Experts confirm climate is narrowing leaves
Data analysis Australian scientists have defended their conclusion that climate change is decreasing the width of leaves.
The research by Dr Greg Guerin, of the University of Adelaide, and colleagues, found a link between increasing temperatures and a narrowing in leaves of a particular species of plant that grows in the Mediterranean climate of South Australia.
"If it was hot but also moist, the plant could evaporate water out of its leaves to keep cool but because it's so dry, especially in the Flinders Ranges in summer, the plants can't afford to do that," says Guerin whose research was published recently in Biology Letters.
"Another way to keep cool when you can't use water is to have narrower leaves that don't get as hot and cool down quicker."
But in this week's issue of the same journal, Dr Richard Duncan from Lincoln University in New Zealand reanalyses the data supplied by Guerin and colleagues and comes to a different conclusion.
"There is ... no evidence of a link between changes in mean leaf width and recent climate warming," writes Duncan.
Guerin and colleagues originally measured leaves from herbarium specimens of the narrow-leaf hopbush (Dodonaea viscosa subsp. angustissima), collected between 1882 and 2001.
In the first study of its kind they found leaves growing closer to the equator were narrower than those growing further away and leaf width had declined over the study period, coinciding with a recent warming of temperatures.
But in his analysis, Duncan split Guerin and colleagues' data into two time periods - prior to about 1950 when state temperatures were fairly steady, and after this time, when they were on the rise.
He says leaf narrowing occurred prior to about 1950 before the onset of warming and remained constant after that time, a period during which temperatures increased.
Guerin says he can see Duncan's point.
"There's lots of different ways to skin a cat so if you look at it that way with that particular data set, it's quite a good point," he says.
But in an invited reply to the criticisms, Guerin and colleague Professor Andrew Lowe defend their original findings.
They say that leaf data prior to about 1950 is very sparse and this means that it's not possible to split the data into the two periods Duncan used and get "statistically robust" results.
"There were only about 30 collections between 1880 and 1950," says Guerin. "There are not enough data points."
Guerin and Lowe used alternative data splits and statistical methods to re-test their original findings.
In addition, they included an extra 10 years' worth of new data on leaf width from extra specimens collected up to 2011.
"With alternative sub-sampling and new data, our original interpretation is clearly supported," says Guerin.
While he says it's not possible to rule out other factors contributing to leaf narrowing, it is not clear what these might be.
"The trends are what we predict with climate change," he says.
"Other data sets are now required to see if trends like this are occurring elsewhere."