Dynamics of savanna ecosystems

Abstract. Theories concerning the factors involved in the dynamics of savannas, particularly the tree-grass interface, are reviewed. Emphasis is put on factors related to soil moisture, soil nutrients, fire and large herbivores. The distinction between external (independent) and internal (dependent, interactive) environment is discussed and it is explained how this distinction is affected by the scale of observation.     

Small-scale plant mobility in a species-rich grassland

Abstract. Plant mobility was studied in a species-rich grassland in S. Moravia (Czech Republic) at scales from 0.0025 to 2.25 m². Cumulative species numbers, cumulative species frequencies and the distribution of distances between sites of occurrence in particular years were established using data collected from 1991 to 1997. The observed values were compared with null models of completely random and restricted random movement of the plants. Most plants persisted at a spot more frequently than expected from the completely random model and with a few exceptions they also spread to neighbouring subplots more often than expected. Cumulative species numbers were between the ranges predicted by the two random models and increased linearly during the 7-yr period at all scales, indicating a large species pool. The role of clonal spreading and of generative reproduction depended on the growth form of the species. I conclude that a high species richness is not necessarily linked with a high plant mobility, even if coexistence of plant species may be promoted by plant mobility.     

基于MODIS黄河三角洲湿地NPP与NDVI相关性的时空变化特征

应用EOS/MODIS卫星遥感资料,分析2001-2006年黄河三角洲地区植被净初级生产NPP及NDVI时空变化特征,在到海岸线不同距离的区域内,讨论了年植被净初级生产力( aNPP)与年均归一化植被指数(ANDVI)的相关性,并建立一元回归分析模型.可以看出,黄河三角洲湿地生态系统分布自海向陆具有明显的空间异质性,滩涂湿地分布于沿海地区,生产力高,该地区aNPP值高且与ANDVI呈显著正相关;其他区域由于是盐碱荒地或农业生态系统,因土壤盐分含量高而系统生产力低,或农耕地受人类干扰严重,一年总的生产力下降,而导致这些区域ANDVI与aNPP呈负相关.利用NDVI进行黄河三角洲生产力监测的可行区域,在距离海岸线小于6_km的范围内,所研究的6a回归模型均通过α=0.01的显著水平的F检验,模型的R2值为0.86-0.97.同时在生产力无法估测的区域,可以使用NDVI有效管理黄河三角洲生态系统,提高其生产力水平.     

Lichen elemental content bioindicators for air quality in upper Midwest,USA: A model for large-scale monitoring

Our development of lichen elemental bioindicators for a United States of America (USA) national monitoring program is a useful model for other large-scale programs. Concentrations of 20 elements were measured, validated, and analyzed for 203 samples of five common lichen species. Collections were made by trained non-specialists near 75 permanent plots and an expert near nine air monitoring sites. Flavoparmelia caperata (most frequent) and Physcia aipolia/stellaris between them represented the full range of local forest cover and pollution load. Evernia mesomorpha (values saturated at intermediate pollution), Parmelia sulcata, and Punctelia rudecta (both difficult for non-specialists) were less useful. Conversion models (GLM or regression) rendered elemental data equivalent between species. Al, Cr, Cu, Fe, Hg, N, and S, plus composite indexes from them, were linked with local air pollution based on correlations with directly measured N and particulate matter as well as from PCA; elements were weakly correlated with modeled pollution estimates. Lichen Hg had no other useful surrogates. Invoking multiple causation and scale-dependence helped address several issues of interpretation, for instance conflicting bioindicator value of Al and Fe from literature.