmifolia has more sclerenchyma and xylem with a narrow diameter than Sapium glandulosum on the midrib and petiole. More sclerenchyma and xylem increase water transport and are found in xeromorphic leaves.

The presence of these plant traits is indicative of the difference in riparian environments in which they occur and grow. So, the choice of species and adaptations must be guided by the project site’s environment and other native vegetation.

Application of Plant Traits

Most of the riparian regions of various scales are degraded. To protect and rehabilitate these crucial riparian adaptations, we can use our knowledge of riparian adaptations to help stakeholders choose species and lifeforms best suited for specific riparian areas.

Researchers studying these ecosystems can use several plant science tools to help them. CID Bio-Science Inc. produces several tools. Some that can be used for riparian research are as follows:

• Minirhizotrons for root imaging to estimate mortality, damage, width, length, etc.
• Leaf area meter for SLA calculations.
• Canopy imager for canopy cover and PAR estimations.

Contact us to learn more about CID Bio-Science Inc. devices for your research.

Sources

Clary, W.P. & Medin, D.E. (1999).  Riparian Zones—The Ultimate Ecotones? Retrieved from

https://www.fs.usda.gov/rm/pubs/rmrs_p011/rmrs_p011_049_055.pdf

García, M., & Jáuregui, D. (2021). Morphoanatomical Characteristics in Riparian Vegetation and Its Adaptative Value. IntechOpen. doi: 10.5772/intechopen.94933

Lamb, E. G., & Mallik, A. U. (2003). Plant species traits across a riparian‐zone/forest ecotone. Journal of vegetation Science, 14(6), 853-858. https://www.jstor.org/stable/3236982

Liang, S. C., Liu, R. H., Rong, C. Y., Chang, B., & Jiang, Y. (2019). Variation and correlation of plant functional traits in the riparian zone of the Lijiang River, Guilin, Southwest China. Chinese Journal of Plant Ecology, 43(1), 16.

Mallik, A. U., & Richardson, J. S. (2009). Riparian vegetation change in upstream and downstream reaches of three temperate rivers dammed for hydroelectric generation in British Columbia, Canada. Ecological Engineering, 35(5), 810-819.

Purohit, S, & Khan, I.Y.D. (2022, Dec). Riparian Ecotones and Why Conserving them is Absolutely Essential. Retrieved https://wotr.org/blog/importance-of-and-threats-to-riparian-ecotones/

Surmacz, B., Foremnik, K., & Pielech, R. (2024). Along the river: Longitudinal patterns of functional and taxonomic diversity of plants in riparian forests. Journal of Vegetation Science, 35(1), e13225.

Wollny, J. T., Otte, A., & Harvolk-Schöning, S. (2019). Riparian plant species composition alternates between species from standing and flowing water bodies–Results of field studies upstream and downstream of weirs along the German rivers Lahn and Fulda. Ecological Engineering, 139, 105576.