| The region of Loess Hilly is characterized by its relatively fragile ecosystems. Serious soil erosion and drought are the two major ecological problems in this region. Sustainable restoration of vegetation is the basic way for the ensurance of the regional ecological environment security. According to the contradiction between revegetation and water deficiency occurred during the process of vegetation restoration, three typical forests in Ansai, the pure forest of Pinus tabulaeformis, pure forest of Hippophae rhamnoides, mixed forest of Pinus tabulaeformis and Hippophae rhamnoides, were selected in this study.Thermal dissipation probe(TDP) was used to measue the sap flow density of three stand of Pinus tabulaeformisand Hippophae rhamnoidesand environmental variables were simultaneously measured(on April 18-November 8, 2015). The influencing factors of sap flow of each sample tree in the three stands were analyzed systematically. The average water use of three typical forests in the growing season was assessed. Meanwhile, the leaf scale characteristic of photosynthesis and water potential of sample trees and the influenced factors were studied using the LI-6400 XTportable photosynthesis system and pressure chamber method. These studies could provide the theory bases for choice of tree species, soil moisture suitability evaluation of forest land. The main results are as follows:(1) Sap flow characteristics and its influenced factors of Pinus tabulaeformis and Hippophae rhamnoides of the pure and mixed forestThe sap flow density showed a significant circadian rhythm change, and the diurnal variation of sap flow density exhibited single-peak or bimodal curve(Hippophae rhamnoides) of Pinus tabulaeformis and Hippophae rhamnoides of the pure and mixed forest in the whole growing season. The daily maximum of sap flow density of these two species during the beginning(May to June) higher than that during the medium(July to August) and end(September to October) of the growing season. These may be associated with changes of soil moisture conditions and the different periods of growing season.The sap flow density were influenced by the photosynthetic available radiation, vapour pressure deficient and soil moisture content of Pinus tabulaeformis and Hippophae rhamnoides of the pure and mixed forest in the most growing season. The contribution rates of factors were different in different periods or stands:the environmental factors were the main influenced factors to the sap flow density in the different periods of pure forest of Pinus tabulaeformis, the contribution of rate were 84.32%, 38.78%, 71.37% individually; the contribution of rate were 80.57%, 74.57% individually, and 71.17% in the end of growing season separately of the mixed forest; soil moisture was the main factor to sap flow density of two stands of Hippophae rhamnoides in the beginning and mid of growing season, the contribution of rate between 35.81~77.24%, the effect of environmental factors were weak in the end, these may be related to the later of the growing season or some physiological dysfunction of leaves.(2) Physiological characteristics of Pinus tabulaeformis and Hippophae rhamnoides of the pure and mixed forestThe difference between predawn and midday leaf water potential of these two species were all show the highest in the middle of the growing season, these may be related to the different water demands of the atmosphere or soil moisture conditions.The diurnal courses of water use efficiency of leaves of these two species were single-peak curve or multi-peak curve during clear days of growing season. The PAR separately explained 36.94% and 36.80% of the WUE in the early and mid growing season; Gs and VPD jointly explained 95.52% of the WUE of pure forest of Pinus tabulaeformis; The PAR separately explained 50.12% of the WUE in the early growing season; Gs and VPD jointly determined 96.21% of the WUE of mixed forest of Pinus tabulaeformis; there was a similar trend in the pure and mixed forest of Hippophae rhamnoides, PAR separately explained 39.16%(pure forset) and 40.37%(the mixed) of the WUE in the early growing season; Gs and VPD jointly determined 81.41%(pure forset), 80.69%(the mixed) and 93.91%(pure forset), 93.47%(the mixed) of the WUE in the mid and end growing season.(3) A comparative analysis of water consumption and physiological characteristics of Pinus tabulaeformis and Hippophae rhamnoides of the pure and mixed forestThe daily water consumption of a single Pinus tabulaeformis and Hippophae rhamnoides was in accordance with the change of its sap flow density in the growing season, that is, Water consumption of Pinus tabulaeformis was gradually reduced; The water consumption of Hippophae rhamnoides was strongly influenced by precipitation, water consumption increased in response to rising precipitation. There were a similar regular change of water consumption in the different periods of growing season and the different stands in the same month, but great difference in the same stand of different months, these may be related to the different biologic characteristics of these two tree species or differences of environmental factors of the different periods of growing season.Comparative analysis water physiological characteristics and root distribution, showed that there were different adaptive strategy when the water conditions changed, that is, Pinus tabulaeformis could be considered as an isohydry plant, mainly by reduce stomatal conductance as soil water decreases and atmospheric conditions dry, maintaining a relatively constant midday leaf water potential regardless of drought conditions. Hippophae rhamnoides could be considered as an anisohydric plant, by contrast, allow midday leaf water potential to decline as soil water declines with drought, and these difference in water demand ensured their water use stability and sustainable. |
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