CREAF, Bellaterra (Cerdanyola del Vallès), Catalonia, 08193, Spain.
Pacific Northwest National Laboratory, Richland, WA, 99354, USA.
New Phytol. 2023 Sep;239(6):2083-2098. doi: 10.1111/nph.19130. Epub 2023 Jul 23.
Frequent observations of higher mortality in larger trees than in smaller ones during droughts have sparked an increasing interest in size-dependent drought-induced mortality. However, the underlying physiological mechanisms are not well understood, with height-associated hydraulic constraints often being implied as the potential mechanism driving increased drought vulnerability. We performed a quantitative synthesis on how key traits that drive plant water and carbon economy change with tree height within species and assessed the implications that the different constraints and compensations may have on the interacting mechanisms (hydraulic failure, carbon starvation and/or biotic-agent attacks) affecting tree vulnerability to drought. While xylem tension increases with tree height, taller trees present a range of structural and functional adjustments, including more efficient water use and transport and greater water uptake and storage capacity, that mitigate the path-length-associated drop in water potential. These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence for height-dependent increased vulnerability to hydraulic failure and carbon starvation, and their coupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Further research is needed, particularly at the intraspecific level, to ascertain the specific conditions and thresholds above which height hinders tree survival under drought.
在干旱期间，人们频繁观察到较大的树木死亡率高于较小的树木，这引发了人们对与树木大小相关的干旱诱导死亡率的日益关注。然而，潜在的生理机制尚不清楚，通常暗示与高度相关的水力限制是驱动干旱脆弱性增加的潜在机制。我们对物种内影响植物水碳经济的关键特征如何随树木高度而变化进行了定量综合分析，并评估了不同的限制和补偿可能对相互作用机制（水力衰竭、碳饥饿和/或生物因子攻击）产生的影响，这些机制会影响树木对干旱的脆弱性。虽然木质部张力随树木高度的增加而增加，但较高的树木会出现一系列结构和功能上的调整，包括更高效的水利用和运输，以及更大的吸水和储水能力，从而减轻与路径长度相关的水势下降。这些适应使较高的树木能够承受间歇性的水分胁迫。然而，仍然缺乏关于与高度相关的水力衰竭和碳饥饿增加脆弱性的决定性证据，以及它们与防御机制以及害虫和病原体动态的耦合。需要进一步的研究，特别是在种内水平上，以确定在何种具体条件和阈值下，高度会阻碍树木在干旱条件下的生存。
