Yang Da, Goldstein Guillermo, Wang Miao, Zhang Wei-Wei, Wang Ai-Ying, Liu Yan-Yan, Hao Guang-You
Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110010, China.
College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China.
Tree Physiol. 2017 Apr 1;37(4):501-510. doi: 10.1093/treephys/tpx018.
Mistletoes absorb water from the vascular system of their hosts and thus the water use of mistletoes can be influenced by the water status of their hosts besides abiotic environmental conditions; however, there is a lack of studies on the dynamics of mistletoe water utilization in relation to both types of controlling factors. By building a canopy platform at 20 m above the ground, we monitored the dynamic changes of sap flow of Viscum coloratum (Kom.) Nakai (Loranthaceae) in combination with continuous measurements of microclimatic variables and volumetric water content (VWC) of its host tree branch xylem. We found that the host tree VWC exhibited substantial fluctuations during sunny days but lower VWC of the host did not negatively affect the sap flow of V. coloratum. Hourly and daily mean transpiration rates (Esap) of V. coloratum calculated from sap flow measurements showed strong positive correlations with photosynthetic photon flux density (PPFD) and vapor pressure deficit (VPD) measured in close vicinity to the point of mistletoe attachment. The mean Esap of V. coloratum was substantially higher than that of their host during clear days (4.55 ± 0.54 vs 2.01 ± 0.15 kg m-2 day-1). Moreover, the mistletoe-to-host transpiration ratio was not constant but became increasingly larger with the increase of PPFD or VPD on both hourly and daily bases, suggesting a weaker control of water loss in the mistletoe in comparison to its host species. The strong dependence of mistletoe Esap on micrometeorological variables and its decoupling from the host tree xylem water status suggests that the development of dense tree canopy functions as a potential mechanism for the host trees in reducing the competitive water use of mistletoes. These findings have important implications for the interactions between mistletoe species and their host trees in temperate forests.
槲寄生从寄主的维管系统中吸收水分，因此，除非生物环境条件外，槲寄生的水分利用还会受到寄主水分状况的影响；然而，关于槲寄生水分利用动态与这两种控制因素关系的研究却很缺乏。通过在离地面20米处搭建树冠平台，我们监测了槲寄生（桑寄生科）的液流动态变化，并结合对其寄主树枝木质部微气候变量和体积含水量（VWC）的连续测量。我们发现，寄主树的VWC在晴天期间呈现出大幅波动，但寄主较低的VWC并未对槲寄生的液流产生负面影响。根据液流测量计算得出的槲寄生每小时和每日平均蒸腾速率（Esap）与在槲寄生附着点附近测量的光合光子通量密度（PPFD）和水汽压差（VPD）呈强正相关。在晴天，槲寄生的平均Esap显著高于其寄主（4.55±0.54对2.01±0.15千克·米⁻²·天⁻¹）。此外，槲寄生与寄主的蒸腾比率并非恒定不变，而是在每小时和每日尺度上均随着PPFD或VPD的增加而变得越来越大，这表明与寄主物种相比，槲寄生对水分损失的控制较弱。槲寄生Esap对微气象变量的强烈依赖性及其与寄主树木木质部水分状况的解耦表明，茂密树冠的形成是寄主树木减少槲寄生竞争性水分利用的一种潜在机制。这些发现对温带森林中槲寄生物种与其寄主树木之间的相互作用具有重要意义。
