Testing climate-informed seed sourcing strategies in black spruce: Comparing local, climate-forward, and mixed provenances

Climate change is increasing uncertainty about which seed sources will maintain productivity in boreal reforestation. Mixed-source deployment may reduce reliance on any single future-adapted provenance, but empirical field evaluations remain limited. We compared local sources, individual climate-forward provenances, and site-specific mixtures of local and climate-forward provenances in 12-year-old black spruce ( Picea mariana ) assisted-migration trials at Dryden, Kakabeka, and Sault Ste. Marie, Ontario. Plot-level height, survival, and productivity index (PI = mean height × survival) were analyzed using mixed-effects models with block as a random effect. Provenance significantly affected height at all sites and survival and PI at two of three sites, explaining 10.3–22.5% of height variation, 8.6–49.9% of survival variation, and 7.3–14.5% of PI variation. Productivity varied widely among individual climate-forward provenances, with model-adjusted PI ranging from 140.9 to 186.8 at Dryden, 156.6–204.3 at Kakabeka, and 155.7–252.8 at Sault Ste. Marie. Mixed-source PI values were 156.6, 211.9, and 191.1 across the three sites, compared with 147.6, 171.5, and 192.7 for local sources. Thus, mixtures fell within or above the climate-forward single-source range, whereas local sources were near the lower end at Dryden and Kakabeka and intermediate at Sault Ste. Marie. Pairwise contrasts detected few significant differences between individual provenances and mixtures, but equivalence tests did not support broad equivalence in growth and survival. Overall, individual climate-forward provenances included both high- and low-performing sources, while mixtures maintained moderate to high productivity across sites. These results suggest that portfolio-style seed sourcing may help balance early productivity and risk in climate-adapted reforestation, although longer-term monitoring is needed to determine whether these patterns persist as stands mature and climates continue to warm.