| Chinese jujube(Ziziphus jujube Mill.) is native tree species in Northern Shaanxi Province. The area of cultivation has been continuously expanding since China implemented the policy of "Conversion of cropland to forestland" in 1999, drought and water shortage have been the bottleneck of restricting the sustainable and healthy development of the local jujube industry. The planting mode of long-term extensive and the excessive consumption of soil moisture in forests, leading to the deterioration of ecological environment and the appearance of permanent soil moisture dry layer. Water consumption of jujube orchard changes complexly, influenced comprehensively by meteorological factors, soil water status, site conditions and its growth and development of their own situation. To grasp the regulation of jujube transpiration and regulate its water consumption process effectively, it is significantly important to realize the dynamic balance of soil moisture in( jujube) forest and the sustainable management ecological economic forest(jujube forest).In this paper,pear jujube widely cultivated in Yulin of Northern Shanxi Province, was the studying object. Used the equipments of TDP(Thermal Diffuse Probe) sap flow monitoring system, CNC100 neutron moisture meter, TDT(Time Domain Transmission) soil moisture monitoring probe and leaf area index instrument to observe jujube dynamic transpiration, deep soil moisture condition(0-10m), meteorological factors, tree growth indexes consecutively for three years. Water consumption and its variation of jujube and jujube forest, the response of them to the influencing factors were analyzed, and water saving regulation strategy was put forward. Research results are as follows:(1)The variation of jujube’s sap flow during the day presented unimodal trend overall. The start time, the magnitude of the peak, the time of exuberant transpiration, the time began to recede and the time stagnated basically changed with the growth period and other factors. Jujube transpiration was low at the beginning and the end of the growth period in May and October, respectively. while was high(80~105mm) from June to September. Transpiration of jujube in the growth period occurred mainly during the daytime, but the sap flow phenomena also existed at night, jujube night transpiration accounted for about 10%-25% of the daily transpiration. A major cause of night transpiration phenomenon was root pressure absorption in trunks. Significant differences were found in the change regularity of sap flow measurement parameters during the jujube growth period and dormant period. Based on this feature, the growth period of jujube could be demarcated more accurately. Found by comparing, the length of growth period based on sap flow characteristics was the same as determined by the observation of sprout and defoliation, both are about 160 days. But the date of growth period based on the sap flow characteristics would be about 5 days in advance. Sap flow of jujube trees started earlier than sprout, and defoliation started after sap flow basically stopped. After the sap flow in early growth stage of jujube trees started, it would experience gradually enhanced process, about 22-34 days. And 2-3 days after the liquid flow stopped, defoliation would occur on a large scale. And the complete recession of sap flow only need 2-3 days.(2) The transpiration of jujube was closely related to its growth and development indexes. Sapwood area of jujube tree was inversely proportional to the instantaneous transpiration(sap flow rate), but proportional to the daily transpiration. Leaf area(LA) and leaf area index(LAI) of jujube in the whole growth period both changed as quadratic curve, while the LA was small(LA < 2.1m2), it was significant proportional to the transpiration(R2=0.84). The law of water consumption of jujube of different ages was basically the same, in the early growth stage, jujube of different ages did not show significant difference in water consumption. With the increase of age in the late growth stage, water consumption would significantly increase. The maximum of daily transpiration usually appeared from July to September, the change law of different aged jujube were significant different. The transpiration variable coefficient(R2) of different tree-age changed in accordance with quadratic curve in 2012 and 2014(R22012=0.6061, R22013=0.3196), differences were small at the beginning and the end of the growth period but large in the middle of the growth period(C.V = 40%- 60%). In 2013, the variation of evapotranspiration among trees with different ages were quite small both in early and later growth period, consistent with the cubic curve(R22013=0.3483), it might be caused by the frequent rainfall and low radiation intensity at the end of the growth period.(3) The relationship between jujube transpiration and meteorological factors was significant, the main effect of daily transpiration, monthly transpiration and transpiration of the whole growth period were different, response degree(R2) and mode(K) of the same impact factor vary with time. VPD and PAR overall are two main impact factors of transpiration, which are also two main driving factors of jujube transpiration. Analyzed the relationship between sap flow varying and two main driving factors, found that there were highly significant correlations between both two main factors and sap flow, and the correlation between sap flow and PAR is higher than that between sap flow and VPD. The daily transpiration of jujube showed significant correlation with two factors, while the correlation coefficient decreased with the expansion of time scale. The monthly transpiration in the growth period also showed significant correlation with two factors, and the response of sap flow to PAR is more significant than that to VPD.4) The response of jujube to two transpiration driving factors showed "time lag " obviously. At the start stage of sapflow, two factors and transpiration started almost at the same time, the peak time of transpiration was earliest, PAR was the second, and VPD was the last. In the decline process of sap flow, PAR change first, transpiration was the second, and VPD was the last. Threshold phenomena existed in the response of jujube to PAR and VPD. In the sap flow rising stage, transpiration would maintain its value stability and no longer increase when two factors beyond a certain threshold. Whereas in the sap flow decline stage, sap flow would decrease significantly until two factors decreased below the threshold. The thresholds of VPD were stable at around 3.5Kpa during the whole stage, while the thresholds of PAR were different at the increase and decrease stage(1000μmolm-2 s-1~1200μmolm-2 s-1). This kind of lag phenomenon was the important sign of jujube drought resistance characteristics, which avoided excessive water loss of jujube effectively. The threshold responded of two factors changed with affect factors such as temperature in environment.5) The change laws of soil moisture in depth 0-1m of jujube forest monitored by neutron moisture gauges and soil moisture probe were completely consistent. The value of soil moisture monitored by TDT probe was lower than the fact when the soil moisture is low. Jujube forest soil moisture fluctuated fiercely due to the rainfall, it increased significantly(soil water storage +28mm) in a wet year(2013), waned a little(-5.5mm) in a slight year(2012), and waned a lot(-40mm) in a dry year(2014). The fluctuation of soil moisture slowed down with the increase of the soil depth(0~10m depths). From top to bottom it could be divided into soil moisture variation layer(0~2.6m), soil moisture dry layer(2.6~6m), and soil moisture recover layer(6~10m). In different rainfall years, transpiration of jujube would increase with the rise of soil water content. Soil moisture condition could not only affect the daily transpiration, but also could affect the instantaneous transpiration(sap flow). The increase of soil moisture would make the minimum value of the sap flow appeared ahead of time, the appearance of the maximum value of the liquid flow delayed, and shorten the time of “lunch break”, and vice versa.(6) The comprehensive analysis of meteorological factors, soil moisture and growth factor on different time scales affecting jujube transpiration, found that the correlations(R2) between factors and transpiration all high in the growth period. The dominant factors of monthly transpiration in the growth period were PAR(R2max=0.68) on hours scales, RH(R2max=0.78) and WS(R2max=0.81) on days scales, and LAI(R2max=0.82) and LA(R2max=0.73) on ten day scales. On small time scales(hours and days scale), the annual transpiration was significantly correlated with climatic factors(p<0.01); while on large time scales(ten days and monthly scale), the annual transpiration was only significantly correlated with LAI, LA and SW(p<0.01). Soil moisture was in significant correlation with jujube annual transpiration, but not correlation with monthly transpiration. That is to say, annual transpiration was mainly affected by the soil moisture status, monthly transpiration was mainly affected by its own the growth and development stage. With the scale of time enlarging, the correlation between meteorological factors and transpiration gradually lowered, and the correlation between crop growth status and transpiration gradually heightened. Main affect factors of monthly transpiration and annual transpiration were different on the scales of the same time.7) In order to explore the effective methods of pruning to limit the jujube transpiration and water consumption of forest land, controlled the gradient of jujube tree body size by pruning. Found that pruning can reduce the transpiration of tree water consumption significantly. Differences in transpiration caused by pruning: daytime > nighttime, sunny day > cloudy day. In the growth period, transpiration and water consumption caused by pruning were not significant different in the germination stage, while in the stage of blossom, fruit setting, fruit enlargement, maturity and leaf fall stage, the differences of transpiration and water consumption were all significant different. After taking pruning control of two years consecutively, the transpiration of severe pruning decreased by 68.6% compared with that without pruning(CK), and the soil moisture increased by 40.5mm. Thus, reducing the tree size by pruning could reduce soil water consumption and increase soil water storage significantly. Rain-fed jujube production and water consumption equation could be constructed, and it determined the maximum of mountain jujube production is 16181.2 kg/hm2 theoretically, while the water use efficiency was only 17.46 Kg/hm2/mm at this time. With the increase of the production of jujube tree, water use efficiency would decrease sharply. With the water supply upper limit of years average rainfall 450 mm, and calculated by the equation of production and water consumption, jujube forest production target of sustainable management could be realized and identified: about 800 kg per mu, water use efficiency would increase to 26.25 kg/hm2/mm. The “water decide production” could be implemented preliminarily, and the relationship between production and tree specification factor could be evaluated comprehensively by the evaluation of linear and nonlinear methods, combined with the actual production, determined that of crown volume and new slightly length were the "water-saving pruning" indexes to regulate and control, which would lay the foundation for size(tree specification) decided by production. |
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