Improved Finnish birch material delivers 40% higher production in central and northern Sweden

This article is a summary of key findings and results from Skogforsk report 1259-2026 "Finnish bred birch seeds outperform local seeds in northern Sweden"

Key findings

A comprehensive 20-year evaluation of birch planting material across central and northern Sweden (60-65°N) reveals significant production differences between Finnish and Swedish improved material, with important geographic limitations.

Finnish improved birch material achieved 162 m³/ha at age 20, substantially outperforming Swedish material (117 m³/ha) and Finnish stand material (138 m³/ha). Critically, stem and branch quality remained similar across all material types, and survival rates showed no significant differences.

However, the Finnish material's production advantage diminished sharply at higher latitudes. At the northernmost test site (65.4°N), Finnish and Swedish materials performed equally, indicating transfer distance limits for tested Finnish seed orchards.

Production results

Volume production at 20 years by material type

• Finnish improved: 162 m³/ha
• Finnish stand material: 138 m³/ha (18% lower than Finnish improved material)
• Swedish material: 117 m³/ha (39% lower than Finnish)

These production differences emerged early. At age 10, Finnish material had already established a 29% advantage over Swedish material. Importantly, no height differences were detected at age 3, confirming that production gains reflect genetic improvements rather than nursery effects.

The production advantage stemmed from consistent improvements in both diameter and height. At 20 years, Finnish material averaged 11.5 cm diameter compared to 10.3 cm for Swedish material and reached 14.7 m height versus 12.7 m for Swedish material.

Geographic performance patterns

Material performance varied significantly across the latitudinal gradient, revealing critical genotype-environment interactions:

Southern sites (60.3-62.3°N):

• Finnish material: 170-220 m³/ha
• Swedish material: 113-151 m³/ha
• Finnish advantage: 46-50%

Northern site (65.4°N):

• Finnish material: 108 m³/ha
• Swedish material: 109 m³/ha
• Finnish advantage: 0%

At the best-performing site (62.3°N), Finnish material reached 220 m³/ha – the highest production recorded. At the northernmost site, this advantage disappeared completely. Swedish material showed more stable performance across the entire gradient, though at lower absolute production levels.

Quality assessment

No significant differences in wood quality were detected between material types. Statistical analysis showed:

• Stem straightness: p = 0.45 (not significant)
• Fork frequency: p = 0.62 (not significant)
• Branch quality: p = 0.38 (not significant)
• Tree vitality: p = 0.51 (not significant)

Approximately 70-80% of trees across all materials achieved veneer quality with no major defects. Quality variation was driven by geographic location rather than genetic origin, with northern sites showing increased fork frequency due to shorter growing seasons affecting apical dominance.

Survival and establishment

Survival rates at age 20 showed no material type differences (p = 0.85):

• Finnish improved: 75%
• Swedish improved: 77%
• Unimproved stand: 77%

Strong site effects were observed (p < 0.001), with survival ranging from 87% at the southern site to 54% at one northern site, reflecting local environmental conditions rather than genetic adaptation.

Seed orchard variation

Significant variation was observed among individual Finnish seed orchards. These differences were not apparent at age 10 but became pronounced at age 20, indicating that assessment periods exceeding 10 years are necessary for reliable evaluation of seed sources under Nordic conditions.

This finding aligns with recent Finnish research showing volume improvements of 1-31% among individual seed orchards compared to local stand material. Forest owners should specify seed orchard origin when procuring planting stock rather than ordering generic "improved material".

Practical implications

For planting sites south of 63-64°N: Finnish improved material from high-performing seed orchards is recommended. Expected production gains of 40-50% over stand material can be achieved with no quality penalties and equivalent survival rates.

For planting sites above 64°N: Swedish or locally adapted material should be considered. Finnish material loses its production advantage at these latitudes, and Swedish sources demonstrate more stable performance across the gradient.

For all material types: Early thinning, targeting trees with quality defects (excessive forks, poor straightness), will produce high-value sawtimber stands. Improved materials provide the additional benefit of 20-40% greater volume production.

Study background

This study represents the first formal evaluation of Finnish birch material performance under northern Swedish conditions. Four field trials were established in 1997 across central and northern Sweden (60-65°N latitude), testing 15 seed lots comprising Finnish improved, Swedish improved, and unimproved material. Measurements were conducted at ages 3, 10, and 20 years both for production traits (stem height, stem diameter at breast height, stem volume), survival, and quality traits (branching quality, stem straightness and forking tendency). These results provide evidence-based guidance for material selection and support the development of Swedish seed orchards particularly adapted to latitudes above 63°N.

Conclusions

1. Finnish improved birch material delivers significant production advantages (39-50%) over Swedish material in central Sweden and southern Norrland (south of 64°N), with no trade-offs in wood quality or survival.
2. Finnish material has geographic adaptation limits. At latitudes above 64°N, production advantages disappear as transfer distances from seed orchard origins (61-64.5°N) exceed optimal limits.
3. Swedish improved material shows more stable but lower production across the latitudinal gradient, making it suitable for northern sites where Finnish material underperforms.
4. Individual seed orchard performance varies substantially. Forest owners should specify seed orchard origin rather than ordering generic "improved material" to maximize production gains.
5. Quality traits show no material type effects but respond to environmental conditions. Early thinning of defective trees is effective for all materials and maintains stand value.

The results confirm that material selection significantly impacts long-term productivity in birch forestry and that transfer distance limitations must be carefully considered when deploying improved material across Sweden's latitudinal gradient.

Study details

Four field trials (60-65°N), 15 seed lots, measurements at ages 3, 10, and 20 years. Funded by Svea Jansson's Forest Foundation and conducted within Skogforsk's breeding programme.