Forecasting tree planting for the future

Traditionally, trees are selected based on their local plant hardiness zone, the geographic regions defined by climate conditions that determine a species’ ability to survive winter. However, to anticipate future warming, forward-thinking foresters are now intentionally planting species that naturally thrive one or two zones further south.

Locally, this proactive strategy, known as assisted migration, is already yielding impressive results at the Elmira Nature Reserve, where recently introduced sycamore trees are thriving.

“The sycamores are Carolinian, and we have managed to put them ahead of where they’d naturally go. As our climate warms here, these trees are going to be well adapted to that, and be quite happy here in the decades ahead,” said Kevin Thomason, vice chair of the Grand River Environment Network.

Sycamore trees were planted at the reserve a few years ago. While the trees are not native to the area, they are doing well today, a clear sign they are in the right place at the right time.

“Those sycamores are thriving because we found that rich, loamy, moist clay soil that they love, and they’ve just taken off and blown everyone away with what they’re doing when you get it right,” said Thomason, adding that the success of a tree really comes down to soil and having the right conditions for the tree.

Every tree species requires specific climate and soil profiles. Sycamores are uniquely equipped to handle the dense, compacted clay soils found across many parts of southern Ontario, flourishing particularly well in moist bottomlands and floodplains. This resilience makes them an ideal ecological replacement for native ash trees, which once dominated these wet environments before disease and pests devastated them.

“The sycamore is kind of proven to be maybe a good replacement tree from a lot of those areas, and it’s a massive canopy tree too. It grows really quickly, so it’s a large-stature tree that provides a lot of habitat and similar function to that large ash that we used to have,” said Braedon Witt, operations manager at ReForest Woolwich.

He added that, when forecasting tree planting for climate change, soil is one of the biggest predictors of what can and cannot be planted in different areas.

“Typically, most people will only go like one or two zones to the south and try and bring that up to the current location to see if, because it’s kind of hard to predict how dramatic climate change will be, but at least you know having a bit of a headstart is very helpful,” said Witt.

“The soil is a big dictator of what you can move up to. You can’t start pushing stuff far, far north in Ontario where there’s poor soil development.”

Looking ahead, the primary challenge for conservationists will be discovering which species can successfully adapt to fill the voids left by decades of blights, disease, and deforestation

“We’ve lost so many trees. We used to have elm. We used to have chestnuts. We used to have so much ash. So, it’s going to be a challenge in the future to try to figure out what trees we can get because we don’t have all of them now. We are trying to bring back some of the chestnuts, and there are some researchers in the province who are trying to find chestnut blight-resistant chestnuts, but these are slow processes, and we’ll try to seed what we can in,” Thomason said.