Individual study: Effects of sand burial depth on seed germination and seedling emergence of Pitcher's thistle Cirsium pitcheri: a greenhouse study using seed collected from Manitoulin Island, Ontario, Canada
Chen H. & Maun M.A. (1999) Effects of sand burial depth on seed germination and seedling emergence of Cirsium pitcheri. Plant Ecology, 140, 53-60
Pitcher's or dune thistle Cirsium pitcheri is a perennial endemic of sandy beaches and dunes around three of the North American Great Lakes; Huron, Michigan and Superior. Its populations have dwindled primarily due to habitat loss (shoreline development), recreational use of beaches, fluctuations in lake levels, infestation by plume moths, and browsing by white-tailed deer Odocoileus virginianus. It is considered 'threatened' in both Canada and USA. A greenhouse study was conducted to determine the effects of sand burial on seed germination and seedling emergence of the thistle.
About 400 seeds were collected on August 4-5, 1993 from a thistle population in Providence Bay, Manitoulin Island, Ontario, eastern Canada. These seeds were germinated in March 1994, and grown in a green house for 5 months; in August they were planted out in an experimental garden at the University of Western Ontario. Plants flowered in July/August 1995 and seeds of healthy appearance were selected. A germination test showed that their viability was 100%.
In October 1996, seeds were sorted into three groups by weight (small, medium and large) and buried at 2, 4, 6, 8, 10 and 12 cm depths in plastic pots filled with unsterilized, sifted (to remove any seeds and debris) sand from Lake Huron sand dunes. These pots were placed outside in sand boxes for natural stratification. In early February, the pots were brought into the greenhouse (16 hr photoperiod; 23ºC during the day, 14ºC at night). Pots were watered daily. Emerged seedlings were counted daily over a period of 8 weeks.
Germination: Percent germination was not affected by seed size but was greatly influenced by burial depth, being much higher at shallower depths (2 cm – 72%; 4 cm – 55 cm; 6 cm – 49%; 8 cm – 3 %; 10 cm <1%; 12 cm – <1%).
Seedling emergence: Seedling emergence (percent with cotyledons appearing at the sand surface) were not related to seed size but emergence was negatively correlated with planting depth. Most seedlings emerged from 2 cm depth; followed by 4 cm and 6 cm. Only one seedling (out of 30 seeds planted) emerged from 8 cm depth and none at 10 or 12 cm depth. Seedlings from large seeds produced longer roots than those of small seeds. The authors consider that a larger root system would probably enhance seedling establishment.
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