A team of researchers from the University of Arizona may have developed the next best thing to a crystal ball that will inform ecologists how future forests will react to the changing climate.
Image Credit: University of Arizona.
By integrating tree-ring data with U.S. Forest Service inventory data on Arizona's ponderosa pines, the team generated a fuller picture than conventional models have provided of what triggers the future growth of trees. The scientists forecast a 56 to 91% drop in individual tree growth, according to a new study reported in the journal Global Change Biology.
The growth declines we're forecasting will mean less uptake of atmospheric carbon dioxide in the future by Arizona's forests. 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.
Kelly Heilman, Lead Study Author and Postdoctoral Research Associate, Laboratory of Tree-Ring Research, University of Arizona
Forests eliminate carbon dioxide from the atmosphere, which balances a few greenhouse gas emissions worldwide and helps to alleviate climate change.
"It's a free service that forests provide, so forests have been touted as one of the many natural climate solutions that countries rely on to offset their emissions," Heilman said. "But competition between trees, droughts, and disturbances can reduce forest carbon uptake. Knowing how much carbon forests take up globally is essential to addressing the climate crisis and planning for a resilient future."
A number of countries, including the United States, keep national forest inventory programs wherein foresters take a census of trees in 1/6-acre plots to monitor forest status and change. These censuses are taken as often as every five years, but in the western United States, they are carried out every decade. The data gathered includes the number of trees, their diameters and the quality of soil.
Ten years doesn't provide enough resolution to look at year-to-year variability and extremes—both things that are exacerbated by climate change.
Margaret Evans, Study Co-Author and Assistant Professor of Dendrochronology, Laboratory of Tree-Ring Research, University of Arizona
Dendrochronologists can use tree rings more or less like a rain gauge or thermometer to study the variability of climate.
"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," Evans said. "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."
When the scientists pooled the tree-ring data with the census data, they could deduce the size of the trees each year and see how the trees reacted to climate variables such as year-to-year variation in temperature and rainfall, as well as ecological features such as competition with other trees, tree diameter and soil quality.
"Past studies have focused just on climate and excluded other patterns in the data because so much of the science of dendrochronology is focused on climate, specifically reconstructing past climate," Evans said. "We're using the tree-ring data in a much more ecological way and thinking about all the things affecting the tree at the same time."
The reasons for deteriorating tree size are complex and varied, but the key culprit is the fact that ponderosa pines in Arizona grow less as the temperature rises. This is particularly true for the largest trees.
A tree has to work against gravity to get water to its top, and the tallest trees have to work the hardest. If you turn up the temperature, the water-transport system of the tree is under even greater pressure and often is damaged. Warming means trees are more drought-stressed, and growth is reduced.
Margaret Evans, Study Co-Author and Assistant Professor of Dendrochronology, Laboratory of Tree-Ring Research, University of Arizona
While taller trees are more susceptible to drought triggered by high temperatures, scientists also learned that small trees are more susceptible to drought triggered by insufficient water. Their smaller roots, which span a smaller area compared to the roots of larger trees, find it hard to extract moisture from the soil.
The third interaction is between forest density and the climate variables. These interactions show that denser forests fare worse when it's hotter and drier, which is generally what we'd expect for the species, but this is concerning given the recent densification of these forests.
Kelly Heilman, Lead Study Author and Postdoctoral Research Associate, Laboratory of Tree-Ring Research, University of Arizona
The study has implications for forester managers, who can now have the opportunity to alleviate climate-triggered stress on trees, Evans said.
"Foresters can't influence the climate, but they can change forest density to reduce competition for the remaining trees," Evans said. "If you have both an overly dense forest and climate warming happening at the same time, that's a double whammy. But if you thin the forests, you can remove one source of stress."
To enhance their tree growth prediction to a greater level, the scientists hope in future work to take into consideration variables such as insect disturbances or past wildfires.
"We analyzed different sources of uncertainty about future tree growth," Evans said. "Knowing where the uncertainty comes from is the grist for scientific improvement."
Journal Reference:
Heilman, K.A., et al. (2022) Ecological forecasting of tree growth: Regional fusion of tree-ring and forest inventory data to quantify drivers and characterize uncertainty. Global Change Biology. doi.org/10.1111/gcb.16038.