High responsiveness of wood anatomy to water availability and drought near the equatorial rear edge of Douglas-fir

In trees, wood anatomy is a more precise and informative measure of hydraulic responses to drought than radial growth. Tree populations located near the equatorial-range edge of their distribution (rear edge) are used to monitor responses to changes in hydroclimate; however, few studies have assessed the wood anatomy of these populations. We measured tracheid lumen diameter (LD) and cell wall thickness (CWT) in a rear-edge Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) population. We also evaluated the formation of intra-annual density fluctuations (IADFs), characterized by earlywood (EW) tracheids with narrow lumens. We expected that EW LD would be particularly sensitive to hydroclimate variability. We found that EW LD was directly related to precipitation during the previous year’s late winter and the current year’s spring. Wet conditions were associated with wider EW tracheids and improved growth. These correlations peaked when considering cumulative precipitation from January to May. EW IADFs were formed in response to dry conditions during March and April, suggesting that dry spring conditions limit tracheid expansion. The described associations in a rear-edge Douglas-fir population confirm that wood anatomy is highly responsive to drought. We conclude that EW LD is a valuable proxy for hydroclimate reconstruction in Douglas-fir.