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Clone-Level Growth Performance
Top 5, 10, and 15 clones were compared to the baseline mean height of 16.2 m in measurement year 22 (Table 2). The top 5 clones achieved the greatest gain, with a mean height of 18.2 m (+2 m above baseline), showing a highly significant difference (p < 2.2 × 10⁻¹⁶, t = 12.109, df = 61) but limited provenance diversity (2 provenances). As more clones were included in the selection, the mean height gains diminished slightly with 17.7 m (+1.5 m) for the top 10 and 17.4 m (+1.2 m) for the top 15, while provenance diversity increased to 5 and 6, respectively. These results highlight a trade-off between maximizing height gains and maintaining genetic diversity in clone selection. Height development of all clones over time is visualized in Figure 15.
Table 2: Comparison of the top 5, 10, and 15 clones of height in measurement year 22 to the baseline mean height. Top clones show substantial gains (+2 m for the top 5) with highly significant differences (p < 2.2 × 10⁻¹⁶) to baseline height. Greater selection diversity reduces height gain slightly while increasing provenance representation.
Figure 15: Height development of Aspen clones over 22 years. Clones are sorted from best height growth (top left) to worst height growth (bottom right). Colors account for the provenances of each clone. C1 = DMI, C3 = WEY, C4 = MWI, C5 = SLP, C6 = ALPAC, C7 = SOLFSJ, C8 = AIN.
Table 3: Top 5 clones under Cold & Dry conditions (2003–2005) based on year 3 mean heights compared to the baseline height of all clones (1.24 m) in year 3. Clone C8012 leads with a height of 1.56 m (+0.33 m), followed by C3014 (1.56 m, +0.32 m). Clones C3043, C6024, and C4029 show gains of 0.26–0.18 m, all statistically significant. These results identify C8012 as the best-performing clone in this climate.
Cold & Dry Climate Conditions
Under Cold & Dry conditions (2003–2005), the top 5 clones showed significantly greater heights at the end of the cold & dry period (measurement year 3) when compared to the baseline mean height of 1.24 m (Table 3). Clone C8012 performed best with a mean height of 1.56 m (+0.33 m), followed closely by C3014 at 1.56 m (rounded, +0.32 m). Clones C3043, C6024, and C4029 achieved gains of 0.26–0.18 m, all with significant differences from the baseline (p < 0.05). These findings highlight the superior adaptation of C8012 and C3014 to Cold & Dry conditions.
Warm & Dry Climate Conditions
Under Warm & Dry conditions (2005–2007), the top 5 clones demonstrated significantly greater heights in measurement at the end of that period (year 5) when compared to the baseline mean height of 2.08 m (Table 4). Clone C3014 stood out as the top performer with a mean height of 3.25 m (+1.16 m). Clone C4029 followed with a height of 2.85 m (+0.77 m), while clones C4028, C6024, and C8012 achieved gains of 0.5–0.53 m, all statistically significant (p < 0.001). These findings underscore C3014's superior growth under Warm & Dry conditions.
Table 4: Top 5 clones under Warm & Dry conditions (2005–2007) based on year 5 mean heights compared to the baseline height of all clones (2.08 m) in year 5. Clone C3014 led with a height of 3.25 m (+1.16 m), followed by C4029 (2.85 m, +0.77 m). Clones C4028, C6024, and C8012 showed gains of 0.5–0.53 m, all highly significant. C3014 emerged as the top performer.
Table 5: Top 5 clones under Cold & Wet conditions (2010–2014) based on year 12 mean heights compared to the baseline height of all clones (7.4 m) in year 12. Clone C3014 led with 9.9 m (+2.46 m), followed by C4029 (9.4 m, +1.95 m). Clones C6028, C6024, and C8013 gained 1.18–1.66 m, all significant. C3014 emerged as the top performer.
Cold & Wet Climate Conditions
Under Cold & Wet conditions (2010–2014), the top 5 clones showed significantly greater heights in measurements at the end of that period (year 12) when compared to the baseline of 7.4 m (Table 5). Clone C3014 performed best with a mean height of 9.9 m (+2.46 m), followed by C4029 (9.4 m, +1.95 m). Clones C6028, C6024, and C8013 demonstrated gains ranging from 1.18 m to 1.66 m, all with significant p-values. These results emphasize C3014’s superior growth under Cold & Wet conditions.
Sex-Based Differences in Height Growth
Sex-based analysis indicated some notable differences in height among Aspen trees within specific provenances (Figure 16). In the AIN provenance, male trees were approximately 1.27 meters shorter than female trees, with confidence intervals excluding zero, suggesting a significant difference (Table 6). Conversely, DMI provenance showed a striking reduction in height for non-blooming trees compared to females, estimated at around 6.05 meters shorter, although this finding may be influenced by a limited sample size for non-blooming individuals (n = 4). Other provenances exhibited weaker trends in sex-based height differences, with most confidence intervals including zero. These results highlight potential sex-based growth variations, which may be relevant in selecting specific clones or sexes for reforestation efforts aimed at maximizing growth.
Figure 16: Differences in height of Aspen provenances 22 years after planting, depending on sex.
Table 6: Confidence intervals for the effect of sex on height growth between provenances 22 years after planting. The female clones were used as reference.
Discussion and Conclusion
This study highlights clear trade-offs in clone selection for maximizing height growth, adapting to climate conditions, and maintaining genetic diversity. Selecting the top 5 clones provides the greatest height gains (+2 m above baseline), but at the cost of provenance diversity. Expanding to the top 10 or 15 clones increases diversity but reduces gains slightly. Companies focused on yield should prioritize top performers like C3014, while those aiming for long-term resilience should diversify their selections. The best trade-off between height growth maximization and genetic diversity can be achieved when choosing the top 10 clones (C3014, C3055, C3043, C3009, C4029, C8013, C1010, C8006, C4028, C6028).
Climate-specific strategies are essential. C3014 excelled across all climates, but clones like C8012 (Cold & Dry), C4029 (Warm & Dry), and C6028 (Cold & Wet) also performed well. Planting diverse clones tailored to local climates can mitigate risks and ensure productivity. Sex-based growth differences, especially in provenances like AIN and DMI, suggest that selecting female trees may further enhance yields.
In conclusion, companies must balance short-term gains with genetic and climatic adaptability. Diversifying clone and sex selections will create more resilient and productive forests for the future.
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