北方土石山区典型树种耗水特征及环境影响因子

摘要

Knowledge on the transpiration and environmental factors of forests facilitates the management of self-sustainable vegetation ecosystems in northern rocky mountainous areas,and provides scientific vegetation construction strategies.We surveyed forest transpiration and environmental factors in a mixed stand of Robinia pseudoacacia and Pinus tabulaeformis from July 11 to October 31,2016 in the experimental forest of the Beijing Forestry University,West Mountain.A thermal dissipation probe (TDP) system was applied to monitor the transpiration.To study the effects of the environmental factors on transpiration,volumetric soil water content,and meteorological variables were also monitored simultaneously,including solar radiation (Rn),air temperature (T),relative humidity (RH),wind speed (W),and precipitation (P).This study aimed to (1) analyze forest transpiration processes and the transpiration responses to environmental factors,and (2) determine the canopy coupling of the forest and quantify the contribution of different factors to transpiration.Results showed that (1) the two species exhibited different seasonal transpiration trends despite their similar diurnal patterns.The transpiration of R.pseudoacacia decreased in autumn.By contrast,the transpiration of P.tabulaeformis was maintained throughout the growing season.The two species demonstrated similar transpiration patterns,but the transpiration of P.tabulaeformis was higher than that of R.pseudoacacia;(2) Both species demonstrated hysteresis between transpiration and Rn.But R.pseudoacacia had a clockwise hysteresis loop,whereas P.tabulaeformis had an anti-clockwise loop.Hysteresis was also observed between transpiration and VPD(vapor pressure deficit).Both species showed clock-wise hysteresis loops with much higher transpiration rates in the morning than afternoon;(3) The main environmental factors influencing transpiration included Rn (P ＜ 0.01),VPD (P ＜ 0.01),and W (P ＜ 0.01).Among them,VPD induced higher transpiration than Rn;(4) the two species were well coupled to the ambient environment (Ω ＜ 0.1);thus,their stomatal control over transpiration was efficient,and the effect of Rn was highest before noon.The sap flow was observed to start earlier under high VPD conditions to avoid xylem cavitation or embolism owing to intense transpiration;(5) The soil water condition of shallow layers(0-50 cm) was not a major influencing factor on plant transpiration.Therefore,further study should include monitoring deeper soil layers to locate the soil water supply source.Based on our results,the transpiration of this mixed stand could be reduced through the abatement of canopy conductance.Thus,water restrictions would be alleviated and the survival rate would be increased.Both species could efficiently physiologically control transpiration.Therefore,in practice,the water consumption of this forest could be managed by reducing the canopy conductance.This could be carried out by pruning,or selecting a species with a low canopy conductance to increase the survival rate of the forest under limited water conditions.%研究北方土石山区植物耗水特征和环境影响因子对于构建稳定的植被生态系统具有重要意义,能够为当地植被恢复策略提供科学指导.在北京林业大学西山试验林场于2016年7月至10月利用热扩散探针的方法,结合同步观测的土壤含水率和气象因子,对刺槐和油松人工混交林进行蒸腾观测和分析.结果表明:(1)尽管刺槐和油松蒸腾的日变化规律相近,但二者蒸腾的季节变化规律不同;(2)两个树种蒸腾与VPD(饱和水汽压差)成顺时针时滞.刺槐蒸腾与太阳辐射成顺时针时滞,油松则成逆时针时滞;(3)二者与大气环境的耦合程度均较高(Ω＜0.1),其气孔活动能够有效地控制蒸腾;(4)影响植物蒸腾的主要环境因子为太阳辐射(P＜0.01)、VPD(P＜0.01)和风速(P＜0.01),其中由VPD引起的蒸腾量高于太阳辐射;(5)浅层土壤(0-50cm)的水分条件可能并不是影响植物蒸腾的重要因素.研究表明,在实际管理中可以采取调控气孔导度的手段来减少刺槐和油松人工林的耗水量,来降低水分这一人工林成活的限制因子,从而提高造林成活率.