How to cope with drought and not die trying: drought acclimation across tree species with contrasting niche breadth

Worldwide drought events have been reported to cause tree growth decline and mortality, thus altering the carbon (C) balance of forest ecosystems. While most of the attention has been focused on the physiological mechanisms associated with drought-induced tree responses of a few species at specific locations, the ecological attributes of these species, like their niche breadth, may be also important in determining species' sensitivity or resilience to drought. We postulated that wide-niche breadth tree species should be more drought-resilient than narrow-niche breadth species. 2. Using the most severe 2015-2016 El Niño drought event in the last 70 years in Patagonia, we determined pre- and post-drought growth (BAI, basal area increment), C reserves in the form of non-structural carbohydrates (NSCs = starch and soluble sugars), wood isotope (δ13C, iWUE and δ18O) signaling, and xylem anatomy (mean vessel diameter, mvd) in eight angiosperm tree species of contrasting niche breadth across a sharp precipitation gradient in southern Chile. 3. All species responded in unison after the drought with a non-water-conservative response, maintaining BAI and NSCs concentrations, decreasing δ13C, and increasing both mvd and the soluble sugars:NSCs ratio relative to pre-drought time. Contrary to previous results reporting species-specific drought responses, our results show unequivocally a functional coordination of organisms' vital traits associated with a non-water-conservative strategy, and a drought-induced acclimation based on starch conversion into soluble sugars in all of the tree species we examined, regardless of their niche breadth and habitat preference. 4. We state that abiotic drivers such as drought may have selected similar interspecific responses provided that they operate at the community level rather than at the species level. These findings mark the need to revise current views about the ultimate interspecific functional coordination of organisms' vital traits when facing more frequent and intensive drought events.

据报道，全球范围内的干旱事件会导致树木生长衰退与死亡，进而改变森林生态系统的碳（C）平衡。尽管既往研究多聚焦于特定区域内少数物种受干旱诱导的生理响应机制，但这些物种的生态属性——如其生态位宽度（niche breadth）——或许同样是决定物种对干旱敏感性与恢复力的关键因素。本研究提出假说：生态位宽度较宽的树木物种，其干旱恢复力应强于生态位宽度较窄的物种。
2. 依托巴塔哥尼亚地区近70年来最严重的2015-2016年厄尔尼诺干旱事件，我们在智利南部沿陡峭降水梯度选取了8个生态位宽度存在显著差异的被子植物树种，测定了干旱前后的生长指标（断面积增量（BAI, basal area increment））、以非结构性碳水化合物（NSCs = 淀粉与可溶性糖）形式存在的碳储备、木材同位素信号（δ¹³C、内在水分利用效率（iWUE）与δ¹⁸O）以及木质部解剖结构（平均导管直径（mvd, mean vessel diameter））。
3. 干旱后所有树种均表现出一致的非节水型响应：与干旱前相比，其断面积增量与非结构性碳水化合物浓度保持稳定，δ¹³C值降低，同时平均导管直径与可溶性糖/非结构性碳水化合物比值均有所上升。与既往报道的物种特异性干旱响应结果相悖，本研究结果明确显示：无论受试树种的生态位宽度与生境偏好如何，所有研究物种均呈现出与非节水型策略相关的生命功能性状协同性，且均发生了基于淀粉向可溶性糖转化的干旱诱导适应性调整。
4. 本研究认为，若干旱这类非生物驱动因子以群落尺度而非物种尺度发挥作用，则可能筛选出趋同的种间响应模式。本研究结果表明，当面临愈发频繁且强度更高的干旱事件时，当前关于生物体生命功能性状种间终极协同性的主流认知亟需修正。