While rapid genetic improvements have been made in livestock and numerous plant species, the most elusive and positive traits of Christmas trees can take decades to improve, and that’s too long for growers to see good economic returns.

However, research with problems in other tree species, such as the mechanism of resistance of ash trees to emerald ash borer, has helped Dr. Justin Whitehill, North Carolina State University, realize the need for developing genetic solutions to combat challenges in tree and forest systems.

Whitehill recently explained the Christmas Tree Genome Project to Rapidly Advance Genetic Improvement, a cooperative effort that received funds from the Real Christmas Tree Board. The NCSU program includes breeding programs for Fraser, Turkish and Trojan firs and Virginia pine.

“Tree systems are difficult to manage for several reasons – mainly their long lifecycles and having to continually treat them in a forest or plantation setting, which isn’t always economically feasible,” said Whitehill.

Fir species dominate the U.S. Christmas tree industry, with Fraser fir comprising half of those. “Douglas fir is shrinking due to Swiss needle cast,” said Whitehill. “Other (popular) trees include grand fir and Concolor fir.”

Fraser fir accounts for about 40% of all trees sold throughout the U.S. annually, but there are serious production challenges.

Whitehill explained that Fraser fir is native to North Carolina, with the six naturally occurring island-like populations of this species found in the highest peaks of the southern Appalachian Mountains. Fraser fir is considered endangered in the wild, especially after invasive insect pests such as the balsam woody adelgid killed many trees in the landscape in the 1950s.

“The reason those fir populations are only found at the tallest peaks of the tallest mountains is they are a ‘great glacial relic’ species,” said Whitehill. “In the last Ice Age, fir were all over the place – temperatures were conducive to survival of fir species. But as temperatures started to rise, the species retreated north into boreal forests in Canada, and where they didn’t retreat up to Canada, they retreated to the tops of mountains.”

Trees became isolated on mountain peaks and adapted to cool, moist environments. However, fir species planted as Christmas trees are growing at a lower elevation and out of their natural environment, which induces stress and can result in disease and undesirable trees.

One ongoing challenge with Christmas trees is needle retention. Whitehill explained that if trees don’t undergo a cooling dormancy period during winter, they are still physiologically active and needles fall off rapidly. Whitehill’s research has focused on identifying trees with “elite needle retention,” which means trees harvested without having gone through dormancy will still have good needle retention.

A major challenge for Fraser fir growers is the root rot pathogen Phytophthora. “We also have insect challenges … We’re dealing with elongated hemlock scale,” said Whitehill. “That’s a regulatory pest that challenges a lot of growers. Deer browse is also a big issue.”

Whitehill said the classic approach of genetic improvement involves identifying superior trees, seed collection and tree evaluation, followed by lengthy production cycles. One approach to hasten the process is to develop genetically modified organisms or through genome editing.

Cultivating a long-cycle crop means growers must select the best available seedlings and manage them for up to 10 years before maturity. Somatic embryogenesis can accelerate the breeding and improvement process for conifers. Photo by Sally Colby

Growing challenges related to the long growing period of Christmas trees are being met through genetic improvement, which Whitehill described as “the application of genetic principles to generate or develop plants that are more useful to humans.”

“In conifers, new genetics have been developed with the classic approach,” said Whitehill. “Gradual population improvement through quantitative genetics where trees from wild populations are domesticated over time.” The other approach, which is somewhat controversial, is genetic modification or genome editing such as CRISPR.

For conifers, Whitehill said there’s only one method for rapid genetic manipulation. “We can go into a seed and dissect out the embryo,” he said. “On media, we can start to develop individual asexual reproduction of the tree through a process called somatic embryogenesis. When the embryo is placed on the media, it begins to make thousands of copies of itself.”

This is an important application for the Christmas tree industry and for preserving endangered plant species, Whitehill noted.

Genetic improvement in conifers is extremely slow, primarily because Christmas trees have not historically held importance as a food source and because breeding cycles are slow. Whitehall said the NCSU program has recently achieved one solid round of Christmas tree improvement, which required 30 years of research by his predecessor.

Because genetic improvement is slow, the current laboratory focus is on genomics.

“Genomics combines three fields of study,” said Whitehill. “Biology, genetics and computer science. A genome is a complete set of genes, or genetic material, of an organism. Genetics is the science of genes, heredity and genetic variation within organisms. Genomics is the study of the entire DNA of an organism and associated patterns across the genome of a given species.”

He explained that while genomics speeds up the improvement process, it still takes years to evaluate tree performance. “If we apply somatic embryogenesis,” he said, “we can start propagating the best trees more rapidly.”

Genomics sets the stage for a reduced testing phase. Instead of planting a tree in the field and waiting until it grows or dies, smells good or has good needles, genomic selection or other genomic tools can be used to examine at the DNA of the plant and identify trees as soon as there’s enough DNA to predict their function.

“The Christmas Tree Genome Project to Advance Genetic Improvement was funded by the Christmas Tree Promotion Board and has been helping to finish the sequencing program,” said Whitehill. “We’re working primarily on Fraser fir, but everything done in that species will apply to Canaan fir and Balsam fir.”

Traits for improvement include more rapid growth (to shorten time until harvest), improved tree form and quality (to reduce hand labor) and better aroma and needle retention. Work to achieve resistance to diseases such as Phytophthora root rot is also underway.

This research will benefit the U.S. Christmas tree industry via funding from a grant submitted to the USDA-NIFA Specialty Crop Research Initiative. This research phase will engage American growers and university cooperators through on-farm studies.

“We want to work with growers throughout the U.S. and focus on fir species,” said Whitehill. “If we know the source of seeds of trees being grown on farms throughout the U.S., we can work with those growers to develop genomic marker systems to identify which trees have the best genetics and what should be planted in the future. The goal is to develop genomic tools to help provide more guidance to growers across the U.S. to improve their economic returns on investment.”

by Sally Colby