潮虫消耗木本植物凋落物的可选择性试验

近年来,越来越多的学者关注外来植物入侵对土壤生态功能的影响效应及其相应反馈机制的探索与研究,然而本地原生土壤生物群落对不同入侵程度下的外来植物种以及本地原生植物种之间是否存在消耗差异却尚不明了.通过等足目潮虫的选择性喂养试验来测试10个本地种、5个非入侵性外来种和5个强入侵性外来种之间的适口性差异,试图求证外来植物的入侵性是否与植物落叶被消耗率呈现必然联系.数据分析结果显示潮虫对本地种、非入侵性外来种和入侵性外来种的消耗并无显著差异；而潮虫对不同生活型下木本植物的取食却存在显著差异,即灌木消耗率显著高于乔木.其次,通过植物初始性状指标(包括木质素、纤维素、半纤维素、碳、氮含量)与相应消耗率的相关比较,消耗量总体上与植物凋落物的氮含量呈正相关关系(R2 =0.358).由此,研究结论强调植物落叶的降解速率并不一定与植物入侵性或入侵阶段呈绝对相互关连,但是氮含量,抑或各种形式的植物氮元素成分都可能在一定程度上参与并影响着外来植物的入侵进程.

Woody plant leaf litter consumption by the woodlouse Porcellio scaber with a choice test

A growing number of researchers are investigating the impact of exotic plant invasion on soil ecological processes. This includes the mechanisms of feedback, especially in litter degradation and nutrient cycling. Recent studies indicate that the litter of exotic plants potentially decomposes faster and releases more nutrients if they are nitrogen-fixing plants, or if their leaves have a higher specific leaf area and maintaining higher concentration of nutrients. But slower decomposition of alien species than native species is found when alien plants have higher lignin content, higher C/N ratio and/or unique secondary compounds. However, few studies have concentrated on the comparison between native plant species and different levels of invasive exotic plant such as non-invasive alien species or invasive alien species. In addition, it is not clear whether local soil organism present consumption differences in their litter and food preference. Nor is it clear if generalist decomposers show any associations with the invasion status or particular plant traits. Therefore, we tested leaf litter palatability for ten native plant species, five non-invasive alien species and five invasive alien woody plant species in Switzerland by a selective food-feeding experiment in the laboratory. The generalist consumer used was Porcellio scaber (Isopoda) which is typically employed as a primary generalist decomposer in forest leaf litter decomposition. Each plant species underwent 42 times replications during the test, and the percentage of mass loss determined the consumption rate. Meanwhile, initial traits including lignin, cellulose, hemicellulose, carbon and nitrogen for each plant species were measured for comparison with their consumption rates. Leaf litter compounds were analyzed for the percentage of dry matter and the C/N ratio was calculated afterwards. First and most importantly, our results showed that there are no significant consumption differences (P>0.05) among native, non-invasive alien and invasive alien species as food resources for the woodlice in our study. However, taking the architectural form as a functional group, the consumption rate was significantly higher in shrubs than in trees (P<0.01). However, the comparison of lignin, cellulose, hemicellulose, carbon and nitrogen content among native, non-invasive alien and invasive alien species or between trees and shrubs did not show any significant differences. And over all the traits, only nitrogen concentration of litter was positively correlated with consumption rate (R²=0.358). Our conclusion therefore emphasizes that the consumption rate of woody species is not necessarily associated to the plants'' invasion status. But nitrogen content or any forms of nitrogen composition may play an important role and may affect the biological processes of invasion to in some extent. Functionally speaking, an increasing presence of alien woody plants should not influence the decomposition of plant material and matter flux negatively. Ecosystem dynamics are altered through a variety of interacting, mutually reinforcing mechanistic pathways, and additional studies on the ecosystem-scale impacts of invasions are needed.