Abstract

在6、7年生三倍体毛白杨(triploid <em>Populus tomentosa)</em>纸浆林中研究了地下滴灌(SDI)下不同土壤水势(-25、-50、-75 kPa,即灌溉起始阈值)对林木生长及生理特征的影响。结果表明:与不灌溉(CK)相比,SDI使6、7年生林分的生产力分别平均提高24%和28%;其中,-25 kPa使6年生林分的生产力达到39.90 m<sup>3</sup> hm<sup>-2</sup>a<sup>-1</sup>,较CK极显著提高43.5%(<em>P</em><0.01)。各水势处理间,-25 kPa的生产力在林分6年生时分别较-50和-75 kPa极显著提高20%和31%(<em>P</em><0.01),在7年生时分别提高13%和14%(<em>P</em>>0.05)。能在毛白杨速生期内(4-7月)大幅提高土壤含水率(20和50 cm处分别平均提高35%和27%)、树干日平均液流速率(46%,SF<sub>mean</sub>)、黎明前叶水势(41%,<em>ψ</em><sub>pd</sub>)是SDI促进林木生长的重要机制。灌溉起始水势阈值的差异对毛白杨SF<sub>mean</sub>和<em>ψ</em><sub>pd</sub>无显著影响(<em>P</em>>0.05)。3个水势处理中,-25 kPa的平均SF<sub>mean</sub>和<em>ψ</em><sub>pd</sub>均最高,且其能使根系活动剧烈的表土层(0-20 cm)的水分有效性有最大幅的提高,这可能是其对林木生长有最大促进作用的主要原因。综上,应在毛白杨纸浆林培育中大力推广SDI,并在应用时可将距滴头10cm、地下20 cm处的土壤水势达到-25 kPa作为灌溉起始阈值。另外,在与试验地环境相似地区栽植毛白杨时,应于4-7月灌溉,8-10月一般可不灌溉。;Subsurface drip irrigation (SDI) is an intensive silvicultural practice which has the potential to increase productivity of triploid <em>Populus tomentosa</em> pulpwood plantations. In order to efficiently apply SDI in the cultivation of <em>P. tomentosa</em> pulpwood plantations, it is necessary to determine irrigation timing. A field experiment was conducted to investigate the effect of soil water potential (<em>ψ</em><sub>soil</sub>) on the growth and physiological characteristics of 6-7 year old trees under SDI in a <em>P. tomentosa</em> pulpwood plantation. The experiment included three <em>ψ</em><sub>soil</sub> treatments, which initiated irrigation when the <em>ψ</em><sub>soil</sub> at 20 cm soil depth and 10 cm distance from a drip emitter reached -25, -50, and -75 kPa, respectively. A control non-irrigation treatment (CK) was also included. Meteorological factors, <em>ψ</em><sub>soil</sub>, groundwater level, and tree growth were monitored hourly, daily, every 1-10 days, and monthly for two years, respectively. Soil water content at 20 and 50 cm depth, pre-dawn leaf water potential (<em>ψ</em><sub>pd</sub>), and trunk sap flow rate were measured in selected periods. Results showed that relative to CK, SDI could improve diameter, height, and individual volume increment of <em>P. tomentosa</em>. Irrespective of SDI treatment, SDI on average increased annual volume growth of the 6- and 7-year-old plantations by 24% and 28%, respectively over the CK treatment. Annual volume growth of the 6-year-old plantation following the -25 kPa treatment reached 39.90 m<sup>3</sup> hm<sup>-2</sup>a<sup>-1</sup>, which was 43.5% higher than the CK treatment (<em>P</em><0.01). Relative to the -50 and -75 kPa treatments, annual volume growth in the -25 kPa treatment was 20% and 31% higher (<em>P</em><0.01) in the 6-year-old plantation, respectively, and 13% and 14% higher (<em>P</em>>0.05) in the 7-year-old plantation, respectively. Relative to CK during the fast growing period (April-July) of <em>P. tomentosa</em>, SDI increased the soil water content at 20 and 50 cm depth by 35% and 27%, respectively; increased average daily trunk sap flow rate (SF<sub>mean</sub>) by 46%; and increased <em>ψ</em><sub>pd</sub> by 41%. The <em>ψ</em><sub>soil</sub> threshold for initiating irrigation had no significant effect (<em>P</em>>0.05) on the SF<sub>mean</sub> and <em>ψ</em><sub>pd</sub> of <em>P. tomentosa</em>. However, among the three <em>ψ</em><sub>soil</sub> treatments, the average SF<sub>mean</sub> and <em>ψ</em><sub>pd</sub> values were highest following the -25 kPa treatment. The increase of soil water availability in the 0-20 cm soil layer (i.e., active rooting zone) was also greatest in the -25 kPa treatment. These results may explain why the -25 kPa treatment had the highest tree growth rate among the three <em>ψ</em><sub>soil</sub> treatments. Overall, the observed increase in soil water content, SF<sub>mean</sub>, and <em>ψ</em><sub>pd</sub> indicate that these are the mechanisms by which SDI significantly improves <em>P. tomentosa</em> tree growth. In conclusion, our findings suggest that SDI should be promoted in the cultivation of <em>P. tomentosa</em> pulpwood plantations. To realize optimum tree growth, irrigation should be initiated when the <em>ψ</em><sub>soil</sub> at 20 cm depth and 10 cm distance from a drip emitter reaches -25 kPa. Furthermore, it is recommended that irrigation should be applied between April and July, and terminated between August and October when planting <em>P. tomentosa</em> in regions similar to our experimental plantation.

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