LINDELL BEACH, B.C. – A discovery at the University of Toronto could be good news for prairie farmers worrying about the spread of purple loosestrife.
University scientists have found that the challenge of Canada’s shorter growing season places a severe reproductive penalty on the invasive weed.
“Bigger plants produce more seeds,” said researcher Rob Colautti with the university’s ecology and evolutionary biology department.
“That larger size may have a fitness trade-off, or constraint, and in this case it’s delayed flowering. When a purple loosestrife plant begins its transition to flowering, cells switch from producing vegetative structures to producing attractive flowers with lots of nectar and, eventually, seeds.”
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Colautti said there is a genetic correlation between flowering time and plant size. The earlier the plant switches from growing leaves and stems to producing flowers, the smaller the plant will be and the fewer seeds it will produce.
“Genes that cause early flowering also reduce plant size, so early flowering and small size evolve together,” he said.
“Smaller size results in lower seed production, which is likely to limit the spread of purple loosestrife in northern regions.”
To test their hypothesis, researchers studied 20 purple loosestrife populations along a 1,200 kilometre stretch from Easton, Maryland, to Timmins, Ont.
They also studied plants grown in an experiment at the U of T’s Koffler Scientific Reserve to measure genetic variation in flowering time and size.
The results of the study were published in the Proceedings of the Royal Society of London and featured in the journal Nature.
Researchers theorized that plants that flowered too early in the season ran out of resources and stopped flowering while those that flowered too late produced flowers after pollinators were gone and when cold weather could kill any seeds produced.
Season length determines how early or late a plant flowers. The further north the plant grows, the shorter the growing season and the earlier the plant needs to flower.
However, it will do so with genetically skewed consequences as a result of its smaller, early-season size.
“As plants flower earlier and earlier they might get to a point where they are so small that the vegetative structures they produce are just barely enough to maintain plant survival with nothing left over for making flowers,” Colautti said.
“The result is a threshold, meaning that no plant can flower before a certain date.”
It also means that plants trying to spread north are trapped in their own genetic limitations and max out their reproductive fitness.
“These results are consistent with adaptive evolution in response to different season lengths at different latitudes,” he said.
However, Colautti said global warming could change this scenario. If climate change extends Canada’s growing season, which scientists have predicted, the weed could be in a better position to extend its seasonal growth, produce more seeds, expand its invasive range and displace more native plants.
“This could increase the rate of invasion,” Colautti said.
“But we have to be careful (in this prediction) because there are a lot of other things that could affect plant invasions besides seed production.”
Loosestrife on the loose
- Purple loosestrife is an aggressive wetland species that chokes out native vegetation, clogs irrigation canals and displaces plants that provide food, shelter and breeding areas for wildlife such as waterfowl, songbirds, turtles and muskrats.
- It is associated with the decline of the native swamp rose mallow and other indigenous plant species. In urban areas it blocks drainage ditches.
- The plant arrived in eastern North America from Europe in the early 1800s. It invaded Canada no later than 1814 and was a significant weed in Quebec by the 1930s. It is now widespread across Canada, particularly in the St. Lawrence-Great Lakes region, southern Manitoba, southwestern British Columbia and the Maritimes.
- While purple loosestrife is predominantly a wetland species, it can on rare occasions establish itself on dry soil. However, the plant has run into challenges and may have finally encountered its limits to growth in Canada’s northern climate.
- It is an attractive, hardy perennial that can grow to more than two metres tall. The flower spikes are made up of many individual pink-purple flowers with small yellow centres.
- Like many exotic species, it has few pests or diseases and resists various control methods. Plants can produce as many as three million seeds a year, which are easily spread by water, wind and wildlife.
