北京山区元宝枫夜间液流活动特征及影响因素

树木夜间会维持部分气孔开放,从而能够在一定环境驱动因子的情况下进行夜间蒸腾。夜间液流作为储存水的重要来源,能够补充植物白天的水分亏缺,使其恢复水分储备,对植物生长发育有重要意义。采用TDP热探针法测定了位于八达岭林场的元宝枫树干液流密度,同步监测了主要环境因子,以深入揭示树木夜间蒸腾耗水规律和植被应对环境胁迫的调控机制,为山区植被建设、森林健康经营和挑选节水树种提供理论依据。结果表明:以0:00为界区分前半夜和后半夜,元宝枫夜间液流速率前半夜较后半夜活跃,且前半夜夜间累积液流量占夜间累积液流量的53.85%—64.10%,而后半夜夜间累积液流量占夜间累积液流量的35.9%—46.15%。5月的夜间累积液流量最大,平均夜间液流通量为5月6月8月9月7月。存在水分胁迫的条件下降雨之后夜间液流会增大,而当土壤水分条件较好,土壤水分不再是夜间液流的限制因子时,夜间液流通量并不高。不同树木形态的夜间液流通量有显著差异,在一定范围内,胸径树高冠幅越大的样木,夜间液流通量越大。用于夜间蒸腾的夜间液流通量与饱和水汽压差、温度、空气相对湿度、风速相关,其中夜间蒸腾存在于前半夜,表现为前半夜夜间液流通量与环境因子的相关性相较后半夜相关性较为显著,后半夜则以补水为主,补水量取决于土壤含水量和日蒸腾强度。存在干旱胁迫的条件下,夜间液流既用于夜间蒸腾,又有一部分用来补水;而土壤水分条件好时夜间液流则主要用于补水,此时夜间树干液流与环境因子相关性不高。元宝枫夜间液流通量的日蒸腾贡献率5、6月份大于7、8月份,即干季比湿季贡献率更高。夜间液流通量的日蒸腾贡献率与白天总蒸腾量相关性较高,并与累积太阳辐射成负相关。

Influencing factors and characteristics of nighttime sap flow of Acer truncatum in Beijing mountainous area

Trees can maintain a partial stomatal opening at night, allowing night transpiration under certain environmental driving factors. As an important source of storage water, the nighttime sap flow can remedy a daytime water deficit and restore the water reserve, which is of great importance to the growth and development of plants. The stem sap flow of A. truncatum in Badaling Forest Center was measured by TDP thermal probe. In addition, we simultaneously measured the main environmental factors, including air temperature, relative humidity, and soil moisture content, to determine the law of water consumption at night and the regulation mechanism of vegetation in response to environmental stress. This research should provide a theoretical basis for vegetation construction, forest health management, and the selection of water-saving tree species in mountainous areas. The results showed that the first half of the night and the latter half of the night were distinguished by 00:00; the nighttime sap flow velocities of A. truncatum were more active in the first half of the night than in the latter half of the night. In the first half night, the cumulative sap flow accounted for 53.85%-64.10% of the nighttime cumulative sap flow, with the latter half of the night accounting for the remaining 35.9%-46.15%. The cumulative nighttime sap flow rate was largest in May and the average nighttime sap flow flux followed the order May > June > August > September > July. Under conditions of soil drought, the nighttime sap flow was increased, but when the soil water condition was better and the soil moisture was no longer the limiting factor of the nighttime sap flow, the amount of the nighttime sap flow was not high. There was a significant difference in nighttime sap flow between different tree morphologies. Within a certain range, a larger diameter of breast height, tree height, and crown size was indicative of greater nighttime sap flow. The nighttime sap flow used for night transpiration was related to air saturation vapor pressure deficit, air temperature, relative humidity, and wind speed; night transpiration occurred in the first half of the night, and the correlation between nighttime sap flow and environmental factors was more significant than in the latter half of the night. In the latter half of the night, the stem water recharge was dependent on soil moisture content and daily transpiration intensity. Under conditions of soil drought, nighttime sap flow was used not only for night transpiration but also for stem water recharge; however, when the soil water condition was good, nighttime sap flow was mainly used to supply water and, in this case, the correlation of nighttime sap flow with environmental factors was lower. The ratio of nighttime sap flow to the total transpiration of A. truncatum was higher in May and June than in July and August; thus, it can be considered that the ratio of nighttime sap flow to total transpiration was higher in the dry season than in the wet season. There was a strong correlation between the ratio of nighttime sap flow to total transpiration and the daily total transpiration. In addition, the ratio of nighttime sap flow to total transpiration was negatively correlated with cumulative solar radiation.