Tree diversity: A popular concept but not without concerns

Story by Wilbert Ronald, photos courtesy of Jeffries Nurseries

The buzz word in tree production and planting is the word “diversity” and it is impacting what trees are grown and planted as tree producers try to expand the range of trees available for use, particularly on city boulevards.

There is consensus among tree specialists that overuse of a limited range of tree species can make for extreme vulnerability to disease and pest losses. We can see this confirmed as we look at fungal diseases causing losses to native American elm from Dutch elm disease, to Shubert cherry from black knot and to the columnar aspens from bronze leaf disease. A range of insect pests as well are on the horizon, as with the emerald ash borer introduced from northeast Asia.

There are some important concerns as the range of tree species is expanded and the limitations of certain species becomes more noticeable. We discuss some of the limitations that a number of trees have and how careful use can help in managing these limitations. We will start at the roots and move up to the fruit and discuss the issue of hardiness.

Transplant ease

Some of the trees such as oaks, buckeyes and the butternut and black walnut group have deep coarse roots which make transplanting and tree establishment a challenge. Nursery growers have responded by developing procedures for root pruning to develop fibrous roots, by using larger root balls when transplanting hard to establish trees and by growing more trees in large containers to help reduce transplant shock.

One danger of using containers for slower growing trees is that trees can become root bound in the container as they spend 2-3 years growing to saleable size. Any encircling roots need to be severed and the root ball somewhat trimmed so that the long-term planting success can be assured.

We now see many of our oaks and buckeyes grown for three to four years in the field and then potted for one to two years to give a five- to six-foot tree ready to transplant. Larger oak trees and other tap rooted species are moved or dug with a tree spade quite successfully.

Different moisture requirements

A number of trees such as the silver and sugar maples as well as all linden species require higher moisture levels than previously planted elms and ash species. Everyone must be educated to provide additional watering. Some Amur cherry and cherry hybrids such as ‘Ming’ like good moisture levels but will not take excessive moisture or roots sitting in water. These plants cannot survive where basement sump pumps leave root zones flooded so be careful to choose the right location.

Winter damage always takes its toll on tender trees.

Soil pH and salts

Maple species in general do not tolerate highly alkaline soils whereas elms and ash are very tolerant of alkalinity. This means that maples may see limited use in areas where soil pH above 7.8 is not uncommon. Likewise, salt spray drift from speeding traffic or the ocean can affect trees such as lindens much more than the elms. Lindens are often recommended more for lower traffic and slower traffic streets including residential and collector streets.

Excessive fruit fallen from a Ussurian pear tree.

Small fruit on ‘Starlite’ flowering crabapple are great.

Large fruit, fruit drop and excessive seeds

Several tree species such as Ohio buckeye, butternut, black walnut, bur oak and Ussurian pears produce large volumes of larger fruit which drops to the ground when mature. Some homeowners don’t mind cleaning up the fruit, but others would regard excessive fruit as a really negative issue. Obviously if we use these trees in larger green spaces and parks the fruit will be less of an issue.

Some trees such as Amur maple and particularly boxelder maple produce such large quantities of seed that they can be regarded as weed trees. There is even a danger that excessive seed can lead to invasiveness issues in non-native trees. Tree breeders have responded by breeding trees in some species that have no seed either because they are mostly sterile or are male individuals bearing no seed. In a number of cases trees have been developed that have small fruit that are retained on the tree avoiding any mess.

Marginal hardiness

As the tree producers have widened their tree production the natural inclination is to look south and try to find more species which may prove to be marginal in hardi-

‘Autumn Blaze’ maples sold in Zone 3, where it is marginal, for example. Remember that trees must survive many climatic extremes over many years as they mature.

Tree guards are the best investment you can make.

Animal, bird and rodent damage

American elm and ash were reasonably resistant to most of this damage but as a wider group of trees are used it produces a bigger “buffet” for wildlife. For over 10 years at my nursery we have used and recommended tree guards which expand as the tree grows and effectively limit most rabbit and mouse damage which can be severe on flowering crabapples.

Another advantage of tree guards is found in their value as trunk protection during the summer season when mowers and weed eaters can cause trunk damage. If you wish you can put the tree guard away for the summer but make sure it is reinstalled by November. Sap suckers can be a problem to the trunks of birch and alders and must be controlled.

Many of the above issues should be evaluated when choosing what tree to plant and where to plant. In summary, there are trees suited to certain sites and not to other sites. Using container stock solves many problems for “hard to transplant” trees. Hardiness and adequate moisture remain are key factors that determine what tree to plant. A sheltered site may entice you to try some out-of-Zone trees, but a windswept site will demand more hardy trees.

Whatever you do, keep planting trees and work through any difficulties you encounter. To obtain diversity it is going to require a range of trees on different sites to reach your goal. There are some good trees including northern sugar maples, seedless ‘Silver Cloud’ maple, ‘Ming’ cherry, Asiatic elms that are Dutch elm disease-resistant, Amur cork tree, new oaks and linden hybrids which are increasing in use. Just remember there are few trees as tough as elm or ash and be prepared to give that extra care to establish your favorite tree.