The size and age of plants have more impact on productivity than temperature and precipitation, according to a landmark study by University of Arizona (UA) researchers.

UA professor Brian Enquist and postdoctoral researcher Sean Michaletz, along with collaborators Dongliang Cheng from Fujian Normal University in China and Drew Kerkhoff from Kenyon College in Gambier, Ohio, have combined a new mathematical theory with data from more than 1,000 forests across the world.

The theory suggests that climate has a relatively minor direct effect on net primary productivity, or the amount of biomass – wood or any other plant materials – that plants produce by harvesting sunlight, water, and carbon dioxide.

“A fundamental assumption of our models for understanding how climate influences the functioning of ecosystems is that temperature and precipitation directly influence how fast plants can take up and use carbon dioxide,” said Enquist, professor in the UA Department of Ecology and Evolutionary Biology, whose research lab led the study.

“Essentially, warm and wet environments are thought to allow plant metabolism to run fast, while cold and drier environments slow down metabolism and hence lower biomass production in ecosystems,” Enquist said.

“This assumption makes sense, as we know from countless experiments that temperature and water control how fast plants can grow. However, when applied to the scale of entire ecosystems, this assumption appears to not be correct.”

To test the assumption on the scale of ecosystems, the team developed a new mathematical theory that assesses the relative importance of several hypothesized drivers of net primary productivity.

The theory was then evaluated using a massive new data set assembled from more than 1,000 forest locations across the world.