This story was researched and written by Vanessa Buzzard, a graduate student in the UA Department of Ecology and Evolutionary Biology, as part of the course "Communicating Science," offered through the UA College of Science.
The study was funded through an NSF MacroSystems award (1065861), a fellowship from the Aspen Center for Environmental Studies, and through support by the National Natural Science Foundation of China (31170374 and 31370589) and the Fujian Natural Science Fund for Distinguished Young Scholars (2013J06009).
The size and age of plants have more of an impact on their productivity than temperature and precipitation, according to a landmark study by University of Arizona 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 to show 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.
The findings were made available as an advance online publication by the journal Nature on Sunday.
"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, a 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," he 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."
Net primary productive are made visible in the animation below. Tan colors represent areas with low net primary productivity and become darker green as net primary productivity increases. This means that the tan areas may be responsible for releasing more carbon into the atmosphere than the plants absorbed. As the seasons change, the areas with green or higher net primary productivity shift across the globe. (Source: NASA)
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. That theory was then evaluated using a massive new data set assembled from more than 1,000 forest locations across the world.
The analysis revealed a new and general mathematical relationship that governs worldwide variation in terrestrial ecosystem net primary productivity. The team found that plant size and plant age control most of the variation in plant productivity, not temperature and precipitation as traditionally thought.
"This general relationship shows that climate doesn't influence productivity by changing the metabolic reaction rates underlying plant growth, but instead by determining how large plants can get and how long they can live for," said Sean Michaletz, lead author of the study and a postdoctoral researcher in the UA Department of Ecology and Evolutionary Biology. "This means that plants in warm, wet environments can grow more because their larger size and longer growing season enable them to capture more resources, not because climate increases the speed of their metabolism."
The finding does not, however, mean that climate is unimportant for plant productivity, the researchers noted.
"Climate is still an important factor, but our understanding of how it influences ecosystem functioning has now changed," Michaletz said.
The team's findings suggest that mathematical models used for predicting the effects of global climate change can be improved by accounting for the effects of plant size and plant age on net primary productivity.
"Understanding exactly how climate controls net primary production is important for understanding the plant-atmosphere feedbacks that control climate change," Michaletz said.
Enquist added: "In other words, to better predict how ecosystems will change with climate, we need to understand what influences the amount of plant biomass in a given area as well as its age."