Cromwell Park Wetland Mitigation – Year 5 Summer Monitoring
November, 2015
Prepared for:
Melissa A. Ivancevich
Surface Water Quality Specialist
City of Shoreline
Public Works Department
17500 Midvale Avenue North
Shoreline, WA 98133-4905
Prepared by:
Nelson Salisbury, Ecologist
6310 NE 74th St, Suite 201E
Seattle, WA 98115
(206) 322-9296 x101
Table of Contents
INTRODUCTION .......................................................................................................................... 3
MAP 1 – CROMWELL PARK WETLAND MITIGATION AREAS .......................................... 4
PERFORMANCE STANDARDS .................................................................................................. 5
MONITORING STATION METHODOLOGY............................................................................. 5
RESULTS ....................................................................................................................................... 6
DISCUSSION ............................................................................................................................... 10
RECOMMENDATIONS .............................................................................................................. 12
REFERENCES ............................................................................................................................. 13
APPENDIX A – Photo Monitoring .............................................................................................. 14
APPENDIX B – Plot Location Photos ......................................................................................... 14
Page 2
INTRODUCTION
In 2009-2010, the 8.5 acre Cromwell Park was re-developed with the primary goal of detaining
stormwater runoff in the headwater area of the north branch of Thornton Creek.
As part of this project, a 4,221 ft2 Class III wetland was filled and an 8,442 ft2 wetland and an
additional 7,885 ft2 of upland buffer were created to mitigate for this loss (Map 1). City and
federal regulations require a minimum of five years of monitoring for wetland mitigation sites.
The Mitigation Monitoring Plan for Cromwell Park, prepared in December 2010 by Touchstone
Ecoservices calls for two monitoring visits during years one and two, and one annual visit during
the following three years. The goal of the project is to “Create 8,442 square feet of wetland
habitat surrounding the abandoned two-cell stormwater pond in order to provide
increased forested wetland area with improved functions for water quality, habitat
diversity, aesthetics and environmental education opportunities.”
The first monitoring inspection on the site was conducted by Touchstone Ecoservices on April
26, 2011. EarthCorps conducted the second Year 1 monitoring visit in 2011 and has conducted
subsequent monitoring visits for years 2-4 as outlined in the Mitigation Monitoring Plan for
Cromwell Park. Year 4 monitoring was completed by EarthCorps in August, 2014.
This report describes the Year 5 (2015) final monitoring visit and assesses the site according to
the Performance Standards as described in the Mitigation Monitoring Plan. As per the
Monitoring Plan, permanent monitoring stations were established in year three in order to
estimate vegetation cover and survival. All wetland and upland buffer plots were resampled in
Year 5 to determine if the final performance standards were met.
This report describes the final plot-based monitoring of the upland buffer and wetland mitigation
sites and the qualitative monitoring of the entire site. Cover estimates for all current vegetation
were taken at each plot and observations are compared to previous data and against the
Performance Standards outlined in the Monitoring Plan. In addition, all woody tree or shrub
species were enumerated in each plot. These data are compared to previous year’s results and
are used to estimate native plant survival in Year 5 as called for in the Mitigation Monitoring
Plan.
This report details findings and recommendations resulting from the early September, 2015 visit.
Overall, the site generally meets or exceeds all of the Year 5 performance standards as described
in the 2010 Mitigation Monitoring Plan for Cromwell Park. Annual maintenance is
recommended to ensure that the site continues to meet the functional and aesthetic goals called
for in the plan. See the Recommendations section below for more details.
Page 3
MAP 1 – CROMWELL PARK WETLAND MITIGATION AREAS
Map 1. Cromwell Park Wetland Mitigation Project and Vicinity Map
Page 4
PERFORMANCE STANDARDS
Specific performance standards as stated in the Mitigation Monitoring Plan for the wetland and
buffer areas applicable to Year 5 are as follows:
OBJECTIVE 1: Establish a species-rich and structurally-diverse native, scrub-shrub wetland
community and adjacent vegetated upland buffer community per the approved planting plan.
Performance standard 1C: In Years 2, 3, 4 and 5, at least six native shrub species will
occur in the wetland creation area. No single species will represent more than 50 percent
presence. Appropriate volunteer native species can be counted toward species richness.
Performance standard 1D: In Years 2, 3, 4 and 5, at least seven native woody species
will occur in the buffer area with no single species having more than 50 percent cover.
Appropriate volunteer native species can be counted toward species richness.
Performance standard 1F: In Year 5, planted material in the wetland creation and
buffer areas will have at least 80 percent survival rate, or if 80 percent survival of planted
stock is not achieved, appropriate species of native volunteer woody plants that are
healthy and at least 12 inches tall will be counted toward this performance standard AND
native understory or emergent* vegetation will have at least 80 percent cover (per SMC
20.80.350 G.3.c)
*“woody” was replaced with “understory or emergent” from the Mitigation Monitoring
Plan for performance standard cover requirements (1F) in order to be more consistent
with the referenced SMC City code (20.80.350 G.3.c): “Vegetation success shall, at a
minimum, equal 80 percent survival of planted trees and shrubs and 80 percent cover of
desirable understory or emergent plant species at the end of the required monitoring
period”.
OBJECTIVE 2: Control invasive species within the mitigation area.
Performance standard 2A: In any year during the monitoring period, invasive species
will not exceed 20 percent cover within the overall wetland creation and buffer areas.
MONITORING METHODOLOGY
As called for in the Mitigation Monitoring Plan, a total of nine monitoring stations were installed
within the mitigation area (five in the wetland and four in the buffer) (Map 1). Circular plots 12
feet in diameter (113 ft²) were randomly stratified throughout each area resulting in an
approximate 6.2% sample of the entire 16,327 square foot mitigation area. Each plot center was
marked with a two foot piece of rebar with an orange cap inserted flush with the ground. See the
Year 3 monitoring report for more details regarding plot establishment and monitoring protocols.
The Mitigation Monitoring Plan specifies that both the wetland mitigation and upland buffer
mitigation areas are to be formally monitored in Year 5. All nine plots were resampled in early
September (09/04/15) and qualitative and photo monitoring were conducted across the entire
mitigation site. Results and recommendations are presented below.
Page 5
RESULTS
Wetland Shrub Species Richness (Performance Standard 1C)
More than six species of native shrubs were identified within the Wetland Mitigation Area
during the Year 5 site inspection. Species observed include the following 11 species: black
hawthorn, Douglas’ spirea, hooker’s willow, Pacific ninebark, Pacific willow, red elderberry,
red-osier dogwood, salmonberry, Scouler’s willow, twinberry, and western crabapple. None of
the shrub species comprised more than 50% of the total shrub cover in the wetland mitigation
Area. Performance standard 1C is currently being met.
Upland Buffer Woody Species Richness (Performance Standard 1D)
More than seven native woody species were observed in the Upland Mitigation Area and include
the following 16 species: bigleaf maple, bitter cherry, black cottonwood, evergreen huckleberry,
Indian plum, mock orange, Nootka rose, oceanspray, Pacific ninebark, red alder, red elderberry,
red-flowering current, serviceberry, snowberry, vine maple, and western red cedar. Additional
woody/vining groundcover species noted include blackcap raspberry, dull Oregon grape, salal,
and trailing blackberry. None of the shrub species comprised more than 50% of the total shrub
cover in the upland buffer area. Performance standard 1D is currently being met.
Native Cover and Plant Survival (Performance Standard 1F)
Native Cover
Overall native understory and emergent cover across all nine monitoring plots (both wetland
mitigation and buffer areas averaged 83% (Table 1). Cover for the five wetland plots averaged
98% in Year 5 (Table 1) indicating an increase from 76% average estimated across these plots in
Year 3. Overall native understory and emergent cover across the four upland buffer plots
averaged 64% in Year 5, an increase from 37% from Year 4.
Table 1. Average percent cover of native and invasive species by functional vegetation group*
from nine plots sampled in the wetland creation and buffer areas at Cromwell Park, 2015. N=
the number of plots in each area.
Zone
Year
Overall
Native
Cover
Woody
Species
Understory
Trees
Shrubs
Wetland Creation Area (N=5)
5
98.2
85
52.4
58
65.8
5
Upland Buffer Area (N=4)
5
63.75
40.75
12.25
36.25
30
5
Combined (N=9)
5
82.89
65.33
34.56
48.33
49.89
5
Herbaceous Invasive
*Cover was estimated for the following functional vegetation groupings:
 Overall; combined native understory plant cover (includes both woody and
herbaceous/emergent native plant species, measured as total aerial coverage regardless of
overlapping strata)
Page 6





Woody; native woody cover (includes both native understory tree and shrub species)
Understory Tree; native understory tree cover
Shrubs; native shrub cover
Herbaceous; native herbaceous/emergent cover
Invasive; combined invasive species cover (includes both woody and herbaceous
species).
The performance standard for Year 5 vegetation cover indicates that vegetation within the
wetland creation and buffer areas will have at least 80 percent cover as defined by SMC
20.80.350 G.3c, which indicates that “desirable understory or emergent plant species” should be
considered. As discussed in the Year 4 plan, the intent of the code appears to specifically
address wetland habitats and is not necessarily intended for upland areas. For example, it does
not take into consideration that shrub cover is generally much lower in upland vs. wetland
habitats or the value that the existing overstory trees provide in the upland habitat. Furthermore,
the code states that “desirable” species need to meet the criteria, indicating that all considered
species do not necessarily need to be native. The plots lacking the most overall native cover in
the upland buffer area (plots 8 and 9) have moderate to high cover of non-native grass species
that would arguably be considered as “desirable” following the intent of the code. Because of
these reasons, it is recommended that it may not be reasonable to acquire an 80% cover of
understory vegetation in the buffer mitigation area. The current vegetation cover and structure in
the upland buffer appears to represent a generally properly functioning upland habitat and
continues to show increases in cover and structure as the site develops. The lack of understory
cover does not appear to be negatively affecting the goals of the mitigation project or the overall
intent of the Cromwell Park redevelopment that occurred in 2009-2010, which according to the
Mitigation Monitoring Plan was to:




Reduce downstream flooding in residential areas
Reduce flashy hydrology in Ronald Bog and downstream reaches of Thornton Creek
Improve water quality in Ronald Bog and downstream reaches of Thornton Creek
Re-establish the park as a groundwater recharge area
The overall assessment of the structure and cover of native and desirable vegetation in both the
wetland mitigation and upland buffer areas suggests that this performance standard as it relates to
cover has been met.
Survival
A full census of the mitigation site indicated that plant survival was adequate through Year 2. As
called for in the Mitigation Monitoring Plan, plant survival was estimated using plot level data
beginning in Year 3. Results indicate that plant density increased in both the wetland mitigation
and upland buffer areas during this time (Table 2). Density increased from 74 to 111 stems in
the five wetland mitigation area plots and from 15 to 28 in the four upland buffer plots. These
increases appear to be a result of both natural recruitment (especially in the wetland creation
areas) and the addition of newly planted material. This performance standard as it relates to
plant survival has been met.
Page 7
Table 2. Counts of all planted and volunteer woody species (greater than 12” tall) by plot present
in the wetland creation and buffer areas at Cromwell Park, 2015
Plot
Scientific Name
Common Name
#
Wetland Creation Area
1
Alnus rubra
red alder
1
Rubus spectabilis
salmonberry
2
Alnus rubra
red alder
2
Betula sp.
birch
2
Cornus sericea
red-osier dogwood
2
Crataegus douglasii
Pacific hawthorn
2
Lonicera involucrata
twinberry
2
Populus balsamifera ssp. trichocarpa black cottonwood
2
Rubus spectabilis
salmonberry
2
Salix hookeriana
Hooker's willow
3
Physocarpus capitatus
Pacific ninebark
3
Salix lucida ssp. lasiandra
Pacific willow
3
Spiraea douglasii
hardhack
4
Cornus sericea
red-osier dogwood
4
Lonicera involucrata
twinberry
4
Populus balsamifera ssp. trichocarpa black cottonwood
4
Salix hookeriana
Hooker's willow
4
Salix lucida ssp. lasiandra
Pacific willow
4
Spiraea douglasii
hardhack
5
Physocarpus capitatus
Pacific ninebark
5
Populus balsamifera ssp. trichocarpa black cottonwood
Total Wetland Density
Upland Buffer Area
6
Amelanchier alnifolia
serviceberry
6
Acer macrophyllum
bigleaf maple
6
Pseudotsuga menziesii
Douglas fir
7
Cornus sericea
red-osier dogwood
7
Philadelphus lewisii
mockorange
7
Physocarpus capitatus
Pacific ninebark
7
Ribes sanguineum
red-flowering currant
8
Acer circinatum
vine maple
8
Holodiscus discolor
oceanspray
8
Oemleria cerasiformis
indian plum
8
Symphoricarpos albus
snowberry
9
Acer circinatum
vine maple
9
Oemleria cerasiformis
indian plum
Total Upland Buffer Density
Overall Density
Page 8
Count
2013
Count
2014
1
2
1
1
2
2
11
1
1
1
38
1
1
16
1
2
3
3
3
1
1
5
1
28
2
3
1
5
1
111
1
1
33
1
5
5
74
2
1
1
2
1
1
2
2
2
1
15
89
Count
2015
2
1
1
4
3
2
1
3
4
4
2
1
28
2
1
1
2
3
3
2
2
2
1
6
2
1
28
139
Invasive species (Performance Standard 2A)
King County non-regulated or weeds of concern (considered target invasive species) were
present in all nine plots at an average cover of 5% (Table 1). Species recorded included
Himalayan blackberry, hedge false bindweed, reed canarygrass, and creeping buttercup.
Creeping buttercup was the most dominant species in both the wetland mitigation site and the
upland buffer area with substantial cover noted in plots 3 and 7 (Map 1). This species is listed as
a King County “weed of concern”, indicating that “This list is for educational purposes only;
these species are not classified as noxious weeds in Washington State. These species often
impact and degrade native plant and animal habitat. Control is recommended where possible
and new plantings are discouraged”
(http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/laws/ofconcern.aspx). Creeping buttercup is able to establish and quickly spread in mulch and has
become prevalent in moist areas along the buffer. Given the overall establishment of native
shrub and herbaceous cover and structure throughout the mitigation site, this species is likely to
persist but does not pose a significant threat to the general functioning of the site. Himalayan
blackberry and hedge false bindweed (King County non-regulated weeds) were present in
negligible quantities within monitoring plots. Reed canarygrass continues to persist in some
areas, and should be considered for further management. Hedge false bindweed remains heavy
along the southeast portion of the property, especially along the chain-link fence and adjacent
areas.
King County Non-Regulated Noxious Weeds





Scotch broom (Cytisus scoparius)
Herb Robert (Geranium robertianum)
English ivy (Hedera helix)
Reed canarygrass (Phalaris arundinacea)
Himalayan blackberry (Rubus armeniacus)
King County Weeds of Concern



Hedge false bindweed (Calystegia sepium)
Bittersweet nightshade (Solanum dulcamara)
Creeping buttercup (Ranunculus repens)
Potentially invasive species




Bird’s foot trefoil (Lotus corniculatus)
Broad-leaved dock (Rumex obtusifolius)
Velvet grass (Holcus lanatus)
Nipplewort (Lapsana communis)
At the time of this monitoring visit, the combined cover of all invasive plant species was
estimated to be less than the 20% threshold in either the wetland or upland buffer areas.
Performance standard 2A is currently being met.
Page 9
Photo Monitoring
Photos were taken at the established points along the perimeter of the mitigation area (Map 1)
and are attached in Appendix A. Photographs of each monitoring plot are included in Appendix
B.
DISCUSSION
Overall, the mitigation site is meeting all objectives as called for in the plan. Both the wetland
mitigation area and the upland buffer continue to develop diverse native plant communities and
are meeting all Year 5 performance standards.
No formal planting or maintenance was necessary following a low level invasive plant sweep in
the summer of 2014, indicating that the vegetation on site has become well established and less
intensive maintenance is becoming necessary. Native shrub and emergent species have infilled
the wetland mitigation area and continue to mature (see photo 1). Invasive species cover is
generally sparse and made up predominantly of low level “weeds of concern” that are not
formally designated by the King County Noxious Weed Control Program, primarily hedge false
bindweed and creeping buttercup.
Photo 1: Area between existing wetland cells at Plot 4
showing well established slough sedge and spreading
shrubs. Photo was taken on Sep. 04, 2015.
Page 10
As discussed above and in the Year 4 report, the upland buffer area in the vicinity of Plots 8 and
9 have a comparatively low cover of native understory vegetation. However, cover provided
from both the existing overstory bigleaf maple tree and the established shrub species that
continue to mature appears to be adequately providing the necessary function and structure of the
buffer in this area (see Photo 2 below). In addition, the establishment of non-invasive grass and
other herbaceous plants are also providing structure and cover to the area. No substantial areas
of exposed soil or signs of erosion were noted and the buffer appears to be healthy and functional
from both a storm water and habitat perspective. While it is recommended that this area
continue to be informally monitored for continued establishment, it is currently meeting the
intended goals and objectives of the mitigation and the overall redevelopment of the park.
Photo 2: Area beneath bigleaf maple showing existing shrub and herbaceous establishment. This is photo monitoring
point 1.3 taken Sep. 04, 2015.
A small area in the south end of the wetland mitigation area just southwest of Plot 2 continues to
have reduced native structure (see Year 2 summer monitoring report). This area is poorly
drained and exhibits evidence of compacted soils. However, native and non-invasive plants have
now well colonized the area (see Photo 3 below) and the vegetation here is expected to continue
to expand and mature.
Page 11
Photo 3: Area at the southern tip of the wetland mitigation effort showing vegetation establishment. This photo was
taken Sep. 04, 2015.
All Year 5 performance standards have been met. The site is progressing very well towards a
mature wetland surrounded by a healthy upland buffer. Woody and emergent species in the
wetland mitigation area are well developed and diverse and have shown robust establishment and
growth over the monitoring period. The upland buffer area is also developing a diverse and
robust native woody plant community with low cover of invasive species. Overall, this site is
suitably meeting the goals of the mitigation plan in improving water quality and providing
habitat, diversity, aesthetics and environmental education. The completed wetland mitigation
provides an attractive and functioning addition to Cromwell Park.
RECOMMENDATIONS
Regular maintenance should continue to occur in order to maintain the health and function of the
mitigation area and to prevent the spread of invasive species. The following actions are
recommended to occur yearly as necessary, and informal monitoring should occur in order to
determine specific tasks and overall site management in conjunction with other areas of the park.
Recommendations:
 Maintenance and removal of target invasive species throughout the wetland and upland
buffer mitigation areas. Target species include: Himalayan blackberry, reed canarygrass, English ivy, bittersweet nightshade, and hedge false bindweed.
o Other non-native plant species present (see “Invasive species (Performance
Standard 2A)” above) are considered a low priority and should be monitored for
excessive spreading and addressed as necessary.
Page 12
REFERENCES
EarthCorps, 2011. Cromwell Park Wetland Mitigation – Year 1 Summer Monitoring Report.
Seattle, WA
EarthCorps, 2012. Cromwell Park Wetland Mitigation – Year 2 Spring Monitoring Report.
Seattle, WA
EarthCorps, 2012. Cromwell Park Wetland Mitigation – Year 2 Summer/Fall Monitoring Report.
Seattle, WA
EarthCorps, 2013. Cromwell Park Wetland Mitigation – Year 3 Summer Monitoring Report.
Seattle, WA
EarthCorps, 2014. Cromwell Park Wetland Mitigation – Year 4 Summer Monitoring Report.
Seattle, WA
King County Noxious Weed Control Program, 2015. King County Noxious Weed List. Seattle,
WA
Touchstone Ecoservices, 2010. Mitigation Monitoring Plan, Cromwell Park, Shoreline,
Washington.
Page 13
Appendices
APPENDIX A – Photo Monitoring: September 28, 2014
APPENDIX B – Plot Location Photos: September 04, 2015
See final PDF report for monitoring photos.
Page 14