How Effective is Plant Cover for Soil Erosion Control?

• Plant cover is the most cost-effective and efficient way to control soil erosion in large areas affected by deforestation, mining, etc.
• Soil erosion measures can also be incorporated in farmlands to prevent topsoil loss.
• Plant canopy parameters like extent, height, and architecture will determine the efficiency of soil erosion control.

Soil erosion is a major global environmental problem in terrestrial ecosystems that can impact land productivity, nutrient and carbon cycling, and socio-economic conditions.

Establishing plant cover is one of the easiest and most effective ways of controlling soil erosion and regenerating lost soil properties and functions. However, not all canopy covers are equally helpful. Find how the canopy cover’s function and benefits can vary.

Figure 1: “Flow diagram of land use changes and their effects on the soil loss estimates. The circular forms refer to three major land use and land cover groups. The arrows indicate the amount (million km2) of global land use and land cover change between 2001 and 2012. Insets a–c demonstrate the net change of the land surface (million km2) and soil loss (Pg yr−1),” Borrelli et al. 2017. (Image credits: https://doi.org/10.1038/s41467-017-02142-7)

Why Soil Erosion Control is Needed

Soil erosion occurs due to various human activties and landuse changes such as deforestation, mining, conversion to croplands, climate change, urban settlements etc, see Figure 1.

Water is the main factor causing soil erosion. Along with the topsoil, nutrients, organic matter, and biota are lost, reducing soil fertility and the ecosystem’s ability to support plants and animals. Soil erosion will also affect rainfall drainage, infiltration, and storage, leading to water scarcity or waterlogging.

The FAO estimates that soil erosion will reduce crop yields by 50%. Tropical regions like India and Central-South Africa will be worst affected, followed by Indonesia and China. Even temperate regions like Europe will have less productivity and economic gains.

Soil erosion degrades ecosystem functions like nutrient and carbon cycling and increases hydrogeological risks like landslides and floods, urban infrastructure damages, human population displacements, and severe biodiversity losses.

Rainwater directly reaches the soil by falling on the soil or indirectly over vegetative covers, such as leaf drainage or stem flow (see Figure 2).

Figure 2: “The role of trees in the rainfall interception process,” EPA. (Image credits: https://www.epa.gov/soakuptherain/soak-rain-trees-help-reduce-runoff)

Effect of Plant Cover

Plant cover prevents erosion by absorbing the raindrops’ kinetic energy. Ground and canopy cover act as a cushion against the rainfall’s energy. A stable and year-round cover will slow water runoff, taking soil away. Without plant cover, raindrops hit the bare ground, dislodging soil particles and carrying them away. The roots and litter keep the soil porous and help in water percolation, so fewer water volumes are left to form runoff. The percolated water traveling deeper will recharge the groundwater supply. Transpiration by leaves, water evaporation from wet canopies and litter, and water absorption by roots also reduce runoff water volumes, see Figure 2.

Three vegetation parameters determine the impact of cover on soil erosion.

The extent of plant cover: According to FAO, soil losses shoot up from 1 to 1000 tonnes when plant cover is reduced from 100% to 0%. Increasing plant cover can significantly reduce soil erosion. A 10% plant cover limits erosion to 78%, a 20% cover to 60% erosion, and a 50% plant cover to 30% of soil loss from bare land.

Plant height: Most raindrops fall nearly vertically and have an angle of incidence less than 25°; however, during extreme events like storms, the rain can come at a slant with an angle up to 45°. So, the height of a plant will influence the effectiveness of even a 100% canopy cover. As the plant height increases, some ground remains exposed, and its efficacy in controlling erosion decreases. If the plant cover is four meters high, 75% of erosion, seen on bare plots, will still occur. If the plant cover is two meters high, soil erosion is 50%, and a height of 50 cm has only 18% erosion.

Plant architecture: Plant architecture and leaf shapes influence rainfall interception and erosion control. Umbrella-shaped architecture directs rainwater flow to the periphery of the crown and scatters its energy, as in banana plants. Other types of trees have leaves that channel the rainwater to the petiole and down branches to the stem. Crops with this architecture are pineapple and maize.

Soil erosion control through vegetation can be permanent or temporary.

Figure 3: “Effect of various types of permanent vegetation and cover on soil splash erosion” Xishuangbanna, SW China. (Image by ZHU Xiai).” (Image credits: https://en.xtbg.ac.cn/rh/rp/202208/t20220802_310095.html)

Permanent Canopy Cover

Canopy cover that is present the year around is provided by natural ecosystems like

old closed forests, secondary forests, tree-savanna, shrublands, grasslands over a year old, natural fallows, and perennial crops. Perennial tree orchards or plantations like rubber are not continuous and will need intercrops for cover or mulch to control soil erosion, see Figure 3. The lowest runoff and soil erosion occur from forests and natural vegetation without human disturbance.

Permanent and continuous canopy cover: Forests have several layers of foliage, bushes, and litter of twigs and leaves that form a mulch and provide year-round cover. Rainwater moves between soil and mulch, with its flow slowed by unevenness in soil created by holes left by decayed roots and animal activities, which will also collect water and reduce runoff volumes.

However, exceptions exist. Forests in steep areas, such as those with a 65% slope, could still suffer soil erosion.

Permanent and discontinuous canopy cover: Soil erosion control in tree savannas and old fallows is nearly as good as in closed forests. The discontinuous tree and shrub canopy cover can form patches of spots or bands. Spatial heterogeneity can change plant dynamics, inhibiting or encouraging annual plant growth and altering hydrological processes. Also, the species of the dominant perennial plants can be crucial in soil erosion control success. Fires, especially at the end of the season, can reduce erosion control effectiveness.

Grasslands: Grass is useful for quick erosion control over large areas across the globe, as it can establish enough cover within a year. Grass distribution must be even, and densities of at least 10,000 stems/m2 are necessary to provide adequate cover. When used as a buffer strip, a band of grass 10-12 meters wide traps soil sediments from leaving eroded areas, and around three meters is enough for farmlands.

Temporary Canopy Cover

Temporary canopy cover is provided for some part of the year by annual crops grown for food, fiber, fodder, or cover. The erosion occurring in these temporary canopies depends on crop species, planting date, rainfall time, crop spacing, cropping and plowing techniques, and slope. The determining factor is the amount of land not covered by plants.

Species: Crops like maize and groundnut with less canopy cover are less effective at erosion control than denser canopies of yam and cassava.

Timing: Crops need two to five months to develop 80% of their full canopy. During the early crop period, the exposed soil is susceptible to erosion, especially in wide rows and when planted long after rains. Grass species are best as they grow faster and provide more coverage sooner.

Cropping techniques: Traditional agriculture, where farmers plant immediately after the first rains, has less erosion. Also, traditional and alternate regenerative farms that use intercropping and several crop species simultaneously have better year-round cover than industrial farms. These two types of farms are also likely to use less or no heavy plowing, reducing soil disturbances.

Mulching can help in soil erosion control, with increasing mulch giving better soil protection- 20% reduces soil erosion by 40%, and 80% mulching reduces erosion by 90%. However, the effect of mulch is enhanced when combined with plant canopy cover. For example, the 20% mulch with 100% canopy cover can reduce erosion by 70%.

Experts leverage the variations in species and cropping methods to control soil erosion. Closer spacing, intercrops and crop rotations, and grass buffer zones ensure less soil erosion in farmlands.

Over the years, these measures have been successful, and conservation agriculture has produced the most significant soil erosion control in South America (16%), followed by Oceania (15.4%) and North America (12.5%). Regions with less success are Europe (1.5%), Asia (1.2%), and Africa (1.1%).

Measuring Plant Canopy Cover

The most important criterion is how much soil is covered and not covered by the plant canopy. Forest managers, hydrologists, soil scientists, and researchers working on finding and establishing adequate plant canopy cover for soil erosion control need to be able to measure the cover percentage. One way to measure canopy is by using high-precision tools like the CI-110 Plant Canopy Imager produced by CID Bio-Science Inc. It uses the Gap Fraction Method to find plant canopy parameters. Researchers and managers can use it in the field for real-time measurements and track changes in canopy cover to ensure better soil erosion control.

Sources

Borrelli P., Robinson D.A., Fleischer L.R., Lugato E., Ballabio C., Alewell C., Meusburger K., Modugno, S., Schutt, B. Ferro, V. Bagarello, V. Van Oost, K., Montanarella, L., Panagos P. (2017). An assessment of the global impact of 21st century land use change on soil erosion. Nature Communications, 8 (1): art. no. 2013 2013. https://doi.org/10.1038/s41467-017-02142-7

 

EPA. (n.d.). Soak Up the Rain: Trees Help Reduce Runoff. Retrieved from https://www.epa.gov/soakuptherain/soak-rain-trees-help-reduce-runoff

 

European Commission. Global Soil Erosion. Retrieved from https://esdac.jrc.ec.europa.eu/themes/global-soil-erosion

 

FAO. Global Symposium on Soil Erosion. Retrieved from  https://www.fao.org/about/meetings/soil-erosion-symposium/key-messages/en/

 

FAO. (n.d.). Effects of plant cover. Retrieved from https://www.fao.org/4/t1765e/t1765e0h.htm

 

Mohammad, A. G., & Adam, M. A. (2010). The impact of vegetative cover type on runoff and soil erosion under different land uses. Catena, 81(2), 97-103.

 

Morgan, R. P. (2007). Vegetative-based technologies for erosion control. In Eco-and Ground Bio-Engineering: The Use of Vegetation to Improve Slope Stability: Proceedings of the First International Conference on Eco-Engineering 13–17 September 2004 (pp. 265-272). Springer Netherlands.

 

Vásquez-Méndez, R., Ventura-Ramos, E., Oleschko, K., Hernández-Sandoval, L., Parrot, J. F., & Nearing, M. A. (2010). Soil erosion and runoff in different vegetation patches from semiarid Central Mexico. Catena, 80(3), 162-169.

 

Xiai, Z. (2022, Aug, 2). Controlling soil splash erosion may benefit sustainable development of rubber plantation. Retrieved from https://en.xtbg.ac.cn/rh/rp/202208/t20220802_310095.html

 

Zuazo, V. H. D., & Pleguezuelo, C. R. R. (2009). Soil-erosion and runoff prevention by plant covers: a review. Sustainable agriculture, 785-811.