MSU researchers investigate changes in the composition and structure of western forests
Hunter Stanke is leading a team of researchers investigating how forests of the western United States have changed in response to recent shifts in disturbance regimes, climate and other environmental drivers.
The research was funded by National Science Foundation grants, USDA Forest Service Northern Research Station, the US National Park Service, and MSU AgBioResearch. Stanke is working on the project alongside Dr. Andrew Finley and Dr. David MacFarlane of MSU, Dr. Grant Domke of the USDA Forest Service, and Dr. Aaron Weed of the National Park Service.
“The nature of forest disturbances and forest-climate interactions are changing, but we still know very little about the consequences for forest dynamics at broad spatial scales,” Stanke said. Previous studies have established that tree mortality has increased across the western U.S. in the last century, but mortality is just one piece of a much bigger puzzle.
“We’re interested in the net result of that mortality after considering other demographic processes like recruitment and growth. How have western forests actually changed, in terms of their composition and structure, and how different is this from what we would expect given stand development alone,” said Stanke.
The team drew on data collected by the USDA’s Forest Inventory and Analysis (FIA) program. FIA collects information on the status and change of forests on a large network of permanent plots across the U.S., opening the door for large-scale studies of forest dynamics.
“Tree growth, recruitment and mortality rates vary by species and fluctuate as forests age. Since forest landscapes are made up of a mosaic of different forest community types and stand ages, it’s very difficult to use these rates as indicators of forest health,” Stanke said.
Aiming to solve this challenge, the team developed a new demographic index called the Forest Stability Index (FSI), that compares actual changes in tree density to those expected, given changes in tree size distributions within a stand.
The team provided evidence that suggests the composition and structure of forests in the western U.S. have changed considerably in the last two decades. Importantly, the team argues that such change is, on average, not consistent with large-scale reversion of forests toward historically stable conditions.
“The FSI allows for questions to be asked in a quantitative way,” Stanke said.
The index allows forest owners to utilize a theoretical working model to understand if changes in tree density are more or less than would be expected in a “stable” setting. The index can also help researchers disentangle the drivers of change in tree populations.
For example, fire acts as a natural thinning agent in forests and often contributes directly to forest management objectives in the western U.S. The FSI can be used to quantify the importance of fire as a driver of change in forest structure relative to other thinning agents, like tree harvesting.
The team’s findings offer a stark warning of changing forest structure and composition across the western U.S., likely arising from novel combinations of anthropogenic and natural stressors.
As the western U.S. encompasses a very broad spatial and climatic domain, these findings may further indicate large-scale transformation of temperate forests, which make up 25% of the world’s forest cover in other regions across the globe.
Stanke and team’s research was published by Nature Communications on January 19, 2021. Their research can be used by forest owners and managers to help them make predictions and better manage forests in the face of changing climate, disturbance regimes and novel stressors.
This work was supported by: National Science Foundation grants DMS-1916395, EF-1253225, EF-1241874; USDA Forest Service, Region 9, Forest Health Protection, Northern Research Station; US National Park Service; and Michigan State University AgBioResearch.