氮水平和竞争对互花米草与芦苇叶特征的影响

 互花米草 (Spartina alterniflora) 和芦苇 (Phragmites australis) 是滨海盐沼湿地的多年生草本植物，从世界范围来看，它们二者具有区域性的相互入侵特征，因此研究生境条件对两物种互侵机制的影响是一个十分有意义的生态学命题。该文运用随机区组实验设计方法，模拟海滩环境、构建人工种群、控制可变因子，研究了外来种互花米草与本地种芦苇分别单种和混种时，叶特征对不同氮水平、不同植株密度的响应。结果表明：随着氮水平的升高，互花米草和芦苇的叶面积无论是在单种还是混种情况下都显著增加 (p<0.05)，但混种条件下芦苇的叶面积在高氮水平下增幅减少，这与高氮状况下互花米草与芦苇的竞争加剧有关；氮水平对单种中两种植物的叶数影响最显著 (p<0.01)，对混种中互花米草的叶数和芦苇的叶宽影响最大 (p<0.05)。植株密度增加导致种内和种间竞争加剧，无论在单种还是混种处理下，都造成两种植物叶面积的显著减少 (p<0.05)。单种处理中， 两物种的叶数受密度的响应最显著 (p<0.05)；而混种处理中芦苇对互花米草的竞争显著减小了互花米草的叶宽和叶数(p<0.05)，互花米草对芦苇的竞争则显著减小了芦苇 的叶长、叶宽和叶数 (p<0.05)。两种植物的竞争结果受到氮营养的调控，低、高氮水平下互花米草的种间竞争能力大于芦苇，中氮水平下则是芦苇的种间竞争能力大于互花米草。高氮水平下互花米草通过叶面积的快速增加抑制了芦苇的叶生长，使其叶面积减少，从而在竞争中占据优势，这可能是互花米草入侵我国海滩芦苇种群的机制之一。

EFFECTS OF NITROGEN AVAILABILITY AND COMPETITION ON LEAF CHARACTERISTICS OF SPARTINA ALTERNIFLORA AND PHRAGMITES AUSTRALIS

Aims Spartina alterniflora, originating from North America, has become an invasive species in Europe and China. Meanwhile, Phragmites australis, a species experiencing `die-back’ in Europe, has invaded coastal ecosystems in North America. Each species is invading the other’s native habitat. We studied changes of leaf characters for the two species under different nitrogen and planting densities in the greenhouse to 1) compare the relative competitiveness and invasive capacity of the two species and 2) reveal potential mechanisms that determine successful invasion in different regions. Methods We grew artificial populations of Spartina alterniflora(S) and Phragmites australis (P) at three different densities in monoculture (S, SS, SSS and P, PP, PPP) and mixed-culture (SP, SPP and PSS), and under three levels of nitrogen (0, 60 and 120 mg·kg^-1). Plants were harvested after 15 weeks, and their leaf characteristics, including area, length, width, thickness and number were measured.  Important findings Nitrogen addition increased leaf area in both species whether in monoculture or mixed-culture (p<0.05), but the change in leaf area of P. australis in mixed-culture decreased with high nitrogen level, which may be due to greater interspecific competition from S. alterniflora. In monoculture, the effects of nitrogen addition on leaf number were greater than on the other leaf traits (p<0.01), while the effects on leaf number (S. alterniflora) or leaf width (P. australis) were greatest (p<0.05) in mixed-culture. Plant densities decreased leaf area of the two species in all treatments (p<0.05). In monoculture, the effects of plant densities on leaf number were greatest (p<0.05). However, in mixed-culture, P. australis mainly reduced leaf width and leaf number of S. alterniflora (p<0.05), while S. alterniflora reduced all the parameters of P. australis (p<0.05). The intensity of competition which S. alterniflora imposed on P. australis was greater than the reverse with low and high nitrogen levels, but this outcome was reversed with medium nitrogen level. At high nitrogen levels, S. alterniflora dominated interspecific competition, with its increased leaf area restraining the leaf growth and reducing the leaf area of P. australis; this may be a mechanism for the successful invasion of S. alterniflora into P. australis populations.