Fast-growing hybrid aspen (
Testing of promising candidate clones within the expected deployment area is an essential phase in practical breeding, prior selection of clones for commercial use. Thus, several field tests including the same clones are normally established to get information about the climatic adaptation at different sites. Such a design makes it possible to test genotype by environment (G × E) interactions.
According to a review published recently by Li et al. (2017), a high level of G × E interaction is usually found for growth traits of different tree species. For hybrid aspen, significant G × E interaction for growth has been observed for 25 clones on four sites in Finland (Yu and Pulkkinen 2003). Similarly, a strong G × E interaction for height growth was found in a study of two tests with poplar and aspen hybrids in Lithuania (Pliūra et al. 2014). On the other hand, only a weak G × E interaction was derived from a series of clonal field trials with hybrid aspen and poplars in Sweden (Stener and Karlsson 2004; Stener and Westin 2017). In addition, no significant interaction was found by Nielsen et al. (2014) in two poplar clonal tests in Denmark.
A G × E interaction means that genotypes are ranked differently in different environments, that is, clones may perform well at one site but poorly at another site. Moreover, different traits may exhibit different patterns of G × E interaction (Li et al. 2017). This complicates the selection of well-performing clones for commercial use. High-yielding genotypes that perform well in a wide range of environments, that is, with high level of stability and plasticity, are to be preferred (Gullberg and Vegerfors 1987; Karlsson et al. 2001).
The climate, especially spring or autumn frost weather condition could be a main driver for G × E interaction as, for instance, shown for height growth of Norway spruce in southern and central Sweden (Chen et al. 2017). For Scots pine in northern Sweden, strong pattern of G × E for both tree vitality and height was detected due to differences in temperature sums among trials (Calleja-Rodrigues et al. 2019). The climate change will create new growing conditions, where higher temperatures are supposed to increase growth (Jylhä et al. 2009). However, there may be a risk that growth will start too early or end too late, increasing the risk of frost damage and thus, making clones with high plasticity and/or high stability more favourable.
The objectives of this work were to 1) study genotype × environment interactions for hybrid aspen clones and 2) estimate stability and plasticity parameters for height growth and survival.
The study is based on data from two series of clonal field trials with hybrid aspen. Series 1 consists of 10 trials planted in 2010–2011 from latitude 56° to 64° in Sweden (Swe 1–Swe 10; Stener and Westin 2017) and one trial at latitude 61° in north-west Russia (Ru 1). Series 2 consists of three trials planted in 2002 in southern Finland (Fi 1–Fi 3; Tab. 1).
Description of the hybrid aspen field trials over Sweden (Swe), Finland (Fi) and north-west Russia (Ru) included in the study
Trial | Name | Type of land | Planted year | Lat [Nº] | Long [Eº] | Alt [m] | Temp. [sum, d.d] |
---|---|---|---|---|---|---|---|
Swe 1 | Sturup | agricultural | 2010 | 56º34’ | 13º23’ | 120 | 1527 |
Swe 2 | Malteröd | forest | 2010 | 56º07’ | 13º37’ | 114 | 1501 |
Swe 3 | Toftaholm | forest | 2010 | 56º59’ | 14º03’ | 162 | 1405 |
Swe 4 | Remningstorp | agricultural | 2010 | 58º16’ | 13º22’ | 133 | 1359 |
Swe 5 | Harg | agricultural | 2010 | 60º05’ | 18º15’ | 29 | 1356 |
Swe 6 | Karön | forest | 2010 | 60º10’ | 18º13’ | 25 | 1347 |
Swe 7 | Pommac | forest | 2011 | 63º07’ | 17º27’ | 265 | 961 |
Swe 8 | Ed | agricultural | 2011 | 63º15’ | 17º14’ | 75 | 1124 |
Swe 9 | Degerbyn | agricultural | 2011 | 64º47’ | 20º49’ | 20 | 1086 |
Swe 10 | Degerbyn | forest | 2011 | 64º47’ | 20º49’ | 20 | 1086 |
Ru 1 | Syktyvkar | agricultural | 2010 | 61º39’ | 50º44’ | 157 | 1075 |
Fi 1 | Sääksjärvi | agricultural | 2002 | 60°36’ | 25°22’ | 70 | 1274 |
Fi 2 | Sävträsk | agricultural | 2002 | 60°32’ | 26°05’ | 35 | 1322 |
Fi 3 | Laukonsaari | agricultural | 2002 | 61°49’ | 29°20’ | 85 | 1235 |
The Swedish trials included in total 109 clones and consisted among others of 29 genotypically selected clones for commercial use in southern Sweden and 23 clones originating from the Finnish breeding programme. In the Russian trial, totally 42 hybrid aspen clones and 8 common aspen clones were tested, and 17 of these were in common with those in the Swedish trials. The Finnish trials included in total 45 hybrid aspen clones out of which 12 were in common with those in trial series 1. All the clones resulting from crosses between Finnish origin
The Swedish trials were planted in a randomized block and single tree plot design with 6–8 blocks (mainly 1–2 ramets per clone and block) with a spacing of 3×3 m. To avoid problems from vegetation, the agricultural sites in Sweden were treated with glyphosate herbicide (Roundup) before planting and the forest land sites were prepared by inverted soil scarification. Damage by deer and moose was prevented by fencing. The Russian trial was established in the same manner but without herbicide treatment and fencing. The Finnish trials (series 2) consisted of a randomized complete block design with 5 × 5 tree plots for each clone in four replicates, where a spacing of 3×3 m was used. Herbicides were not used in the Finnish trials but soil scarification was done in patches using an excavator and they were all fenced. All trials were established in spring using dormant, 1-year old container plants that had been produced by the research organizations in each country respectively.
The total height of each individual tree alive was measured in all the trials. This was performed after four growing seasons in series 1, that is, year 2013 in Swe 1–Swe 6 and Ru 1, and 2014 in Swe 7–Swe 10. The trials in series 2 were measured after 12 growing seasons (year 2013). Dead trees were registered as well at all the sites, in order to get a measure of survival.
The statistical analysis was based on clone means of tree height and survival for each trial. Since height and survival deviated from normal distributions according to Shapiro and Wilk-tests (Sabin and Stafford 1990), both traits were analysed by nonparametric tests. The clones were ranked based on mean heights and survival within each trial. To study the stability of individual clones across trials of series 1, variance of ranks (
where:
The plasticity index (
The relationship between clonal ranks for height and survival at the age 4 (series 1) and 12 (series 2) years was tested with the Spearman correlation analysis, as an additional way to describe the G × E interactions. The statistical package Statistica 6.0 was used for all the statistical analyses (SAS/STAT User’s Guide 1999).
Tree height at age four averaged 25 dm over the 11 trials of series 1 and varied from 16 dm (Ru 1) to 47 dm (Swe 3). Average survival after four years in field was 80% and varied from 52% (Swe 8) to 96% (Swe 3). In series 2, tree height after twelve years of growth averaged 96 dm, ranging from 90 to 119 dm and average survival was 88% and varied from 80 to 94% (Fig. 1).
The stability parameter (
For the 12 clones included in both series, there was an intermediate, positive and significant correlation (r = 0.65;
In most of the 14 test sites, the hybrid aspen clones survived well, reflecting a good initial adaptation to the environmental conditions (Fig. 1). Three of the four northern Swedish trials (Swe 7–Swe 9) diverged from this pattern and showed lower survival. This can partly be attributed to vole and hare damages, which were quite frequent in these trials especially at Swe 8 (Ed) (Stener and Westin 2017) but it is also likely due to a transfer effect.
There is no contradiction in the stability (
High values of stability combined with high growth performance of hybrid aspen clones were reported by Pliūra et al. (2014) in two field tests in Lithuania. Significant clonal variation in height as, observed in this study, is in accordance with the results in the previous studies (Stener and Karlsson 2004, Rytter and Stener 2005), and suggest that there is still a great potential to increase yield further by selecting the most productive clones. According to Yu and Pulkkinen (2003) selection on the overall mean is all that is necessary to assure the largest overall gains.
In general, variances of ranks were higher for survival than for height (Figure 2), indicating that survival is a less stable parameter than height. In addition, less
Taking into account that height growth and survival are usually the primary selection traits in tree breeding in the boreal zone, we conclude that tree height is a more stable and more plastic parameter in comparison to tree survival. In this respect, our results seem to be consistent with data obtained for hybrid aspen by Stener and Karlsson (2004) who found higher broadsense heritabilities for height than survival. The study was based on many sites but with few clones measured at an early age. Still it was considered to get rough but relevant estimations of stability and plasticity of hybrid aspen clones.