Data from: Pushing the limits to tree height: could foliar water storage compensate for hydraulic constraints in Sequoia sempervirens

1. The constraint on vertical water transport is considered an important factor limiting height growth and maximum attainable height of trees. Here we show evidence of foliar water storage as a mechanism that could partially compensate for this constraint in Sequoia sempervirens, the tallest species. 2. We measured hydraulic and morpho-anatomical characteristics of foliated shoots of tall S. sempervirens trees near the wet, northern and dry, southern limits of its geographic distribution in California, USA. 3. The ability to store water (hydraulic capacitance) and saturated water content (leaf succulence) of foliage both increased with height and light availability, maintaining tolerance of leaves to water stress (bulk leaf water potential at turgor loss) constant relative to height. 4. Transverse-sectional area of water-storing, transfusion tissue in leaves increased with height, while the area of xylem tissue decreased, indicating increasing allocation to water storage and decreasing reliance on water transport from roots. 5. Treetop leaves of S. sempervirens absorb moisture via leaf surfaces and have potential to store more than five times the daily transpirational demand. Thus, foliar water storage may be an important adaptation that helps maintain physiological function of treetop leaves and hydraulic status of the crown, allowing this species to partially compensate for hydraulic constraints and sustain turgor for both photosynthesis and height growth.

1. 树木垂直水分运输的限制是制约树木高生长与潜在最大树高的关键因素。本研究针对现存最高树种北美红杉（Sequoia sempervirens），证实叶片储水（foliar water storage）可作为一种补偿机制，部分抵消这一运输限制带来的不利影响。
2. 我们在美国加利福尼亚州，分别在北美红杉地理分布区的湿润北部边界与干旱南部边界，对高大个体的带叶枝条开展了水分生理与形态解剖特性的测定。
3. 叶片的储水能力（水力容度，hydraulic capacitance）与饱和含水量（叶片肉质性，leaf succulence）均随树高与光照可获得性的提升而增加，同时维持叶片水分胁迫耐受性——即膨压丧失时的总叶水势（bulk leaf water potential at turgor loss）——与树高保持相对恒定。
4. 叶片中储水转输组织（transfusion tissue）的横截面积随树高增加而增大，而木质部组织（xylem tissue）的横截面积则随之减小，这表明北美红杉随树高增长，会将更多资源分配至储水功能，同时降低对根系水分运输的依赖。
5. 北美红杉的顶梢叶片可通过叶面吸收水分，其储水潜力可达每日蒸腾需求的五倍以上。由此可见，叶片储水是一项重要的适应性策略，可维持顶梢叶片的生理功能与树冠的水分生理状态，帮助该树种部分抵消垂直水分运输的限制，维持光合作用与高生长所需的膨压。