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Building a better tree: When the goal is creating tougher trees for urban areas it means focusing on more than just parentage.

Are living things a product of their genetic make-up or of the environment? The answer is both. When it comes to making it in the world of trees, who you are is no more important than where you come from.

This is the second of a two-part series on trees and genetics. In the first part, Trees, Environment. and Genes (Summer 03), I discussed how and whether it's possible to recreate the "perfect tree." There's been much talk about whether it's possible to clone a tree and be assured that its offspring will grow up to be exactly like the "parent." Two identical clones will look diferent when grown on different sites. A tree is as much a product of where it grows as the genes from which it begins life.

Genetics plus environment equals phenotype is "one of the first things you learn in 'tree improvement 101,'" says Bert Cregg, a professor at Michigan State University. Put simply, two trees can be genetically identical but look very different because of the environmental conditions on the sites where they're growing. The concept of phenotype--the tree you see---provides a foundation for the initial and repeated research that must be conducted to confirm a scientific finding.

I was introduced to the concept of phenotype during a course in forest dendrology, the taxonomy of trees. It was there that I heard a fascinating story about the Scotch pine in America, relayed by the professor as a way to help us recognize the species. It also turned out to be a lesson in phenotype.


The Scotch pine (Pinus sylvestrus) is widely grown for a variety of uses in the U.S. and is perhaps best known as a popular Christmas tree. The tree has unique, easily recognized orange bark on branches and bluish-green needles. It's a landscape specimen that's also grown for lumber production, much to the surprise to many forestry students in the U.S.

I remember rolling my eyes when the professor instructed us to include Scotch pine in the list of timber trees we needed to know. The trees I had seen in people's yards and in the parks of my hometown were far too crooked to be used for lumber. I learned a few weeks later, though, that Scotch pine is actually a well-established timber tree with a huge range that extends from Western Europe to parts of Asia.

What I didn't realize as I reluctantly prepared to add the tree to my timber list, was that I was about to learn a lesson about phenotype, that all-determining combination of genes and environment. The Scotch pine I had grown up around were crooked because of the seeds they grew from. Those seeds came from trees that grew in a specific mountainous area and were short, stunted, and crooked.

So just how exactly did this country get so many crooked Scotch pine when other countries are populated with gloriously straight ones? The answer dates back to the period just after World War I, when a widespread concern over shortages of forest products prompted a push for growing more timber resources.

It was a popular idea but ultimately unsuccessful because it focused on generating a lot of seed rather than a lot of good seed. People interested in making a few dollars energetically gathered bushels of pine cones, but it was easier to collect the cones that came from those short, crooked trees rather than from the tall ones. So people searched out the short, squatty ones for easy picking.

I saw my first tall, straight Scotch pine on a class trip to an experimental seed orchard. I remember the shock of seeing not just one but a whole plantation of these legal trees covering acres and acres.

Horticulturalist and author Donald Wyman, longtime curator of the Arnold Arboretum at Harvard University, describes the picturesque bluish-green Scotch pine in his book Trees for American Gardens. Although considered to be a valued timber tree in Europe, Pinus sylvestrus has not proved successful in the same capacity in the United States, despite being planted extensively here, Wyman says. Considerable time. money, and effort was expended on the failed attempt to establish the Scotch pine as an important timber tree in the United States, a failure ultimately attributable to a lack of good genetic science.


Wise selection is a central element in the science of urban tree improvement today. In fact, Nina Bassuk, a professor of horticulture at Cornell University, says that most tree improvement efforts underway in horticulture and urban forestry today focus on selection and not breeding. One reason for that is a lack of money. Tree improvement efforts receive little funding nationally, and selecting superior trees from the existing gene pool is less expensive than breeding new varieties.

Another reason for the focus on improvement over breeding is abundance. The gene pool has been diversifying and expanding on its own for a couple hundred million years, so there's a lot of genetic material to choose from. And even though finding the good varieties is like looking for the proverbial needle in a haystack, it's an easier task than breeding new ones.

In some cases a little good science can go a long way because there are a lot of tree varieties available that are better than the ones we plant most frequently. Homeowners and sometimes city officials often choose trees for nonscientific reasons--availability or price, perhaps. Norway maples and London plane trees, for example, were a popular species choice in thee Northeast and Siberian elm was popular in the dry parts of the West. When buying a tree, homeowners in particular may not have known to ask for more information on the characteristics of the different varieties so they could purchase the one that ultimately would be the best variety of the species for their space.


No tree was more beloved for city streets and backyards than the American elm, which fell victim to Dutch elm disease and all but vanished from the urban landscape. Denny Townsend, a research geneticist at the National Arboretum in Washington, DC, is widely known for his efforts to breed American elms that would be resistant to the disease.

Townsend's successes along that line to date include the varieties known as Valley Forge and New Harmony. Being able to breed an American elm that could flourish again in our cities would be a huge contribution to the urban forests of this country since the American elm is widely recognized at the best urban tree ever to grace our streets.

Another champion of city trees, the late Frank Santamour, who was also a research geneticist at the National Arboretum, searched for trees that could survive tough urban conditions and then tried to simulate those stresses at the arboretum--creating wounds, for example, to see how they healed--to see how the trees would fare as potential city species.

Researchers need to do that for urban species, because trees in the city have a tough life. They're expected to grow and thrive in cramped growing spaces, tolerate usually poor soil, and survive car door bangs and bicycle chains. Santamour once told me that nurseries traditionally select trees more for their human interest than their suitability for urban conditions. That doesn't necessarily bode well for the trees when they're thrown into the stresses of city life.

So what will we need to create the best possible urban forests for the future? Thomas Ranney, a professor of horticulture at North Carolina State University, says the need for better urban trees is escalating rapidly. At a recent meeting organized by Chicago's Morton Arboretum, research geneticists came together to identify tree improvement priorities for urban forests. At the top of the resulting list were pest resistance and stress tolerance.

As Ranney said, "Those of us that work on breeding and developing new trees are generally concerned with developing plants that have greater pest resistance, improved adaptability (stress tolerance), reduced environmental impacts {reduced invasiveness, reduced pesticide use) and superior commercial potential (in other words, they're more efficient to produce with greater environmental and ornamental merit). When you consider the tremendous diversity of trees in the urban landscape, there are great opportunities for developing improved urban trees."


In the near future, will we look to the field or the petri dish to get those improved trees? The next most logical step in advancing tree improvement efforts seems to be to focus on breeding programs that take advantage of existing genetic material. That's different than developing genetically manipulated trees--trees in a petri dish. The science of genetic manipulation offers many opportunities but also poses the risk of creating new varieties that have unknown problems as well as potential benefits.

Another option, cloning, takes into account genetics but not environment. That's the case with an effort underway to clone trees deemed by AMERICAN FORESTS to be the largest known of their species--trees that appear on our National Register of Big Trees. These big tree champions triumph in large part because of their growing conditions and a lack of competition for light and nutrients. While we revere and champion the protection of these trees, we know that--as with the former national champion American elm, which recently died from Dutch elm disease--their time at the top is tenuous. Therefore, AMERICAN FORESTS does not endorse any effort to clone national champion trees.

A better idea would be to take the scarce funding that exists for urban tree improvement and focus it on scientifically significant research that has a positive effect on the quality of the environment where we live.


Progeny = offspring

Phenotype = combined effect of genetic and environmental conditions

Provenance = geographic place where trees originate

Tree selection = picking trees for propagation from existing tree stock

Tree breeding = taking genetic material from one tree and crossing it with genetic material from another. This is usually done by taking pollen from one tree and using it to pollinate the flowers on another.

Gary Moll is vice president of AMERICAN FORESTS' Urban Forest Center.
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Article Details
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Title Annotation:Heartwood
Author:Moll, Gary
Publication:American Forests
Article Type:Column
Geographic Code:1USA
Date:Sep 22, 2003
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