Assessment of Plant Vigor and Survival at an Urban Restoration Site in Relation to Climate, Topography, Geology, and Soil Properties
Cole D. Gross
Factors that affect site productivity include, but are not limited to: climate, topography, geology, and soil properties. Soil properties of an urban restoration area on the University of Washington (UW) Seattle Campus were examined and analyzed in relation to native plant species vigor and survival at the site. Climate, topography, and geology were also taken into account to assess applicable native plant associations for the site and to instruct future plantings. The findings support a change in protocol regarding the selection and installation of native plant species for establishment at the site. In general, native plant species well adapted to the soil properties at the site experienced greater success than less adapted species; however, the topography of the site can be used to the advantage of some native plant species less adapted to certain soil properties. Future considerations for monitoring native plant species success at the site include the creation of equally-sized plots and the installation of predetermined native plant species with equal sampling sizes of at least 30 plants per species.
The Whitman Walk Restoration Area (WW) is a small forest tract (approximately 0.25 ha) located on the University of Washington (UW) Seattle Campus between Denny Field and McCarty Hall (Figure 1). The restoration site is currently managed by the Society of Ecological Restoration UW Chapter (SERUW). WW has been traditionally divided into multiple zones as a means of monitoring the installed plants; however, this study will focus on only Zones 5a, 5b, 5c, 6, and 7, as the remaining zones will be disturbed or lost entirely during the upcoming North Campus Housing construction (Figure 2). While the construction boundaries of the North Campus Housing project essentially encompass all WW zones, the zones examined in this study will be spared according to the lead landscape design architects for the project (R. Roark and J. Henson, personal communication, October 16, 2015).
Since 2008, UW students and volunteers have worked at WW to remove invasive plant species such as English ivy (Hedera helix L.) and Himalayan blackberry (Rubus armeniacus Focke), which had dominated the understory at the site (SERUW 2015). However, according to plant monitoring data collected by Cronan and Saari (2015), successful establishment of native plant species at the site has been varied. During the summer of 2015, Cronan and Saari (2015) documented a 14% death rate of plants installed at the zones of interest, with another 45% of plants experiencing moderate to severe drought stress (Table 1; Figure 3).
One soil pit was manually excavated to 50 cm at WW Zone 5c (Figure 4; Figure 5). The location of the soil pit was selected in order to provide a representative dataset while also minimizing any harm to the ecosystem and restoration work as a result of the excavation. Soil horizons, horizon depth, and horizon structure were identified in the field, and bulk density samples were taken in the middle of succeeding soil horizons using a soil corer of known volume. After collection, the samples were sealed in plastic bags and returned to the lab.
Bulk density (Db) results were obtained by: 1) weighing the samples; 2) drying the samples in an oven at 105o C for 24 hours; 3) reweighing the samples; 4) separating the coarse and fine soil fractions using a 4.75 mm sieve; 5) weighing the coarse soil fraction (if applicable); and 6) obtaining the volume of the coarse soil fraction by submerging said fraction in a known volume of water and measuring displacement. The following formula was used to calculate the fine soil fraction Db:
Following bulk density calculations, a representative sample of the fine soil fraction was weighed, placed in a ceramic vase, and heated at 550o C for four hours in a muffle furnace. The following formulas were used to calculate % organic matter (O.M.) and % carbon:
Percent moisture and pore space were calculated according to the following formulas, using a solid particle density (Dp) value of 2.65 g/cm3 as the average value for a mineral soil:
Horizon color, texture, and pH were determined in the lab using the fine soil fractions following bulk density calculations. A Munsell soil color chart was used to identify moist soil color. Soil texture was determined using the cast, feel, and ribbon method. Soil pH values were measured using a pH meter after saturating the soil and allowing it to sit undisturbed for 30 minutes.
Climate is a vital component in determining appropriate plant associations for a specific site. Microclimates can also play an important role in the dominance of certain plant species over others, as well as in the increased or decreased productivity of nearby or adjacent sites. The topic of microclimates will be discussed further in the next section on topography. The majority of the plants installed at WW were salvaged from local sites slated for development or construction; however, climate varies considerably within a 40 km radius of Seattle. Many plants were collected about 30 to 35 km southeast of Seattle, where the annual precipitation is approximately 82% greater (U.S. Climate Data 2015) (Figure 6). On the other hand, average annual temperatures, including average high and low temperatures, are very similar between the two locations. As a result of the annually higher precipitation average southeast of Seattle, 34% of the plant species collected and subsequently installed at WW require or prefer moist soils year round and have low to no drought tolerance (Lady Bird Johnson 2015; Missouri Botanical Garden 2015; NatureGate 2015; Pendergrass et al. 2008; Plants For A Future 2012; Shebitz 2003; USDA, NRCS 2015; Washington Native Orchid Society 2011; Washington State Department of Natural Resources 2015; World Public Library 2015) (Figure 7). The high incidence of drought stress in the WW plants reported herein was observed during the summer of 2015. In Seattle, the months of July and August account for only 4% of the average annual precipitation (U.S. Climate Data 2015) (Figure 8). The significant difference in average annual precipitation between Seattle and many of the collection sites, coupled with the dry summer, is partially responsible for the high incidence of drought stress and mortality of the installed native plants at WW.
WW consists of multiple narrow tracks of land, most of which have slopes cutting through them horizontally (as the elevation contours in Figure 5 show). These slopes help to create microclimates, in which the upslope conditions are significantly different than the downslope conditions. Areas upslope often lose both water and topsoil to the areas downslope. Erosion on the slopes at WW is especially a concern due to the disturbed nature of the site and lack of plant and native plant cover. The slopes have a southeast aspect, which tends to create warmer and drier conditions since the cumulative intensity of solar radiation increases with equator-facing slopes (McCune and Keon 2002, as cited in Swanson 2006). Some of the plants installed at WW were salvaged from riparian areas where topography (as well as proximity to a stream or river) would have played a large role in providing additional water and nutrients to the plants in their original habitats (J. Cronan, personal communication, September 12, 2015). The topography of WW encourages stormwater runoff into the zones east and southeast of the paved paths. Some of the plants surviving or thriving at WW despite having higher water demands were installed downslope and in slight depressions, and are therefore exploiting the microclimates created by topography.
During the Vashon stade of the Fraser glaciation approximately 14,000 years ago, Seattle was covered by an advance of ice (Troost and Booth 2008; Thorson 1980). Glacial materials deposited during this time consist of “proglacial lacustrine silt and clay, advance outwash sand and gravel, till, ice-contact deposits, and recessional deposits including outwash sand and gravel and lacustrine silt and clay” (Troost and Booth 2008). WW consists of Vashon stade till closely bordered by Vashon stade outwash deposits to the east, followed by deposits of a pre-Fraser glaciation age and peat deposits, all within approximately 200 m (Troost et al. 2005) (Figure 5). According to Troost and Booth (2008), Vashon stade till is “predominantly a massive matrix-supported mixture” of sand, silt, and subrounded to well-rounded gravel. Thickness of the Vashon till varies from 1 to 30 m, with an average thickness of about 10 m based on over 20,000 borings (Troost and Booth 2008). Dense till overlies the landscape and topography of a significant portion of Seattle, and even extends to and below sea level in certain areas; however, Troost and Booth (2008) also report that discontinuities within the till (i.e., fractures and intercalated sand lenses) are common and abundant and increase the permeability of the till by several orders of magnitude. The upper meter of the Vashon till is generally weathered and less dense than the underlying unit (Troost and Booth 2008).
The soil sampled at WW is a very coarse, sandy soil with well-rounded gravel and moderately acidic conditions (Table 2). Soil structure is very poor to nonexistent, leaving the soil susceptible to erosion. The high bulk density of the Bw horizon (1.42 g/cm3) reduces pore space and further damages soil structure; although the bulk density is not high enough to restrict root growth in a coarse-textured soil, even when the soil is dry. Soil moisture content was extremely low throughout the profile. High sand content (i.e., coarse soil texture) results in the relatively quick downward percolation of water through networks of macropores (>0.08 mm pores) in the soil; water movement in this case is dominated by gravitational forces rather than matric forces (Brady and Weil 2008). The coarse texture of the soil therefore only exasperates the issue of extended periods of low soil moisture. Low clay content results in less surface area and fewer charged sites in the soil to hold, attract, and bind water molecules. The acidity of the soil increases the number of protons (H+) in the soil, and thus the number of protons adsorbed to cation exchange sites provided by the humus; this results in the displacement of cations and essential nutrients such as K+, Mg+2, Ca+2, and NH4+, making them highly susceptible to leaching out of the soil profile with the water. Therefore, despite a reasonable percentage of organic matter in the A horizon, WW is likely a poor soil nutrient site. Native plant requirements of nutrient rich sites and high soil moisture availability appear to positively correlate to plant mortality at WW (Figure 7). Adaptation to soil texture seems to be less of a factor in plant mortality at WW, perhaps because all of the native plant species studied are at least adapted to loamy, if not coarse textured, soils. Ultimately, many factors – only some of which are discussed herein – affect plant vigor and mortality (see Appendix for plant species data for both installed and naturally regenerating native plant species at WW).
Although native plants were collected from nearby sites, climate is highly variable within the Puget Sound. Local climate at plant salvages should be considered prior to installation of plants at WW. However, native plants from differing climates may still be viable at WW, but only within certain parameters, such as downslope or in depressions where the soil will receive and maintain higher percentages of moisture.
As mentioned above, slope and aspect can be exploited to provide microclimates for native plant species that might otherwise fail to thrive at WW. In general, drought-tolerant plants should be installed on slopes and ridges with drought-intolerant plants installed downslope and in depressions. Erosion concerns associated with slopes also need to be taken into consideration, with deep-rooting, soil-stabilizing plants installed on slopes.
The glacial material deposited at WW is moderately dense to dense and may be more superficial in areas. According to Troost and Booth (2008), till covers the surface of approximately 40% of Seattle. Portions of WW may therefore be less permeable by water than others, depending on the depth of the underlying moderately weathered to unweathered dense glacial till. Monitoring WW surface conditions following heavy rains, as well as collecting more soil samples at greater depths, will help to determine if isolated conditions of higher soil water retention exist due to the geology of the site. Similar to microclimates created by topography, the variability of conditions created by underlying geologic layers can also be exploited to improve native plant success.
The soil sampled at WW has low moisture-holding capacity and poor soil structure. Furthermore, due to acidity and high sand content, the site likely has low nutrient availability. Multiple methods can be used in an effort to improve the soil conditions of the site and thus improve productivity. As mentioned above, deep-rooting plants work especially well to restore or create soil structure and to stabilize slopes. For example, bigleaf lupine (Lupinus polyphyllus Lindl.), is a deep-rooting forb/herb that works well to stabilize slopes and disturbed soils. This species is well-adapted to the soil type at WW (see Appendix). While only one bigleaf lupine has been planted thus far, it is thriving (Figure 7). More importantly, this native species fixes atmospheric nitrogen into ammonia within root nodules via symbiosis with Rhizobium soil bacteria (Brady and Weil 2008; USDA, NRCS 2015). According to Brady and Weil (2008), lupines (Lupinus) can fix 50-100 kg of nitrogen per hectare per year. Nitrogen fixation is the process of converting the inert atmospheric gas, N2, to reactive nitrogen; through the nitrogen cycle, this fixed nitrogen becomes available to all forms of life. Studies conducted in the Pacific Northwest – to include the effects of soil nitrogen on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) growth – have shown that the most limiting soil nutrient in the region is nitrogen (Miller et al. 1989; and Carter and Klinka 1990, as cited in Littke et al. 2014). Through nitrogen fixation, bigleaf lupine would improve the soil nutrition, increasing diversity and aiding in the reestablishment and dominance of native plants at the site.
Another factor involving plant vigor and soil may be most evident by examining the native plant species, summer coralroot (Corallorhiza maculate (Raf.) Raf.), which experienced a 100% death rate at WW. Four plants of this species were installed at the site. Members of the genus Corallorhiza are unique in that they contain little or no chlorophyll, and thus cannot provide self-sustenance via photosynthesis; therefore, they are parasitic orchids, deriving nutrients through the consumption of mycorrhizal fungi in the soil (USDA, NRCS 2015; Washington Native Orchid Society 2011). The 100% death rate of this species suggests that mycorrhizal fungi are limited or absent from the soil at WW. According to Boerner et al. (1996), the temporal disturbance of a site is related to soil mycorrhizae presence and infectivity success in tree roots (as cited in Athy et al. 2006). In disturbed areas, mycorrhizae grow through soil gradually, developing an uneven distribution (Athy et al. 2006). It is estimated that mycorrhizae take at least 30 years to reestablish in disturbed soil (Boerner et al. 1996, as cited in Athy et al. 2006). According to Athy et al. (2006), until mycorrhizae are able to reestablish, mycorrhizal-independent plants will prevail. Trees and plants dependent on associations with fungi will have difficulty establishing on disturbed soil unless they are near forest-field margins where mycorrhizae is already well established (Athy et al. 2006). Unfortunately, WW consists of small and isolated tracts of forest which have been highly disturbed over time (as have most soils in urbanized areas). Selecting native plant species from previously undisturbed or only lightly disturbed salvage sites and then installing them at WW – especially with remnants of the original soil still attached to the roots – can help inoculate the site with mycorrhizal spores and may prove highly beneficial in the long term. Mycorrhizal spores are also relatively inexpensive to purchase and could be used experimentally with future installations of summer coralroot at WW.
Adding mulch is another practice that may improve soil conditions and native plant vigor and survival at the site. For instance, a layer of hardwood mulch can narrow the amplitude of soil temperatures throughout the year, help prevent soil water evaporation as well as soil compaction and erosion due to rain, and possibly increase soil nitrogen and carbon levels. The beneficial modifications of soil properties by mulches have been shown to increase plant growth and seedling survival rates (Athy et al. 2006). According to Athy et al. (2006), hardwood mulches are less likely to decrease soil pH levels as a result of the relatively slow decomposition rates of these mulches in comparison to other mulches. Hardwood mulch should be added to the soil surface and not mixed into the mineral soil, as mixing the mulch into the soil will increase microbial nitrogen consumption and thus further deplete soil nutrition (Brady and Weil 2008).
IMPLICATIONS FOR SITE MAINTENANCE AND FURTHER RESEARCH
The most representative plant association for WW is probably Douglas-fir – pacific madrone/oceanspray/hairy honeysuckle (Pseudotsuga menziesii – Arbutus menziesii/Holodiscus discolor/Lonicera hispidula; abbreviated: PSME-ARME/HODI/LOHI) (Figure 9). The plant species in Table 1, Figure 3, Figure 7, and the Appendix are partially ordered in accordance with this plant association, which can help guide native plant species selection for the site. Continued monitoring of native plant species success at the site will include the creation of equally-sized plots and the installation of predetermined native plant species with equal sampling sizes of at least 30 plants per species. Soil will be sampled by depth to at least 50 cm at 10 or more locations selected based on topography and to provide an overview of the entire site. Installed native plants will be monitored quarterly. Native plant species selection and installation will be performed in consideration of multiple factors, including climate, topography, geology, soil, water, nutrition, and plant species.
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 According to Harrison et al. (2003), sampling that discards the >2 mm soil fraction, as well as sampling that ignores the deep soil layers, underestimates total carbon pools. While this study is not intended to estimate total carbon pools, the dataset of this study could potentially be expanded upon to estimate total carbon pools if deeper sampling at the sites is conducted.
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Table 1. Vigor of native plant species installed at WW. NDS = no drought stress; MDS = moderate drought stress; SDS = severe drought stress.
Table 2. Soil properties at WW (soil sampling marker #7 in Figure 5).
Figure 1. Map of University of Washington Seattle Campus showing the location of WW. Note: the area highlighted in green includes zones of WW not included in this study; WW has expanded south of the highlighted section into areas also not included in this study. Figure credit: SERUW.
Figure 2. Map of WW zones. Figure credit: SERUW.
Figure 3. Vigor of native plant species installed at WW. NDS = no drought stress; MDS = moderate drought stress; SDS = severe drought stress.
Figure 4. Sampled soil profile (left) and relative location of soil pit (right).
Figure 5. Geologic map of WW and surrounding area.
Figure 6. A comparison of Seattle and Snoqualmie, WA, climates.
Figure 7. WW plant species death rate in comparison to soil suitability variables.
Figure 8. Graph showing average precipitation by month in Seattle, WA.
Figure 9. Plant association most representative of the overall site conditions at WW. Figure credit: Washington State Department of Natural Resources.
Plant Species Data for both Installed and Naturally Regenerating Native Plant Species at the Whitman Walk Restoration Area