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  • Writer's pictureMauricio Sagastuy

Riparian buffers - How to design them? (Agroforestry)

Updated: Apr 12

Riparian buffers are strips of grass, shrubs or trees planted next to streams or other waterbodies. Their main function is to prevent pollutants from agricultural runoff to enter the water as well as to enhance wildlife habitat. Riparian buffers are usually split into three different zones, each with its specific purpose for filtering runoff and protecting the adjacent aquatic system. 


In this post I will give you an overview about riparian buffers by covering the following topics:




1. The benefits and limitations of riparian buffers


BENEFITS

 

Water quality: Riparian buffers act as natural filters by absorbing and filtering animal wastes, nutrients, sediments and pesticides from crop and rangelands.


Wildlife habitat: Riparian buffers provide habitat and travel corridors for diverse plant and animal species. They offer food, shelter and breeding grounds for fish and aquatic organisms, as well as terrestrial wildlife. The shade that the trees and shrubs provide regulates the water temperature, which is especially important for aquatic cold-water species. 


Erosion control: The vegetation in riparian buffers stabilizes streambanks and shores, preventing erosion caused by water flow and reducing the loss of soil. 


Flood mitigation: Riparian buffers slow down surface runoff and absorb excess water, which reduces peak flows during flood events and lessens downstream flooding. 


Additional income: The vegetation in the riparian buffers can increase the land value and provide extra sources of income. The rows of trees can produce profitable timber or non-timber products while at the same time maintaining buffering capacity. Additionally, the increase in wildlife habitat may attract “game animals” to your farm which allows you to lease fees for hunting.  


Riparian buffer in Iowa, United States

Riparian buffer protecting a farm´s stream in Iowa, United States

Source: Riparian buffer protecting a farm´s stream in Iowa, United States, picture taken by the USDA. 


LIMITATIONS


Loss of productive land: Establishing riparian buffers will reduce the amount of land that can be cultivated or used as pasture. However, riparian areas are subject to periodic flooding and have a shallow depth to the water table, which results in lower agronomic yields. Riparian buffers can offset the loss in yields, and even increase them, by having vegetation that favors pollinators and crop pest predators [1].


Increased management: Riparian buffers require ongoing management, especially in the early years. The amount of management time you will need to put in your system will depend on the chosen plant species and its design. Typical management tasks for a riparian buffer include weed control, pruning, replanting, protection against wild animals, and irrigation (in the early years). 


Financial costs: Establishing and maintaining riparian buffers may require significant investments, such as planting, protection from wild animals, and increased management time.


Possibly, lower yields in the riparian buffer: Riparian areas often experience periodic flooding and the soils may have shallow depth to the water table, which poses risks to harvestable products. Select plant species adapted to floodplain hydrology in order to mitigate these risks.


2. The purpose of the different riparian buffer zones


Riparian buffers are usually divided into 3 different vegetation zones. Each vegetation zone has a specific purpose for filtering agricultural pollutants and protecting the adjacent aquatic system. The 3 zone-system allows for an effective management while at the same time maximizing the buffers environmental benefits. 


The 3 zone-system of riparian buffers

The riparian buffers 3 zone system

Source: USDA National Agroforestry Center


Zone 1: Is the zone that directly borders the water body. Its main purpose is to stabilize stream banks, shade the water source, and provide habitat for aquatic organisms. Additionally, the vegetation in this zone produces leaf litter which serves as food source for macroinvertebrates, which are in turn eaten by fish. 


This zone is composed primarily of native that are adapted to floodplain hydrology. This area should not be disturbed by extensive management practices such as grazing, logging or driving of heavy machinery. These practices may affect the ecology of the system, which in turn affects the environmental benefits provided by this zone.


Zone 2: Is the transition zone between grassland and forest. Its primary purpose is to filter pollutants, reduce sedimentation and provide habitat for wildlife. This zone consists of native trees and shrubs that can tolerate periodic flooding. This area can also be managed for additional income from timber, nuts, fruits, berries or woody floral products.


Zone 3: Is the area adjacent to crop fields or grazing lands. It acts as the first line of defense by taking up nutrients, providing high infiltration, filtering sediments, and slowing water runoff. This zone is composed of native grasses, wildflowers or other herbaceous plants that must be removed to provide effective nutrient sequestration. Removal may be in the form of grazing or mowing but it is imperative for maintaining the vigor of the plant community.



3. Design considerations


The same principle that applies to any type of agroforestry system applies to riparian buffers as well: how you design your system depends on the outcomes you want to achieve. Riparian buffers can be planted for many different purposes and the function(s) you are looking after will influence its design. Below you can see a figure that shows the recommended width for your riparian buffer depending on the benefits you want to achieve [2]. For most of these benefits, research information is limited, so the widths indicated represent best estimates [2].


Figure 1 - Estimated buffer width required for providing each specific benefit

Estimated buffer width required for providing specific benefits

Source: Schultz et al. (2022) Riparian and upland buffer practices [2] 


As shown above, the width of your riparian buffer will influence its function and the outcomes you want to achieve. The wider your riparian buffer, the more functions it fulfills. Therefore, a good rule of thumb for riparian buffers is “ the wider it is, the better”.


Besides the width of the riparian buffer, the plant species you choose will also determine the function of your buffer. Different plant species will impact the function of your riparian buffer in different ways via their roots, organic matter, the nutrients they absorb, and the way they grow. Thus, you should choose plant species that will help you achieve your desired outcomes and plant them strategically in each buffer zone. Below, you can see a table that shows you how effective grasses, shrubs and trees are at helping you achieve your desired outcomes [3].


Table 1 - Specific benefits that different types of vegetation provide

Specific benefits that different vegetation types provide

Source: Adapted from Dosskey, Shultz, & Isenhart (1997) [3]


The third aspect that will influence your design is how you divide the different zones of your riparian buffer. Designing your riparian buffer according to the 3 zone system allows you to manage it efficiently while at the same time maximizing its environmental benefits. Of course you do not have to strictly follow the 3 zone system in order to achieve your desired results. However, the more you can align your design to the 3 zone system, the more you will retain the buffer´s critical environmental benefits. 


Below, I describe some key design considerations for each zone. The following recommendations are general guidelines [2, 4] that can be adapted to fit your specific context and desired outcomes. 


Zone 1:


  • This zone should be at least 5 meters wide

  • Select tree species that can withstand the frequency, magnitude and length of the flooding regime of your site.

  • Select tree species that are well adapted to the depth of the water table during the growing season. 

  • Native species should be chosen for this zone, due to their importance for aquatic and terrestrial wildlife.

  • Fast growing tree species that are adapted to floodplains are preferred for this zone, especially if the channel is incised or if you want to quickly stabilize your stream bank. Examples of such tree species are willow, cottonwood, green ash and silver maple.

  • Shrubs or deep-rooted grasses and herbaceous plants can also be planted in zone 1, however they do not provide the same strength to steep banks because of their more rapid turnover.


Zone 2:


  • This zone is usually at least 18 meters wide.

  • It is recommended to have 4 to 5 rows of trees in this zone.

  • Trees in rows 1 and 2 (nearest the stream zone), and possibly 3, should be selected for their ability to quickly develop roots and be adapted to floodplain hydrology, which increases bank stability. The tree species recommended for these rows are similar to those of zone 1, such as willow, cottonwood, green ash and silver maple. 

  • The outer area of the tree zone (rows 3 to 5) can be managed in order to generate additional income. Tree species that produce bioenergy, firewood, high-value timber, nuts or fruits are great choices for these rows. 

  • If the water table is at least 1 meter below ground for most of the growing season, plant tree species that require good drainage. For sites with poor drainage, select tree species more tolerant of wet conditions.

  • Shrubs also develop a perennial root system and they can fulfill similar functions as tree species (even though trees usually provide more benefits). Extra rows of shrubs can be planted outside of the tree zone in order to add diversity, provide more habitat for the wildlife and produce additional products.  Additionally, multi-stemmed shrubs slow floodwater and trap floating debris within the buffer keeping it out of the adjacent crop field or pasture

  • If desired, herbaceous plants and wildflowers can also be established in this zone. Thus, increasing the biodiversity of the system. 

  • It is recommended to plant many different plant species in this zone. This increases the resiliency of the system, creates more habitat for the wildlife and prevents the loss of benefits if one species fails.


Zone 3: 


  • This zone is usually at least 6 meters wide. 

The main purpose of this zone is to filter sediment and pollutants. The best way to achieve this is by planting grasses that have the following characteristics:


  • dense and stiff stems, which slow the overland flow of water, increase infiltration rates and block the sediments carried by the water

  • large and deep root system, which improves infiltration rates and increases the soil organic matter

  • high rates of biomass production, which further improves the soil quality by providing large amounts of organic matter. 


  • Native forbs and wildflowers may also be part of the mix, especially if they are seeded in clumps with other native grasses. 


Finally, it is important to note that riparian buffers alone may not always be sufficient to prevent pollutants from entering water streams [2]. In such cases, it may be necessary to implement upland buffers and filters, especially in complex landscapes with long and steep slopes. These buffers could be either grass or grasses and forbs only or a combination of grass and trees, or grass, shrubs, and trees similar to the designs discussed above. In most cases, these upland buffer systems will be narrower than the riparian buffers.


Have you ever established a riparian buffer in your land? Tell us about your experiences below!


References:


[1] Pywell, R. F., Heard, M. S., Woodcock, B. A., Hinsley, S., Ridding, L., Nowakowski, M., & Bullock, J. M. (2015). Wildlife-friendly farming increases crop yield: evidence for ecological intensification. Proceedings of the Royal Society B: Biological Sciences, 282(1816), 20151740.


[2] Schultz, R. C., Udawatta, R. P., Isenhart, T. M., Simpkins, W. W., & Schultz, P. L. (2022). Riparian and upland buffer practices (pp. 205-279). North American agroforestry: An integrated science and practice 3rd edition


[3] Dosskey, M., Schultz, R., & Isenhart, T. (1997) How to design a riparian buffer for an agricultural land (Agroforestry Note 4). Lincoln, NE: National Agroforestry Center


[4] University of Missouri (2018) Training Manual for Applied Agroforestry Practices – 2018 Edition


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