Hybrid or Native?

January-February 2009

THERE ARE A LOT OF VOICES in the “tree” community talking about native species versus hybrids. On both sides of the debate, some see no room for compromise. Others see a mosaic of trees in Ontario. The issues are complex. But while the arguments continue, consumers are making the decision with every tree they plant. These choices will determine how we weather climate change and what we will pass on to our children.

In my view, if there is good evidence of native resistance, there is no reason to go exotic,” says Judy Loo, researcher with the Canadian Forest Service, Atlantic Region. Loo considers herself to be a “purist” and acknowledges many of her colleagues disagree with her views. Her own work is with native beech but she does recognize sometimes hybrids are not only acceptable but necessary.

Loo is testing and searching for native beech trees that are resistant to beech bark canker.

She believed native resistance was “out there” because she was personally aware of trees in natural forest settings that were not affected by the canker. Never having heard of a successful beech hybrid, native resistance seemed the best and obvious route to preserve the historic form and majesty of Canadian beech.

The team found four to five percent of the beech in the research area showed no symptoms of the disease complex. The exact percent of beech with some resistance is unknown but the guess is around 50% of the disease-free trees or 2-3% of all the beech in the area. The other healthy looking trees have just not contacted the fungus yet. No one knows how many genes are involved in resistance nor do we know how these genes behave but these questions are under study.

I asked Loo, “Would you plant a native beech on your own front lawn?”

She replied, “Not yet.”

“In my view,” Loo continued, “if hybridization is the only way to save a gene pool then it may be justified, but I wouldn’t plant a straight hybrid without a lot of backcrossing so it is mostly native.” Don’t stop with the first backcross or even the second. You can end up with a tree that is less then one sixteenth exotic and the rest native. An example of a backcross is the chestnut cross being developed by the Chestnut Council. They are selecting for form to keep it as close as possible to the native. With the chestnut they searched for many years for a native resistance and no chestnut trees were found resistant to the disease. Crossing is the only way known to save the species.

Loo’s opinion of the white pine is different. The native white pine has degrees of resistance to blister rust. One of the dangers of hybridization is the risk of pollination with native species. “This danger is underestimated and we must be conscious of it.” Any work done on hybrids must include a risk analysis to determine if the hybrid will spread to areas outside the native range. This is to ensure the hybrid does not become invasive. The quality and quantity of the seed on a hybrid may also be different then the native tree. With multiple backcrossing, this risk of altering a wildlife food source is decreased.

Judy doesn’t have secure funding outside of the federal government for the beech project – and government funding is never guaranteed.

Stephen Woodley, Chief Ecosystem Scientist with Parks Canada, said they supplied genetic material to the research efforts looking for native beech. Their mandate “is to preserve native species and ecosystems.” Thus even multi-backcrossed hybrid species of trees are not planted on national park lands. Restoration is only done with native species. So on policy, they will not be accepting Lu Pengxin’s, Ontario Forest Research Institute (ORFI), new hybrid multi-backcrossed white pines developed to be resistant to white pine blister rust.

White pine is Ontario’s provincial tree, and an economic species. We’ve grown accustomed to its misshapen silhouette and almost forgotten the beauty of a healthy white pine. But our forestry industry needs better survival rates for white pine and the hybrids now exist and are being planted in Ontario. Not being allowed in national parks is not slowing progress. Lu says, “Our forests have changed permanently. We can’t eradicate white pine blister rust, we have to find ways to cope.”

Sean Fox is Assistant Manager at the University of Guelph Arboretum. He says he is pro-native but believes there are specific urban locations where choices can be limited for suitable and enduring native trees. “In these cases, it may be better to make the responsible choice of an appropriate exotic than have no tree at all.” Regarding the hybrid white pines being planted in plantations, he said there are two ways to look at the situation. First, if this tree is being planted for harvestable plantations, then it is much like a farm product just like another exotic, corn. However, what must be considered is how these trees will interact genetically with the surrounding natural populations. “There are far more variables involved than we can possibly conceive. Forests have endured many hardships over the millennia, and genetically-diverse, natural populations are part of nature’s survival strategy.”

According to Abby Obenchain, OFRI, Lu’s hybrid white pine is the only hybrid presently under study in their facility.

“My program (developing hybrid apple trees) is not being funded anymore and the new four lines may be the last releases from our centre,” says Shahrokh Khanizadeh of Agriculture Canada in Quebec. You may not know Khanizadeh’s name but he knows his apples. “Eden” was one of his most widely publicized trade named fruit trees – a reduced browning apple bred for the organic market or as he says, “to encourage people to eat more apples.”

Khanizadeh says it is difficult to judge hybrids as a group. Each has advantages and disadvantages. They are usually selected to improve hardiness, disease resistance or to make a tree more suited to a specific industry like his new “Diva” bred for cider or ice cider production. His work with apple hybrids is all about encouraging both the farming and horticulture Canadian industries by developing new species to increase the market share available to Canadians.

Khanizadeh knows an apple a day is about more than just keeping doctors away. His newest developments aren’t for the lunch pail and they don’t have descriptions or names yet. They are ornamentals. They hold on to their cherry sized or smaller fruit. This reduces the annual raking chore many homeowners dread and makes the fruit a valuable winter food source for birds – which delights the backyard bird watching community. They are also all scab resistant and winter hardy when protected from drying winds.

Cheryl Hampson from BC is another Agriculture Canada researcher working on hybrid development of commercial apples. She says, “All apples are hybrids, as are many wild plants because they can’t self pollinate.” The seeds in a single apple will not grow trees that are (in human terms) twins but rather half siblings. Public funding for tree breeding is falling while at the same time public interest is increasing not just for “new” but “better” hybrids. Her funding is federal with no outside assistance.

The domestic apple is not native to Canada but it does have two relatives listed in Native Trees of Canada by Hosie. They are Malus coronaria and the western crab apple Malus diversifolia. The native ranges are the extreme western coast line and the far south of Ontario. However, anyone who has travelled in Ontario knows the apple is widely dispersed. Technically, the fruit that keeps doctors away is an alien invasive tree adding a complex twist to the debate.

Barb Boysen, Forest Gene Conservation Association, is pro-native species but not completely against exotics. One of her roles is supporting the search for a native resistant butternut; and not just relying on hybrid walnuts that can be more resistant to the canker fungus. She explains that trees have a landscape effect – good and bad – and it’s often the planting location of exotics she believes can be wrong. When you are planting in an area beside a river, a natural area or in a park, try to choose natives and avoid invasive exotics like Norway maple. It is also sad to see exotics in ecology parks planted by well meaning people and nurseries not able to identify the species. Several of our native species can be confused with their exotic relatives. Our white birch can be confused with European birch; our native highbush cranberry for the European species. The list goes on and on.

Boysen says, “We promote native species because they are proven performers on the landscape – they are adapted to local sites, climates and communities of species. Also, in an urban setting, planting natives helps the urban island contribute to the gene flow between adjacent natural areas, rather than become a barrier, or worse contribute exotic material. Exotic material is rarely an immediate problem. It usually only becomes one after it becomes hard and expensive to eradicate. Problems can include poor growth, short life span, reduced diversity, and increased insect and disease problems, not to mention replanting costs. There are the really harmful effects if they invade native communities with the problems of hybridizing and reducing ecological functions.”

Boysen continues, “Where the urban sites themselves are highly disturbed and one could argue no longer ‘native,’ the introduction of exotics is still an experiment. What is the origin of the clones or populations you are introducing? How different is the source from your site, are they really adapted to our sites? Will they be long lived, healthy?”

The Canadian Chestnut Council (CCC) is presently functioning without federal or provincial funding. The development of this backcrossed hybrid is progressing on the determination of 200 volunteers with past financial support from the Ontario Trillium Foundation and continuing support from members of the CCC.

Previous work on the chestnut was conducted in the US and the CCC admits they are following in the footsteps of the American Chestnut Foundation (TACF) which is about a generation behind in developing a version for the Canadian climate. They crossed the scrubby Chinese chestnut with a Canadian-grown American chestnut and are in the process of multiple backcrossing to ensure the traditional shape and characteristics of the original American chestnut in a hybridized 92% American chestnut having blight resistance. They have two planting sites in Ontario.

Charles Hooker, Secretary of the Canadian Chestnut Council, says he believes in the hybrid-chestnut approach instead of alternatives for two reasons. First, the chestnut family hybridizes readily, and Chinese chestnuts are growing in the province so they will eventually cross anyway. Secondly, no one has discovered an original native American chestnut tree that has survived the blight. Chestnut blight kills the tree back to the roots and shoots sprout from the roots. These shoots again die back, but few reach the seed-producing level. Blight resistance is therefore unlikely to develop and it is almost certain the tree will become extinct without the development of a blight-resistant hybrid.

The American chestnut is both federally and provincially listed as “endangered.” It is technically illegal to possess an endangered species in Canada or to hybridize it, but the CCC continues its efforts to make the Chinese-American crosses. So far, appeals by the CCC for government assurance of protection from prosecution have not been answered.

Hybridizing native species is not the biggest concern pro-native tree supporters fight, says Fox. People tend to choose the same species over and over, like the sugar maple or in the past we chose the American elm. It was native but we planted it on almost every street. When disease struck, it got most of the trees. Our biggest problem is over planting for specific ornamental or economical attributes. We sometimes ignore environmental factors or other problems that could arise from a lack of biodiversity.

Fox also said introducing hybrids or exotics into wild areas is not entirely responsible. We don’t know the full impact in the long-term, especially with a changing climate. While some exotics may not be invasive in the landscape, they can still create a biodiversity void where interactions with soil microorganisms, insects and other types of fauna are lacking. It is now tough to find a pure red mulberry due to the escape of the exotic white mulberry into the wild and the subsequent hybridization that has occurred. Removing one species from the forest doesn’t seem to make a big impact. But forests developed over thousands of years to be resilient to steady changes. With the increased movement of exotic plants and pests and all of the other human-induced impacts being placed on our forests, we are certainly putting a lot of stress on our trees at a far more rapid rate than ever before.

Richard Ubbens, Forester for the City of Toronto, says he’s seen a lot of improvement in the last 20 years. “We’ve been harping at growers and in seminars to encourage people to put more energy into collecting seed and growing native species.” Through development approval processes on properties adjacent to ravines, conditions are set requiring ravine improvements. And prescribed burns were done in High Park to encourage rare black oak regeneration. Yes, some species like native mulberry are rare because these species are often not appreciated in urban areas where there is little habitat left for them. But invasive species are sometimes a bigger threat than concrete. “Don’t give way,” Ubbens says, “to the urban condition. Enhance the soil. Increase diversity and enhance the condition of plants.”

I called a local senior real estate agent and asked about public requests concerning trees when buying a home. He didn’t seem to know the terms hybrid or native as they apply to trees. He replied, “Sure, people like trees, but if they like the house, they don’t ask about tree species or maturity. They can plant what they like after the purchase.”

I tried again, “Do people prefer a home with a 100 year-old oak or a mature apple?”

“People only care if they like the house.”

“Is there any change in what people are looking for?”

“Not in my experience.”

I didn’t like his answers so I did a MLS search and discovered in 250 house listings, only one mentioned tree species and that house included a white spruce reforestation area. There might be a difference between the preferences and priorities of urban versus rural homeowners but I can’t confirm if this is true.

If the experts I interviewed for this article are correct and we don’t deal with the potential results of planting hybrids versus natives, there will be lots of work for arborists in the future. Not one, but many native species are challenged (and don’t forget ash). Hybrids and exotics have a real place in our society but they can’t replace natives. The real estate market is telling us the message isn’t getting out. People are still planting trees that look good in the nursery or on the internet and not necessarily the species that are appropriate for the environment or even their site. These trees struggle on but never reach their potential and often require arborist assistance. But the bottom line is that if Ontario residents don’t get the message about the importance of planting the right tree, they just won’t pay, as tax payers or consumers, to maintain a healthy urban forest.

Boysen gets the last word. “Exotics are one big experiment. Why don’t we experiment with more native species? People need to care that when they plant a tree it will live for decades and fit into the whole landscape and contribute to a healthy, green infrastructure.”


Ornamental Apples
PGO-11 or SJC59A76-08, PGO-12 or SJC59A79-01, and PGO-14 or SJC5791-01 (developed by Shahrokh Khanizadeh). Descriptions of trees developed by Agriculture Canada can be found at the Plant Breeder’s Office in Ottawa. Licensed operators can obtain bud wood for minimum costs by contacting Dan Thompson at the Canadian Food Inspection Agency.

Apple Trees
Developed by Cheryl Hampson. Plant material is marketed through the website www.picocorp.com.
• Aura Golden Gala: a crisp juicy apple with a long storage life
• Nicola: late harvest, bi-colour fruit
• Sabena: for backyard gardeners, an old fashioned European flavour with slow browning
• September Sentinel: for small backyard gardens, free of many diseases
• SPA493: a tart apple awaiting more testing

Multi-backcrossed hybrid white pines from OFRI average 93.75% parentage from Pinus strobus. Field trials are established.

First Canadian hybrid backcrossed chestnuts are estimated to be ready in five years.

New Brunswick native resistant beech is at the greenhouse planting stage. No potential release date has been set.

More Canadian Grower Information


Species Basics
• Celtis occidentalis
• Medium-sized tree
• Similar form to elm
• Attracts bluebirds, brown thrasher, cardinal, flicker, mockingbird, robin and 29 other bird species
• Zone 3b
• Native to deciduous forests
• Yellow fall colour
• Full sun
• Moist to well drained rich soil best but tolerates moderately wet, dry and alkaline.
• Not affected by Dutch elm disease
• Tolerant to urban environment
• Does not tolerate soil compaction
• Does not tolerate flooding
• Edible fruit
• Rapid growth

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