Interannual variations in needle and sapwood traits of Pinus edulis branches under an experimental drought

1) In the Southwest United States, recent large-scale die-offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural response to droughts. 2) We hypothesized that piñon pines (Pinus edulis) respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a seven-year experiment in central New Mexico with three watering treatments (irrigated, normal and rain exclusion). 3) We analyzed how variation in ‘evaporative structure’ (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (d), defining the yearly ratios SA:LA and SA:LA/d. 4) Needle length (l) increased...