In the forests of the Rocky Mountains, fewer trees are growing back after recent wildfires because of climate change. That’s what a team of researchers discovered after studying seedling regeneration at 1,500 sites in five different states.
University of Montana fire ecology Professor Philip Higuera is a co-author of the study. He joins us now.
Philip Higuera: The reason the study is important is because we’re starting to see a lack of tree regeneration, in particular at lower treeline.
Think about the fires that occurred last summer, the summer of 2017. We might expect at some of the lower elevations in those fires, that we’re not going to have trees return. At the mid and high elevations, we should expect to see trees return.
Nora Saks: How did you and your colleagues go about analyzing tree regeneration? Tell me a little about your field and your lab methods.
PH: One of the cool things about this study is that it combines previous data from literally a half-dozen studies. In general, what we were doing with each of these studies was going out to plots on the landscape that had burned in large wildfires in between 1988, so all the way back to the ‘88 Yellowstone fires and other fires of that year, up through 2011.
Within a wildfire, we’d pick a spot randomly and we would search that area within the transect for seedlings. If there were seedlings there - we would count them, we would identify the species, and then from that, we’d get an estimate of seedling density at each site.
NS: How many seedling specimens did you end up gathering?
PH: So among all of the studies, there were just over 63,000 seedlings sampled.
NS: 63,000 seedling samples?
PH: Yes, across a large area.
NS: Can you tell me why seedlings? Why is that what you were focused on?
PH: That life stage of a tree is particularly sensitive to climate. And of course, if you don’t pass that life stage, then you can’t become a tree. When we think about the impacts of climate change on forests, and the interaction between climate and wildfires, and how that’s going to impact future forests - that initial stage - seedling establishment and then survival - we think is going to be particularly important. And we think those trees, the seedlings, are going to be more vulnerable to warm dry climate conditions than the trees that are well established, that are 50 or 100 years old.
NS: Another finding that popped out was that tree regrowth has decreased in particular in areas that have burned since the year 2000. I asked Professor Higuera what changed at the turn of the millennium.
PH: The main difference is that moisture stress has been significantly higher after 2000. And what that means from the seedling’s perspective is that not only is the average condition, the average summer that a seedling experiences warmer and drier, but what we see occurring less and less, or not occurring at all, is these individual years or sets of years where conditions are cool and moist.
The reason that they can regenerate, and you can see large ponderosa pine in warm dry regions, is because they’ll regenerate in these individual or few years where there are cooler or wetter conditions than average. That’s all they need to be able to establish. Once they establish and make it through that first life stage - they can hang on and make it through warmer and drier conditions. But when we look at the climate data across the study region, we don’t see those conditions happening in the 21st century.
NS: What are some takeaways for managers or other people who work in forestry in terms of replanting or forest recovery?
PH: So at the simplest level, what this study tells us is that there are some areas where planting tree seedlings could be useful, and there are some areas where planting tree seedlings is not going to be useful.
Work from this group, and work in the lab, where we’re going now is trying to help identify the specific climate conditions where we would expect a tree, whether the seed got there with or without human assistance, whether that seedling would be able to grow or not.
There are going to be some locations where we can plant as many seedlings as we want, but because of the warm dry conditions in the summer, they’re going to be drought-stressed and they won’t survive.
NS: It might be hard for some folks here to wrap their head around the idea that the forests that are so beloved here are changing, potentially rapidly. What’s your suggestion on how to engage with changes in our forest?
PH: One of the things that my longer-term work instills or reinforces is that really change is the rule. With respect to the findings of this study, and what we think we’re starting to see, a decrease in forest regeneration at lower treeline, and perhaps a decrease in forest density at the mid and high elevations, it reminds me that a century ago, many of these forests were a lot less dense than they are today. The decrease in forest density, or transition from forest to non-forest ecosystems at low elevation, in a way it’s going to be taking these systems back to something that they were like sometime in the past.
I think rather than throwing our hands up and being like “oh no, we can’t let it change,” it’s more productive and I think more efficient to understand how they’re changing, make our best anticipation of what that change is going to look like, and then see how that is or is not consistent with what we we want to get out of the landscape and the way that we value or manage these landscapes.
NS: Philip Higuera is a professor of fire ecology at the University of Montana. I spoke with him about a study he co-authored on declining forest resilience to wildfires whose results were published in December in Ecology Letters.
Read the study here: Evidence for declining forest resilience to wildfires under climate change