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

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 un-assessed.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 water-holding capacity of an ecosystem.Water status of soils and major tree species from a primary evergreen broadleaf forest in Ailao Mountain were monitored in the 2010 dry season.Soil and litter water-holding capacities of a primary forest and a secondary Yushania-Pteridium shrub-land formed after logging and burn were compared.Inter-annual and seasonal variations in soil groundwater table and relative water content of the forest and the Yushania-Pteridium shrub-land 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 Yushania-Pteridium shrub-land,suggesting better water availability for plants in the primary forest compared to the secondary Yushania-Pteridium shrub-land.The relatively high soil water availability of the primary evergreen broadleaf forest during the rainfall anomaly event is because of its good water-holding 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 water-holding capacity,especially noncapillary water-holding capacity of the evergreen broadleaf forest soils in Ailao Mountain was higher than that of the Yushania-Pteridium shrub-land 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 water-storage-pool size of the forest than that of the Yushania-Pteridium shrub-land.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.