Slim-Leaf Onion (Allium Amplectens) in Oak Haven Park
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Slim-leaf Onion (Allium amplectens) in Oak Haven Park Natasha d’Entremont April 1st, 2014 ER390 Independent Project Restoration of Natural Systems Diploma Table of Contents Abstract 2 Introduction 2 Oak Haven Park Site and Description 2 Site Value 5 Slim-leaf Onion (Allium amplectens) 6 Identification 6 Habitat 7 Reproduction and Dormancy 8 Methods 9 Site Inventory 9 Removal of Scotch broom (Cytisus scoparius) 9 Results 11 Discussion and Recommendations 13 Slim-leaf Onion Population monitoring 13 Trail removal and fencing 14 Scotch Broom (Cytisus scoparius) 16 Acknowledgments 16 1 Abstract Oak Haven Park has been identified as an excellent example of a Garry Oak plant community with high conservation values. Slim-leaf onion (Allium amplectens) is a native species in Garry Oak ecosystems and is Blue listed in British Columbia as a vulnerable species to focus on for preventative conservation. The population of Slim-leaf Onion in Oak Haven Park was surveyed and found to be comparable in size with other populations in BC, and adds to the conservation value of the Park. Removal of broom (Cytisus scoparius) was undertaken in the vicinity of the Slim-leaf Onion population to preserve the habitat quality and open up more Garry Oak meadow for possible expansion of the Slim-leaf Onion Population. Recommendations for future health of the Population are the active discouragement of a walking pathway which tramples the plants, and continuing removal of Scotch Broom which will re-grow near the population site. Introduction Oak Haven Park Site location and description Oak Haven Park in Central Saanich (Figure 1) is within the coastal Douglas-fir biogeoclimatic zone (CDFmm), which covers an area including the southwest coast of Vancouver Island, several gulf Islands, and a sliver of the sunshine coast on the mainland of BC (Meidinger and Pojar 1991). This area is characterized by wet winters and cool dry summers, with some of the mildest climates in Canada (Meidinger and Pojar 1991). Oak Haven Park was established in 1997 and has a conservation covenant on it to ensure protection of the conservation values of the park. The park is managed jointly by the District of Central Saanich and Habitat Acquisition Trust. 2 Figure 1. Oak Haven Park in its regional setting on the Saanich Peninsula. Oak Haven Park is 10.2 ha of mostly forested hilltop, with a mix of Douglas fir forest in the lower elevation areas and scrub Garry oak in the mid to higher elevations. The higher elevations are a patchwork of open meadow, shrub vegetation, and oak forest. There are no permanent pools or streams in the park but there are seasonally wet areas during winter and spring. The park elevation ranges from 90-125 m above sea level, with a slope of 14-40% in its main northerly aspect, and a steeper slope of up to 80% on a southeast aspect from the highest point (Park Management Plan 1999) (Figure 2). 3 Figure 2. Topography and local setting of Oak Haven Park. Benvenuto Avenue is the road in yellow to the north. Although Oak Haven is a small park, it can be passed through by many people as part of larger hikes. Oak Haven Park is part of a larger ecosystem associated with Tod creek and the Gowland Range, it has trails connecting to Gowland-Tod Provincial Park, and regional Saanich peninsula trails (Park Management Plan 1999). Management of the park is focused mainly on conservation, with non-pedestrian use discouraged. In keeping with the conservation values, the best potential for human activities in the park has been identified as walking and nature appreciation, which have a low impact. Oak Haven Park provides walkers the potential to enjoy a relatively undisturbed Garry oak ecosystem. The housing lots surrounding the park have minimum lot areas, and the conservation covenant has restrictions on lot subdivision and removal of oak trees in an effort to reduce the impact of development on the park (Park Management Plan 1999). 4 The Oak Haven Park Management Plan has a detailed plant list for the park, including a list of invasive species. The species of focus for this project is the Slim-leaf onion (Allium amplectens), a locally rare vascular plant which is blue-listed in BC (Figure 3). Figure 3. Location of Slim-leaf Onion (Allium amplectens) within Oak Haven Park. Benvenuto Avenue is the road in yellow to the north. GPS Coordinates UTM 10N, 467084 E, 5379142 N. Site value The scrub oak forest and Garry Oak Meadow ecosystem is considered a rare habitat in BC and Canada, as the habitat has been largely converted to urban and agricultural uses (Vellend et al. 2008). Conversion of natural habitat to urban and farming use and the suppression of fire has affected about 90% of the historic Garry oak landscape (Bjorkman and Vellend, 2010, MacDougall, 2004). The lack of regular fire in the landscape has facilitated the growth of Douglas-fir into previously open canopy areas, and invasive plants grow densely in many areas that do still have an open tree canopy. Garry oak meadows that do remain are often shallow soil sites where it is more difficult for Douglas-fir trees and invasive plants to grow densely, capitalizing on the drought tolerance of many Garry oak meadow species (MacDougall, 2004). Though historically oak savannah habitat was predominantly found in flat deep soil sites 5 (Vellend et al. 2008), the reduction in habitat has meant that rocky shallow soil sites are crucial refuges for Garry Oak meadow species, and Oak Haven Park has been evaluated as an excellent representation of a Garry oak plant community (Park Management Plan 1999). The dominant invasive plants in Oak Haven Park are Daphne (Daphne laureola) in the Douglas- fir forested sections, and Scotch Broom (Cytisus scoparius) in the open mossy areas and scrub oak forest. Daphne is visible throughout the Douglas fir forest areas with low to medium density and a mix of young and mature plants. Scotch broom is present in all open canopy areas with medium to high density being the most common. Scotch broom is well established with mature plants as well as seedlings. Broom is identified as having significance in Garry oak ecosystems with high potential for damaging the habitat (GOERT 2002). Scotch broom is focused on for removal in this project due to it having the most potential impact on Slim-leaf Onion and the rare Garry Oak plant community. Slim-leaf Onion (Allium amplectens) Identification Slim-leaf onion (Allium amplectens) is a spring flowering onion with perennial bulbs. It is rare in BC and is blue listed as a vulnerable species. The range of A. amplectens extends from Middlenatch Island in BC to southern California (BC conservation data 2014, Hawrynski 2002). All BC populations are found in the coastal Douglas-fir Biogeoclimatic zone, on dry rocky bluffs or in Garry oak meadows (GOERT 2011). The flowers are on an umbellate, hemispheric inflorescence, and are a shade of white to pink (Figure 4) (Wheeler 2006, GOERT 2011). The light pink and white flowering populations have been found to be triploid, while the dark pink variant is tetraploid (Wheeler 2006). The population in Oak Haven Park was observed to flower very pale pink or white, and is of the more common triploid variety. The flowers have six tepals, six stamens, and one style (Wheeler 6 2006). ). Slim-Leaf Onion is very similar to Hookers onion (Allium acuminatum), but the tepals do not turn back at the tips as Hooker’s onion does (Pojar and Mackinnon 1994). Leaf senescence often occurs before the flowers are fully developed (Hawrynski & Allen 2011). Figure 4. Slim-leaf onion flowers in Oak Haven Park, June 213. Habitat Slim-leaf onion is a locally abundant rare species, found in dense but small populations, in a specific habitat type over a large range (Hawrynski 2002). In BC Slim-leaf onion is at the northern end of its range and is restricted to the Garry oak ecosystem in southwest BC, which has high densities of human population (Hawrynski 2002). In a 1996 survey approximately 30 populations were found in Southwest BC (Hawrynski 2002). Previously known populations of Slim-leaf onion in BC, which were no longer extant when surveyed in 1996, were largely located 7 in the Victoria area on southern Vancouver Island, where there is a large human development density (Hawrynski 2002). Slim-leaf onion can grow on a variety of substrates as long as there is reliable moisture in spring. Populations of Slim-leaf onion are usually found on southern or southwestern exposures, in full sunlight or light shade. There is often evidence of moisture from spring seepages on the site. The soil can range from very rocky shallow soil to deep rich soils (Hawrynski 2002, Hawrynski et al. 2011). Reproduction & Dormancy Slim-leaf onion reproduces by seed and vegetative offset (Wheeler 2006). Plants produce 1-3 seeds per flower, and in favorable conditions large bulbs can produce up to 8-12 offsets per year (Wheeler 2006). Most BC populations of slim-leaf onion are triploid which reduces sexual fertility (Comai 2005, Wheeler 2006). Vegetative propagation is likely the main mode of reproduction in most BC populations, with seed production minimal in relation to offsetting in the BC populations studied (Hawrynski 2002, Wheeler 2006). Clonal growth is likely much more important for a slim-leaf onion population growth rate than seedling recruitment due to this reduced fertility (Hawrynski 2002, Wheeler 2006). Slim-leaf onion plants are often dormant for a prolonged period, with an average yearly dormancy in some populations of 38%. Plants are frequently dormant for multiple years (Hawrynski 2002, Hawrynski et al.