哀牢山常绿阔叶林水源涵养功能及其在应对西南干旱中的作用

Water-holding capacity of an evergreen broadleaf forest in Ailao Mountain and its functions in mitigating the effects of Southwest China drought

为了解云南哀牢山中山湿性常绿阔叶林在2010年初西南地区特大干旱中是否遭遇水分胁迫,及其水源涵养功能在应对干旱中的作用,测定了该森林土壤和主要树种在2010年旱季的水分状况,并对比研究了原生林和森林经砍伐烧垦后形成的毛蕨菜-玉山竹群丛的土壤持水、凋落物持水、水面蒸发和土壤水分季节动态。结果表明:常绿阔叶林主要树种在2010年初西南特大干旱中并未遭受水分胁迫(最旱月叶片凌晨水势高于-0.4 MPa)。虽然常绿阔叶林土壤含水量和地下水位在最旱月都达到了有观测以来的最低点,但主要根系分布区的土壤水势仍不低于-0.5 MPa,并高于毛蕨菜-玉山竹群丛。森林较好的水分状况和原生常绿阔叶林较好的水源涵养功能有关。常绿阔叶林的土壤总持水量尤其是非毛管持水量要显著大于毛蕨菜-玉山竹群丛,同时也大于云南地区的一些次生林和人工林。常绿阔叶林地表丰富的凋落物通过持水和抑制土壤蒸发也对水源涵养有一定作用。哀牢山常绿阔叶林良好的水源涵养功能,充足的土壤地下水储存弥补了旱季和特大干旱中降水的不足。结果指示原生林在水源涵养中不可替代的作用,以及加强原生林保护在提高区域抗干旱能力中的重要意义。

Southwest China experienced the most severe drought in 100 years in the late 2009 and early 2010. The effects of this severe drought event on forest ecosystems in this region were unassessed. The aims of this study were （ 1 ） to assess if the evergreen broadleaf forest in Ailao Mountain, Southwest China, experienced water deficits in the 2010 rainfall anomaly event; （2） to understand the contribution of forest water storages in mitigating the effects of rainfall anomaly events; and （3） to assess the effects of logging on the waterholding capacity of an ecosystem. Water status of soils and major treespecies from a primary evergreen broadleaf forest in Ailao Mountain were monitored in the 2010 dry season. Soil and litter waterholding capacities of a primary forest and a secondary YushaniaPteridium shrubland formed after logging and burn were compared. Interannual and seasonal variations in soil groundwater table and relative water content of the forest and the Yushania Pteridium shrubland were also analyzed. Trees in the evergreen broadleaf forest did not experience water deficits even in the driest month （ predawn leaf water potential was higher than 0.5 MPa）, and the photosynthetic performance of the trees did not show symptom of long-term depression （ predawn maximum quantum efficiency of Photosystem II was higher than 0. 80）. Soil water content and groundwater table depth of the forest reached an historical minimum during the 2010 Southwest China rainfall anomaly event, however enough soil water was still available for plants during the driest month of 2010 （water potential of the soil layer where the roots were growing was still higher than 0.5 MPa）. The soil water potential of the evergreen broadleaf forest was higher than the YushaniaPteridium shrubland, suggesting better water availability for plants in the primary forest compared to the secondary Yushania Pteridium shrubland. The relatively high soil water availability of the primary evergreen broadleaf forest during the rainfall anomaly event is because of its good waterholding capacity, which allows sufficient rainfall in the wet season to be stored in the soil, and to be available to plants in the dry season. The soil waterholding capacity, especially noncapillary waterholding capacity of the evergreen broadleaf forest soils in Ailao Mountain was higher than that of the Yushania Pteridium shrubland soils, and higher than those of some other secondary forests in this region. In addition, the abundant litters in the forest floor also contribute to the water storage function of the forest through holding water, preventing evaporation, and decreasing surface runoff. The seasonal dynamics in ground water table and soil relative water content also suggest a larger soil waterstoragepool size of the forest than that of the Yushania Pteridium shrubland. In conclusion, the evergreen broadleaf forests in Ailao Mountain possess a strong water storage function, and the sufficient ground water and soil water storages mitigated the shortage of rainfall in dry seasons and in severe rainfall anomaly events. Therefore, our study suggests the irreplaceable role of primary forests in mitigating the effects of rainfall anomaly events, and the importance of protecting primary forests in enhancing the capacity of this region in coping with rainfall anomaly events.