Short-rayed alkali aster (Symphyotrichum frondosum) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search efforts

Fieldwork was conducted in the Okanagan over a period of ten days in June and August, 2002, and over four days in 2003.  Known populations were checked and confirmed at Vaseux and Skaha Lakes. Additionally, intensive targeted searching of most saline lake and pond shorelines was conducted from Osoyoos in the south Okanagan to Salmon Arm in the north.  The exception was non-public areas of Indian Reserve lands on the east shoreline of Osoyoos Lake, where additional sub-populations of the species have been reported.  Timing of these searches was important, so that sufficient drawdown would have occurred that would expose the species’ habitat.  For the most part, most lakes and ponds had receded considerably by the time searches were conducted in each year; however, additional drawdown could occur, and plants may yet be found in some spots.  The discovery of the new Penticton population in the second week of September 2003 indicates additional searching for the species is warranted.

Sections of the shoreline of the following lakes were checked: Deadman Lake, Gallagher Lake, Mahoney Lake, Okanagan Lake (including Sun Oka Provincial Park), Osoyoos Lake, Prather Lake, Shannon Lake, Skaha Lake, Tugulnuit Lake, Twin Lakes, Vaseux Lake, White Lake, and others, plus associated wetlands. 

Additionally, some limited searching for this species was conducted in the lower Fraser River valley (2002-2004).

Abundance

Abundance data are limited for this species and are summarized in Table 1 (below).  Abundance for this species fluctuates with annual climatic variation and water levels so data from single years do not provide a true picture of the species.

 

(All counts are for mature plants, no seedlings are reported.)

Table 1.  Summary of abundance data for Symphyotrichum frondosum in Canada (2005)
Location Sub-population 1939 1992 1993 1994 1995 1999 2000 2002 2003 2004
Osoyoos Lake   Collected by Eastham, label data indicates abundant                  
Osoyoos Lake Douglas #1
1 km SE of Mica Creek
        X (no data) 40+ plants over 200 m2   no data no data no data
Osoyoos Lake

Lomer  #2

Cottonwood Park/ pump house

  X (no data)         1 plant over 1 m2 Extirpated    
 Osoyoos Lake

Lomer  #3

Mica Creek

    50-70 plants (no area given)         No data    
 Osoyoos Lake Sears et al. #4 .             5 plants over 1 m2    
Vaseux Lake             Approx.70 plants over 20 m2   100 plants over 20 m2 12 plants under white sweet-clover over 4 m2  
Skaha Lake             4900 plants over 500 m2   525 plants over 500 m2 No count, but  plants covered 7 m.  
Penticton*                   8 plants, no area given  
Surrey         “small patch”       extirpated    

*This population was reported by Eastham (1947) as “locally abundant in wet saline soil, Penticton and Osoyoos”.

Fluctuations and trends

An assessment of the biogeographic patterns of this species leads the writers to believe that shoreline use (including recreational use and development, such as campgrounds, boating facilities, etc.), site maintenance activities, and managed lake levels at three sites have led to a decline in a species that was once more abundant within this region (Eastham 1947).  Specifically:  One reported sub-population at Osoyoos Lake is extirpated (Lomer pers. comm. 2003).  The Skaha Lake population has declined in numbers since 2002 as a result of site maintenance.  Decline was also noted in numbers at Vaseux Lake in 2003, where white sweet clover had invaded in 2003.

Note that most of the eastern shoreline of Osoyoos Lake was not searched, and additional populations may occur in this relatively undeveloped section.  However, development proposals are evolving for this area.

Rescue effect

It is quite possible that meta-population dynamics are in play with this species, and that the BC populations are connected via currents, wind or wave action, or via dispersal by waterfowl.   Given that the rivers that connect these populations flow south, and seed dispersal happens in the fall, the probability of long distance transport of seeds from populations in Washington into Canada is low.

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