Can We Use Existing Trees More Efficiently To Help Maximize LID Potential?

Introduction The development of modern stormwater mitigation practices in the form of various LID techniques is a critical step forward for the implementation of safeguards against some of the damaging effects of urbanization. Federal, state , local governments, as well as conscientious businesses are employing these techniques to reduce the deleterious effects of excess stormwater. When describing the improved stormwater management techniques of LID, most literature explains that the preservation and use of existing plant material is an important component of successful design. However, even when LID techniques are implemented, in practice too many other facets of development often come before the preservation of existing plant materials. In particular, existing desirable trees have the potential to provide significant benefits regarding stormwater mitigation. It is well established that trees can provide significant rain interception, temperature moderation, nutrient load removal, and move large amounts of water from the soil back into the atmosphere through transpiration (1,2). In addition, when trees are incorporated directly into infiltration trenches or rain gardens, conflicts can arise such as concerns over root intrusion into pipes and loss of stability due to root confinement. Thus, LID sites may focus on herbaceous swales to mediate stormwater issues rather than utilize the many benefits that trees could provide in these systems. We propose that a greater emphasis should be placed on utilizing trees, specifically desirable, existing trees, as stormwater mitigation tools. Trees are an important component of successful low impact development and stormwater mitigation practices, yet, ironically, trees are often removed or severely damaged to create these very same stormwater systems.

Preliminary Case Study Figures 1 ,2 &3 illustrate an “improvement” project that reflects a general lack of understanding of the value of tree preservation. There is currently no drainage problem at this 100 year old site, but additional paved parking space is needed. Pervious pavements and other more advanced LID techniques would have been useful here, but the site engineer and landscape architect chose to accommodate the additional generated runoff by utilizing infiltration trenches and piping. To move the water to the trench on the left of figure 2, approximately 400 feet of piping will snake through the site. The original development plan called for the removal of only two trees. Only after the original plan was rejected due to insufficient tree information was a consulting arborist brought in to assess the impact of the project on the existing mature trees. The consulting arborist has indicated the following:

16 trees with 12” or larger diameter (DBH) that are proximal to the LOC will be lost due to stormwater management

Of these 16 trees only 4 are currently deemed undesirable.

In effect, the proposed construction project will eliminate the vast majority of the mature trees on the site.

Code/Stormwater/Tree conflicts Tree removal is often necessitated to accommodate municipal codes regarding stormwater mitigation. Codes often change from locality to locality. These changes may be influenced by particularly sensitive watersheds like the Chesapeake Bay, but often even cities/counties within one watershed will have different codes. However, what seems to be nearly universal is that tree preservation is not a major priority. In practice, it is often easier for developers to meet tree canopy requirements through planting new trees on or off site when other code requirements are more restrictive. Consequently, massive amounts of stormwater mitigation effects can be lost with the removal of mature trees.

Figure 2: Aerial view of the site with limits of clearing outlined

Figure 3: Tree Conservation Plan for the Northern Virginia site

The Green Industry Perspective The green industry plays an active role in LID practices. Maintenance of stormwater BMPs often falls to landscape contractors. Horticulturists, arborists, and foresters are less frequently consulted during site planning and construction, when their experience could be more valuable. Preliminary findings from the ongoing Water Quality and Sustainable Landscape Practices in Virginia Survey indicate the green industry representatives are highly attuned to stormwater mitigation issues:

83% indicated that they have a very good or better understanding of stormwater runoff concerns, with 46% indicating they have an in-depth understanding

75% indicated they have a very good or better understanding of water quality issues, with 36% indicating they have an in-depth understanding

73% indicated that they would like their business to be more green/sustainable

66% indicated that they practice conserving site resources like water, soil, and plants

72% indicated that they practice minimal site disturbance while landscaping

Nearly half of the respondents who indicated that tree removal is part of their business have removed trees to accommodate stormwater management systems

Respondents indicated that 3 of the 4 most important practices for achieving a green/sustainable site were site design, reducing sedimentation and erosion, and stormwater management

Respondents were primarily landscape designers and contractors, with some arborists. Preliminary results indicate that the landscaping and arboriculture industries are legitimately concerned about meshing current landscaping practices with stormwater BMP engineering practices. We believe communication between experts in these fields could be improved to the benefit of all stakeholders.

Acknowledgements: A special thanks to Ed Milhous, consulting arborist on the preliminary case study site and Laurie Fox, for allowing the use of a portion of her preliminary survey results.

Thomas Martina, PhD. Candidate and Dr. Susan Daya, b, Assistant Professor a Department of Horticulture b Department of Forest Resources & Environmental Conservation Virginia Tech, Blacksburg, Virginia

This project is partially supported by the Tree Research Education & Endowment Fund. We are grateful for their support. For more information : Thomas Martin thmarti2@vt.edu Susan D. Day sdd@vt.edu

www.hort.vt.edu & www.urbanforestry.frec.vt.edu

Figure 1: The affected site in Northern Virginia. Three of the four large trees pictured surrounding the church will be lost to account for the stormwater management system, the fourth has a “good” chance of surviving. An additional 9 desirable trees will be lost. These trees presently are effective at mitigating stormwater issues at this site and in some cases could be saved with alternative plans and a change in the municipal code to reflect the huge benefit trees can have with regards to stormwater mitigation.

Conclusions & Next Steps Trees and stormwater BMPs both have value and are essential components of urban landscapes. Working towards the resolution of the conflicts between stormwater management and healthy vegetation and soils can improve the outlook for both. We would like to conduct an additional survey which would include stormwater engineers, municipal representatives, arborists, and green industry representatives that would focus on the use of trees and potential conflicts in LID sites. The information gathered in these surveys will potentially allow for further research to help resolve the issues raised. Please leave your card if you would like to be included in this survey.

Figure 4: A completed stormwater mitigation site in Blacksburg, Virginia illustrates the conflicts between stormwater mitigation practices, existing trees and landscapes. This site was stripped of all existing vegetation, graded, and the stormwater channeled to the area shown. The soil is heavy clay and gravel is visible on the surface as a remnant of construction. Thus dense subsoils were exposed by grading and soil pH increased by construction debris—both conditions hostile to plant establishment and sustaining healthy soils. Thus, the prognosis for the installed trees is not good. Additionally, the use of barberry (a species that is not only invasive but known for its ability to collect trash) in the retention area is questionable.

References 1) Bartens, J.et al. 2008. Can Urban Tree Roots Improve Infi ltration through Compacted Subsoils for Stormwater Management? J.

Environ. Qual. 37: 2048 -2057.

2) Xiao, Q., and E. McPherson. 2003. Rainfall interception by Santa Monica’s municipal urban forest. Urban Ecosyst. 6:291–302.

At this building site, 16 trees more than 12 inches in trunk diameter will be removed to make way for stormwater management