Status of the threatened plants: Lake Huron Tansy (Tanacetum huronense Nutt.) and Pitcher’s Thistle (Cirsium pitcheri T.&G.) at the Antrim Creek Natural Area

 

by Hal C. Reed, Ph.D. and David Mahan, Ph.D.

Au Sable Institute of Environmental Studies

Based on 2004 Conservation Biology student reports

 

Four teams from the Conservation Biology and Field Botany classes examined the status of the two threatened plants (Lake Huron Tansy, Tanacetum huronense Nutt. and Pitcher’s Thistle, Cirsium pitcheri T.&G.) at the Antrim Creek Natural Area.  One team studied the tansy population in a 25 x 25 ft plot just north of the line of boulders to the beach entrance (section 16, Fig. 1), another team studied the thistle population in a 25 x 25 ft plot approximately 450 m south of the tansy plot (section 7, Fig. 1), a third team examined the microhabitat in a 1 m radius around each of the budding thistles, and the final group was the transect team that conduct a walking survey of all budding or flowering plants of both species from the southern end at Antrim Creek to the northern end of the beach (sections 1-27, Fig. 1).  This team also counted thistles of all ages in the north end sections 1-10 where all the thistles were located. Each 25 x 25 ft plot has been examined in mid June since 1998 for the number of plants, age of plants (1st year plants, medium non-flowering plants, and flowering plants), and spatial distribution within the plot.  We utilized this plot method to sample the population as we did not have the time and resources to sample the entire preserve. The transect team conducted a survey once a summer since 2001 in order to get a better estimate of the total number of flowering plants in the preserve.  GPS readings were taken of each corner of the two plots, each 50 m point along the transect, and each budding thistle examined in the microhabitat study.  These points should facilitate tracking these plots and plants in future years.  The 2004 work required two trips of two classes (31 students + 3 faculty) and additional 3rd trip of two students. 

 

In 2004 the thistle plot team counted the highest total number of individuals of all 3 age classes (262) in the study plot as compared to the previous 7 years.  This total exceeds the previous high of 207 individuals in 2001 and to 159 individuals in 2003.  However, in 2004, only 3 budding/flowering plants were found in contrast to 6 budding/flowering plants last year.  Also, the number of budding/flowering plants is considerably less than 22 flowering plants in 1999 and 15 flowering plants in 2001.  The average number of buds per plant has also been steadily declining in this plot.  These two factors together indicate an overall decrease in productivity and potential reproduction for the thistle population.  However, since these plants may require 5-8 years to flower, the plot population may be in a low reproductive period of their life cycle. 

 

The 2004 thistle microhabitat team examined the associated plants, slope and other thistles in a 1 m radius surrounding each budding/flowering thistle in sections 1-10.  Three zones of the dunes were clearly recognized from the lake shore to the forest edge. The beach strand is the zone adjacent to the lake; next is the fore dune zone stabilized by dune grasses; and the blowout zone that is dominated by steeper slopes of sand on the leeward side of the foredunes.  The foredunes occupied the largest area of the three regions at Antrim Creek.  Only 8 budding/flowering thistles were located in the beach strand, while the fore dune and blowout zones had 53 and 33 budding/flowering plants, respectively.  Budding/flowering thistles in this fore dune area had the highest number of buds per plant, highest average number of buds/plant and had more other thistles in the surrounding 1 m area than did budding/flowering thistles in the other two dune zones.  The blowout zone (eastward toward the forest from sections 3-6 at the south end of the area) had a thriving population of thistles with little competition from the dune grasses that predominate in the fore dune zone. The most common plants near the budding thistles were Marram or beach grass, wormwood,  bearded wheatgrass, horsetail, and in the beach subregion, Baltic rush and balsam popular were commonly encountered.  Budding thistles in the beach strand were often in moist or wet sand and were wilted. 

 

In 2004 the tansy plot team counted nearly the same number of individuals of all 3 age classes (225) in the study plot as compared to the total of 2003 (224).  These totals are the highest number since the study was initiated.   However, only 5 budding/flowering plants were counted in 2004 which is similar to the 4  budding/flowering plants that were found in the past two years in contrast to 21 flowering plants in 2001 and 11 flowering plants in 1999 in this same study plot.  Interestingly, these peak years in tansy flowering coincide with the high number of flowering thistles in 1999, a drastic decrease in 2000, and another increase in 2001.   Thus, growing conditions in these two peak years may have been good for plant reproduction of both dune species.

 

The transect team results in 2003 and 2004 showed that the Lake Huron tansy had a higher population and was more widely distributed at Antrim Creek Natural Area than the Pitcher’s thistle population.  However, there was another decrease of flowering tansies from 280 plants in 2003 down to 93 plants this year.  The tansy flowering population was 836 plants along this transect in 2001 and thus, we have seen nearly a 10 fold decrease in flowering tansies in a 4 year period. It is unknown whether this decline is due to a decrease in the overall population or due to the cyclical nature of their flowering population.  The thistle population primarily inhabits the fore dune and blowout zones of the south end of the area in sections 3-10 while the tansies primarily occupy the more stabilized dunes in the middle and northern end of the area in sections 14-23.  In contrast to the decrease in the number of flowering tansies, flowering thistles along the transect increased from 34 plants in 2003 to a 4-year high of 57 plants this year. This team also did a complete count of all age classes of thistle at the south end of the area (sections 3-10), including the blowout zone, and found a total of  65 budding plants and 243 younger, non-budding plants.  This data can be used for comparison for future total counts in this same area.

 

The tansy population is certainly higher and more widely distributed in the Antrim Creek Natural Area than the thistle population.  It appears that the tansy and thistle populations (of all age classes) within the two study plots are stable with some increases in the past few years.  However, the number of budding/flowering plants is quite low in each study plot raising concerns about the future of these populations.  The number of younger plants has been somewhat stable and thus, there should be another increase in flowering plants of both species in the next few years. 

 

Along the transect in 2004, the number of budding/flowering tansies was only 1/3 of the number of plants found in 2003, while the budding/flowering thistles increased in 2004 over the totals of the past two years.  The tansies occupy primarily the foredune area that is more established with dune grasses and do not occur in the south end of the Antrim Creek area and the blowout zone (sections 5-10) where the thistles predominantly are found.  The microhabitat study revealed that the foredune zone contained the highest number of thistles and the most productive individuals (in terms of number of buds/plant) as compared to the other zones.  However, the blowout zone has a significant population of budding/flowering thistles (33) since the blowout is significantly smaller in area than the foredune zone.   The population fluctuations during this 8 year period could be due to varying lake levels, cyclical nature of their phenology and changing dune landscape.  Further in-depth studies are need if these population trends are to be further clarified.

 

One of the most significant ecological features of the Antrim Creek Natural Area is the presence of these two federal/state-listed species of plants.  In order to insure their survival, specific management actions will be critical.  Such actions could include: interpretative signs focusing on the plants; more effective barriers to vehicular traffic on the beach; rope or wire barriers to limit human and pets from walking in sites with high concentration of the listed plants (e.g. the blowout zone and the section 7 thistle plot area); monitoring for invasive species (e.g. baby’s breath and spotted knapweed) occurrence in this dune system as has happened at Sleeping Bear Dunes National Lakeshore.


Text Box:

Fig. 1. Map of Antrim Creek Natural Area.  Sections 1-27 along the beach represent 50 m sections.  The Pitcher’s thistle is exclusively found in sections 3-10 and the study 25 x 25 ft plot is section 7.  The Lake Huron Tansy is predominately found in sections 14-23 and the study 25 x 25 ft plot is in section 16.