Non-native forest species 'extending growing season'

Autumnal Acer leaves (Image: BBC) Species that display an extended growing season can have a big impact on a habitat's ecology

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Non-native plant species are extending the growing season in eastern US forests by an average of four weeks, a study has suggested.

There was no difference in the start of growing during the spring, but the report found a noticeable difference between native and non-native species in the autumn.

This could have a profound impact on forest ecosystems, such as how soil nutrients are absorbed, the paper says.

The findings are published by Nature.

"There is a bit of a saying in these parts that if you go for a hike in March and you see something green, then it is an invader," said author Jason Fridley, an ecologist at Syracuse University, US.

"So I thought I would invest little bit of my time to quantify that if the invaders were waking up a little earlier in the spring, and were keeping their leaves longer in the fall, what was the significance to their ecology and their ability to get into the forests."

Prof Fridley said that his experiment, carried out over three years and involving more than 70 species, actually revealed that there was not a signal of non-native species coming into leaf earlier than native species during the spring.

"It turns out that the real difference is in the autumn - nobody was expecting this - it turns out that our native species in the east of the United States really don't do anything after October, but the invaders were still very active," Prof Fridley told BBC News.

Whether the later finish to the growing season gave the non-native species an advantage was an area that requires further investigation, he explained.

"It turns out that although the invaders are doing something very different from native species, our calculations suggest that they are not getting more food over the course of the year.

"We think this is because the native species have a slight edge in the spring, so when the year is taken as a whole there is no real net benefit."

Experimental plot of plants under artificial shade (Image: Genevieve Fridley) It could be that "Old-World" species have not experienced so much evolutionary disruption

Prof Fridley added that that holding on to the leaves for longer was likely to have an impact on the area's ecosystems.

"The invaders are actually losing nice green leaves to that fall to the forest floor, and those nutrients are feeding the microbes and feeding the nitrogen cycle," he said.

"So we do think they are having a pretty big impact on what is happening beneath the ground.

He added that there was a possibility that the extended growing season among the non-native species could have a wider impact on the environment.

"This opens up a lot of interesting questions for the food chains, such as: are there insects or mammals that are taking advantage of the fact that there are more things to eat very late in the season?" he observed.

"We have real problems with deer in the eastern parts of the US, and the longer that things stay green in the [autumn], then it is a brighter outlook for deer trying to get through the winter.

Prof Fridley said that the species that were displaying the later leafing behaviour were primarily from China, Japan and Korea, with a number from the UK and Europe.

"I would love to know what they are doing in their native environments," he said.

"I have a sneaking suspicion that the plants coming from the Old World to the New are actually better adapted than New World species, and it could be that the New World experienced some pretty major disruptions over the past two million years during the last Ice Ages."

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