Wednesday, October 21, 2020

To Harvest Or Not To Harvest: Recent Insight On Riparian Buffers

By Dr. Tyler A. Groh, Penn State Assistant Research Professor

Surface runoff and subsurface seepage from agricultural fields can contribute to excess nutrient and sediment (from soil erosion) loading to adjacent water bodies. 

These excess nutrients and sediment are typically a byproduct of growing annual row crops like corn and soybean instead of having a more perennial crop system. 

Elevated levels of nutrients and sediment can cause downstream issues like eutrophication, the dense growth of algae and other aquatic plants. 

Eutrophication is especially of concern in the Chesapeake Bay Watershed, where the United States Environmental Protection Agency (US EPA) issued a nutrient diet called a total maximum daily load (TMDL) in 2010. 

This TMDL puts a limit on the total amount of nitrogen, phosphorus, and sediment allowed to enter the Chesapeake Bay from its contributing streams if the health of the Bay is to recover.

Pennsylvania plays an important part with this nutrient diet for the Chesapeake Bay since it covers approximately 35 percent of the Chesapeake Bay Watershed area (PDEP, 2020). 

Further, the main portion of Pennsylvania that is in the Chesapeake Bay Watershed drains to the Susquehanna River, supplying approximately half of the total freshwater to the Chesapeake Bay. 

Fortunately, farmers and landowners have been and continue to be able to contribute to the health of the Chesapeake Bay through conservation practices. 

The main conservation practice promoted throughout the Chesapeake Bay Watershed is riparian buffers. These buffers are perennial vegetation alongside surface water bodies, typically streams, that help protect surface water quality from upslope land practices. 

These systems are effective at removing nutrients and sediment with removal efficiencies ranging from 19-65 percent for nitrogen, 30-45 percent for phosphorus, and 48-60 percent for sediment (Belt et al., 2014).

There are many state and federal riparian buffer cost assistance programs for landowners that want to install this conservation practice. Most have strict guidelines on buffer size, length of contract, and maintenance. 

One of the most common programs is the Conservation Reserve Enhancement Program (CREP) through the United States Department of Agriculture. 

The minimum width of buffer for CREP is 15 meters (49.2 feet), and this program limits the harvesting of vegetation from these areas due to potential negative water quality impacts associated with harvesting. 

This uncertainty is the focus of a new study that came out of Penn State University, published in the Journal of Environmental Quality (Jiang et al., 2020).

Drs. Jiang, Preisendanz, Veith, Cibin, and Drohan conducted a study that assessed six buffer designs and their ability to remove nitrogen, phosphorus, and sediment. These designs were--

-- A. 10 meters of grass,

-- B. 15 meters of grass,

-- C. 15 meters of deciduous trees,

-- D. 30 meters (98.4 feet) of grass and deciduous trees,

-- E. 30 meters of grass and trees which had a portion of the trees furthest away from the stream harvested every three years, and

-- F. 30 meters of grass and trees that had the grass portion closest to the field harvested every year.

These buffer designs were chosen to represent a variety of options and to compare their water quality benefits. 

For buffer A, 10 meters of grass represented a narrower buffer than the CREP preferred width but in accordance with landowner desires, while buffers B and C were as wide as the CREP minimum for buffers and were chosen to compare vegetation types (grass vs trees). 

Buffers D, E, and F were 30 meters wide, made up of a 6 meter (19.7 feet) wide grass portion next to the field edge and a tree section composed of a middle 19 meter (62.3 feet) wide section and a stream edge section that was 5 meters (16.4 feet) wide. 

This design specification is in accordance with Pennsylvania DEP’s Stormwater BMP Manual. 

Buffers E and F were designed to have portions (6 meters of grass for E and the middle 19 meters of trees for F) of the buffer harvested to determine if harvesting would impact buffer water quality benefits.

The researchers ran a 16-year long model using both the Soil and Water Assessment Tool (SWAT) and the Riparian Ecosystem Management Model (REMM) to model nutrient and sediment loading into the buffers as well as the processing of these nutrients and sediment through the buffers relative to common central Pennsylvania agricultural and riparian soils. 

They determined that grass buffers were more efficient at removing both nutrients and sediment than tree buffers. Further, wider grass buffers removed more than their narrower counterparts (buffer B vs buffer A). 

The authors went on to explain that the largest removal of nitrogen, phosphorus, and sediment loads occurs in the section of buffer closest to the field edge when assessing the 30-meter-wide buffers. 

This does not mean that nutrient and sediment removal does not occur closer to the stream, but rather that removal as water moves across the buffer is not equal (not linear) across the entire width. 

The 30-meter-wide buffer also did not have significantly more or less removal of both nutrients and sediment when compared to the 15-meter-wide grass buffer, indicating that land-owner preferred grass vegetation at the current 15 meter CREP width suggestion can be sufficient for nutrient and sediment removal.

When assessing the difference between harvested and not harvested buffers, the authors stated that removing vegetation did not significantly affect the buffer’s capacity for nutrient removal. 

The model harvested vegetation during peak biomass in late September, so the vegetation was able to grow all year before harvesting and was able to act as a nutrient sink. This harvesting of vegetation therefore eliminated some nitrogen and phosphorus from the buffer, increasing total buffer nutrient removal by 2-3 percent.

These results are very positive for those landowners interested in harvesting vegetation from buffers, especially when considering multifunctional buffers ("Multifunctional Riparian Forest Buffers – More than Just Trees"). 

However, many current state and federal financial buffer incentive programs have regulations that limit what landowners can do for management and harvesting. 

If the restrictions of these programs were loosened to include harvesting and greater flexibility for vegetation type to match landowners’ interests, it is thought that there could be a faster rate of buffers planted throughout the state of Pennsylvania. 

This could have very positive implications for water quality in the Chesapeake Bay. 

Overall, the data analyzed in this paper was the first step to understand riparian buffer harvesting, and how riparian vegetation management and water quality can go hand-in-hand.

Click Here for references.

[To learn more about buffers and available assistance, visit the DCNR Forest Buffers Along Waterways webpage.

[Grants Available

[DEP is accepting applications for EPA Section 319 Watershed Restoration Grants until October 23.  Click Here for all the details.

[Department of Agriculture Farm Conservation Excellence Grants are available for Lancaster and York Counties.  Learn more here.

[Contact your local county conservation district for landowner financial and technical assistance options.  Find your district.

[Apply now for U.S.D.A. Natural Resources Conservation Service landowner assistance in designing and funding farm conservation projects.  Click Here for more.

[Need funding for a park, trail, riparian buffer or conservation project? The Department of Conservation and Natural Resources Bureau of Recreation and Conservation is hosting virtual regional grant workshops in November.  Click Here for all the details.

[The DCNR website includes information about the Community Conservation Partnerships Program grants]

(Reprinted from Penn State Extension Oct. 21 Watershed Winds newsletter.  Click Here to sign up for your own copy.)

Related Article:

-- Harvesting Vegetation From Multi-Function Riparian Buffers Barely Reduces Water Quality Benefits

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[Posted: October 21, 2020]  PA Environment Digest

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