In my last column two weeks ago, I described the first half of some research conducted this past summer looking at projected climate change impacts to our urban forest. This week, I want to unpack the second half of that research, which was conducted by a recent Goshen College graduate, Aidan Friesen, and began by comparing our public tree inventory data to the USDA U.S. Forest Service Climate Atlas. The “Climate Atlas” can be found at https://www.fs.fed.us/nrs/atlas/tree/373. Detailed information about projected climate change impacts to Indiana can be found at https://ag.purdue.edu/indianaclimate/indiana-climate-report/.
The first half of this research showed that the economic, social and ecological value of our urban forest is projected to decline between nearly 20% and more than 40% by the end of the century due to the way in which a warming climate will have adverse effects on the current set of tree species in Goshen. In other words, many of our trees may not thrive as they do today. The research cited maple trees (more than 50% of all our trees) as especially vulnerable. This decline registers as an annual drop in value of about $430,000 (2020 dollars) for Goshen — i.e. dollars which we don’t currently spend on things such as air conditioning, stormwater services and air pollution mitigation because of the presence of trees, and which we will have to spend in the future for similar services in the absence of trees.
The second part of the research focused on what we can do to adapt our urban forest for the projected changes. In a word, it says, diversify. While diversifying our forest has been a key principle of our work throughout the past decade (as we’ve grown to understand the risks of relying too heavily on a few species), we haven’t had a clear idea about how to focus this principle. The research begins to point the way.
For example, analysis suggested that while our aggregate urban forest will suffer in the future, some species currently in Goshen are likely to fare better in a warming climate than they do now. Black locust, American beech and shagbark hickory are a few of these which appear to have a brighter future. Bur oak, American hornbeam and hackberry also show good tolerance for future warming. Some of the species which may not do so well are white pine, red oak, and sugar, silver and red maples — as already mentioned.
Useful as this analysis of our own tree species is, we were also thinking about the fact that there are certainly other cities and towns in climate zones further south than ours which must have tree inventories. We wondered whether comparing some of those inventories to our own might reveal further directions for us to explore as we think about our future urban forest. Bloomington, Indiana, generously allowed us access to their tree inventory data in order to ascribe importance values for their trees in comparison to ours.
The results were interesting. While there was no definitive answer about which tree species from Bloomington might do well in Goshen (possibly meaning that I need to refine the questions, certainly meaning that more research can be done!), a small number did check some of the boxes we were considering: sweet gum, pin oak, hackberry, honey locust, swamp white oak, red bud. These species all answered portions of the questions we were asking about climate zones, climate change and projected economic value.
So what do these different data sets mean? I think the first thing to take away is that we are on the right track, asking the right sorts of questions: we are trying to understand how to diversify our urban forest so that it is beautiful, healthy and productive 80 years from now. The second thing to glean is that while more research is necessary, this first step gives us an idea about a starter-kit of species which we should be planting more of — black locust, American beech, shagbark hickory, sweet gum, hackberry, honey locust, pin oak, American hornbeam, red bud — and some which we should plant less. We can begin to use this information as we work toward our tree canopy goal.
The research doesn’t suggest that we should stop planting maples in the Maple City. Maples should have an important place in Goshen for a long, long time — just not as exclusively prominent. Actually, I think our maples will become more valuable to us in the future, will require greater care, and will give us a deeper kind of pride and appreciation for their presence among us. We just need to plant lots of other species alongside them. As people planted trees 80 years ago with no idea how we would enjoy them — for instance, the amazing fall color of the past week — we should be thinking and planting carefully for people who will love them 80 years from now.