Knowing dangerous climate change is on the horizon and planning for said changes are two different scenarios. Even with increased environmental research and simulations over the past few years, scientists can still only guess how forests of the future will respond to the changing climate.
A new study from University or Arizona researchers has captured a more complete picture of future tree growth by combining tree-ring data with U.S. Forest inventory data on the state’s ponderosa pines.
"While Arizona's forests are relatively small in terms of their contribution to the total U.S. carbon sequestration, our approach can be used to make the same predictions for forests around the world," said lead study author Kelly Heilman, a postdoctoral research associate in the Laboratory of Tree-Ring Research. “Knowing how much carbon forests take up globally is essential to addressing the climate crisis and planning for a resilient future."
Many countries, including the U.S., maintain national forest inventory programs in which foresters take a census of trees in 1/6-acre plots to track forest status and change. In the western U.S., the census is done every 10 years—which is not often enough to provide good data on climate-induced changes.
To get a better understanding of tree growth amidst climate state, Heilman and her colleagues combined the census data with their tree-ring data.
"The annual resolution data you get from tree rings strongly complements the forest inventory measurements when you combine them in the statistically robust way that we have here," said study co-author Margaret Evans, assistant professor of dendrochronology in the Laboratory of Tree-Ring Research. "We're using tree rings in a new way to think about how the whole forest ecosystem is behaving and how carbon sequestration is influenced by climate variability."
The researchers were able to infer the size of the trees each year and see how the trees responded to climate variables, such as year-to-year variation in rainfall and temperature, as well as ecological characteristics such as competition with other trees, soil quality and tree diameter.
According to the study results, published in Global Change Biology, the researchers predict a 56 to 91% decline in individual tree growth. While all the reasons for the decline are varied and complex, the main culprit is the fact that ponderosa pines in Arizona grow less as temperature increases. This is especially true for the largest trees.
While taller trees are more vulnerable to drought driven by high temperatures, the researchers discovered that small trees are more vulnerable to drought driven by lack of water. Their smaller roots, which cover a smaller area than the roots of larger trees, struggle to extract moisture from the soil.
Additionally, the study results showed that denser forests fare worse when it's hotter and drier, a find Heilman characterized as “concerning given the recent densification of the forests.”
To improve their tree growth prediction even more, the researchers intend to consider other variables, such as past wildfires or insect disturbances, in future studies.
"[In this study], we analyzed different sources of uncertainty about future tree growth. Knowing where the uncertainty comes from is the grist for scientific improvement,” said Evans.
Photo credit: University of Arizona