Friday, June 3, 2022

Trees for a Changing Climate

My ex-wife gave me a shirt that reads “Change is Good. You Go First” when our divorce was finalised, a much-appreciated bit of humour in the midst of a challenging time. It’s hard to find the mirth in some changes, especially when we don’t have a say in them. Climate change is a good example.

Global temperatures are rising at an ever-increasing rate. Extreme weather events are becoming more frequent and severe with time, and no amount of denial will make it go away. We have to learn to roll with this one. We can’t stop climate change tomorrow, but we can “trick” it by updating the kinds of trees we consider for home and community planting. A warmer world affects trees in a myriad ways: Record wet seasons like in 2013, 2017, and 2019 allow normally weak foliar pathogens to spread and flourish, becoming primary agents of mortality.

Needlecast diseases are killing more conifers than ever before, while anthracnose of maples, walnuts, and other deciduous species is also on the rise. Recent drought years (e.g., 2012, 2016, 2018) caused the lowest soil-moisture readings ever recorded. Root systems died back greatly, making trees more vulnerable to pests and diseases for 3-4 years afterward. In some places, extensive death of red oaks and sugar maples occurred when drought years followed on the heels of defoliation by tent caterpillars or spongy moth. Longer seasons and milder winters allow invasive forest pests to move north faster than was thought possible. Hemlock woolly adelgid, oak wilt, and emerald ash borer are but a few examples.

Solutions: Sometimes it’s important not to do stuff – I always like that strategy. Colorado blue spruce (Picea pungens) is incredibly vulnerable to needlecast diseases. In fact, about ten years ago, a senior Cornell Extension educator suggested all Extension educators tell the public not to plant them, period. Katsura trees (Cercidiphyllum japonicum) are one of my favorites, but can’t weather
even moderately dry conditions. They’re also very salt-intolerant, making them especially bad for street planting. The majority of white-barked landscape birches are sitting ducks for bronze birch borers, which are lethal unless caught early and treated systemically. European weeping birch (Betula pendula), white or paper birch (B. papyrifera), and grey birch (B. populifolia) should be used only rarely in most landscapes.

Syringa reticulata. Wikipedia Commons photo.

More Solutions: Thinking outside the box-elder is a key strategy. Years ago, I was firmly in the “Native Species Only” camp. It took a while to realize that taking a hard line on native plantings was no longer wise in our fast-changing climate. Below are some non-regional trees and tough local characters that deserve to be more widely planted. The ginkgo (Ginkgo biloba) can withstand severe drought, high soil pH, salt, and air pollution. Once trained, it is low-maintenance in terms of pruning. The Japanese tree lilac (Syringa reticulata) is another familiar urban tree that is often underrepresented in our landscapes. Aside from its lush blooms, it is perfect for under utility wires. In addition, it is largely pest and disease-free, and requires little maintenance pruning.

The Norway spruce (Picea abies) is not drought-tolerant, but is very resistant to needlecast diseases that are decimating Colorado blue and other spruce species, and is OK with marginally wet sites. The ever-popular crabapples (Malus spp.) face much higher disease pressure from apple scab and fire blight. It’s imperative to get disease-resistant varieties exclusively, and to only plant on full-sun sites. In spite of its name, the Kentucky coffeetree (Gymnocladus dioecious) is cold-tolerant to USDA Hardiness Zone 3b, and is fantastically drought-resistant. It also withstands air pollution and  soil pH up to about 8.2, and occasional waterlogged conditions. Plus, its coarse bark and ascending branch habit give all-season interest. While the northern catalpa (Catalpa speciosa) also developed a bit to our south, it is nonetheless good to Zone 4a. It is moderately drought-tolerant and will survive intermittent flooding as well.

The thornless common honeylocust (Gleditsia triacanthos var. inermis) is overplanted in some urban locales, and possibly with good reason. It laughs at deicing salt, drought and occasional flooding alike, and manages fine in compacted soils too. The native hackberry (Celtis occidentalis) is hardy to at least Zone 3b. Often passed by because they lack conspicuous flowers and flashy fall color, hackberry trees handle severe drought and occasional flooded soil. The bur oak (Quercus macrocarpa) may top my list of favorites. With a species lifespan of 800+ years, it is a true legacy tree. It’s super drought-tolerant, yet fine with seasonal flooding. It can be hard to get established, so the smaller the better for transplant stock. River birch (Betula nigra) are variously listed as hardy to Zone 3b or 4a. In any case, they are a good substitute for white-barked birches, as they are more accepting of landscape conditions and are rarely attacked by bronze birch borers. Not drought- tolerant, but great for seasonally wet sites.

Attention to Detail: Choices around planting make a difference, too. Everyone wants fast results, but large-caliper trees are an even bigger gamble today than they used to be. New research indicates that 5 cm (2”) should be the largest caliper size considered for planting. Studies show removing wire and burlap in the planting hole is more important than we once believed. We also should emphasize to customers the need for multi-year supplemental watering for transplants.

And finally, remember that we are planting for future generations!

First published in Ontario Arborist magazine, Summer 2022.

Paul Hetzler is an ISA-Certified Arborist and a former Cornell Extension educator.

Photo at top: Ginkgo biloba tree. (Wikipedia Commons photo.)

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Paul Hetzler

Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.

You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World




14 Responses

  1. Boreas says:

    I have to agree with the Thornless Honeylocust. I received a tiny sapling about 20 years ago from the National Arbor Society. After a year a deer took the apical meristem and messed with its shape a little, it now has a 6″ diameter trunk and is a beautiful, shady specimen with leaves that couldn’t be raked in fall if you wanted to.

    Another favorite I have is a Flowering Plum. Grew like a rocket but has yet to spread much. Loaded with white blooms every spring. Very healthy after about 10 years.

    Another surprise were two Baldcypress trees I planted about 10 years ago. Large, healthy, and no leaves to rake. Very unusual to see planted up here. Nice color in fall.

    I have also planted three American Chestnuts over the years. I won’t be around long enough to see if they are successful, but I figgered they ain’t gonna grow where they ain’t planted. Hopefully they survive.

    • nathan says:

      i planted two american chestnuts and like you hoped, 4 years and looking wonderful, then …..i really hope yours fair better. i too had hoped to see those chestnuts i used to collect as a kid and roast on the woodstove with family.

      • Boreas says:

        The oldest is still healthy after 15 years or so. SLOW grower though. The other two are fairly recent, tiny, and have to be protected from deer and gypsy moths.

    • JohnL says:

      I have a smallish lot in a small Central NY town and have planted (from seeds) 2 Gleditsia triacanthos and 2 Quercus rubra. 30 years later they are doing beautifully. Maybe too much so as they are in need of some serious trimming attention. I too have planted 3 Castanea dentata saplings that I got from my alma mater (SUNY-ESF) at a cancer survivors event. Only 1 has survived and it is growing so slooooooo ly. I too, will certainly not be around to see if it is successful, so I’m going to take the chance of replanting it at my daughters house in rural Vermont. Hopefully the next 2 generations will see the results of this grand experiment.

      • Boreas says:

        I suspect the best-case scenario for the chestnuts is to outlive a few generations of humans, but still die in “early life” from blight. But at least a few more generations will be able to see some fairly large chestnuts – unlike my generation. In days gone by, many rural people fed their families and livestock on chestnuts. Wouldn’t that be nice again!

  2. Nathan says:

    picking trees to weather the storm of seasons, wet, drought, bugs, fungus, invasive everything is getting tougher. i try to plant 2 of each variety of tree, roughly 8 per year, i figure if i always plant much more varieties, then i only loose a few to some disasterous killer what ever it is. i always am looking for trees tolerant of what ever blight killed them off, i am especially looking for elm, black walnut, butternut and american chestnut. great american trees that have all but disappeared or have disappeared. Just keep trying variety folks

  3. Brian Joseph says:

    The author needs to read Apocalypse Never by Shellenberger and stop spewing misinformation about “extreme weather events”.

  4. mike says:

    “Global temperatures are rising at an ever-increasing rate.”
    With all the droughts and wet seasons trees will never grow anyway. The only solution is more lithium batteries, electric cars, and solar panels from China.

    • Boreas says:

      Too late for that – our bed is made. Burn the fossils we have left as soon as we can – it will certainly make our descendants’ choices easier once that resource is gone.

      • nathan says:

        especially burning more fossil fuels, make electricity and send through wires at up to 40% loss to recharge those battery cars that are in themselves a enviromental disaster to mine and refine. or get the electric mower that will need a $100 battery every few years, or the simple gas mower at 5 gallons a year of fuel.

  5. Richard Fentonf says:

    Paul, I hope you will reconsider your recommendation for Japanese tree lilac. It is proving very invasive. Check out the information from the Capital Region Partnership for Regional Invasive Species Management (PRISM). I spoke recently with a representative who is monitoring a rapidly growing population along rivers.

  6. Neil Luckhurst Neil says:

    You mention Box-Elder in a pun but what do you think of it for the NE? Up here in Canada it is seen as a weed tree but it can grow to be very stately.

  7. Rebecca Bernacki says:

    The Adirondack Park Invasive Plant Program (APIPP) urges landowners to avoid planting Japanese tree lilac. This species becomes invasive in riparian areas and has been found escaping from ornamental plantings into natural habitats throughout the Adirondack Region. APIPP is monitoring a large infestation that is altering native habitat along almost four miles of river corridor, as well as many smaller invasive populations. Although this species is commonly planted as a street tree, we urge landowners to seek alternatives that cannot escape cultivation and negatively impact natural areas. Paul lists several great native alternatives including river birch and bur oak.

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