紫茎泽兰和飞机草的形态、生物量分配和光合特性对氮营养的响应

EFFECTS OF SOIL NITROGEN LEVELS ON MORPHOLOGY, BIOMASS ALLOCATION AND PHOTOSYNTHESIS IN AGERATINA ADENOPHORA AND CHROMOLEANA ODORATA

比较研究了紫茎泽兰(Ageratina adenophora)和飞机草(Chromolaena odorata)的形态、生物量分配、生长和光合特性对氮营养的可塑性反应,探讨其与入侵性的关系。结果表明:1)两种入侵植物对氮营养变化表现出很高的可塑性。随供氮量的增加,两种植物的根冠比、根生物量比降低,叶生物量比(LMR)、叶面积比和叶根比升高。低氮时,增加吸收器官的生物量分配,有利于养分吸收;高氮时,更多的生物量投入同化器官,有利于碳积累。相比之下紫茎泽兰对氮素的适应性更强。2)两种入侵植物偏好较高的氮营养环境,土壤氮含量升高利于紫茎泽兰和飞机草的入侵。在较大的氮范围内,其相对生长速率(RGR)、总生物量、株高、分枝数、叶面积指数、最大净光合速率和光合色素含量都随供氮量的增加而显著增加,过量氮素对上述参数的抑制不显著。在本地种基本停止生长的干季,紫茎泽兰和飞机草仍维持较高的RGR,这与它们的入侵性密切相关。3)在决定RGR对氮营养的响应过程中,平均叶面积比和净同化速率同等重要。LMR对两种植物的RGR有重要的影响,是决定处理间和种间RGR差异的重要因素。随氮素的增加,紫茎泽兰的比叶面积(SLA)降低,飞机草的SLA升高,但在所有氮水平下,前者的SLA都高于后者,紫茎泽兰SLA的变化规律更利于植物适应氮环境。

Nitrogen availability is a major determinant of successional patterns in many ecosystems. Increased levels of soil nitrogen, caused by atmospheric nitrogen deposition, continuously fertilize a large （and growing） portion of the terrestrial biosphere. Increased nitrogen deposition onto natural ecosystems is disadvantageous to slow-growing native plants that have adapted to nutrient-poor habitats by creating environments favorable for faster-growing plants, such as grasses. In this paper, two invasive plant species, Ageratina adenophora and Chromoleana odorata, were studied. Both of them were planted under five soil nitrogen levels for more than four months. By investigating their traits related to morphology, biomass allocation, growth and photosynthesis, we compared their phenotypic responses to nitrogen. Our main objectives were to l ） explore how the two species acclimate to soil nitrogen availability, 2） evaluate which plant traits were associated with the invasiveness of the two species, and 3） determine whether the increased levels of soil nitrogen could facilitate their invasion. The two species were very plastic in their response to nitrogen availability. They exhibited considerable （nitrogen-acclimation abilities. With an increase in nitrogen levels, their root mass ratio and root mass/crown mass decreased, but their leaf mass ratio （LMR ）, leaf area ratio and leaf area to root mass ratio increased. At lower nitrogen levels, more biomass was invested into the root system, a nutrient absorbing organ, which could enhance nutrient-capture ability. At higher nitrogen levels, more biomass was invested into the leaves, an assimilative organ, which could increase their carbon accumulation and improve their competitive abilities. A. adenophortun could acclimate better to nitrogen environments than C. odorata. Tne two invasive plant species could benefit from high nitrogen levels, which were usually excessive and/or harmful for most native species. Under a wide range of nitrogen levels, relative growth rates （ RGR）, total biomass, branch numbers, leaf area index, maximum net photosynthetic rate, chlorophyll and carotenoid content increased significantly with increasing nitrogen levels, and did not decrease significantly at over-optimal nitrogen levels. The two species could maintain relatively higher RGR in the dry season when native plant species almost stopped growing. Having the ability to use resources at times when native plants could not, their competitive abilities and invasiveness were promoted. Mean leaf area ratio （equal to IMR/SIA （specific leaf area）） and net assimilation rate were coequally important in determining the response of RGR to nitrogen levels in A. adenophora and C. odorata. IMR was a very important determinant of RGR, which played the most important role in determining differences in RGR among nitrogen treatments and between species. With an increase in nitrogen levels, the SIA decreased in A. adenophora whereas it increased in C. odorata. But under all nitrogen levels, SLA was higher in A. adenophora than in C. oclorata. The higher SLA of A. adenophora compensated this species for its lower LMR and was favorable to its growth. The response trend of SLA to nitrogen levels in A. adenophora was more profitable than in C. odorata. In conclusion, our results indicated that the two invasive plant species were able to acclimate to a wide range of nitrogen environments and could grow better in higher nitrogen environments, suggesting that enhanced soil nitrogen levels might promote their invasion.