不同土壤水分梯度下桉树整树水分利用的年龄效应与水力调节

Large-scale cultivation and short rotation of eucalypt trees based on economic considerations have led to an excessive consumption of water and serious depletion of nutrients in soils. Such practice makes the recharge and rehabilitation of soil difficult, which has largely raised concerns on regional water balance and environmental effects in the eucalyptus cultivation zone. On the other hand, the uneven distribution of regional rainfall induced by climate warming has intensified the contradictive relation between soil moisture supply and the strong transpiration of eucalyptus trees. Previous researches have emphasized more on the relationship between transpiration of fast-growing eucalyptus and environment factors, plant density, and cultivation way, whereas ignored the age effect on eucalyptus’ water transpiration, which might lead to misunderstanding of water use of eucalyptus. We argue that the adjustment of hydraulic structure which is closely related to tree age and determines water transportation efficiency, could indirectly affect the response of tree water use to soil moisture. Accordingly, we propose that there is an age tradeoff between transpiration and growth of trees under different soil moisture conditions. To illustrate age effect on the response of whole-tree water use to changes in soil moisture and to reveal the relevant regulatory mechanisms, the sap flow of different age eucalyptus trees, their hydraulic conductance, canopy stomatal conductance, growth indexes, and water use efficiency will be examined under different natural soil moisture gradients. These results would provide experimental support for management of eucalyptus cultivation and for prediction of eucalyptus regional water balance under future climate change, which are of academic and practical importance.

桉树的大面积种植和基于经济效益考虑的现行轮作模式过度消耗水分和营养，土壤无法得到有效补充和恢复，人们对林区的水量平衡和环境效应产生担忧。气候变暖导致区域降雨分配不均，桉树持续的非保守性蒸腾与土壤水分供应的不协同越发突出。至今，相关研究偏重桉树蒸腾与环境、种植密度及耕作方式的关系，忽略树龄对蒸腾耗水的影响，对桉树水分利用时间规律的认识出现偏差。我们认为，与树龄紧密相关、决定输水效率的水力结构的调整，间接影响树木水分利用对土壤水分梯度的响应，据此提出“不同土壤水分梯度下桉树的水分利用与生长存在年龄权衡”的观点。本项目将设计自然土壤水分梯度，研究不同龄级桉树的树干液流、水力导度、冠层气孔导度、生长参数和水分利用效率的变化，阐明桉树整树水分利用响应土壤水分变化的年龄效应和水力调节机理，为种植管理措施的设计和预测未来气候变化情景下桉树林区的水文动态提供实验支撑，有明显的学术价值和实践指导意义。

华南地区桉树的大面积种植和短期轮作模式过度消耗了土壤水分和营养，加剧了人们对桉树林区水量平衡和环境效应的担忧。因此，研究华南地区桉树水分利用与其生长之间的平衡显得尤为重要。基于此，我们假设桉树树龄和土壤水分是调节该平衡的重要影响因子，并通过研究三种处在典型年龄阶段的尾叶桉人工林（即3-4年的速生期，6-7年的成熟收获期，及大于25年的稳定老龄期）沿丘陵斜坡形成的三个土壤水分梯度下的水分利用与生长以检验该假设。我们展开了为期两年（2018.01-2019.12）的桉树树干液流连续监测，并结合环境因子和定期测量的叶片水势（黎明前叶片水势Ψpd和正午水势Ψmd）、叶面积指数（LAI）、生长指标（胸径和树高）等因子，阐述不同年龄层桉树在不同土壤水分梯度下水分利用的水力调节，并通过整树蒸腾和生物量增长情况计算水分利用效率。研究结果表明，1）桉树年龄对其生长和水分利用具有重要影响，相对于速生期和成熟期的桉树，老龄桉树的蒸腾更为旺盛；2）三种年龄桉树的生长速度随着年龄增大而降低，因而速生期桉树相对老龄桉树具有更高的水分利用效率；3）桉树水分利用和生长速率在不同土壤水分梯度下并无显着差异，表明华南地区土壤水分梯度并不主导桉树生长与其用水间的权衡关系；4）我们认为，蒸腾量随树龄的变化与树木边材面积和林分LAI值的变化有关，且Ψpd和Ψmd在不同时期的值表明，即使该地区处于相对干旱的时期，土壤表层水分的波动仍然较小。总体而言，我们的研究为中国华南区域桉树人工林的管理和生态系统稳定性提供了重要的实验支撑。