Talking Trees with Davey Tree

How are Trees Affected by Climate Change?

July 15, 2021 The Davey Tree Expert Company Season 1 Episode 27
Talking Trees with Davey Tree
How are Trees Affected by Climate Change?
Show Notes Transcript

Dan Herms, vice president of research and development for the Davey Institute, talks about how climate change will affect your property, such as the trees you plan to plant, what pests you may start or stop seeing, and how hardiness zones are being affected across the country.

In this episode we cover:

  • When Dan realized something was changing (0:49)
  • Changes Dan has seen (2:30)
    • Hardiness zones (3:16)
    • Increased forest productivity (4:01)
  • Winners and losers (4:37)
  • Affects on pests (6:05)
  • Mitigating the effects of climate change (7:25)
  • Plant the same species or new ones? (8:51)
  • Think globally, but act locally (11:47)
  • iTree software (12:08)
  • Scientist's point of view (13:30)
  • How Dan started his job (16:38)

To find your local Davey office, check out our find a local office page to search by zip code.

To  learn more about your hardiness zone, check out our hardiness zone map Dan Herms created with the Arbor Day Foundation to show how hardiness zones will shift in the coming years: Hardiness Zone map.
To explore the iTree MyTree software, go to

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LinkedIn: The Davey Tree Expert Company

Have topics you'd like us to cover on the podcast? Email us at We want to hear from you!


Doug Oster: Welcome to the Davey Tree Expert Company's podcast Talking Trees. I'm your host Doug Oster. Each episode showcases one of Davey's certified arborists sharing advice with everyone about caring for your trees and landscapes. We'll talk about everything from introduced pests, seasonal tree care, deer damage, how to make your trees thrive and much, much more. Tune in every Thursday to learn more because here at the Talking Trees Podcast, we know trees are the answer.

This week I'm joined by Dan Herms. He's Vice President of Research & Development for the Davey Tree Expert Company and we're talking all about the effect of climate change on trees. Dan, welcome to the show.

Dan Herms: Thank you. It's great to be here. I appreciate the opportunity.

Doug Oster: When thinking about climate change in all the research you've done, was there a time or epiphany early on where you realized something is changing?

Dan Herms: I think my first awareness of climate change came in 1988. I was a PhD student at Michigan State University and I was studying the effects of drought stress on tree resistance to bronze birch borer. 1988, many people may recall was extremely hot, extremely dry year, severe drought, and it triggered a natural outbreak of bronze birch borer in the Great Lakes that killed 20% of the Birch trees.

About that time to NASA scientists began testifying to Congress about the impacts of climate change and increasing carbon dioxide concentrations in the atmosphere and all of those things really converged to trigger my awareness on the topic. In the early 90's as a post-doc I started studying the effects of elevated carbon dioxide atmospheres on tree insect interactions and it just has gone from there.

Doug Oster: Well, I remember that summer living near Kent, Ohio where the Davey offices are and having well water and very worried about watering my trees and watering my garden during that drought. Since then, what have you seen as far as a change as we've moved forward here and climate change has become a big issue for all of us.

Dan Herms: The climate has warmed steadily. The average surface temperature in the earth has increased a little more than two degrees Fahrenheit since 1900, but most of that warming has occurred since 1980. It's been very steady with some natural ups and downs, but a steady increase. Over those last 40 years, there's been some very dramatic changes in our weather and our climate system.

When as an undergraduate horticulture student at Ohio State University, for example, Ohio was a hardiness zone five. Which means that the minimum winter temperatures were between -10 and -20 Fahrenheit. Well, since then, Ohio has become a zone six state. The winters are getting warmer in Ohio, precipitation has increased, but in other parts of the country, it become dramatically drier.

A mega drought for example occurring over the last decade or so in the Western United States. That's led to massive tree mortality, bark beetle outbreaks. In the Northeast United States for example, the increased precipitation has led to increased forest productivity and tree growth. As the growing season has gotten longer, carbon dioxide has increased, precipitation has increased, but there's winners and losers. The Northern adapted trees, spruce trees, the white pines are becoming stressed and the more Southern oaks and maples are winning, other trees are losing and that's going to be a pattern.

Doug Oster: Talk a little bit about the winners and losers. Why are these trees winners or losers? Do we know?

Dan Herms: Well, the winners are adapted to the warmer temperatures and wetter temperatures. In areas where they were limited by those factors, their growth is increasing. The losers, the story varies depending on species, but trees that are losing the most tend to be at the Southern limit of their distribution. For example, birch in Michigan, not tolerant of heat, not tolerant of drought and it's experiencing more of those conditions. White pine in New England is experiencing increased disease pressure because of the increased precipitation and humidity.

Foliar needle diseases, canker diseases. Changing climate can upset the phenology of pollination and seedling establishment, these kind of things. Fire in the West is changing the species composition in many areas. It is complicated and it depends on the species, but typically trees are adapted to the climate that they've experienced. When that climate changes, they find themselves in a situation that they're not accustomed to.

Doug Oster: Then how does global warming affect pests? Do we see different pests as the climate has changed?

Dan Herms: Global warming affects pest in a couple different ways. We're seeing certain insect pests migrating north into areas where they really didn't survive well in the past because the winters were too cold. For example, the Southern pine beetle which is a native bark beetle in the Southeastern United States is killed by temperatures that get down to zero or so, has migrated northward into areas it has never been before.

For example, the pine Barrens of New Jersey into New York, even Massachusetts. Now, another impact that happens is that some insects can have multiple generations in one season if the growing season is long enough. We're seeing that happen as well. For example, oyster shell scale, which can be a serious pest of trees in Northern Ohio and Michigan. It historically had one generation per year. Now it's having two generations per year, so the populations can build up faster and higher and exert more pressure.

Doug Oster: In your work, is there anything that you're finding that you can do to mitigate some of this?

Dan Herms: Yes. We have studied that the effects that trees can have on mitigating the effects of climate change and for contributing to climate resilience. Trees provide services that we call ecosystem services to society by for example, providing shade that reduces electricity consumption, air conditioning. They reduce storm water runoff and increased precipitation and heavy rain events have increased as a result of climate change. Trees can dramatically reduce the storm water runoff in urban environments.

Tree sequester and store carbon. There is, some of your listeners may be familiar with the Trillion Tree Campaign which is receiving a lot of attention and something that you can Google, Trillion Tree Campaign. Efforts going on across the United States and the world to repopulate the urban forest and natural forest with trees. Now that won't be the only solution, but it'll be part of the solution.

Doug Oster: Planting trees, that sounds great to me. With a change in, like you said, Ohio going from five to zone six, can we plant different things that we couldn't plant 20 years ago or do we stick with the same things?

Dan Herms: Well, we do need to give thought to which trees to plant, and not just for today's climate, but for tomorrow's climate. For example, by mid-century 30 years out, 40 years out, 50 years out, well within the lifespan of a tree that's well taken care of, Ohio is projected to become a zone seven states as the winter continue to warm. There are are many trees that are growing here now that will continue to thrive under those conditions. There are some trees that won't do so well. Sugar maple is predicted to become less important in Ohio for example. Northern species like birch we'll probably just forget about, spruce is not going to do well.

If you look at the distribution of trees, Ohio as many species that were either like, right dead in the heart of their distribution. They extend quite a way South, or were at the northern at edge of their distribution. Things like sweetgum will do well in Ohio for years to come. Bur oak will do well in Ohio. Red maple, black gum, honey locust. These trees extend far to our gum South.

Then there are trees that are native to the South, they're not native to Ohio, but we know from experience that they do well here. For example, bald cypress, it's not native to Ohio but we plan it here it grows well. River birch is native to Southern Ohio, but it grows very well in Northern Ohio. These Southern species that we know from experience will do well. At our daily research plots, we're going to be trialing trees at are zone six and zone seven species to see how they perform moving forward to help us inform our recommendations.

Doug Oster: Well, it's funny that you up sugar maple because it's one of the trees that when I ask an arborist, "Hey, what's one of your favorite trees to plant in the landscape?" I usually expect something strange and something completely different, but sugar maple comes up a lot. That's sad to think that that's not going to be as important a tree 30 years down the road. Is there anything else we can be doing to help slow this down as a homeowner?

Dan Herms: Well, there's the adage think globally but act locally. Planting trees can help with your home energy costs and Davey Tree and US Forest Service collaborate on a tool of software, or package of software tools called i-Tree. i-Tree is free software that will quantify the benefits that a tree or a population of trees provide as well as the economic value of those benefits.

There is a module that is called iTree MyTree, and you can just Google iTree MyTree and you can use that to determine the value of the services that the trees in your yard are providing. You can also use that to select the location to plant trees that will maximize the benefits that those trees provide if you're interested, those kind of things. Then of course anything that you can do to reduce your carbon footprint and encourage policies that do so. Because the biggest question in terms of how warm the climate is going to get in the future, is what will be the future trajectory of greenhouse gas emissions.

Doug Oster: Well, from a regular person's standpoint it's scary and it's depressing, from a scientist's point of view. How do you look at it?

Dan Herms: I try to look at it empirically. I've been studying and publishing on effects of climate on insects and trees since the early 1990s. I try to look at it in a objective, non passionate way. I try to view the evidence like a juror would, rather than as an attorney that's trying to argue a case in a particular direction. As the evidence has accumulated over the years, I published my first review article on climate change in 1999 and I go back and review those conclusions and they've just gotten stronger. The questions have been answered.

The models that predicted climate change that were published in 1990, looking back at those predictions, they were accurate. Those models have only gotten stronger and more powerful as super computing power has increased and our understanding of the climate system has increased with the tens of thousand of articles that have been published. I try to look at the overwhelming evidence that the climate is warming and what is the cause? Those are really the two fundamental scientific questions.

Doug Oster: If you're in a group of people and they don't know who you are and what you do, and you overhear this conversation and you have all the science, do you step in and say something or do you step away and roll your eyes?

Dan Herms: I don't step away and roll my eyes. I don't embed myself in political discussions in parties and so forth, but if people ask, I'll provide an answer to their questions and I try to focus on the science. What I try to do is separate the science from the politics. There's two aspects to this. One, again, the scientific questions is the climate warming and why? And then there's the policy questions. All right, what should we do about it? What are the ethical implications?

These are value judgements, political questions and so forth. I have a lot of people that are very smart, that I highly respect that see the world in a different way than I do. I'll provide an overview of the scientific evidence when they ask questions about it.

Doug Oster: I always like to ask, how you got into this and especially now with what you're doing, what do you get out of it because it seems like very important work to me?

Dan Herms: Well, I got into horticulture because my family had a greenhouse business and a floral shop in Portsmouth, Ohio where I grew up. That led me to Ohio state university, where I got a degree in horticulture and I became interested in plant protection. Went on to get a master's degree in horticulture and entomology and a PhD in entomology. The path into climate was, I don't know, serendipitous, I suppose I was studying the phenology of insects and plants and the effective heat and temperature on their life cycles since 1980s.

That's become an important discipline phenology of climate change. I was studying more from a pest management standpoint but you couldn't avoid that hard turn into climate change. Then the same with drought stress, when I started studying drought stress, climate change was not on my radar, but again, that became so intimately entwined with climate change that I couldn't avoid that, it was a natural progression.

What I hope to get out of this is, I hope to increase our understanding. I hope to make impact. I hope to help Davey Tree Expert Company become or strengthened their leadership in the area of tree health and its relationship to our environment and to contribute to developing environmental solutions focused on trees.

Doug Oster: Well, Dan, I'm going to leave it right there. That is great stuff. I certainly appreciate all this great information. It's wonderful to be able to reach out to somebody and get the science of this. I like the way you present that as the juror who is looking at the facts, it's enlightening. Thanks for what you're doing as far as your research too, I think it's wonderful.

Dan Herms: Thank you, Doug. It's been a pleasure.

Doug Oster: Tune in every Thursday to the Talking Trees Podcast from the Davey Tree Expert Company, I'm your host, Doug Oster. Next week, are you buying a house, working on a new plan for the yard? We'll talk about what not to plant and reveal some red flags on existing landscapes. As always, we'd like to remind you on the Talking Trees Podcast, especially after listening to what Dan had to say today, trees are the answer.


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