By: DeJenae Smith
If you want insight on the health of a body of water, having a laboratory or expensive equipment is not a necessity. Often, all you have to do is look up – not at the sky or its clouds, but at the trees. Unlike migrating birds, fish, or other highly mobile organisms, trees stay rooted in one place, living alongside or near a stream, creek, or river for as long as the water flows, and often for many centuries after. Their presence, structure, and diversity can offer powerful information about the health of a watershed.
Over the past few months that I’ve spent as a Drexel University co-op with WCT, I have learned a lot about the process of ‘reading the landscape’ and its value; and I quickly realized how much of that understanding depends on trees.
While trees serve many functions for an ecosystem and planting more in a space is beneficial, it is important to plant a tree in the appropriate conditions to increase their chances of survival. As I’ve walked through Kirkwood and Ashbridge Preserves and participated in riparian tree plantings, I have learned from my mentors about three key “zones” for trees and shrubs as it relates to wetlands.
Zone 1 is located closest to the stream’s edge. Examples of trees WCT has planted here are silver maple (Acer saccharinum), buttonbush (Cephalanthus occidentalis), American sycamore (Platanus occidentalis), and swamp white oak (Quercus bicolor) (Figure 1). These species are very tolerant to wet soils and are key in keeping a stream bank together (Figure 2).
Alongside structural support, trees in Zone 1 provide other functions. As they grow and create canopies, they provide shade over a water body, keeping the water cool for fish and invertebrates. Trees are also capable of cladoptosis, the process of shedding their branches. When branches fall into water, they can provide habitat, hiding places, or transportation for small aquatic organisms (Figure 3).
About 15 feet from the water is Zone 2, also known as the “Transitional Zone.” Trees here are tolerant of moist conditions, but not to the same extent as plants in Zone 1. The primary purpose of trees and shrubs in this area is to absorb excess nutrients and potential contaminants, preventing them from entering the water.
Examples of trees that thrive in this zone are chokeberry (Aronia melanocarpa), redbud (Cercis canadensis), and pin oak (Quercus palustris). A densely populated forest offers more habitats and food sources, also acting as a rest stop for migrating birds.
Going even further, Zone 3 is located about 50 feet from the water, typically contains fewer large trees and is full of native grasses, shrubs, and wildflowers. Plants in Zone 3 are the first line of protection for the stream, filtering runoff that may carry sediment, nutrients, or pesticides. Zone 3 also helps prevent flooding. When there’s heavy rain, stormwater can rush into a watershed at once. Dense vegetation in Zone 3 slows that water before it reaches lower zones. As water takes longer to travel, more of it soaks into the soil and helps refill the groundwater supply.
Groundwater is essential for drinking, farming, and keeping land stable. If too much is removed, the ground can sink – a problem called subsidence. Zone 3 plants help prevent this by allowing rainwater to seep into the ground. Some of the shrubs WCT has added here include red osier dogwood (Cornus sericea), witch hazel (Hamamelis virginiana), and common ninebark (Physocarpus opulifolius).
Each layer of the riparian zone plays a unique and vital role in keeping our watersheds healthy. While these areas may seem like simple scenery, the trees, shrubs, and plants that grow there work hard to stabilize streambanks, filter pollutants, support wildlife, and show us the health of our watersheds. Learning to read these green spaces during my time with WCT has deepened my appreciation for the overlooked power of riparian zones and the trees that stand not in stillness, but in silent service to the land and its ecosystems.
Funding for this project was awarded through the “Protect Your Drinking Water” grant program, administered by the Pennsylvania Environmental Council with funding from Aqua, an Essential Utilities company.
References
Multifunctional Riparian Forest Buffers: More Than Just Trees. (n.d.). Extension.psu.edu. https://extension.psu.edu/multifunctional-riparian-forest-buffers-more-than-just-trees
Penn State Extension. (2005, February 11). Riparian Buffers for Wildlife. Penn State Extension. https://extension.psu.edu/riparian-buffers-for-wildlife
Plant Materials | Natural Resources Conservation Service. (2024, March 7). Www.nrcs.usda.gov. https://www.nrcs.usda.gov/plant-materials
Riparian Tree Plantings – Western Pennsylvania Conservancy. (2023, October 20). Western Pennsylvania Conservancy. https://waterlandlife.org/trees/riparian-tree-plantings/
Trees, Shrubs, and Groundcovers Tolerant of Wet Sites. (2022). Psu.edu. https://extension.psu.edu/trees-shrubs-and-groundcovers-tolerant-of-wet-sitesUS Department of Commerce, National Oceanic and Atmospheric Administration. (2019). What is subsidence? Noaa.gov. https://oceanservice.noaa.gov/facts/subsidence.html
