Tree rings, wood density and climate-growth relationships of four Douglas-fir provenances in sub-Mediterranean Slovenia

Key message The relationships between growth rates of examined provenances in the sub-Mediterranean change between juvenile and adult growth phase, while wood density is approximately similar in all four examined provenances. Tree rings, wood density and the climate-growth relationship of four Douglas-fir provenances were analysed separately for the juvenile and adult phases. Four provenances were selected from an existing IUFRO provenance trial planted in 1971 based on their diameter at breast height and vitality. Increment cores were extracted from individual trees, on which we measured tree-ring widths (RW), earlywood widths (EWW) and latewood widths (LWW). Wood density was assessed in standing trees using resistance drilling. The climate-growth correlations were calculated between provenance chronologies of RW, EWW, LWW and latewood share, and the day-wise aggregated Standardised Precipitation-Evapotranspiration Index (SPEI). The analysis was done separately for the juvenile and mature phases of growth. Provenances 1064 (Jefferson) and 1080 (Yelm) exhibited larger annual radial increments than provenances 1028 (Merrit) and 1089 (Cathlamet). The two provenances with the highest annual radial increment in the juvenile phase did not exhibit the same trend in the adult phase. In all provenances, RW, and consequently EWW and LWW, were wider in the juvenile than in adult phase. The share of latewood was in all cases higher in juvenile wood than in mature wood. All four provenances had similar wood densities in both analyzed growth phases. Our analysis showed that when selecting the most promising provenance for planting, possible changes in relative growth rate from the juvenile to adult phase need to be considered.