Process And Mechanism Analysis Of Leaf Water Absorption And Reverse Migration Of Platycladus Orientalis In Beijing Mountainous Areas

The forest vegetation in Beijing mountainous area is mainly planted forest.However,in recent years,the ecological system is fragile with climate warming has become increasingly obvious in North China.It is of great significance to study the strategy of foliar water uptake for the management and maintenance of the fragile ecosystem.In this study,the typical coniferous Platycladus orientalis was taken as the research object.In the rainy season and the rainy season(April-November),through field comparative quadrat control experiment and pot experiment,natural precipitation and artificial simulated rainfall experiment were used,revealing that the relationship between process and influencing factors in foliage water uptake,studying the occurrence and characteristics of reversal of sap flow in trees,clarifying the process and mechanism of reversal of sap flow in trees,and providing experimental support and scientific basis for further understanding of the process and mechanism of forest.The major research results were as follows:(1)When the Platycladus orientalis has been under drought stress for a long time,the leaves can use most of the rainfall and benefit from it.Leaf water absorption capacity was positively correlated with rainfall intensity and negatively correlated with soil moisture content.Under severe droughtthe leaf water absorption of Platycladus orientalis was the most obvious under 15 mm/h rainfall.The maximum water absorption per unit area could reach 8.47±1.00 mg/cm2,and the maximum change of leaf water potential was 1.18±0.17 Mpa.When the soil water content was higher than 20.8%,the leaf water absorption did not occur.(2)Leaves of Platycladus orientalis under long-term drought stress will transfer water absorbed by leaves downward to stem or even to root and rhizosphere soil area through reverse water potential gradient after absorbing water.Under the same rainfall conditions,the lower the soil water content,the greater the reverse recharge of leaf water to branches and rhizosphere soil area;and under the same soil water content,the greater the rainfall intensity,the greater the reverse recharge of leaf water to rhizosphere soil area.Under the condition of the lowest soil moisture content(3.9-6.5%)and the highest rainfall intensity(15mm/h),leaf water absorption was the most significant.After rain,leaf water absorption could supply 5.92±0.27%of reverse xylem transport to branches,and 5.66+0.57%of soil.(3)The recharge of leaf water and branch water increased with time and then decreased,and reached the maximum replenishment at 1h and 2h respectively;Basically decreased to below 2%after 8h.In about 1h reverse sap flow moving to the rhizosphere soil area,and the volume gradually increased,the recharge rate reached the maximum at 4h,then it gradually decreased.In the experiment,the effect of reverse migration of water absorption on the water of plants generally does not exceed 24h.(4)Under the extreme conditions of severe drought and sufficient rainfall intensity,the maximum leaf water uptake of Platycladus orientalis is 78.77±3.98g,and the maximum water absorption of leaves account for 3.15±0.16%.The water transported to the xylem of the branches is up to 4.05±0.09g,and the water transport to the rhizosphere soil is up to 1.47±0.04g.At the same time,5.14±0.03%of the water absorbed by the leaves supplement the xylem of the branches,and 1.87±0.05%replenish the rhizosphere soil area.