Trees lap up the city heat
Monday, 7 May 2012
Canterbury University researchers have taken part in a US study showing city heat encourages trees to grow much more quickly than those grown in the countryside.
The study involved growing common US native red oak seedlings from acorns for one season at four sites from Central Park in urban New York City to a cooler, rural setting in the Catskill Mountains in upstate New York about 160km away.
The urban location had average maximum temperatures 2.4 C warmer than the rural location, and minimum temperatures 4.6 C higher.
By the end of summer, the city trees had put on eight times more biomass than those raised outside the city mainly by putting out more leaves, the study, published in the journal Tree Physiology, found.
The researchers also grew similar seedlings in the lab under identically varying temperatures, which showed much the same result.
That confirmed the researchers' hypothesis that the increased growth was due to warmer night temperatures in the city - the so-called urban heat island effect.
The urban heat island is a well-known phenomenon that makes large cities hotter than surrounding countryside; it is the result of solar energy being absorbed by pavement, buildings and other infrastructure, then radiated back into the air.
'Some things about the city are bad for trees. This shows there are at least certain attributes that are beneficial,' lead author Dr Stephanie Y Searle said. She carried out the research as part of her PhD at the University of Canterbury.
'The seedlings grew much larger in the city, with decreasing growth as you get farther from the city.'
Due to air pollution the city also had higher fallout of airborne nitrogen-a fertilizer, which could have helped the trees as well, Searle said, but temperature seemed to be the main factor.
Searle's supervisor and co-author on the paper, Prof Matthew Turnbull from Canterbury University, said the urban heat island effect may be having an impact on tree development in New Zealand, especially in densely populated areas such as central Auckland.
Other experiments done in Japan and Arizona have shown that higher temperatures, especially at night, may promote growth of rice plants and hybrid poplar trees.
A 2011 study showed that conifers in far northern Alaska have grown faster in recent years in step with rising temperatures, The Earth Institute at Columbia University said.
Some US eastern seaboard trees also seemed to be having growth spurts in response to higher carbon dioxide levels alone, according to a 2010 study by scientists at the Smithsonian Institution.
Heat could cut both ways; in lower latitudes, rising temperatures and shifting weather patterns appeared to be pushing some species over the edge by causing ecological changes that stressed them. Massive die-offs were under way in the US West and interior Alaska.
Some evidence indicated that with a warming climate, New York area forest compositions were already changing, with northerly species dwindling and southerly ones that tolerated more heat coming in, tree physiologist Kevin Griffin of Columbia University said.