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30, 20, 10 – 5: Rules of Thumb

Article From the City Forester, Joran Viers

Spring in New Mexico

Ah, spring in New Mexico – cool, overcast, rainy…wait, that doesn’t sound right! Every so often, we get a break from the normally hot, dry and windy weather of spring, and get to enjoy a spring like this one. All of that rain and cooler temperatures have benefited plants across the city, which are looking clean, crisp and refreshed. Well, for the most part…

Let It Be

Some of you may have noticed browned leaves and twigs in the lower canopies of trees in open spaces like parks and large yards. Not to worry, that is just a little frost nipping. The trees will put on new leaves to replace those lost to cold. Don’t panic, don’t reach for the chainsaw; give nature some time to work out the issue. I’ve found that often works.

Nursery Update

This spring we planted some new trees at our little City of Albuquerque tree nursery down on Rio Grande Boulevard. Most of the trees in that nursery are a bit overgrown for the equipment we have to dig and transplant with, and most of what is in the nursery are species that we can purchase easily from local vendors. I had some great advice when it came to selecting new stock to plant out – don’t grow what you can buy cheaper than you can grow. So I made my decisions based on a couple of factors.

  • First, I selected species that we know will grow well here, but for whatever reason are not readily available, species like Kentucky coffetree, zelkova, golden raintree.
  • The other selection criteria was things that might do well here, but we don’t know yet - Persian ironwood and hardy rubber tree are some examples.

Species Diversity

There is a concept in urban forestry that involves looking at species diversity. Basically, the idea is that the more different species growing in a given area, the less likely that a new pest or disease can destroy a large percentage of the urban canopy. We have great lessons from the past to reinforce this concept. The magnificent American elm was the main street tree choice across the country until Dutch elm disease came over. Within a very short time, this disease spread widely and devastatingly, denuding towns and cities of their towering elms. So, people turned to green ash as a replacement – a tough tree that tolerates a wide range of conditions, including notoriously unforgiving urban conditions. Green ash was planted heavily and soon became the dominant species in many urban centers. Now, much of those have been lost, and many more are threatened, by the emerald ash borer (EAB for short), a little bitty beetle from the old world that just loves to eat ash trees. Turns out, the trees don’t need to be pre-stressed as is often the case with insect pests. They just need to be ash trees, any species: green, white, Arizona, Raywood…all are susceptible.

Rules of Thumb

When we think about diversifying the urban forest, we have a couple of “rules of thumb” we can look to for guidance. One is the 30-20-10 rule, which states that in a given area (however one wants to define that…), no more than 30% of the trees should be in the same plant family (for instance, Oleaceae – the olive family), no more than 20% from the same genus (for instance, Fraxinus – the ash trees) and no more than 10% from the same species (Fraxinus oxycarpa, the Raywood ash). Following this rule is trickier than it sounds, because of market availability. We can push the nurseries to produce a bigger variety of trees, but if there is no established market for those, the nurseries are taking a huge financial gamble. Plant nurseries are businesses, and they have to meet certain financial obligations in order to stay in business. Investing in a lot of unproved species could leave them holding a lot of stock no one is buying (it could also leave them as the sole-source for hot new items, which is a great business position to be in). Still, it gives us a goal.

However, it may not be a relevant goal. I heard a presentation last summer at the International Society of Arboriculture conference that was an eye-opener. Basically, the presenter showed how many of the feared pests and diseases did not discriminate at the species level, or at the family level. They discriminate at the genus level. Our friend EAB is a great example of this – any ash species is vulnerable, if it’s in the ash genus. Other members of the Oleacea family are not vulnerable. So the beetle is cueing on the genus Fraxinus. From this, the presenter suggested that we don’t need to worry about family or species diversity, but rather focus on genus diversity, and (in the real stunner) he suggested that no more than 5% of the trees in a given area be in the same genus. He also suggested that the least vulnerable genera are those with only one, or very few, species in the genus.

Kentucky Coffeetree: An Example

As an example, he brought up Kentucky coffeetree. The family is Fabaceae, the pea family. The genus is Gymnocladus, the species is Gymnocladus dioicus. This species is the only species in that genus, world-wide. It turns out that this tree has very few pest and disease problems. This pattern repeats in other mono-typic genera (genera with only one species) – very few pest and disease problems. In contrast, genera with many species (oaks, ashes, cottonwoods, etc.) tend to have a bigger set of potential problems, and those are potentially problems across all the species in that genus. Whew, is that enough botanical taxonomy or what?!

The 5% Rule Applied

So, the new 5% rule – take that to the nursery and see how far you get. There simply are not very many monotypic genera trees out there in the market. We still have to rely on the tried-and-true, knowing we are potentially setting the stage for future catastrophe. However, that doesn’t mean the rule is useless. Like many goals, it may not be ultimately attainable but the closer we can get to it, the better. Maybe our diversified planting in the City nursery can help explore that palette and establish new go-to species for use here. Under a climate-change scenario, this exploration becomes even more pertinent.

Take Preventative Measures

It is also important to understand that the majority of threats facing our urban trees are not pest or disease issues. They are environmental stresses – too little water, too much heat, soil that is too hard, etc. We can do a lot to improve the state of our urban forest by concentrating on remediating those conditions, and by doing so, we will also reduce the incidence of pest and disease problems, for those tend to follow the environmental stresses. Just like human health, preventative measures can do more for overall population health than rescue medicine.

Article by Joran Viers, ABQ City Forester

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