Open4st Aspen Breeding Strategy

Overview

Poplars are a diverse collection of species that have significant commercial, wildlife and aesthetic value. The cottonwoods, balsam and aspens comprise the three major groups of Poplars. The cottonwood types and their hybrids (aka “Hybrid Poplars”) are most commonly used in plantation culture, since they are easy to propagate, coppice from the cut stumps and have a rich history of research and development.

The aspens differ from traditional Hybrid Poplars since they are more difficult to propagate but can re-establish themselves from the cut stumps and their root systems.  This capability may allow plantation establishment with less resource inputs and environmental impact.  The re-generated growth may also provide mid-rotation biomass harvests with mutual benefits for the target tree crop, biomass industries and wildlife diversity.  These attributes provide the rationale for the open4st team to develop vigorous, good rooting aspen materials that are durable, more native and effectively sterile.

In the early 1990's Patrick McGovern was involved in an aspen tree improvement project that was funded by the Department of Energy and supervised by Dr. Richard B. Hall of Iowa State University.  Over 207 crosses were produced with aspen material from Canada, Michigan and European sources.  See: "The Early Years" and the Open4st Family and Clone Naming Conventions.

Phase III - P. alba Crosses

In 2010 a Phase III breeding project (P3) was initiated by Brad Bender and Patrick McGovern to leverage previous gains and provide a better foundation for future crosses with American and European aspens.  It consisted of 19 P. alba crosses that included Canadian and European parents and a vigorous male clone, "NFA" that was discovered  in SW Michigan.  Brad Bender planted a cross section of these materials in Michigan's Upper Peninsula.  Patrick McGovern retained them in the nursery and selected 10 families (51 ortets) that averaged 81% dormant stem rooting with 6" cuttings in 2011 (2011-Family-Summary).  These materials will be re-tested with selections made for rooting, vigor, stem form and leaf issues.  The goal is that the final selections will provide diverse alternatives to the vigorous and good rooting family 83xAA04. 

Phase IV - Double Hybrid Backcrosses

Traditional Hybrid Poplars and willows are well adapted to moist sites that are ideal for food crop production.  A 2014 Phase IV project was initiated to develop vigorous aspen genotypes that are more native with good rooting, figured wood and may be  adaptable to a variety of sites.   We hope to leverage variation from double hybrids such as P. canescens x (P. alba x grandidentata) aka CAG crosses.  A secondary strategy is to backcross good rooting (and fertile) CAG selections with other F1 hybrids or native aspens.  The strategy will also involve selecting big tooth clones to create a gene pool of potential parents that are adaptable to upland sites.  Another double hybrid option may involve unrelated AG x GA parents.  

In 2011 open pollinated seed  (cross 1xBW) was collected from a double hybrid (CAG204) obtained from Canada.  This female clone is quite fertile, has more alba characteristics, averages 50% rooting and is fairly vigorous on good sites.  The open pollinated CAG204 progeny resulted in about 70% of the seedlings with P. alba characteristics (tomentose on stems and leaves) and 30% with aspen characteristics with little or no tomentose and intermediate leaf shape.  It is speculated that the male for the aspen types is the native P. tremuloides (aka small tooth aspen) and the male for the alba types is the Canadian P. canescens clone, AE42 which has 5 vigorous trees planted 20 yards directly West of CAG204.  This cross was produced in 2011 and will be compared with other CAG204 and male combinations.

Ten selections of the 1xBW family had 6 aspen and 4 alba "types" that were cut into 6" cuttings and re-planted in the 2012 nursery.  Interesting combinations were observed.  The 6 aspen types averaged 68% rooting.  One aspen type (selection 1BW1) had 100% rooting from 6 cuttings in 2012 and with vigor that was competitive with other nursery P. albas.  This clone also rooted 76% from 75 6" cuttings  (4-12mm dia) in 2013 and 75% in 2014.   Another aspen type had more stem and leaf  tomentose than  1BW1.  Two of the 4 alba types survived in the 2012 nursery with 71% and 50% rooting.  These early results may indicate the potential to produce vigorous aspens that root better than F1 AG hybrids.  It also suggests potential to capture traits from native aspens by breeding them with F2 hybrids to yield some progeny with the desired traits and adequate rooting.

It should be noted that a CAG x G or CAG x T cross is 25% alba with potential to yield good rooting in selected progeny.  Conversely, the cross CAG x A is 75% alba and perhaps fertile selections will serve as interesting and diverse gene pools in future breeding where P. alba is needed.  Also, seed and pollen fertility issues have been observed with diverse F1 and F2 hybrids which could present selection opportunities for good rooting clones without the invasive tendencies of pure alba.

In 2012 the McGovern Breeding Program (MBP) produced a CAG cross (2xB) with the parents - P. canescens C173 x 15AG4MF (aka "AG15").  C173 is an open pollinated selection from Yugoslavia, with predominately P. alba characteristics.  AG15 is an AG selection from a 20 year family field trial in Michigan's Upper Peninsula. The female parent of AG15 is the well formed Canadian A10.  Over 300 seedlings have been planted in the McGovern nursery for screening.  A vigorous selection (2B25) from this family in the 2014 nursery had a median height of 8.95 feet grown from 6" cuttings and 63% rooting.  Another CAG 2xB selection, 2B21 rooted 87% with 5 replications, and 40 survived trees. These new CAG materials may provide long term selection opportunities, while using CAG204 and CAG177 for short term gains with crosses similar to 1xBW.  Going forward it may be possible to mate fertile, good rooting CAG selections (eg. 2B25) with native aspens (via P. grandidentata) to select clones adaptable on a variety of sites and exploit their re-combination characteristics for a variety of tree breeding goals. For more Phase 4 details see the Phase 4 Breeding Strategy Matrix.

Planning for Future Breeding Opportunities

The Phase IV materials represent a variety of pure, F1, F2 and F2 back cross combinations.  Field trials with these materials should include a mix of families, species and clonal combinations to allow future breeding opportunities.  The r4st Pedigree table (see r4st Database) provides a convenient way to view MBP family parents to help make future breeding decisions and avoid inbreeding.  Other strategies include mating MBP selections with wild or native parents. 

The 2016 primary selections includes about 30 clones that are recommended for field trials (see 2016 Open4st Summary).  These clones are represented by 18 different families with  91% dormant cutting survival rate and 5 selection types (Alba=3, Alba Hybrids=8, Aspen Hybrids=8 Hybrids=13). Examples of a future MBP Phase V family level breeding options include the following:

• Any B family mated to: wild or unrelated AGs, P. gradidentata or P. alba

• Any BA or AB family mated to: wild P. grandidentata or unrelated P. alba

• Any BG family mated to: wild P. grandidentata or unrelated P. alba

• Any A family mated to: wild P. grandidentata

• Any G family mated to: wild P. grandidentata

Selecting for Effectively Sterile Clones 

Native aspen regeneration from seed is somewhat rare.  Most native aspens spread from root suckers after harvest or fire.  The White Poplar (P. alba) is native to Europe, interbreeds easily with native aspen and has larger perhaps more durable seed.  This may help explain why F1 progeny from these trees are considered invasive.  However, their hybrid F2 offspring are often less fertile (eg. AG x G) and may not be occurring in the wild (see: "A morphological analysis of populus alba populus grandidentata and their natural hybrids in southeastern michigan"). 

There are many variables to germination rates such as parental combining characteristics, branch health/age, catkin seed set,  seed age and storage conditions.  There were significant germination differences within families listed on the MBP Phase 4 Breeding Matrix that may illustrate potential for selecting less fertile trees.  It may worthwhile to investigate if specific crosses can produce selections with desirable traits including "effectively sterile" progeny to prevent interbreeding with native populations.  Below are MBP aspen germination observations under ideal Rakers Greenhouse conditions with various parental/species combinations:

• Three AxA families had an average germination rate of 53%.

• The male native bigtooth GG12 was also mated to an MBP 1991 selection GG102 yielding 6xGG family with 48% seed germination.  Note that bigtooth seed is very small and may not be as durable as AxA seed.

  • The female AxG hybrid, Plaza and male F1 hybrid 9AG105 had combined 8 cross progeny germination rates under 25%.

• The bisexual hybrid 9AG105 had 35% germination as a female with 5xRB and as a male it had 24%, 4% and 10% germination  with the families 15xB, 9xBR and 3xRR.  All germination rates for this hybrid for 4 crosses were below 36% but varied significantly.

  • The female AxAG hybrid, AAG2002 and its male sibling AAG2001 had 4 cross progeny germination rates under 7%.

• The 2014 11xAB family had 67% germination and the 2014 16xAB family had 35% germination.  Both families had the same CAG177 male parent but different P. alba males.  Note that the male parent 83AA305 had 35% for the 16xAB family and 33% germination for the 106xAA AxA family.

  • The male native bigtooth GG12 was mated to 2 P. albas and the hybrid AAG2002.  While the 24xR family may be an anomaly, the AxG cross 24xR had a high germination rate of 68%. 

  • The high fertility and vigor of 25xR cross may help explain the abundance of AxG wild hybrids and the invasive nature of P. alba near native populations.

  •  The vigorous female AxAG hybrid, AAG2002 was mated to the native bigtooth clone, GG12 (1xARG) with 5% germination and the open pollinated (2xARW) had 6% germination.  These similar germination rates may help validate comparisons between artificial crosses and open pollinated matings.

• Perhaps females could be tested via open pollination in areas with high aspen populations and artificially mated to several males of known high fertility.

• Perhaps males could be tested via artificial matings to several females of known high fertility.

• Research is needed to determine a "safe" germination rate for an aspen family to be considered "effectively sterile".

Research is needed to establish selection and testing protocols for these low seed germination traits.  Perhaps using  effectively sterile trees will have less impact to local native gene pools than native but fertile trees introduced from distant regions.  Perhaps hybrid aspen plantations should also include local native aspens.  They would act as a control, provide selection/breeding opportunities and serve as a native alternative if they outperform the hybrids.

Figured Wood Strategy

Aspen trees can have straight or varying degrees of wavy, undulating grain patterns known as figured wood (FW).  These characteristics may add considerable value as lumber or veneer and have been found to be heritable in aspen.  These traits may be incorporated by breeding figured materials from internal and external sources such as native and European aspens.  Wild selections of highly figured bigtooth or smalltooth aspen could be combined with internal P. alba, P. smithii, CAG, and other native aspens.  We have collected aspen figured wood samples from Michigan, Minnesota and Maine.

There is a lot to learn about figure in aspens but the amount of aspen figure appears to be fairly consistent within each tree and across ramets.  Several 22 year old hybrid aspen AG trees were cut down in March 2013.  The wood was radially split to test for figure at various places of the entire log, upper branches and one year stump shoots.  Two of these trees were siblings, one had low figure the other had none.  The third wild AG clone, AGRR1 had the most figure of the three samples.  However, this clone was tested for rotary and slice veneer but the figure was not readily apparent.  Clone AGRR1 was also grown several miles away and 2" branch samples appeared to indicate a similar amount of figure.  It was observed that this clone had very good peeling and slicing characteristics - better than native aspen.  Therefore, good peeling, slicing and bright white wood color are important veneer traits that will be considered for selection.  The white wood color is important since wood can always be stained darker but never lighter. 

On 12/31/15 we located a highly figured bigtooth aspen tree near Grand Rapids, MI (see GG12 photos).   This tree had high figure in the butt log, at 25', ~40' and in a nearby 3 year old sapling sucker stem.  We were fortunate to obtain flowering branches and mate the GG12 male clone with 4 female aspens (see GG12 Breeding photos and 2016 Crosses).  The seed was grown at Raker's Greenhouse and the seedlings will be raised to about 4' in 2016 and selected for rootability and figure.  Here is a gif image that shows the figure of clone GG12.

We speculate that it may be possible to grow a forest of highly figured aspen clone(s) with predictable figure in every butt log.  While commodity scale markets for figured aspen are not yet developed, perhaps existing markets for Tongue and Groove aspen panelling could be expanded to T&G figured aspen.  A search for figured aspens will be a long term part of this project and will primarily involve educating forest professionals on How to Find Figured Aspens.  We have observed that aspens can have varying degrees of figure and it can be detected in one year aspen shoots.  It may be helpful to grow a number of highly figured clones to develop consistent detection methods.  The current selection strategy will be to select figure in one year seedlings, grow them in the nursery as 12" "mini-stools" then make final nursery selections for figure at age 3. 

Test Strategy

The 4 year, 2 step testing strategy involves planting a set of 1-0 ortet seedlings in a family trial then separately planting 6" cuttings in replicated nursery trials that select for rooting from dormant cuttings, consistent stem and 1 year stool vigor.  The "Vigor Survival" metric is used as a scoring method to  describe this pattern.  It is derived by multiplying the survival rate by the median collar diameter (mm) in a replicated clone set.  The nursery process also involves planting short 1-0 rooted cuttings (aka mini-stools) from the above selections to monitor how they perform as stools.  The final selections will consider including diverse parentage where possible.  Below is a description of the 2 step process:

Four year testing and ramp up to clonal trial material:

• Year 1 - Seedlings: Produce seed from controlled crosses, grow in greenhouse, plant as 3-4" seedlings in nursery and grown to 4' - 5' tall.  Plant in dual rows 4" apart with rows centered 3 feet apart and seedlings 3" apart.

  • Year 1 Winter:  Conduct WASP 4" tip rooting tests on tip samples from each family to estimate family rooting potential.  Also sample some larger stems to observe family percentages for having figured wood.  Use this data to estimate rooting and FW potential at the family level and determine how many ortets to propagate for each family in year 2.  Map the WASP tips back to the 1-0 seedlings if you plan to plant a family field trial.

• Year 2 - Plant a Limited Family Field Trial

  • Include the best rooters from the above WASP test with 1-0 ortet seedlings.  Consider reserving a few WASP selections for the nursery testing.

  • A family field trial should have the highest resource priority.  It may be possible to identify promising families at age 5-6 then recreate those families to produce larger populations for more selection opportunities.

• Year 2 - Nursery First Year Ramp-up:

  • Select a small set of vigorous ortets (5-10) from each family and plant 5-6 - 6" cuttings per ortet.   The first and largest cutting should be cut 1" below the root collar to ensure better survival of that cutting and healthy bud material.  Plant standardized cutting diameters for consistency (eg. 7mm to 12mm), which may not be possible if the stock is only 3' tall.  This will test for good rooting clones via 6" cuttings.

  • Then for the other family ortets, prune the 1-0 ortet to a 2" root and 6" stem and plant it 4" apart along with the next 8" stem.  This could yield 10' of cuttings (6' + 4').  Don't label as ramets as selection will likely occur later.  Plant just the lower 8" stem cutting if space/time resources are limited.

  • In the Fall: Use vigor/survival metrics to make performance selections.

  • In the following Spring: When making cuttings, test the bottom cutting for for signs of figured wood by inspecting for banded patterns in the split sections.

• Year 3 - Second Ramp-up & Selection:

  • Create "1-0 mini-stools" from the above rooted 1-0 trees.  Mini-stools are made by cutting the lateral roots of a 1-0 stock to 1" and the stem to 6".  Plant the 1-0 mini-stools at 9" to 12" apart.  The larger distance should produce a thicker stem.  Each mini-stool should yield about 6 10" field grade cuttings (> 3/8" dia).

  • Create 8" cuttings in 7mm to 12mm diameters from the remaining 1-0 stock and plant them in the same spacing as seedlings in replicated sets.  Potential yield from 2 good rooting 10 cutting replications could produce about 10 plantable 1-0 trees 4-5' tall.  Wider spacings will produce thicker trees.

  • Optionally, use the smaller stem material to make WASP plantlets.  Start them in early spring for nursery planting when the cuttings have just started to leaf out.

  • Use vigor/survival metrics to make selections in the Fall.

• Year 4 - Third Ramp-up and re-test:

  • There should be adequate material per clone to distribute to cooperators, but testing one more year in the nursery would be helpful to verify rooting performance over 3 years.

  • Plant 4 replications of 8" cuttings with standardized cutting diameters (eg. 7mm to 12mm).

  • Measure field trial DBH and heights and compare family rankings to nursery selections at family level.

• Year 5 - Release materials for clonal trials:

  • Release to cooperators

The resulting selections will be made available to cooperators that may plant them into stool beds for observation and as source stock for field clonal trials (see: Open4st Aspen Research Field Trials).  The current strategy is to carry forward about 50 selections and archive about 130 clones representing the best P. alba, CAG and other hybrids (eg, CAG x G or CAG x T) clones.  Over time the older, less favorable clones could be replaced with new selections to maintain diversity and promote improved materials.   If you are interested in testing these materials, please see the document:  Hybrid Aspen Research and Deployment Strategies.

Proposed Activities

Below lists proposed (annual) activities to implement parts of the above breeding strategy.  Residual tasks should carry forward to the next year (click on year tabs).  The 2013 and 2014 tasks reflect high priority "Proof of Concept" crosses (POC) to investigate the potential of these crosses.  The "Nice to Have" crosses and testing ideas could be implemented as resources permit.