Historic Pioneer Square in Seattle contains a boulevard-like setting on 1st Avenue with London plane (Platanus x acerifolia) trees in the median. In Occidental Square and 1st Avenue, the trees are typically mature 15″ diameter stems swaying upwards of 50′, mulched, and tolerating the pollution as they do in cities around the world.
But in recent years, the normally dense crown of these trees has become a porous barrier to sunlight. My sense is they are suffering from drought-stress among other threats. I have not performed a full VTA or Visual Tree Assessment but the stress from low soil moisture, extra transpiration during hot days, and evaporation during warm nights are damaging to tree health. In urban arboriculture, this has become a familiar story.
We are well beyond climate deniers being the problem for taking action on our stressed trees, but why haven’t we taken more corrective action?
In arboriculture, We Know
*cluster plantings improve tree survival by increasing overall soil moisture levels on the deep root netwirk and multiple crowns reduce solar intensity on singular trees
*ground cover reduces moisture evaporation through the upper root network
*irrigation: drip or hand watering can take the edge off of low soil moisture levels when the heat waves are persistent.
I have not seen any of these basic approaches applied to our lovely Pioneer Square trees in the West-coast’s most impressive National Historic District.
What’ll it be?
Looking north a younger London plane open crown for dramatic effect
London plane tree with a type of retrenchment in it’s lower crown; note the bark loss on key branches