Douglas Fir Performance in Romania: Growth Insights from Common Garden Experiments

Abstract

Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) is one of the most important non-native conifers introduced to Europe, valued for its rapid growth, versatile timber, and resilience under climate change. In Romania, the Douglas fir was introduced at the end of the 19th century and later tested in international provenance trials. A total of 61 Douglas fir provenances were evaluated across the three Douglas fir trials established in 1977, representing the complete set of genetic material introduced to Romania. Of these, 38 provenances were common to all three trial sites (Aleșd, Făget, and Padeș). We assessed growth performance using tree volume, slenderness index, and basal area as indicators of productivity and stability. Linear mixed models showed that provenance significantly influenced all traits at the Aleșd and Făget trials, whereas at the Padeș trial, provenance effects were significant only for tree volume. The provenance x location interaction was significant, indicating that provenance varied among sites. Strong provenance × location interactions indicated an instability in growth performance across sites. Among North American sources, interior provenances such as Idaho performed consistently well across sites, while coastal provenances from Washington and British Columbia achieved superior productivity at specific locations. Romanian provenances, including Toplița, Piatra Albă, and Anina Buhui, also ranked among the best performers, underlining their adaptive potential. Correlation analyses showed positive relationships between tree volume and basal area, while both growth traits were negatively associated with the slenderness index. Geographic origin variables explained part of the variation: longitude correlated positively with productivity, whereas higher elevation of origin was linked to slower growth and greater slenderness. Our results emphasise the importance of matching seed sources to planting conditions and highlight the potential of Douglas fir for Romanian forestry.