毛乌素沙地优势灌丛对两种松树幼苗的保育作用

选择毛乌素沙地优势灌丛柠条(Caragana intermedia)、紫穗槐(Amorpha fruticosa)和沙柳(Salix psammophila)作为保育植物,选择2年生油松(Pinus tabulaeformis)和樟子松(Pinus sylvestris)幼苗作为目标植物,将其分别种植在开阔地、柠条、紫穗槐和沙柳4种微生境下,连续3a监测油松和樟子松幼苗的存活和生长动态,从而确定毛乌素沙地优势灌丛是否对植物幼苗的定居有促进作用。结果表明:柠条、紫穗槐和沙柳冠层下光照强度、大气温度和土壤温度都显著低于开阔地,致使土壤湿度增高,油松幼苗总存活率均显著高于开阔地,而樟子松幼苗总存活率仅紫穗槐冠层下显著高于开阔地(P0.05)。紫穗槐冠层下油松和樟子松幼苗总存活率均最高(P0.05),外来种紫穗槐可选为保育植物。保育灌丛下松树幼苗的主枝生长良好,并未受到抑制,但第1年主枝生长长度最小。光照强度和土壤湿度是决定主枝生长长度的重要因素。保育灌丛高度和冠幅面积与油松和樟子松幼苗存活率呈显著正相关(P0.05)。固氮灌丛(紫穗槐和柠条)对松树幼苗存活有持续的促进作用,但柠条冠幅结构紧密,可能因为对光的竞争,减弱了促进作用。沙柳灌丛下松树幼苗的存活率较低。油松幼苗具有一定程度的耐阴性,樟子松幼苗喜光性强,保育灌丛下油松幼苗存活率显著高于樟子松幼苗(P0.05)。总的来说,保育灌丛对松树幼苗的定居有促进作用,保育植物技术可作为一种有效的恢复措施,应用在毛乌素沙地的植被恢复中来。但是,保育灌丛的选择要考虑是本土种或外来种,以及灌丛的高度、冠幅大小、冠幅结构和化学效应,恢复物种的选择要考虑其自身的生物学特性及其与保育植物的相互作用。

Nurse effects of dominant shrubs on the seedlings of two Pinus species in the mu us sandy land in Northwest China

Over-grazing, mining, and inappropriate agricultural management practices combined with repeated drought events in the Mu Us sandy land in northern part of Shaanxi, Northwest China, have caused severe desertification. This landscape-scale degradation has significantly declined the possibility of natural ecosystem recovery. These conditions warrant ecological restoration of these areas to allow the re-development of sustainable ecosystems. Seedling establishment is the most crucial stage in a restoration project. In the Mu Us sandy land, restoration efforts might have been largely limited owing to the soil moisture deficiency, temperature fluctuations, high light intensities, low soil fertility, competition, allelopathy, herbivory, and wind. Several restoration techniques have long been used to alleviate these problems; however, their application has been restricted by low seedling and plant survival as well as high cost. Thus, developing novel, low-cost, and efficient restoration techniques is necessary. Plant-plant interaction effects provided by nurse plants can significantly increase or decrease the seed germination rates and establishment of seedlings under their canopies. Therefore, this study primarily aimed to determine the ability of shrubs to be used as a restoration tool in the Mu Us sandy land. Two-year-old seedlings of Pinus tabulaeformis and Pinus sylvestris were planted under the canopies of three shrub species (Caragana intermedia, Amorpha fruticosa, and Salix psammophila) or in open areas with no shrubs as a control; the objective was to determine whether the presence of shrubs facilitated the establishment of pine seedlings, which was assessed three times over three consecutive growing seasons, in the Mu Us sandy land. The height and canopy spatial area of shrubs were measured. The microclimate conditions (light intensity, air and soil temperatures, as well as soil moisture) were recorded in the four microhabitats, that is, under the canopy of each of the three shrubs and in areas without the shrubs (control). The near-surface light intensity as well as air and soil temperatures were lower under the shrubs, which led to higher soil moisture; because of these conditions, the survival rates of P. tabulaeformis seedlings growing under the canopy of the shrub species were higher than those of the seedlings growing in the control area (P < 0.05). However, the survival rates of P. sylvestris seedlings were higher only under the canopy of A. fruticosa (P < 0.05). The survival rates of P. tabulaeformis and P. sylvestris seedlings were the highest under the canopy of A. fruticosa (P < 0.05). Thus, the exotic species, A. fruticosa, could be used successfully as a nurse plant. Increased plant height and canopy area of nurse shrubs might improve the establishment of pine seedlings. The leguminous shrubs (C. intermedia and A. fruticosa) showed continuous facilitation of seedling establishment during moderate drought stress (summers of 2012, 2013, and 2014), but the presence of a dense and small shrub, C. intermedia, reduced the rate of seedling establishment possibly because of the competition for light. In 2012, S. psammophila also facilitated seedling establishment, but an unknown effect led to a high mortality rate for seedlings growing under their canopy. A noteworthy finding was that the growth of the two pine species was not inhibited when they were planted under the three shrubs. Pine seedlings had the shortest leader shoot lengths in 2012, with light intensity and soil moisture as important factors that controlled their length. The shade-tolerant species, P. tabulaeformis, had a higher seedling survival rate than that of P. sylvestris. In conclusion, nurse-shrub facilitation can be used as an effective restoration strategy in the Mu Us sandy land. However, the successful use of shrubs as nurse plants depends on their origin and morphological characteristics and ecological effects; the selection of target species depends on their tolerance of shade and their interaction with nurse plants.