Thursday, January 14, 2016

Ideology v reality

I was reading the Street Tree Masterplan of a large Perth council this week and came across this:

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Advantages of Australian Species

  • Resilience in local conditions
  • Tolerance to drought and water logging
  • Generally reliable growth rates and habit
  • Reduced maintenance, pruning, reticulation and fertilising
  • Lower water use
  • Less nutrients entering groundwater/water bodies through fertilisers/seasonal leaf drop
  • Reduced possibility of becoming weeds
  • Provide wildlife corridors 

Advantages of Exotic Species

  • Deciduous trees allow sunlight into the street in winter
  • Seasonal interest in some species (ie autumn colour, spring flush)

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Sounds like a no-brainer that you'd go with Australian natives every time, right? I mean, look at the long list of advantages compared to those hopeless exotics.

OK, from someone who has been at the frontline of street tree management for over 30 years, here's my take on the lists:

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Advantages of Australian Species

Resilience in local conditions
My experience has been that Australian trees are often less resilient than exotics in the artificial environment of a suburban verge.

Tolerance to drought and water logging
No difference. There are equally good examples in both camps.

Generally reliable growth rates and habit
I'm not sure who the author of the list consulted when they put it together but I'd say it wasn't an experienced Parks and Gardens person. Because they will tell you there's no way this is the case. Australian trees are often much less predictable than the common exotic street trees.

Reduced maintenance, pruning, reticulation and fertilising
Again, not in my experience. You still need to water and fertilise native trees when they're young. Once they've matured, no - but neither with exotics. There's no advantage in terms of maintenance and pruning either. I have found that Australian trees are, if anything, more likely to need attention as a result of limb drop, splitting, dead patches in canopies and the like.

Lower water use
Nope. Not in a suburban verge where the tree is usually competing with grass. And again, there are many exotics that are just as drought resistant as Australian trees.

Lower nutrient loads entering water through fertiliser use and seasonal leaf drop
No-one fertilises mature street trees, native or exotic. And both types require some fertilising when they're young. Both also drop leaves. Natives just do it all year round instead of once a year. In either case, you just set up your street sweeping regimes to match your fall patterns and leaves entering waterways isn't an issue.

Reduced possibility of becoming weed species
I don't know. Maybe, maybe not. I do know that Australian trees have caused their own fair share of weed problems when used in the wrong areas.

Provide wildlife corridors
Yes, they do - no question. But that doesn't mean exotics don't support any wildlife. They do.


Advantages of Exotic Species

This was a pretty uncharitable list I have to say. Both things on the list are true but there's a few other pretty important things that could have been added. Such as:

  • generally live longer
  • usually more predictable in growth habit
  • typically tolerate suburban "abuses" better

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Bottom line: the stated advantages of Australian trees might be applicable to them in their natural environment, but we're talking street trees here. It's a whole different world.

Bottom line (2): it's really time to drop this whole native vs exotic thing. It's holding us back. It's time to start thinking of trees in cities as "green infrastructure". They're there to do a job, to make the city function better - like roads. Let's get rid of the ideology because it's not serving us. If any tree - native or exotic - doesn't do the job we need it to, it's the wrong tree.

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