Discovery Park is the largest park in Seattle, operated by Seattle Parks and Recreation (“Mulching at Discovery Park”). On July 28, I was one of many volunteers for Mountains to Sound Greenway to assist in native plant restoration in Discovery Park. The day was spent shoveling piles of mulch into buckets, carrying the buckets to a particular part of the forest that had been replanted with native species, and mulching over invasive plant species that would otherwise harm the newly planted trees and shrubs.
Mountains to Sound Greenway has been greatly involved in the restoration of Discovery Park. Their goal is to remove invasive species in the park and rehabilitate regular forest functions by planting native species (“Mulching at Discovery Park”) including coniferous trees like the Douglas-fir (Pseudotsuga), deciduous trees like the Big-leaf Maple (Acer macrophyllum), Snowberry (Symphoricarpos albus), and Nootka Rose (Rosa nutkana) In order to allow such species to mature, Mountains to Sound Greenway has volunteers like myself mulch over nonnative invasive species such as Himalayan Blackberry (Rubus armeniacus), Scotch Broom (Cytisus scoparius), and English Ivy (Hedera helix) (Noufi). The mulching process was fairly simple, but will have tremendous benefits in terms of forest restoration.
Alien species such as the Himalayan Blackberry, Scotch Broom, and English Ivy are harmful to Discovery Park’s biological diversity. Due to their invasive nature, said species have a rapid growth rate in the park. This may be due to the fact that they are not originally from this region, so there are no natural enemies existing in their new ecosystem that can restrain growth.
The species compete with native species for land and nutrients, and this can limit the natural functioning of the ecosystem (Friedland and Relyea 323). Himalayan blackberry, for example, is native to western Europe and was brought to the Pacific Northwest to be used as a crop in 1890. The plant was eventually naturalized by mammals that would eat the berries and disperse of them elsewhere, causing them to spread quickly. Now, they kill native species by shading them out and produce shelter for invasive species such as rats (Boersma, Reichard and Van Buren 30-31). Similarly, Scotch Broom was brought to the Pacific Northwest by British settlers who wanted a piece of home with them. At first, it was used near highways but now it shades out native species and creates high levels of nitrogen in the soil that native plants are not accustomed to (Boersma, Reichard and Van Buren 32-33). Lastly, English Ivy was brought to the Pacific Northwest by early settlers in the 1890s as well. This invasive species poses a more serious threat due to its ability to grow up the trunks of native trees and access sunlight. It can completely damage a forest, partly because it competes for resources but also because of its poisonous nature to other plants through allelopathy (Boersma, Reichard and Van Buren 44-45). Although brought to the region with good intentions, nonnative species can seriously damage and completely destroy forest communities. Part of Mountains to Sound Greenway’s goal to restore Discovery Park to its original state is to rid the area of these invasive species so that native species can once again thrive and grow.
At the event, volunteers used woody mulch to “choke out” or bury the invasive species. This can be more effective than alternatives such as burning or pulling out the plants. For example, researchers in Eugene, Oregon at Hendricks Park wanted to study the impact of various mulch types throughout their restoration work and removal of invasive species. They found that burial of invasive species reduced their growth, meaning typical mulching was more effective than burning or pulling weeds (Lintz et al. 1-4). Mountains to Sound Greenway takes this into account when having volunteers bury unwanted grasses and shrubs with woody mulch, blocking them from the sunlight needed to perform photosynthesis thus making survival difficult. Another study done by landscapers in search of the “perfect” mulch technique found that woody mulch was the most stable and durable organic mulch. It was able to kill off weeds more effectively than mulch textile until native plants could grow in (Ruggeri, Provenzano and Rossini). According to Mountains to Sound Greenway employees, the mulch used would prevent invasive plant growth until trees and shrubs were tall enough to cover the area in shade, halting the growth of any species that needs sunlight to thrive (Noufi).
While ridding the forest area of invasive species, mulch also is beneficial in improving soil quality thus aiding in the regrowth of newly-planted native species like the Douglas-fir, Big-leaf Maple, Snowberry, and Nootka Rose. According to a study done at an Ohio landfill searching to restore degraded soil, mulch has many positive impacts on plant growth such as the regulation of soil temperature, decrease in evaporation rate, and addition of vital nutrients to the soil. Based on their research, soil plots with mulch were much cooler than those without. This indicates that the addition of mulch to the soil decreases the impact that warm weather could have on heating underlying soil, which could otherwise experience high temperatures that could kill newly planted trees (Athy, Keiffer and Stevens). This may explain why Mountains to Sound Greenway focuses mulching efforts in the late spring and throughout the summer, when temperatures are high and rainfall is decreased. Furthermore, the mulch decreases evaporation rate. Moisture in the soil is vital to plant growth mainly due to increased activity in soil microbes that create nutrients for plants (Athy, Keiffer and Stevens). By adding mulch around the native plants in Discovery Park, volunteers were increasing the likelihood of successful plant growth and advancing the restoration process. Lastly, mulch supplies vital nutrients such as carbon and nitrogen to the soil. These nutrients are important for plant growth and most especially the survival of newly-planted seedlings (Athy, Keiffer and Stevens).
One such study done on using mulching for forest restoration using Douglas-fir seedlings in Albany, California found that the major cause of mortality in the newly-planted trees was drought (McDonald, Fiddler and Harrison 4). By adding large sums of mulch around seedlings over a 5-year trial period, researchers found that the mulch provided double the benefits that they had initially thought. Not only did the woody mulch provide benefits like temperature regulation and a decrease in soil evaporation, but it also added more organic matter and moisture to the soil by killing off unwanted vegetation beneath it (McDonald, Fiddler and Harrison 9). With such great benefits, it is a curiosity why environmentalists and gardeners alike would not prefer mulching over weed-pulling and burning. This can simply be caused by the high cost of mulch. According to a leader at Mountains to Sound Greenway, some restoration projects simply do not have enough funding to afford mulch. According to the Albany study done in 1994, costs for what they considered to be “large mulch” added up to almost $2059 per acre in their particular experiment (McDonald, Fiddler and Harrison 8). Without proper funding, using mulch as a resource is not feasible.
Protecting and restoring Seattle’s largest park has become a project of great importance to the people at Mountains to Sound Greenway, and it takes a lot of work. As one can see, native species cannot thrive in what has become a haven for invasive species competing for nutrients and land. The species cannot thrive without the help of volunteers willing to focus their efforts on fixing the high-stress competition for resources that early settlers brought to the Pacific Northwest. Someday, the native species will be big enough to outcompete other species and until then, it is volunteers and funding that must fight for them.