Research paperGrowth performance and stability of hybrid poplar clones in simultaneous tests on six sites
Introduction
The hybrid poplars (Populus spp) represent a promising long-term biomass feedstock for biofuels and the emerging bio-products industry, as well as for traditional forest products, with significant potential for improving rural economies of the Midwestern United States and other regions [1]. Commercial adoption of poplar plantations requires genetic improvements for increased and consistent yield, disease resistance, and broadened adaptability across a range of climates and soil types. Genotype x environment (G x E) interaction is a limitation that relates directly to adaptability, complicating growth performance testing and reducing overall genetic gains [2].
There are several studies on G x E and clone stability in Populus [[3], [4], [5], [6], [7], [8], [9], [10]]. G x E explained from 8% to 25% of the variation in diameter, basal area (BA), or derived tree volume in these papers. The ratio of clone to G x E variance components varied widely, from 2.70 to 0.40. Stable clones (lesser growth response to change in environment) varied from 44% to 67% of the clones tested in these studies, depending partly on the stability measure chosen. It is likely that the wide variability in clone × environment interaction and stability of Populus in the literature is affected by the variation of the growing sites and the composition and size of the clonal populations. Our hybrid poplar breeding and genetic tree improvement program at the Natural Resources Research Institute (NRRI) utilizes replicated clone trials of clones from tested superior full-sib families to select the best clones for testing in yield blocks representing near-commercial conditions. This approach captures both additive and non-additive genetic variation [11]. A clone trial established with the same 69 clones on six sites in central and northern Minnesota in 2008 provides a robust opportunity to identify clones putatively superior for growth rate, stability, and disease resistance. It also can supply information on the adaptability (G x E) of the clones, as well as derive some general principles underlying phenotypic plasticity and G × E interaction for hybrid poplars. Clones present a more powerful means than families or seed sources for detecting G × E interactions and analyzing genotypic stability [5,12]. The trial provides a rich source of data on clonal genetic variation, stability, G × E interactions, and inter-site correlations, essential information for selecting clones to move forward in the testing process, calculating expected genetic gains, and designing an optimum breeding strategy.
The objectives of this study were to: (1) identify hybrid poplar clones that are superior in growth rate, disease resistance, and have acceptable genetic stability and G × E interaction across different environments as candidates for moving into field tests approximating commercial conditions (referred to in this paper as Yield Block tests); (2) investigate the relationship between clonal stability and G × E interaction; and (3) discover general principles for understanding, controlling, and using clonal stability and G × E interactions in hybrid poplar genetic improvement programs.
Section snippets
Study design and breeding system
The experiment was a randomized complete block design with six blocks within each of six sites and a single-tree plot of each clone within each block. The study was a clone trial that is part of our sequential breeding and testing system, as depicted in Fig. 1.
Clone selection for tests
The 69 clones included in the tests reported here (Table 1) were initially selected from our family genetic field trials (Populus FFTs) in Minnesota and from external sources within the region. In addition, non-native parent materials
Means and analysis of variance
Diameter breast height (DBH) and BA after the fifth year averaged 93.5 mm and 72.11 cm2, respectively, over the six trials. Frequency plots of both variables fit a normal distribution. Diameter and height are significantly correlated (R2 = 0.78) in five-year-old poplar plantations. Based on our previous work, a DBH of 93.5 mm equates to an approximate total height of 9.7 m at five years at 3.05 m × 3.05 m spacing [Ht. (m) = 2.65944 + 0.07536 DBH (mm)]. Site means for DBH and BA varied from
Discussion
The linear regressions of clone mean versus site mean will be strongly affected by the leverage effects of the Riewer (slowest growing) and Wheeler (fastest growing) sites. If one of these two sites has missing data, the regression model has higher probability of being insignificant. Twelve clones in the Wheeler site have two observations or less. All 12 of these clones had an insignificant linear fit between clone mean and site mean. This is part of the reason why only 35 of the 69 regression
Acknowledgements
This work was funded by the State of Minnesota appropriations to the Minnesota Hybrid Poplar Research Cooperative (MHPRC), State Special appropriations to the University of Minnesota Duluth Natural Resources Research Institute, Minnesota Agricultural Utilization Research Institute, US DOE BETO Sun Grant Initiative Poplar Woody Crops Program contract # DEFC36-05GO85041, US DOE Idaho National Laboratory contract #184125, USDA NIFA Agriculture and Food Research Initiative Competitive Grants
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