Abstract

Swiss stone pine (Pinus cembra L.) may represent a viable option for afforestation at the upper altitudinal limit of forest ecosystems, due to the species’ adaptability to limiting climatic conditions. The goal is to replace vulnerable spruce monocultures (which are highly affected by windthrows) with mixed stands of spruce and Swiss stone pine.

This study aims to assess phenotypic variability, heritability, and juvenile–adult correlations in two common garden (comparative provenance trial) experiments established in distinct environmental conditions and two separate branches of the Carpathians: the Cârlibaba trial (Eastern Carpathians) and the Cugir trial (Southern Carpathians). The objective is to inform forward selection strategies and determine the optimal age for selection. Twelve provenances were tested, seven from the Romanian Carpathians and five from the Alps (including one from France, one from Switzerland, and three from Austria). The trials were established in the autumn of 1997, using a 2.5 × 2.5 m planting design.

The strong influence of the testing site and the differing responses of the provenances highlight the need for caution when transferring forest reproductive material across regions. The low to moderate quantitative genetic differentiation (Q_ST) among provenances suggests limited genetic diversity, supporting the recommendation to combine high-performing provenances identified in multiple studies for future afforestation programs. The average Q_ST values for phenotypic traits (tree height and DBH) were approximately 50% lower than those for wood traits (ring width, earlywood, and latewood), indicating stronger genetic control over wood properties.

Among the tested provenances, Bluhnbach and Călimani (for the Southern Carpathians), Grächen–Wallis and Gemenele (for the Eastern Carpathians), along with Pietrele (highlighted in both trials and originating from the Southern Carpathians), emerged as promising candidates for forward selection.

In both trials, significant juvenile–adult correlations were observed for growth traits, suggesting that early selection could be effective, with age 17 identified as optimal for selection. Tree height was identified as the most critical trait for improvement, to enhance the competitiveness of this slow-growing species, particularly during the juvenile stage when it faces competition from Norway spruce.

Notably, the Cârlibaba trial demonstrated superior resilience and resistance, as well as a strong recovery capacity across all provenances in both trials. However, the overall low resistance observed—especially in the Cugir trial—is concerning in the context of climate change (Budeanu et al. 2025a, b).

Funding: This research was supported by the National Institute for Research and Development in Forestry “Marin Drăcea” under grant number 71PHE/2024 (UEFISCDI, Carmine project).