地下滴灌下土壤水势对毛白杨纸浆林生长及生理特性的影响

Effects of soil water potential on the growth and physiological characteristics of Populus tomentosa pulpwood plantation under subsurface drip irrigation

在6、7年生三倍体毛白杨(triploid Populus tomentosa)纸浆林中研究了地下滴灌(SDI)下不同土壤水势(-25、-50、-75kPa,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 m3hm-2a-1,较CK极显著提高43.5%(P<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%(P<0.01),在7年生时分别提高13%和14%(P>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%,SFmean)、黎明前叶水势(41%,ψpd)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SFmean和ψpd无显著影响(P>0.05)。3个水势处理中,-25 kPa的平均SFmean和ψpd均最高,且其能使根系活动剧烈的表土层(0—20cm)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 Populus tomentosa pulpwood plantations. In order to efficiently apply SDI in the cultivation of P. tomentosa pulpwood plantations, it is necessary to determine irrigation timing. A field experiment was conducted to investigate the effect of soil water potential （ψsoil） on the growth and physiological characteristics of 6--7 year old trees under SDI in a P. tomentosa pulpwood plantation. The experiment included three ψsoil treatments, which initiated irrigation when the ψsoil 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, ψsoil, 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 （ψpd）, 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 P. tomentosa. 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^3 hm^-2a^-1 , which was 43.5% higher than the CK treatment （P〈0.01）. Relative to the -50 and -75 kPa treatments, annual volume growth in the -25 kPa treatment was 20% and 31% higher （P〈0.01） in the 6-year-old plantation, respectively, and 13% and 14% higher （P〉0.05） in the 7-year-old plantation, respectively. Relative to CK during the fast growing period （April--July） of P. tomentosa, 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 ） by 46% ; and increased ψpd by 41%. The ψsoil threshold for initiating irrigation had no significant effect （ P 〉 0.05 ） on the SFmean and ψpd of P. tomentosa. However, among the three ψsoil treatments, the average SFmean and ψpd 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 ψsoil treatments. Overall, the observed increase in soil water content, SFmean and ψpd indicate that these are the mechanisms by which SDI significantly improves P. tomentosa tree growth. In conclusion, our findings suggest that SDI should be promoted in the cultivation of P. tomentosa pulpwood plantations. To realize optimum tree growth, irrigation should be initiated when the ψsoil 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 P. tomentosa in regions similar to our experimental plantation.