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树木边材液流监测是表征树木蒸腾过程、诊断树木水分状况的有力工具[1-2],也是估算单株蒸腾量的有效手段之一[3]。边材液流速率的准确测定对于研究树木个体的生物学结构、水分利用特征及其对环境变化的响应,系统认识树木抗旱特征及水分利用策略[4-5]具有重大意义。
以插针式为特点的热技术如热扩散(Thermal dissipation probe)、热脉冲(heat pulse methods)、热场变形技术(Heat field deformation)等技术正成为树木液流测定的最主要手段,并得到广泛运用[6-10],然而,越来越多的报道表明,树木边材液流速率可能存在着方位差异[11-12]。对于某一树种而言,方位差异是否都显著,是否有固定的关系可寻,目前仍没有统一的认识。如报道认为西伯利亚落叶松(Larix sibirica Ledeb.)南侧树干液流较高[13],而枣树(Ziziphus jujuba Mill.)西侧液流速率较高[14],旱柳(Salix matsudana Koidz)与小叶杨(Populus simonii Carr.)的方位差异关系不确定,或随季节而变化等[15]。这些研究结果提示我们,从某一方位测定的边材液流速率可能并不足以可靠地用于单株液流通量的估算,另一方面,方位差异可能与边材水力结构的非均质性[16]、树冠几何形状或根系分布的空间不对称性[17-18]等有关联,或与地球公转引起的太阳辐射有规律的年周期性变化而相关联。
黄土残塬沟壑区是典型的旱作农耕区,这里光照充足、辐射能源丰富、昼夜温差大,已经成为全国最佳的苹果优生地区之一[19]。该区生产的苹果色艳质佳、产量高,是主要的经济来源,是当地的主导产业。精准确定果树单株在生长季的蒸腾用水规律是进行果园水肥管理的重要基础[20]。为此,本研究通过对果树不同方位边材液流速率的测定与比较,并分析其与主导环境因子如太阳辐射及大汽水分亏缺间的关系,旨在明确苹果树液流速率的方位特征,从而为通过测定液流来估算单株蒸腾用水量的方法提供更科学的依据。
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研究区选择在属于黄土残塬沟壑区的山西省临汾市吉县,该地区年均气温10.2℃,年均日较差11.5℃,年均降水522.8 mm,大于10℃的有效积温3 361.5℃,霜期年平均172 d,多年平均日照时数2 538 h,属暖温带大陆性季风气候,春季干旱多风,夏季降雨集中;秋季多连阴雨,冬季寒冷干燥[21]。试验地具体选择在东城乡上社堤村,110°35.655′ E,36°04.739′ N,海拔910 m,所选样地为2000年建植的苹果园,面积为1.7 hm2,品种为红富士(Malus pumila Mill),栽植密度为4 m×6 m。果园布设有防雹网,经营管理技术完备(施肥、修剪、人工授粉、生草覆盖、铺设反光膜、套袋、病虫害防治等),果树处于经济成熟期,生长良好。
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在试验林分内架设包括7个要素的小型气象站(空气温湿度,AV-10TH;风速传感器AV-30WS;风向传感器AV-30WD;大气压力传感器AV-410BP; 净辐射传感器NR-LITE2;雨量传感器AV-3665R)(其中AV系列传感器来自于美国AVALON公司,NR系列传感器来自于荷兰Kipp & Zonen公司)。数采器为SQ2020(英国Grant公司)。所有探头均通过主杆与支架安装在果树冠层以上(离地面约3 m处)。采样间隔为10 min,记录间隔为30 min。
大汽水分亏缺(VPD)的计算公式:
$ VPD = 0.611{{\rm{e}}^{\frac{{17.502T}}{{T + 240.97}}}}\left( {1 - RH} \right) $
(1) 式中:T为气温(℃),RH为空气湿度(%)。
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在样地内选择3处(株间、行的两侧带间)监测土壤水分动态。每处均在距表层50、100、150、200、250和300 cm处分别安装ECH2O土壤水分传感器,连接Em50数据采集器,采集间隔为30 min。
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试验苹果树品种为17龄红富士苹果,株行距为4 m×6 m,果树生长健壮,树体长势整齐,修剪量适中。选定3株冠形完整、生长健康的果树为试验样株,样株基本情况见表 1。采用Granier式热扩散传感器(TDP-3 cm,澳大利亚Plantsensor公司)测定果树树干边材的液流速率值。每个样株分别在东、南、西、北4个方位(根据手持罗盘仪确定)各安装1套TDP-3 cm,并连接32通道SQ2040数采器(英国Grant公司)。数据采集间隔为10 min,记录间隔为30 min。采用100 W多晶太阳能板接12V-100AH铅酸蓄电瓶连续供电。TDP安装过程及技术要点见文献[22-23]。边材液流速率的计算采用Granier公式。
$ {J_s} = 119 \times {10^{ - 4}}{\left( {\frac{{\Delta {T_0} - \Delta T}}{{\Delta T}}} \right)^{1.231}} \times 60 $
(2) 表 1 样株基本情况及液流速率统计结果
Table 1. Information of sample trees and statistics for sap flow velocity
样株
Samples胸径
DBH/cm树高
Height/m冠幅Crown width
南-北×东-西
S-N×E-W/(m×m)日平均液流速率值(平均值标准差)
Means and standard deviation of daily sap flow velocity /(cm·d-1)北侧North side 南侧South side 东侧East side 西侧West side Tree 1 20.2 2.4 4.2×4.0 172.0±204.3 140.5±189.0 140.5±173.2 108.3±139.5 Tree 2 21.5 2.5 4.4×4.2 175.2±231.0 122.8±170.5 103.2±154.2 77.4±122.5 Tree 3 20.1 2.4 4.1×4.0 153.0±184.4 136.0±197.3 131.1±170.4 109.1±149.3 式中,ΔT为上下两探针间实际温差(℃), ΔT0为液流为零时上下两探针间的温差(最大值,℃),该值采用两次回归法确定[25-26]。
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对样株在各个方位的液流速率值分别进行平均作为本研究分析的基础数据。Js与VPD,Js与Rn间的关系均采用指数函数(式3)进行拟合。选择典型晴天,对每个晴天液流速率的日变化过程(逐小时)采用Gauss模型(式4)进行拟合,来确定Js的峰值时刻,用同样的方法同步确定VPD与Rn的峰值时刻。不同处理中均值间差异的比较采用单因素方差分析方法,在统计软件OriginPro2017中完成,多重比较采用Tukey HSD post hoc在P < 0.05或0.01为显著性临界水平进行判断。图均在OriginPro2017中完成。
$ y=a-b{c^x} $
(3) 式中a,b,c均为参数。
$ y = {y_0} + \frac{A}{{w\sqrt {\pi /2} }}{{\rm{e}}^{ - 2\frac{{{{\left( {x - {x_c}} \right)}^2}}}{{{w^2}}}}} $
(4) 式中参数xc代表峰值时刻,参数y0代表基值(最小值),A,w分别为参数。
晋西黄土区苹果树边材液流速率的方位差异研究
Study on Azimuthal Variation of Sap Flow Velocity of Apple Trees in Loess Plateau Area, Western Shanxi Province
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摘要:
目的 通过对苹果树不同方位边材液流速率的测定与比较,明确苹果树液流速率的方位特征,为通过液流测定来估算单株蒸腾提供依据。 方法 利用热扩散技术(TDP)对黄土区苹果树主要生长季4个方位边材液流速率及土壤水分、气象因子进行同步、连续监测。 结果 苹果树树干边材液流速率(Js)具有显著的方位差异(P < 0.01),其中北侧Js最高,整个生长季日平均值达189.3 cm·d-1,其次为南侧(为北侧的83%)、东侧(北侧的80%),西侧最小(北侧的63%)。各方位Js总体均表现出5—8月间递增、9—10月间递减、11月基本停止的季节动态,且均与冠层净辐射(Rn)、大汽水分亏缺(VPD)间呈较好的指数正相关关系。在典型晴天,不同方位Js的日峰值时刻均明显提前于VPD的峰值,平均提前约1.6 h(最大2.43 h),且提前的时长与VPD日平均值呈线性递增关系。当VPD高于2.0~2.2 kPa时,Js不再随VPD的增加而上升;不同方位间Js峰值时刻与VPD(P=0.97)峰值时刻间的差异不显著。 结论 苹果树边材液流速率存在着明显的方位差异(P < 0.01),其中北、南方位液流速率较高,东、西方位液流速率较低。不同方位液流传输受大气环境影响的过程具有一致性。为提高果树液流通量估算的精度,在实际测定中应考虑方位差异。 Abstract:Objective To determine the azimuthal character of sap flow velocity (Js) of apple trees. Method the thermal dissipation probe technology (TDP) was used to simultaneously and continuously monitor the sap flow velocity in the four locations of apple trees during the whole growing season in the loess region. The soil moisture and meteorological factors were measured simultaneously, too. Result The Js was significant different among directions (P < 0.01). The highest Js in the sapwood was observed in the north side with an average of 189.3 cm per day over the entire growing season, followed by that in the south side (83% of the north side) and the east side (80% of the north side). The Js from the west side was the smallest (63% of the north side). The Js of all sides showed seasonal dynamics, i.e. increasing in May-August, dropping in September-October, and nearly stopped in November. The Js showed an exponentially increasing relationship with the net radiation above canopy (Rn) as well as with vapor pressure deficit (VPD). On a typical sunny day, the diurnal peak time of Js in different directions appeared earlier than that of the VPD (1.6 hours in average with the maximum of 2.43 hours). The length of advancing time increased linearly with the increasing of the daily mean VPD. The time advancing reflects that the peak Js of apple was limited by VPD while over 2.0~2.2 kPa. The difference of time advancing between Js and VPD in four directions was not significant (P=0.97). Conclusion There are obvious azimuthal differences (P < 0.01) in the sap flow velocity of the apple tree. The Js in the north side and the south side are high than that of the east or the west side. However, the process of sap flow in different directions is consistent with the influence of atmospheric environment. In order to improve the accuracy of estimating sap flux of apple trees, the azimuthal difference should be taken into account in actual measurement. -
Key words:
- sap flow velocity
- / vapor pressure deficit
- / net radiation above canopy
- / apple trees
- / sapwood
- / the Loess Plateau
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表 1 样株基本情况及液流速率统计结果
Table 1. Information of sample trees and statistics for sap flow velocity
样株
Samples胸径
DBH/cm树高
Height/m冠幅Crown width
南-北×东-西
S-N×E-W/(m×m)日平均液流速率值(平均值标准差)
Means and standard deviation of daily sap flow velocity /(cm·d-1)北侧North side 南侧South side 东侧East side 西侧West side Tree 1 20.2 2.4 4.2×4.0 172.0±204.3 140.5±189.0 140.5±173.2 108.3±139.5 Tree 2 21.5 2.5 4.4×4.2 175.2±231.0 122.8±170.5 103.2±154.2 77.4±122.5 Tree 3 20.1 2.4 4.1×4.0 153.0±184.4 136.0±197.3 131.1±170.4 109.1±149.3 -
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