宽窄行栽植模式下三倍体毛白杨根系分布特征及其与根系吸水的关系

采用剖面法对宽窄行栽植模式下三倍体毛白杨(triploid Populus tomentosa)的根系分布特征进行了研究;采用管式TDR系统对土壤剖面含水率变化动态进行了连续观测,并据此计算林木根系吸水速率,以探讨土壤含水率、根系分布和根系吸水分布之间的相关关系。研究结果表明:毛白杨的总平均根长密度在林带两侧和不同径向距离处非常接近(P>0.05);但在不同土层间变化很大(P<0.01),其中0-20和60-150 cm土层为根系主要分布区域,其根系所占比例共达86%;不同径阶间的根长密度差异显著(P<0.01),且其比例关系会随空间位置的改变而发生变化。不同栽植方位下,林带东侧毛白杨根系分布的浅层化程度高于西侧,且在径向240-280 cm内其0-0.5 mm的极细根显著多于西侧(P<0.05)。因此,宽窄行栽植模式下,深度和径阶是毛白杨根系分布的主要影响因子,而栽植方位会对其形态构型产生影响。毛白杨根系吸水模式受细根分布的影响,但会随土壤剖面水分有效性分布的变化而变化:当表土层水分有效性增加时,根系吸水主要集中在表土层;当表土层水分有效性降低时,深层土壤根系的吸水贡献率会逐渐增加;当土壤剖面水分条件异质性较高时,根系吸水主要集中在根系密度与水分有效性均较高的区域;当土壤剖面水分分布均匀且不存在水分胁迫时,根系吸水分布与细根分布最为一致。

Property of root distribution of triploid Populus tomentosa and its relation to root water uptake under the wide-and-narrow row spacing scheme

Property of root distribution and its relation to spatial and temporal patterns of root water uptake were the basis to establish efficient water management strategies for fast-growing and high-yield triploid Populus tomentosa pulp plantation, which should be fully understood. In this study, the spatial distribution pattern of roots of poplar trees growing under the wide-and-narrow row spacing scheme was studied with profiling method in a 5a triploid P.tomentosa pulp plantation, located in Gaotang county, Shandong province. 1620 soil cores of 2000 cm³ volume were taken out around 8 trees. The water contents of soil profiles were observed consecutively for four months with a time domain reflectometry (TDR). The rates of water uptake by roots were calculated based on field measurements. The relationship among soil water contents, distribution of roots, and patterns of water uptake by roots were analyzed to see how root water uptake responded to fine root (< 2 mm) distribution and changes in soil water availability. The results showed that the average root length densities (RLD) of poplar trees at both sides of planting belts and at different radial distances were close (P>0.05). However, RLD varied significantly among different soil horizons (P<0.01). Roots were mainly distributed in soil layers at depths of 0 20 cm and 60 150 cm. Roots in those two layers accounted for 29% and 57% of the total length density of roots respectively. The RLDs of trees in different root diameter classes were significantly different (P<0.01) and the ratio of roots varied with spatial locations. However, in the root system of tiploid P.tomentosa, about 44% of the roots corresponded to a diameter of 0.2 0.5 mm, followed by 35% of roots of 0 0.2 mm diameter, a 15% of roots 0.5 1 mm and only a 6% of roots greater than 1 mm. Among different orientations, the distribution of roots was shallower at east side than that of west side. The roots of 0 0.5mm within 240 280 cm radial distances at the east side were also more abundant compared with the west side. Therefore, under the wide-and-narrow row spacing planting scheme, the soil depth and root diameter class are two main factors affecting the distribution of roots of poplar trees while planting orientation can influence their morphology and architecture. The water uptake by roots is primarily affected by the spatial distribution of fine roots but will change along with the availability of water in soil profile. When the availability of water increases in top soil, the water uptake by roots mainly happens in top soil. The contribution of roots in deep soil layers will increase when the availability of water in top soil becomes low. If the water condition of a soil profile is highly heterogeneous, the water uptake by roots mainly occurred in zones with high fine roots length densities and availability of water. On the other hand, water uptake is in good agreement with the distribution of fine roots when water spreads evenly in soil profiles and the water stress is not imminent.