August 9, 2019

Impact of Soil Mounding in Washington Square Park Woodland


The largest lawn in the northwest corner of Washington Square Park is being restored. Much of the lawn has been spray seeded. Sod was installed in the sunny southeast nook.This area of the park is also the largest wooded area in the park. I am concerned about the grade changes in this area of the park. Most of the trees in this section of the park are large ranging in diameter from 15 inches to 40 inches. I contacted the NYC Parks Administrator for Washington Square Park (wearing my Washington Square Park Eco Projects hat) and expressed my concern about the volume of soil placed within the critical root zones of the trees in the northwest woodland.


The critical root zone (CRZ) of a tree is the circular area of tree roots that are most sensitive to disturbance. A rapid calculation for CRZ is the area below a tree's drip line equivalent to the outer limits of its canopy spread. Another method to calculate CRZ is a diameter:ground ratio. NYC Parks requires a CRZ ratio ranging "from ½ foot per one inch DBH (diameter at breast height) to 1½ foot per one inch DBH. If the species tolerance is unknown, then the 1½ foot per one inch DBH standard is assumed" (NYC Parks Parks Tree Preservation Protocols, 2009). The CRZ for a 20 inch diameter tree, for example, can range from 10 to 22.5 feet radius from the trunk. A tree's roots extend beyond the drip line or CRZ but using either measure provides the minimum protection to a tree's root zone. Disturbance in the CRZ should be limited to none. In NYC parks, "In general, no encroachment of the CRZ shall occur without the written permission of the Agency, and without the on-site presence of the Agency’s representative or an approved arborist" (NYC Parks Parks Tree Preservation Protocols, 2009).

Disturbance includes grade changes. Changing the grade by removing or adding soil can negatively impact the feeder roots of a tree. "Many tree roots occur within the top six to eight inches of the soil" (NYC Parks Parks Tree Preservation Protocols, 2009). Adding soil above a tree's roots alters the oxygen and water exchange and movement. The degree of grade change, the type of soil (clay vs sandy loam), and the tree species contribute to the response that a tree will have to a grade change. Per the NYC Parks Parks Tree Preservation Protocols (2009),
Fill of up to three (3) inches additional depth may be permitted with the written approval of the Agency. Fill exceeding three inches shall not occur without the prior installation of an aeration system or other detail approved by the Agency, such as a tree well, retaining wall, terracing, or other such mechanism.
I don't know the specifics of the plan submitted to NYC Parks for this public project because the agency has not posted any signs at the project site nor is there information about the project on the agency's website.


Let's take a look at one of the trees impacted by grade change in the northwest lawn. The Japanese pagoda tree (Styphnolobium japonicum) closest to the Arch is 20 inches in diameter. The tree's CRZ is at least a 20 foot radius from the trunk. The new mound adjacent to the tree sits within the CRZ. The mound, which peaks at 2.5 feet in height, starts at 5.5 feet from the tree's trunk. The numbers presented here are estimates. The project site is not accessible and the agency has not posted information at the site.

I don't know how the Japanese pagoda tree will tolerate this grade change within its drip line. There are several other instances of grade change within the drip lines of trees in the northwest lawn. Oaks and lindens, which are fill intolerant species, are growing in the lawn.

Often tree decline occurs years after the initial disturbance. Because of the lag time between disturbance and decline, the triggering event is often not tagged as the cause of the tree's demise. Here are symptoms of grade change disturbance:
Symptoms of damage from grade changes can appear as a progressive decline of the crown occurring over a period of several months to several years. Initial symptoms generally include delayed budbreak, reduced growth, stunted light green to yellow leaves, crown thinness, and premature fall coloration and leaf abscission. Epicormic sprouts might form on the trunk and large limbs and twig dieback may occur. This may be followed by dieback of large branches and entire leaders and finally, tree death. (Preventing Grade Change Damage to Trees by E.Thomas Smiley, PhD, Urban Forestry, for Bartlett Tree Experts)
I will visually monitor the trees in this section of the park for these symptoms.


 Update: I had a walk-through with the Washington Square Park Administrator, George Vellonakis, on August 16th. He told me that the grade change was made with fill and not soil. To my concern about drainage within the drip line, he said that fill would permit drainage to the underlying soil. However, I have read - I am no longer a certified arborist - that even fill should not be placed within the drip line of a tree, especially a mature tree. Many of the trees in the northwest lawn could be considered mature trees. The new contours of the NW lawn have created a "swale" in the center of the lawn to capture rain and prevent runoff unto the hardscape. However, some of the large trees sit within this "swale." The water regime of these large trees will be different in the presence of the swale. There will be more water pooling around their trunks and within their drip line. This can create a situation that is similar to overwatering from irrigation systems. In addition, if the lawn area is irrigated for lawn health (shallow and more often) vs. tree health (deeper and less frequently), overwatering can also occur. Too much water in the soil reduces air exchange. Water is essential for trees but so is root access to oxygen.

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