The South Plateau is an isolated upland with a separate, unofficial entrance. From the main body of the park, it is only accessible via a steep social trail that is slippery in the winter and friable in the summer. If you think of North Beach Park as a boot, the South Plateau is the heel. The floor of the plateau is surrounded by steep, short walls.
The South Plateau, at 25,000 square feet, is also the largest flat area in the park. As explained in “Park and Restoration History,” the South Plateau was intensively cleared in the summer of 2012 by an independent forest steward.
For more than a year, the only work done in the South Plateau was by Parks Department Natural Area Crew. In the summer of 2014, forest stewards watered and did some after care for the plants in June and July, and there was a work party in September.
The South Plateau has less than 1% conifer cover, but at least 75% deciduous cover.
The target forest type for the South Plateau is Tsuga heterophylla – Pseudotsuga menziesii/Polystichum munitum – Dryopteris expansa (Western hemlock – Douglas fir/Sword fern – Spreading wood fern; TSHE-PSME/POMU-DREX). The reference ecosystem is Mesic-moist conifer and conifer-deciduous mixed forest.
During the rainy season, water accumulates from NW 85th St. and 26th Ave NW (310 feet) (all elevations from Seattle DPD GIS map). It runs to the north uninterrupted by any green scape or drainage system the length of 26th Ave. to 88th St., where it turns to the west. Once at 27th Ave., it turns again to the north and enters the park. The floor of the South Plateau is at 250 feet, giving this run about a 5% grade.
Figure 1: Path of water flow into the South Plateau.
Before clearing, the dense ivy and blackberry cover dissipated a lot of the energy of this water flow, spreading it out over the surface of the plateau. However, invasive removal caused a serious erosion problem was caused.
The Parks Department has installed rip rap and forced meanders into the water flow using plantings and fascines (water barriers made of bundles of salmonberry live stakes).
Figure 2: Water flow in May, 2014
There is still some water flow control to be done on the South Plateau, and it will have to be studied during rain events of different sizes during the fall and winter.
Water control can be improved in this area by adding meanders to the downstream end of the storm runoff, maintaining the existing meanders and fascines, and working with the stream to slow it down and let the water percolate through the plateau.
During the summer drought, the South Plateau has no water source. This leads the soil to dry and harden, becoming very compact. Plant establishment is very slow, but improving.
For more recent observations on South Plateau water issues, please see Water Flow: South Plateau Street Runoff.
At the start of restoration, the South Plateau was a mix of Acer macrophyllum (Big leaf maple) and Alnus rubra (red alder), with a shrub layer almost exclusively of Hedera helix (English ivy) and Rubus armeniacus (Himalayan blackberry). Other notable invasives included Lamium galeobdolon (Yellow archangel) and Clematis vitalba (Wild clematis).
There is still a fair amount of remnant and resurgent Hedera helix (ivy). Calystegia sepium (bindweed), Lapsana communis (nipplewort), and Geranium robertianum (herb robert) have also made inroads.
A forest monitoring plot following the Green City protocol was established in the South Plateau in July, 2012, and revisited in August, 2013. Note the difference, in Figures 3 and 4 below, in regenerative invasive trees. This is what happens with overclearing followed by neglect.
Figure 3: Invasive regenerative trees, South Plateau, 2012.
Figure 4: Invasive regenerative trees, South Plateau, 2013.
Invasive Removal and Restoration Plan
Figure 5: South Plateau.
Subarea A (outlined in blue in Figure 5, above), at 13,000 square feet, is the largest and driest flat area of the park and the most volunteer friendly. Even though it’s surrounded by Subarea B, it can be accessed by walking carefully down some rip rap. This was the area the independent forest steward and her crew worked in.
The over-clearing followed by neglect has left the South Plateau with a plant community that is still very much out of balance. It’s in better shape than when the ivy and blackberry dominated, but it’s still at risk of an invasive-only plant community.
There is still a lot of invasive removal in Subarea A, including annuals such as Lapsana communis (nipplewort). Subarea A could use a lot of wood mulch, both around the establishing plants, and in large areas of relatively bare ground. In the long term, this would ease the compaction of the soil and aid in plant establishment.
Suggested tasks for Subarea A:
- Mulch around existing plants, and spread mulch to a depth of at least 4” in bare areas of South Plateau.
- Monitor water flow during rain events. Adjust and repair fascines as necessary.
- Add meanders to further reaches of South Plateau. The goal is to slow and spread the water, so it stays on the South Plateau and percolates into the soil.
- Investigate mycelium inoculation as a means of improving soil conditions.
- Forest stewards continue working in South Plateau one day a month for after care and weeding.
- Have two work parties a year (one for planting, one for invasive removal and/or after care).
Subarea B is the walls surrounding the plateau part of the South Plateau. It measures approximately 12,000 square feet. The walls are nearly vertical, making it only available for work by the Parks Department Natural Area Crew. There is a rim of the plateau accessible from 27th Ave NW, but it is so narrow that the best approach is to have the Natural Area Crew work on the rim, and the forest stewards or volunteers do aftercare.
Either the Parks Department Natural Area Crew or the volunteers in the summer of 2012 (or both) have done some work on the western slope. On the eastern and southern slopes of the wall, property lines might be an issue.
Further work on Subarea B will be done by the Parks Department Natural Area Crew. Some of the work could be done at the same time as working on the South or West Slopes (see below).
Suggested tasks for Subarea B:
- Remove resurgent invasives and increase density in cleared areas.
- Remove ivy and put survival rings on trees on the northern edges of the South Plateau.
- Coordinate work on the northern edges of the South Plateau with work done on the South Slope.
Department of Planning and Development. 2007. City of Seattle Department of Planning and Development GIS map. http://web1.seattle.gov/dpd/maps/dpdgis.aspx (Dates of accession various.)
EarthCorps. 2012. North Beach Park South Plateau Baseline Report. (unpublished document). EarthCorps, Seattle.
EarthCorps. 2013. North Beach Park South Plateau Monitoring Report. (unpublished document). EarthCorps, Seattle.
Green Seattle Partnership, 2014. GSP Reference Map on ArcGIS.com. http://www.arcgis.com/home/webmap/viewer.html?webmap=9be9415001144aa383e5b86e481d2c45&extent=-122.5312,47.374,-121.7945,47.7577 (Dates of accession various.)
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