Growth decline and wood anatomical traits in Nothofagus dombeyi populations along a latitudinal gradient in the Andes, Chile

There is evidence of recent declines in tree growth in the temperate forests of South America, due to the ongoing climate change. This study assessed growth-climate relationships and the xylem hydraulic architecture of coihue (Nothofagus dombeyi (Mirb.) Oerst) trees exposed to the warmer and drier conditions of recent decades. We selected four coihue populations along a latitudinal gradient in the Andes, Chile, corresponding to a wide range of variation in growing season precipitation (northern dry to southern wet sites). Tree-ring width was measured in 24–32 adult trees per site during the last 60 years. We measured wood anatomical traits in a subsample of four trees per site during the last 25 years. All data were correlated with climatic variables. During the studied period, SPEI-6 passed from positive to negative in all sites. Basal area increment decreased by 1.7 mm² year⁻¹ over the period 1960–2020. Tree-ring width had a positive correlation with precipitation at the drier sites and a negative correlation with maximum temperature at the wetter populations. We estimated a density of 1.78 × 10^–4 and 1.2 × 10^–4 vessels µm⁻² in the xylem of dry and wetter sites, respectively. Vessel density had a negative correlation with precipitation at the driest site and a positive correlation with maximum temperature at wetter sites. The hydraulic diameter was smaller under drier conditions, reaching 68–75 µm in the driest and wettest sites, respectively. Among the traits measured, vessel density was the most sensitive to climate. Drier and warmer conditions were associated with an increased number of smaller sized vessels, especially at the northern populations. Compared with the southern populations of our gradient, the northern populations growing at the drier sites are more sensitive to the ongoing changes in climate, and potentially more vulnerable to the even drier conditions projected for the future.