The Perfect Invader – The European Sycamore Maple and Vancouver’s City Forests.

Ryan Regier
May 14
6 min read

Previously published on February 15, 2021 on my old Blog.

About a few weeks ago I removed a small sapling of a European Sycamore Maple (Acer pseudoplatanus) from a forest near me in Jericho Beach, Vancouver. It’s considered an invasive, shade-tolerant tree in the park. In fact, a 2014 report on the park was stunned just how much the tree was dominating new growth in the park and outcompeting other trees. It recommended they be actively removed as much as possible, but also grudging admitted that is was so well established that it’s dominance in the park might just have to be accepted and worked around.¹

I figured I could do my part by removing at least one seedling. And my interests weren’t completely selfless, I thought it be interesting to see if I could somehow grow this baby as a house-plant. Maybe try some bonsai? This was mid-January and the tree was leafless at the time (like all deciduous trees are during the winter) so I just tossed it in an old milk carton with some soil, placed it by my window, and planned on forgetting about it till spring. Hopefully it would awake from winter dormancy then, but my hopes weren’t too high.

I was blown away when only a couple weeks later, in the midst of a Vancouver snowfall, it had leaves! Also, perhaps even more amazing, a maple sycamore seed that I had unknowingly collected with my soil had sprouted and was growing quickly!

Sycamore Maples budding and growing inside while snow is outside.

I was incredibly delighted yet also confused by this. I know that trees’ germination and spring growth depends on a number of different factors, but mostly comes down to longer photoperiods (The amount of sunlight each day. Much higher in summer than winter) and higher temperatures. It was my understanding that unless a tree was evergreen, it would always depend on a combination of the two. A deciduous tree could do some serious damage to itself if it started growing leaves in the middle of a winter warm-spell. A drop in temperature below zero and those thin leaves would lose water and die quick.

So I did some research and whoa. Sycamore Maple appears to be one of the very rare cases of a temperate deciduous tree where photoperiod has no impact on bud bursting, only temperature does.²

The potential advantage this gives Sycamore Maple with Climate Change could be pretty substantial. As temperatures warm up, trees that get a head start on growing will be the ones dominating the canopy. Yes, the risk of getting caught in a late-winter/early-spring frost is still pretty high. This is a high risk, high reward strategy, but compared to other trees, Sycamore Maple shoots up quick and produces seeds when still relatively young. It’s rolling the dice, but it’s got a lot of rolls. A lot will die, but some will survive.

There’s a terrible old joke about two friends in the woods coming across a bear. One of them freezes and then looks in confusion at his friend who is stretching his legs. “What are you doing?! You can’t outrun bear!”, he whispers between gritted teeth. “I don’t need to outrun the bear! I just need to outrun you!”, yells his so-called friend as he takes off into the bush. The Sycamore Maple doesn’t need to flourish. It just needs to survive Climate Change longer then other trees around it. And it’s not looking good for some native Vancouver Trees.

Just a few kilometers from Jericho Forest is one of Vancouver’s best city forests, Pacific Spirt Park. Only slightly smaller than Vancouver’s famous Stanley Park, it’s a beautiful, classic BC temperate rainforest full of Western Redcedar, Douglas Fir, and Hemlock. But it’s forest composition is starting to change. Douglas Firs (an incredibly long-lived, quick growth tree that somehow breaks the deal that wood offers: live slow and live long or live fast and die young) are starting to be replaced with more shade tolerant species. Western Redcedars, the provincial tree, are starting to die off in park because of high climate change temperatures and lack of water.³

The time is ripe for the Sycamore Maple. And in the northern section of the park on the south side of Chancellor Boulevard, something is starting to happen.

Typically forest succession on disturbed lands in BC starts with Red Alder growing fast and dying young, all the while using its incredible ability to take inaccessible nitrogen (a key element for photosynthesis) from the air and putting it into the soil. This nitrogen-filled soil then allows the Vancouver’s native Evergreens to come in and grow big to their massive sizes. But in this disturbed land next to Chancellor Boulevard, that’s not what is happening. The trees that are growing under the Red Alders are Sycamore Maples.

It’s the same thing happening in Jericho Forest where I took the Sycamore Maple from. Alders dying. Sycamore Maple beginning to take their place in large numbers. Nitrogen is a key limiting nutrient for Sycamore Maples, and Alder has given them a utopia.

And that’s not all they have going for them. The understory of these Alders in Pacific Spirit Park and Jericho is dominated by invasive vines like Himalayan blackberry and English Ivy. These create dense mats of evergreen leaves under trees that few native species can break through to get sun. They dominate the forest floor and ensure almost nothing else can grow. But once again, Sycamore Maple has an advantage.

These vines are very familiar to the Sycamore Maple from hundreds of years of competing with them in Europe. There’s a clear symbiosis between them that almost appears to be mutualistic. I haven’t been able to find much literature diving into the topic unfortunately ( beyond the fact they are often seen growing together), but I’ve noticed a few things.

Firstly, the evergreen leaves of these vines appear to actually protect sycamore seedlings from colder temperatures and may help them germinate quicker. I’ve dug beneath a couple mats of these vines and came upon sycamore maple seeds beginning to send out roots (In early February!) on multiple occasions.

Secondly, Sycamore maple has this incredibly straight pole-like growth in its first few years. You come across a young tree in the winter and its often just a thin pole with no side branches.⁴ This would allow them to easily poke their way through the vine mats, while other small trees with side branches can get easily entangled.

I also found a fascinating study talking about how Sycamore Maple has actually changed itself quite a lot to become some successful in foreign environments. It is now very different from its ancestors who still live in it’s native range. For example, Sycamore Maple is considered a very shade tolerant tree here. But in it’s native range it isn’t considered to be that shade tolerant. It’s evolved better shade tolerance. There’s also evidence to suggest that the invasive version of Sycamore Maple have evolved faster growth and significant differences in leaf composition.⁵

It’s incredible when you think about it. Sycamore Maple was tree brought over to North America for its ability to survive urban environments. It was brought over here to be a street tree. But it’s tree. It grew, it adapted, it didn’t give up. Now it’s starting to grow where it wants to grow and not only where we plant it.

References

Demontreuil, J., & Ballin, L. (2014). Part 1: Jericho Park Baseline Inventory Report. Retrieved from https://cfs2.sites.olt.ubc.ca/files/2014/04/Jericho-Baseline-Inventory_Part-1_FINAL.pdf
Basler, D., & Körner, C. (2014). Photoperiod and temperature responses of bud swelling and bud burst in four temperate forest tree species. Tree Physiology, 34(4), 377–388. https://doi.org/10.1093/treephys/tpu021
Fisher, B., Ivanochko, T., Lipsen, M., & Kreitzman, M. (n.d.). Tracking Salal ( Gaultheria shallon ) and Western Red Cedar ( Thuja plicata ) Die- off in Pacific Spirit Regional Park. Retrieved from http://hdl.handle.net/2429/74126
Hein, S., Collet, C., Ammer, C., Goff, N. Le, Skovsgaard, J. P., & Savill, P. (2009). A review of growth and stand dynamics of Acer pseudoplatanus L. in Europe: Implications for silviculture. Forestry, 82(4), 361–385. https://doi.org/10.1093/forestry/cpn043
Shouman, S., Mason, N., Kichey, T., Closset-Kopp, D., Heberling, J. M., Kobeissi, A., & Decocq, G. (2017). Functional shift of sycamore maple (Acer pseudoplatanus) towards greater plasticity and shade tolerance in its invasive range. Perspectives in Plant Ecology, Evolution and Systematics, 29(January), 30–40. https://doi.org/10.1016/j.ppees.2017.11.001