IUFRO Breeding theory and progeny testing

First generation testing

As in many temperate regions of the world, forest tree improvement got underway in the Pacific Northwest of the USA in the 1950s, with a number of companies and agencies starting independent tree improvement programs. Graft incompatibility became evident by the early 1960s and dampened enthusiasm for grafted clonal orchards (Silen and Copes 1972). The problem of graft incompatibility was eventually greatly reduced due to work done by Don Copes of the PNW Research Station, but by then had played an important role in shaping tree improvement in the region.

The IFA-PNW “Progressive Tree Improvement System� was launched in 1966 due to the failure of grafted clonal orchards (Silen 1966, Silen and Wheat 1979). The emphasis here was on forming local cooperatives to share costs, and on progeny testing large numbers of trees using wind-pollinated seed in small testing zones. Silen and others felt that due to steep variation in environmental factors (e.g. temperature) caused by the mountainous topography of the Pacific Northwest, it was not appropriate to move Douglas-fir seed far from the source. Improved seed was to be delivered from seed orchards established using full-sib crosses made on the parent trees.

The first-generation testing phase ran from 1967 till 1993, during which over 25,000 first-generation Douglas-fir parents were tested in 109 breeding units, with over 3 million progeny test trees planted. The typical measurement schedule was five, 10 and 15 years from seed. Height was usually measured on each occasion, while diameter was usually measured at age 15 and sometimes at age 10 as well. The incidence of ramicorn branches, forks and stem sinuosity was assessed in some of the later testing programs, as information was collected on the inheritance of these traits (e.g. Adams and Howe 1985, Temel and Adams 2000).

Advanced-generation breeding and testing

Data from first-generation tests were used to draw several conclusions relevant to advanced-generation breeding and testing; other decisions were based on computer simulation and other breeding programs (Johnson 1997, 1998a, 1998b; Johnson et al. 1997). It was recognized that full-sib breeding had several advantages over open-pollinated breeding. There appeared to be little marginal gain per test beyond six successful progeny tests (in terms of ranking families), that two or three crosses would give a reasonable estimate of a parent’s GCA, and that final selection around age 12 years would be efficient. An advanced-generation breeding and testing strategy for coastal Douglas-fir was developed between 1996 and 1997.

Implementation of the advanced-generation testing program has been fairly similar to the proposed strategy with some deviations. First-generation testing programs have been merged in the north-south direction into larger testing zones. To date, 74 first-generation programs have been consolidated into 8 second-generation breeding programs (10 or 11 testing zones). The emphasis has been on lands up to 3,000 feet, but one program is likely to extend to 3,500 or 4,000 feet.

Breeding population size for a second-generation program is at least 200 selections, but most are greater than 300. Within a breeding population, breeding groups of 20-30 selections were constructed, each breeding group was from a single first-generation program. This resulted in sublines to manage inbreeding, and multiple populations to maintain locally adapted gene complexes. The rule of thumb in choosing second-cycle selections has been a 1 in 10 among-family selection intensity. Most selections were made on age-15 height; information on DBH, stem form and wood specific gravity were also considered. The top 10% of selections within a breeding population were assigned to an elite population.

The breeding population for each new testing zone includes selections from the "local" breeding groups from the testing zone, and only the highest-ranked selections from breeding groups originating further away from the testing zone. Selections are used in at least two crosses, and elite selections in up to four crosses. Some elite crosses are made across first-generation zones. Unimproved checklots were included in all tests planted after 2001. Most tests are designed to be thinnable.

In total, the Douglas-fir breeding effort will be comprised of over 2,600 crosses (80% have been completed), 106 tests (51 have been established) and about 300,000 test trees planted. Thus, the total number of trees will be around 10% of the trees planted in the first generation. Between five and six tests are established per testing zone, with 20 trees per cross per site in single-tree plots. Second-cycle test established is to be completed by 2010 or so, while it will be possible to make third-cycle selections in 2006.
Trials established to date have contained from 143 to 283 full-sib crosses, planted as containerized seedlings and fenced for protection against browse. When crosses of a given program are planted in two Phases, the separate Phases are linked by at least 10 common linker crosses. The goal is to keep the tests weed-free for three years after planting, and control harmful competitors (such as aggressive hardwoods) until crown closure. Tests will probably be measured twice, when the trees are 15 and 30 feet (4.5 and 9 meters) tall respectively. This may take only five and 10 years from planting on the most mesic southern sites, but longer on colder, mid-elevation, northern sites.

References

Adams, W.T. and Howe, G.T. 1985. Stem sinuosity measurement in young Douglas-fir progeny tests. P. 147-159 In: Proceedings of the IUFRO Working Party on Breeding Strategy for Douglas-fir as an introduced species. Working Party: S.2.02.05. June 1985, Vienna, Austria.

Johnson, G.R. 1997. Site-to-site genetic correlations and their implications on breeding zone size and optimum number of progeny test sites for coastal Douglas-fir. Silvae Genet. 46:280-285.

Johnson, G.R. 1998a. Breeding design considerations for coastal Douglas-fir. USDA Forest Service PNW Research Station General Technical Report PNW-GTR-411. 34 p.

Johnson, G.R. 1998b. Parental GCA testing: how many crosses per parent? Can. J. For. Res. 28: 540-545.

Johnson, G.R., Sniezko, R.A. and Mandel, N.L. 1997. Age trends in Douglas-fir genetic parameters and implications for optimum selection age. Silvae Genet. 46: 346-358.

Silen, R. R. 1966. A simple progressive tree improvement program for Douglas-fir. USDA For. Serv. Res. Note PNW-45. 13 p.

Silen, R. R. and Copes, D. L 1972. Douglas-fir seed orchard problems – A progress report. J. For. 70: 145-147.

Silen, R. R. and Wheat, J. G.: 1979. Progressive tree improvement program in Coastal Douglas-fir. J. For. 77: 78-83.

Temel, F., and W.T. Adams. 2000. Persistence and age-age genetic correlations of stem defects in coastal Douglas-fir (Pseudotsuga menziesii var menziesii[Mirb]Franco). Forest Genetics 7: 145-153.

Authors: Keith Jayawickrama and Terrance Ye, Northwest Tree Improvement Cooperative, Department of Forest Science, Oregon State University, Corvallis, OR 97331-5752, USA.