Looking Down to Look Ahead
A new study sheds light on how forests are changing in the eastern US
There's a change going on in the forests of the eastern United States. If you've been reading this blog, then you know I've written before about the importance of eastern US oaks-their wood is valuable, they provide crucial habitat and food for wildlife, they store a lot of carbon, and they make up a large percentage of the canopy in eastern US forests. So, if you're in one of these forests and you happen to look up, you're likely to see an oak tree. But, if you look down, you're much less likely to see one in the understory. This is because oaks, across much of the eastern US, are actually in decline.
Ecologists have been studying this oak decline for a few decades now, and there seems to be a combination of factors causing it, including changing fire frequency, precipitation pattern changes, land use changes, and changes in wildlife population dynamics. Even some of my own work has looked at this from the perspective of different growth responses to water availability among oaks. Which is all to say that although it's a complex issue, it's also a well-studied one. And what it means is that as these forests mature, the next generation of trees to fill the canopy will have much lower percentages of oak, and the generation after that will have even less.
Where most studies haven't looked, though, is the forest understory, and specifically at what is going on the there. Forests regenerate when a disturbance–like insect outbreaks, disease, or timber harvesting–causes an opening in the canopy. Once that happens, established saplings can take advantage of that opening by entering the canopy and filling the gap, while seedlings can grow into the next generation of saplings. Oak seedlings and saplings may be less prevalent than they are in the canopy, but they do still appear frequently in many areas, and so examining how they respond to openings in the forest canopy can give us insight into what to expect from aging forests across the eastern US.
Conventional forestry wisdom might suggest that oak seedlings and saplings respond differently to different kinds of disturbances, but there's been surprisingly little research in this area. Which is why a recent study, by Dr. Lance Vickers and co-authors from the University of Missouri and the US Forest Service, examined this exact response. Using data from the USDA Forest Inventory Analysis, Lance and his colleagues analyzed how both the type and magnitude of a forest disturbance affects the regeneration of oak seedlings and saplings in eastern US forests. What they found was interesting.
The results from the study suggest that forest canopy openings due to pest insects, disease outbreaks, and timber harvesting all have similar effects on oak regeneration and sapling growth, causing increases in growth and regeneration as the magnitude of the disturbance increases. What played a much larger role than either disturbance type or magnitude, however, was whether or not already-established oaks were present at a site. That is, oak seedling and sapling regeneration was substantially greater across all disturbance types, and magnitudes, when oaks were already established within the sub-canopy, and much lower when they were not.
This has a couple of important implications. One is that when established oaks are present, forest disturbances can release them from competition and allow the next cohort of oaks to fill the canopy, while the seedlings and sapling grow into establishment. The other side of this, though, is that when oaks are not established, forest canopy openings can allow competitor species to fill the gap, accelerating the ongoing decline of oaks in general. The authors of the study highlight this as a reason to classify low oak regeneration as a forest health crisis, given the ecological and economic importance of oak species.
When I spoke to Lance about these implications, he said, “Often, discussions of forest health concerns draw to mind the acute effects of a particular pest or pathogen – the dead or dying trees. Our study points to insufficient oak establishment as a pervasive, chronic condition underlying many of those acute health crises that can alter the forest for generations.”
A feedback loops starts to emerge here, where lower abundances of oak establishment begets even less establishment, and so on. To counteract this requires active management of forests to produce the conditions best suited for oak regeneration, and that management needs to be informed by more studies such as this one.
There’s no shortage of ecological questions that need answered, and I’ll be using Lance’s work to inform some of my own research going forward.
To cite this article, use the following:
Lockwood, B. (2024). Looking Down to Look Ahead: A new study sheds light on how forests are changing in the eastern US. Brief Ecology Newsletter, 4. Retrieved from: https://benlockwood.substack.com/p/looking-down-to-look-ahead
Dr. Lance Vickers is an Assistant Professor of Forest Dynamics and Management at the University of Kentucky. He was kind enough to provide a quote for this article, and if you want to know more about the kind of work he does, you can find his research here.