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Why Tree Improvement?

Like improved varieties of corn, beans, wheat, vegetables, and fruit trees, researchers can select for desirable traits for trees being planted for conservation purposes. For example, we may want faster growing trees to overcome susceptibility to depredation from mice, rabbits and deer. Or we may want to simply reduce the amount of time it takes a tree to grow to a merchantable size, so it can be sold in a shorter period of time.
Once a trait is identified through testing, the best selections for that trait can then be propagated and planted into seed orchards. Making selections from trees growing within native stands of forest ensures locally adapted genotypes are being captured. This provides a diverse collection of genes to maintain species diversity.

Iowa’s tree improvement program has 3 goals:
1. Gene conservation to maintain biological diversity for desired tree species. 
2. Genetically improved and locally adapted selections of black walnut with the goal to produce above average growing black walnut seedlings from parentage displaying high quality timber properties.
3. Butternut archive to maintain a reproducing population for this species before it goes extinct. Emphasizing native Iowa butternuts, but working in collaboration with other states to develop genetically superior butternut that can withstand butternut canker. 

Bringing seedlings in from out-of-state can cause problems with the timing of pollination with other trees that are the same species, making it difficult to produce seed that is vital for the survival of that species and as a source of food for people and wildlife. An additional risk of transporting seedlings from other states into Iowa occurs when insect and disease pests hitchhike on those seedlings, allowing them to spread and become established in new areas faster.

Moving seed or seedlings out of the range they originated from can cause problems in productivity by decreasing the growing season they are adapted to. Trees that are adapted to a longer growing season than we have in Iowa may not always be dormant when a killing frost occurs, causing those trees to dieback. Growing trees from northern climates here sacrifices growth compared to trees native to Iowa, because they are programmed to shutdown earlier in the growing season. 

Tree Improvement for Black Walnut

Black walnut is Iowa’s most valuable timber tree species. Oftentimes harvesting practices like “selective harvesting” removes the best trees and leaves the rest. Over time the genes that are growing the best trees that are providing the desirable traits that bring top dollar are no longer reproducing on those sites, leaving less desirable trees to reforest the site. Iowa currently (2008) has the third largest volume of sawtimber size black walnut growing in the world. Maintaining Iowa's edge as a leader in the production of one of the world's most valuable timber species is why this tree has been selected for tree improvement. The economic importance of black walnut helps with supporting 30 sawmills (including 1 veneer mill) that employ 380 people in rural areas producing $54 million worth of goods based on 2002 U.S. Census Data.

Without a vibrant, local wood industry the value of all timber in Iowa’s woodlands will decrease because of the increased hauling distance to bring in loggers and their equipment to remove the timber. Less harvesting and less value for wood products means more of Iowa’s forests will transition to shade-tolerant hardwoods like maple-basswood or ironwood. Studies of dietary needs by forest wildlife show shade-tolerant forests do not support the same number or same wildlife species as mast producing black walnut and oak-hickory forests will.

The first trait selected for in black walnut improvement is faster growing seedlings, the advantage of supplying early faster growing black walnut trees:


  • Less weed control needed to successfully establish the trees,
  • Grow larger seedling diameter and height to withstand depredation by mice, rabbits and deer,
  • Shorten the amount of time it takes to grow these trees to desired harvest size.

How are we Testing?

350 high value black walnut trees selected from 65 counties in Iowa to be tested; with 226 selections having been tested for early fast growth. Collecting seed from these trees in the wild only gives us half of the genetic material because we do not know the pollinating parent. The seeds are planted and their performance is measured annually. Fencing is installed and weed control is applied to protect seedlings and optimize growth.

Deer Fence First year growthBrent Olson with Walnut


Seed Orchards

Seed orchards allow for genetic improvement to occur within a species, providing landowners with a better seedling to plant in the future. They allow native forest grown trees to be planted in proximity to one another. Planting conservation seedlings from trees that are from native forest stands of Iowa that are locally adapted to our climate and have desirable genetics that landowners want to sell and loggers want to buy. This would be something no other nursery can provide Iowa landowners. 

Tree Improvement for Butternut

Butternut trees produce valuable wood products used by carpenters for cabinets, flooring and furniture. It is softer wood than walnut, making it easier for woodworkers. Butternut grows on a variety of sites, doing best on well-drained soils in riparian areas. Native to Eastern half of Iowa and can live up to about 80 years. Like black walnut and oak, it is intolerant of shade, so silvicultural regimes that are suitable for oak and black walnut are appropriate for butternut. Butternut produces seed that is desired by people and many different forest wildlife species.

Bruce Blair with mature butternutButternut planting

During the past 40 years, a disease called Sirococcus clavigignenti-juglandacearum (butternut canker) has spread throughout the northeastern region. The spores of the fungus are spread by rain splash and wind, but the rapid spread of the disease suggests that insects also act as vectors. Dr. Bergdahl and his colleagues have found that at least 17 species of beetles closely associated with butternut that can carry the spores. A single beetle can carry as many as 1.6 million spores (just one is needed to cause an infection) and the spores can remain viable on insects for at least 16 days. The fungus can also be carried on the nut; causing some trees to be infected before they even begin to grow.
There is no known treatment for the butternut fungus, so conservation efforts are focused on finding and protecting resistant trees. A challenge with planting more butternuts is knowing if the trees are genetically pure. Butternuts can hybridize with other trees like Japanese walnut, which was introduced to North America in the 1800’s.

Status of Butternut in Iowa

In 1990 Iowa had an estimated 1.4 million butternut trees; by 2008 an estimated 84,000 trees left (94% drop). Of the 84,000 or so remaining no determination has been made of how many of those trees are hybrids because of the difficulty in diagnosing hybrids and native butternuts in the field. There are some physical characteristics that can be used to distinguish between a native butternut and a hybrid, but often times those keys can’t be used for mature trees in a forest setting because the branches and leaves are too far away to reach for closer inspection. The butternut trees found in Iowa are being tested using molecular DNA markers to determine which trees are hybrids and which are native. The native trees are being propagated using scion from the mature butternut trees to increase populations and create seed orchards for butternut. These seed orchards are important because they are increasing the population of known native butternuts. In time the trees showing the best ability to heal from butternut canker can be grouped together in a separate area to allow cross pollination of the gene pool and hopefully improve the genetic resistance ability for future butternuts. If seed orchards are not developed now, the native population of butternut would continue to be lost forever. Reproduction of butternut requires viable seed to fall in areas with adequate sunlight to promote germination. With butternut being a desirable food source for wildlife, a lot of the seed is eaten, leaving few seeds to try and find their way onto a site with enough sunlight to grow to maturity. 

Iowa is in a unique position with respect to the North American butternut range. The eastern half of the state is in the natural range of butternut and the western half is outside of the naturally occurring range. Therefore the Loess Hills State Forest has been designated as a repository for conserving butternut. It is hoped that placing butternut trees just outside their range, but on a suitable growing site, the tree's exposure to the canker killing fungus will be lower. 

What is Iowa doing?

The U.S. Forest Service has made selections of native butternuts throughout the northeastern United States over the past 20 years. Branches (scion) were collected from these trees to capture the exact genetics of those desirable trees. Scion has been grafted onto black walnut root stock to help create seed orchards that can produce more seeds to maintain a viable population of native butternut and to test for resistance to butternut canker. The Forest Service selections were made from butternut trees that survived around other butternut trees that died from canker, giving hope that this is a sign of resistance. Iowa has planted 150 of these seedlings in 2007 & 2008 (41 families) in 2 different regions in the Loess Hills.

In 2009 Iowa along with 4 other states (IN, CT, VT, PA) put together a grant to fund more butternut research. The grant helps to get more butternut surveyed in these states, record with GPS devices the locations of known butternuts, perform DNA testing to determine which trees are native, grafting scion from native selections, perform butternut canker resistance testing through direct inoculations, and plant preserves/orchards of more butternut trees that have the exact genetics of the forest grown survivor trees. The seed orchards will make it easier to collect seeds and grow more seedlings for conservation plantings, hopefully stabilizing the population of butternut in Iowa. 

Winter of 2010, 36 butternut trees were sampled and tested for genetic purity from Iowa. DNA analysis resulted in 25 of the trees being native. This nearly triples the number of known native butternuts that were known to exist previously. These trees are now being grafted onto black walnut rootstock to provide additional seedlings for seed orchard establishment and disease testing. 

Iowa planted an additional 350 butternut seedlings grown from seed by Hardwood Tree Improvement Regeneration Center, HTIRC, in the Spring of 2010 from a widespread northeastern U.S. collection effort to continue to preserve more butternut seedlings. It is easier to collect seed from a wide area and grow them in a nursery bed, compared to collecting branches from trees that sometimes are 50 feet or higher from the ground to get scion for grafting. 
Finally, we are continuously following-up on leads of known forest grown butternuts. We have collected seed from 13 different Iowa butternut trees and established a seed orchard in the Loess Hills. We have begun DNA testing to determine which trees are pure and which are hybrid, so far we know we have 25 pure native butternuts and 14 hybrid, with the other 172 trees still needing to be tested. 

 Butternut seedScion wood from ButternutAron Flickinger measuring a Butternut

  Newspaper article about the Butternut Project at Loess Hills State Forest. 


Aron Flickinger, Special Projects Forester
Iowa Department of Natural Resources
Office: 1-800-865-2477