Feet Firmly on the Ground, Looking Up
Ecologists sometimes refer to a “climax forest”, which is the forest that will come to dominate a landscape if it that landscape is not disturbed for some long period of time. It’s an interesting concept, but ultimately disturbance will happen; change is the only long-term constant. However, in a smaller way, that term might be used to describe many of the parks in our fair city.
Growing Old Together
Go into one of the older historic parks in town, and look up. There may be a tall canopy of Siberian elms, or maybe cottonwood trees, or even sycamores and green ashes. They will probably be very close in age, and in size, to each other. That’s simply because they were all planted at certain, fixed points in the life of the park landscape, likely when the park was first built. They grow old together. Soon, some will begin dying together. All landscapes change over time, even the “artificial” park ones we create inside our urban envelope.
Think of the tall line of trees bordering the park, casting precious drops of shade onto the hot streetscape. As the healthy ones reach the end of their lifespan, they start dying back from the branch tips. Whole limb systems follow, leaving stiff wooden skeleton fingers scraping against the sky. Now, they may well cast not drops of shade, but instead heavy, hard missiles at the innocent citizens parked below. So, in steps our valiant City Forestry crew take down the dangerous trees. How quickly that line of trees can change, can shrink and even disappear. I don’t want to sound cavalier about the historic nature of some of the parks, but let’s be honest about the biological processes at play and the fact that all living things eventually die.
How to Combat this problem?
To combat this problem of even-age tree populations in our parks, we have to plant more new trees. Sure, we do this already, to some degree. But what I’m talking about now is more drastic: in some cases, we need to remove healthy, mature trees – in order to plant the next generation in their place, literally. Those tall elms along the street provide shady parking spots for park users, and working folks catching some lunch time in their vehicle. We need to start growing the replacement canopy now, phase it in over a decade or more. That will make the transition easier, smoother, from our current older canopy into a younger, and less even-aged, canopy.
Managed Landscape Ecosystems
Understanding that our parks are managed landscape ecosystems, and not fixed hard infrastructure, is important. Thinking that way, we see the value in diversity in kind as well as age. Planting more, and new-to-us species, helps increase this kind of diversity. I recently heard a presentation from a leading arboriculture (look it up!) professor in which he said that the species level distinction is not the important one, but rather the genus level. Okay, did I lose all of the non-biology types? Sorry…
Every Crisis is an Opportunity
Think of it this way. If, or when, the emerald ash borer (EAB) hits Albuquerque, it will not care if that is a green ash or an Arizona ash or a Raywood ash…Fraxinus is Fraxinus to the EAB. We can’t diversify our way around this beetle unless we step away from that whole genus Fraxinus. Which is too bad, there are some nice trees in that group. By the way, EAB hails from the Far East, gained its North America foothold in the Detroit area in 2002, and has now reached Colorado despite millions of dollars of intervention. So, it’s a’comin, just you wait.
When it does get here, there is hope, on a tree-by-tree basis. It does appear that systemic insecticides, applied via cambium injection under the bark every one to two years, will keep it at bay if the treatment starts prior to the beetles infesting that tree. Given the cost of the treatment and the number of ash trees in our city, most will not be saved. So, we will replant with something other than ash.
Every crisis is an opportunity. EAB will give us time and space to adjust our tactics. In addition to diversifying the genera used, we can make improvements (even if limited ones…) to the planting sites. City parks usually have decent soils, but not so along our streets. Urban soils are very hard and compacted, so we can physically loosen them. That will do a lot of good for future root growth! We can match our irrigation to the site, ensuring the maximum spread of water in the planting zone so the roots can occupy the maximum soil volume the site affords. That will do wonders for future tree growth! Where a really big, healthy tree is called for, we can go the very expensive but very effective route of soil vaults and suspended sidewalks…there are a number of strategies, and the expected results will match the time and expense pretty closely.
3/4 the Size
Speaking of expected results, how big do trees get in Albuquerque? Well, mostly the answer is along the lines of “about three-quarters as big as the books say”. When trees are planted in poor sites, they don’t grow as big or live as long as reference books suggest. That fact doesn’t, and shouldn’t, stop us from planting trees, but again we need to be honest about the biology – they are not going to get as big as we’d like to believe. Some species come close, being tough survivors. Some of those same species have been banned from the City due to their prolific pollen production – mulberry, and Siberian elm. Great trees for us, except for that nasty pollen issue.
What's good for the trees is good for us.
We can have a nice urban canopy in Albuquerque, but we have to be smart about it, and keep working at it. We have invited these living organisms to share our space, to our benefit, so we owe them a decent chance to grow, as individuals and as a community. What’s good for the trees is good for us.
Parks and Recreation Department
City of Albuquerque
“Trees are poems that the earth writes upon the sky.”
― Khalil Gibran, Sand and Foam