Forest growth season getting longer in northern hemisphere, study says

Satellite images taken over a 25-year period show that, on average, what plant life recognizes in the northern hemisphere as the end of autumn is taking place later in the year, and spring is starting earlier, according to a U.K.-led research team.

University of Southampton geography professor Peter Atkinson and his colleagues say they saw evidence that the return of greening vegetation is now arriving "slightly earlier" in the year, while a more "significant" change has been noted in the delay of when plant dormancy arrives.

Atkinson worked with Dr. Jadunandan Dash at the university and Prof. Jeganathan Chockalingam from the Department of Remote Sensing at the Birla Institute of Technology in India. They looked at images taken from 1982 to 2006, studying in detail the growth cycle of the vegetation and identifying physical changes such as leaf cover, colour and growth.

The team was able to examine the data for specific vegetation types: grassland, shrub-land, forest and cropland; broad-leaved deciduous forest; needle-leaved evergreen forest; needle-leaved deciduous and evergreen forest; mixed broad-leaved and needle-leaved forest; and mixed-forest, shrub-land and grassland.

They analyzed data across all the groups, taking into account human interventions such as farming and fires.

The most pronounced change they found was in the broad-leaved deciduous and needle-leaved deciduous forest groups, showing the end of the growing season (EOS) has become significantly later.

As much as 12 days

For the needle-leaved deciduous forest group, the delay in EOS was found to be as much as 12 days over 25 years, but for other classes, it could be as much as 27 days, depending on the specific groups of trees and location.

Needle-leaved deciduous vegetation, on average, had the maximum decrease in the start of growing season (SOS) of 1.07 days per year. Broad-leaved deciduous vegetation had the maximum delay in EOS of 1.06 days.

Latitudes stretching from 55-65 degrees north experienced the greatest changes in the growing season. The southern line cuts through Glasgow, Copenhagen and Moscow while the northern line cuts through Iceland and the northern half of the Scandinavian countries.

The study, published in the journal Remote Sensing of the Environment earlier this month, mentions a "changing climate," and says the satellite images provided "more reliable estimates" of the trends in annual vegetation growth occurring at latitudes greater than 45 degrees north.

'Significant piece' of climate change puzzle

"There is much speculation about whether our seasons are changing and if so, whether this is linked to climate change. Our study is another significant piece in the puzzle, which may ultimately answer this question,"Atkinson said.

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"Previous studies have reported trends in the start of spring and end of autumn, but we have studied a longer time period and controlled for forest loss and vegetation type, making our study more rigorous and with a greater degree of accuracy," the lead researcher said.

"Our research shows that even when we control for land cover changes across the globe, a changing climate is significantly altering the vegetation growth cycles for certain types of vegetation," he said. "Such changes may have consequences for the sustainability of the plants themselves, as well as species which depend on them, and ultimately the climate through changes to the carbon cycle."