Importance of litter and fire effects during woody species invasion in Transylvanian dry basiphilous grasslands

草原是世界上物种最丰富的生态系统之一；其中包括发生在罗马尼亚特兰西瓦尼亚平原的干燥的亲基草地。这些草原在人类的不断利用下演变为草甸和牧场。在缺乏管理的情况下，这些草原经历了凋落物的积累和灌木和乔木的入侵，这些都降低了草原的植物物种多样性。特兰西瓦尼亚草原不仅受到该地区原生灌木和树木的入侵，而且还受到非本地木本物种的入侵，这进一步增加了自然保护的关注。入侵灌木和乔木对被入侵植物群落的影响可能通过直接和/或间接的影响来调节，后者部分通过植物凋落物转化。凋落物对不同生态系统种子/幼苗性能、生物量和物种丰富度的影响以负向为主。只有在草地上发现凋落物效应也是积极的。但是，这种模式的效应大小和符号受到凋落物质量和数量差异的进一步调节，并且关于本地和非本地物种凋落物对物种丰富的干基草地入侵性的相互作用的认识很少。不同的反应可能与物理、机械、结构和化学差异有关。当生态系统受到非本地物种的入侵时，其影响在一定程度上也是由非本地物种和本地物种相互作用的结果来调节的。在研究区域，在停止常规管理后，广泛使用火来清除凋落物垫和阻止灌木和树木的入侵，这种模式变得更加复杂。关于火灾对物种入侵影响的研究强调了物种反应的特征差异。此外，火灾频率与木本树种之间似乎没有线性相关。除了火灾事件的干扰影响外，火通过热量和烟雾作为直接的发芽线索。这些影响在美洲等自然易发火灾的生态系统中已经得到了很好的证实，但在特兰西瓦尼亚干燥的亲basibous草原等系统中却没有得到很好的理解。本文将探讨以下工作假设：-本地和非本地木本物种的入侵对草原物种多样性的影响不同-凋落物介导了这一过程，因此凋落物类型，即草地与林地，非本地与本地将对草本植物和木本物种的种子萌发和幼苗建立产生不同的影响。由于高温和化学环境的变化，火灾会进一步加剧凋落物的影响。-灌木的成功种苗建立将取决于草本常住草地物种的竞争。在存在火灾的情况下，居住草地物种的负面（竞争）和积极（保护）效应之间将存在权衡。

Grasslands are among the most species-rich ecosystems worldwide; among them are the dry basiphilousgrasslands occurring in the Transylvanian plain in Romania. These grasslands evolved under continuoushuman utilisation, as meadows and pastures. In the absence of management, these grasslands experienced accumulation of litter and invasion by shrub and trees, which in general both decrease plant species diversityin grasslands. Transylvanian grasslands are not only invaded by bushes and trees native to the region but also by non-native woody species, which further increase the concerns of nature conservancies. Effects of invadingbushes and trees on the invaded plant community might be mediated through direct and/or indirect effects, with the latter being in part translated through plant litter. Effects of litter on seed/seedling performance,biomass and species richness of different ecosystems are predominantly negative. Only in grasslands litter effects were found to be also positive. But the effect size and sign of this pattern is further modulated by differences among litter quality and quantity, and knowledge about the interplay of litter of native and non-native species on invasibility of species-rich dry basiphilous grassland is scarce. Distinct responses might be linked to physical, mechanical, structural and chemical differences. Where ecosystems are invaded by non-native species, their effects are consequently in part also mediated by the outcome of the interplay of non-native and resident species. In the study region this pattern gets more complex by the widespread use of fire to remove litter mats and to stop invasion by bushes and trees after the cessation of regular management.Those studies addressing the effects of fires on species invasion highlight trait-based differences in species response. Furthermore, there seems to be no linear correlation between fire frequency and woody species.Apart from disturbance related effects of fire events, fire acts as a direct germination cue through heat and smoke. These effects are well established for naturally fire prone ecosystems of e.g. the Americas, but lesswell understood in systems like the dry basiphilous grasslands of Transsylvania. The following working hypotheses will be adressed:- Invasion of native and non-native woody species differs in its impact on species diversity of the grasslands - Litter mediates this process and therefore litter type, i.e. grassland vs. woodland, and non-native vs. nativewill show differential effects on seed germination and seedling establishment of herbs as well as woody species. Litter effects will be further accentuated by fire through heat and changes of the chemical environment.- Successful seedling establishment of bushes, will depend on competition by herbaceous resident grassland species. In the presence of fire there will be a trade-off between negative (competitive) and positive (protective) effects of resident grassland species.