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

Radial growth, wood density and climate-growth relationship of four Douglas fir provenances were analysed separately for the juvenile and the adult phase. Two pairs of 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 day-wise aggregated Standardised Precipitation-Evapotranspiration Index (SPEI). We calculated the accumulated drought effects by aggregating climatic water deficits into a log-logistic probability distribution to obtain the SPEI index series of different seasons, starting from three weeks to nine months, including the effect of previous growing season. In all provenances, RW, and consequently EWW and LWW, were wider in juvenile period than in adult period. Share of latewood was in all cases higher in juvenile wood then in mature wood. All four provenances have similar wood density in both analysed growth phases. The general effect of wet conditions in current growing season was positive, indicating that Douglass fir’s radial growth was favoured in moist years, and reduced in dry years. The significant positive effect of SPEI on LW was observed also at the beginning of previous growing season. Our analysis showed that when selecting the most promising provenance for planting, it needs to be considered that growth rate may change from juvenile to adult period. Only by combining climate-growth analysis with measurements of external tree features we can compare and assess the suitability of certain provenances for planting in current and future climate.