Lintunen Anna, Preisler Yakir, Oz Itay, Yakir Dan, Vesala Timo, Hölttä Teemu
Institute for Atmospheric and Earth System Research/Physics, University of Helsinki, Helsinki, Finland.
Institute for Atmospheric and Earth System Research/Forest Sciences, University of Helsinki, Helsinki, Finland.
Front Plant Sci. 2021 Dec 20;12:790684. doi: 10.3389/fpls.2021.790684. eCollection 2021.
Drought can cause tree mortality through hydraulic failure and carbon starvation. To prevent excess water loss, plants typically close their stomata before massive embolism formation occurs. However, unregulated water loss through leaf cuticles and bark continues after stomatal closure. Here, we studied the diurnal and seasonal dynamics of bark transpiration and how it is affected by tree water availability. We measured continuously for six months water loss and CO efflux from branch segments and needle-bearing shoots in growing in a control and an irrigation plot in a semi-arid forest in Israel. Our aim was to find out how much passive bark transpiration is affected by tree water status in comparison with shoot transpiration and bark CO emission that involve active plant processes, and what is the role of bark transpiration in total tree water use during dry summer conditions. Maximum daily water loss rate per bark area was 0.03-0.14 mmol m s, which was typically ~76% of the shoot transpiration rate (on leaf area basis) but could even surpass the shoot transpiration rate during the highest evaporative demand in the control plot. Irrigation did not affect bark transpiration rate. Bark transpiration was estimated to account for 64-78% of total water loss in drought-stressed trees, but only for 6-11% of the irrigated trees, due to differences in stomatal control between the treatments. Water uptake through bark was observed during most nights, but it was not high enough to replenish the lost water during the day. Unlike bark transpiration, branch CO efflux decreased during drought due to decreased metabolic activity. Our results demonstrate that although bark transpiration represents a small fraction of the total water loss through transpiration from foliage in non-stressed trees, it may have a large impact during drought.
干旱可通过水力衰竭和碳饥饿导致树木死亡。为防止水分过度流失，植物通常在大规模栓塞形成之前关闭气孔。然而，气孔关闭后，通过叶片角质层和树皮的水分流失仍在继续。在此，我们研究了树皮蒸腾作用的昼夜和季节动态及其如何受树木水分供应情况的影响。我们在以色列半干旱森林的一个对照地块和一个灌溉地块中，对生长着的树枝段和带针叶的嫩枝连续六个月测量了水分流失和二氧化碳排放。我们的目的是弄清楚与涉及植物活跃过程的嫩枝蒸腾作用和树皮二氧化碳排放相比，被动树皮蒸腾作用受树木水分状况的影响程度如何，以及在干燥的夏季条件下，树皮蒸腾作用在树木总水分利用中的作用是什么。每单位树皮面积的最大日水分流失率为0.03 - 0.14 mmol·m⁻²·s⁻¹，这通常约为嫩枝蒸腾速率（以叶面积为基础） 的76%，但在对照地块蒸发需求最高时甚至可能超过嫩枝蒸腾速率。灌溉并未影响树皮蒸腾速率。由于处理间气孔控制存在差异，据估计，干旱胁迫树木中树皮蒸腾作用占总水分流失的64 - 78%，而灌溉树木中仅占6 - 11%。在大多数夜晚观察到有水分通过树皮吸收，但吸收量不足以补充白天流失的水分。与树皮蒸腾作用不同，干旱期间树枝的二氧化碳排放因代谢活动降低而减少（原文为CO efflux，推测是CO₂ efflux，翻译时按二氧化碳排放处理）。我们的结果表明，尽管在非胁迫树木中，树皮蒸腾作用在通过叶片蒸腾的总水分流失中占比很小，但在干旱期间可能产生很大影响。
