Data from: 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 with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness while stomatal diameter was reduced. As a result anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/d showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/d became highly non-linear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. 5) In new branch tissues, the responses of annual functional ratios to water stress were immediate (same year) and do not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.

1）美国西南部近期发生的大规模针叶树死亡事件，引发了人们对干旱条件下针叶树恢复力与死亡率的探讨。迄今为止，学界对干旱引发的年际结构响应仍知之甚少。
2）本研究提出假说：矮松（Pinus edulis）可通过针叶形态调控，逐年降低枝条叶水势的下降幅度，以此响应干旱胁迫。我们在新墨西哥州中部开展了为期7年的控制实验，设置三种水分处理（灌溉组、自然降水组、排雨组）以验证该假说。
3）本研究分析了“蒸发结构”（针叶长度、气孔直径、气孔密度、气孔导度）的变化对水分处理及年际气候变异的响应；此外，通过比较年针叶面积（LA）增量、年边材面积（SA）增量与枝条顶端距离（d），计算得到年SA:LA与SA:LA/d比值，以此分析年际功能调控特征。
4）针叶长度（l）随冬季与季风季供水增加而升高，且在土壤更干燥时表现出更强的年际变异性；气孔密度随干旱程度升高而增加，气孔直径则随之降低。由此，不同水分处理下的解剖学最大气孔导度相对恒定。SA:LA与SA:LA/d比值在各处理间存在显著差异，且与我们的预期相悖：供水减少时该比值更低。在平均降水区间内，各处理的比值对土壤湿度的响应趋势一致；但当极端土壤干旱叠加高水汽压亏缺（Vapor Pressure Deficit, VPD）时，针叶长度、SA:LA与SA:LA/d比值均呈现高度非线性响应，这表明存在高VPD与干旱土壤共同作用的响应阈值。
5）新生枝条组织中，年际功能比值对水分胁迫的响应具有即时性（即当年响应），且并未试图降低叶水势的下降幅度。本研究认为，干旱引发的不利蒸发结构响应，可通过动态气孔调控进行补偿，以最大化光合速率。