'Epidemiological' tree study shows impacts of climate change on forests

Tree growth and fecundity - the ability to produce viable seeds - are more sensitive to climate change than previously thought, according to an 18-year study of 27,000 individual trees by Duke University researchers.

The study, published April 5 in Global Climate Biology, identifies earlier spring warming as one of several overlooked factors that affect tree reproduction and growth, and can help scientists and policymakers better predict which species are vulnerable to climate change, and why.

It also identifies summer drought as an important but overlooked risk factor for tree survival and fecundity, and finds that species within four broad genera of trees - Pinus (pine); Ulmus (elm); Fagus (beech) and magnolia - are particularly vulnerable to variations in climate.

'In a sense, what we've done is an epidemiological study on trees to better understand how and why certain species, or demographics, are sensitive to variation and in what ways,' said the study's lead author, James S. Clark, the H.L. Blomquist Professor of Environment and professor of biology and statistics at Duke's Nicholas School of the Environment.

Clark and his colleagues measured and recorded the growth, mortality and fecundity of each of the 27,000 trees in the study at least once every three years, ultimately compiling an archive of more than 280,000 tree-years of observed data.

The researchers analysed the effects of climate change on the species of trees with spatial climate correlations. This approach allowed them to calculate the relative importance of various factors, such as competition for light and summer drought, alone and in combination, and the effect on the trees.

'As climate continues to change, we know forests will respond. The problem is, the models scientists have used to predict forest responses focus almost solely on spatial variation in tree species abundance - their distribution and density over geographic range,' Clark said.

If all trees of a species grew in the same conditions - the same light, moisture, soil and competition for resources - this generalised, species-wide spatial analysis might suffice. Scientists wouldn't need to worry about demographic variables and risk factors when trying to predict biodiversity losses due to climate change. 'But in the real world, we do,' Clark said. 'That's where this new concept of climate and resource tracking of demographic rates comes in.'

Trees are much more sensitive to climate variation than can be interpreted from regional climate averages. 'By quantifying the effects and relative importance of competition and climate variables, including previously overlooked impacts on fecundity, over both time and space, the model we've developed addresses this need,' he said, 'and can be used to help guide planning.'

The trees studied included 40 different species, located in 11 different forest stands in three geographic regions of the Southeast - the southern Appalachians, the Piedmont and coastal plain. They were subjected to both natural and experimental variations.