Isolated Pasture Trees and the Vegetation under their Canopies in the Chiapas Coastal Plain,Mexico1

The Coastal Plain of Chiapas (southern Mexico) was formerly covered by large tracts of subhumid tropical forests but is heavily deforested at present. In this region, 15 pastures were selected to characterize species composition of isolated trees, as well as to describe species composition, growth form patterns, and patterns of dispersal units in the vegetation growing under their canopies. The 65 recorded pasture trees belonged to 20 species and 11 families, of which Fabaceae and Moraceae were the most species-rich. Coccoloba barbadensis (Polygonaceae) and Enterolobium cyclocarpum (Fabaceae) were the most abundant and frequent trees in the studied pastures. More than half (55%) of isolated tree species were fleshy-fruited. In the vegetation sampled under pasture trees, 134 species and 45 families were found. Fabaceae and Poaceae had the largest numbers of species. Herbs were the predominant growth form (46.3%), followed by shrubs (23.9%), trees (23.1%), and lianas (2.2%); 6 species could nor be placed in any growth form category. Most species of this flora were fleshy-fruited (43.3%), followed by heavy, gravity-dispersed fruits (17.9%). The analysis of dispersal units by growth form category confirmed the prevalence of fleshy fruits, although their predominance was not so obvious among herb species. Almost half (49.2%) of the flora under pasture trees was typical of secondary vegetation; this pattern was true for herbs but not for most woody species, which were typical of primary vegetation. A numeric classification of the vegetation samples taken under pasture trees produced eight floristic groups, all of which were independent of the specific identity of pasture trees. No significant effect of dispersal unit type of pasture tree on the characteristics of the vegetation growing under them was found. Future attempts to re-create the original forest cover using isolated trees in pastures as regeneration foci should pay more attention to the maintenance of a large specific diversity independently of the dispersal types among these components of tropical landscapes.     

Early Plant Succession in Abandoned Pastures in Ecuador1

We compared early plant succession in four abandoned pastures of differing age since abandonment and a nearby secondary forest site in northwestern Ecuador. Two “Open” pastures had no tree canopy covering, and two “Guava” pastures had a well-developed canopy cover of Psidium guajava. No site had been seeded with pasture grasses. All pastures were compared in a chronological sequence; two were monitored for 18 months. Species richness was consistently higher in Guava sites than in Open sites and it continued to increase over time, whereas it remained static in Open sites. Species richness was highest in secondary forest. Recruitment of tree saplings in Guava sites was lower than in secondary forest; however, it was nearly absent in Open sites. The seed bank contained predominantly herbaceous species at all sites, and was highly dissimilar to aboveground vegetation. Dominance-diversity curves for Guava sites showed a more equitable distribution of species that increased over time. In contrast, dominance-diversity curves for Open sites were static and indicated dominance by a few aggressive species. Soil characteristics among sites were variable; however, a principal components analysis on soils isolated the older Open site from all others. The older Open site had the lowest species richness and was dominated by Baccharis trinervis, an aggressive shrub species. The site appears to be in a state of arrested succession and some form of restorative intervention may be necessary to initiate succession toward a forested condition. Succession in Guava sites appears headed toward secondary forest, whereas it does not in Open sites.     

Agrin and acetylcholine receptors are removed from abandoned synaptic sites at reinnervated frog neuromuscular junctions

Changes in the distribution of agrin and acetylcholine receptors (AChRs) were examined during reinnervation and following permanent denervation as a means of understanding mechanisms controlling the distribution of these molecules. Following nerve damage in the peripheral nervous system, regenerating nerve terminals preferentially return to previous synaptic sites leading to the restoration of synaptic activity. However, not all portions of original synaptic sites are reoccupied: Some of the synaptic sites are abandoned by both the nerve terminal and the Schwann cell. Abandoned synaptic sites contain agrin, AChRs, and acetylcholinesterase (AChE) without an overlying nerve terminal or Schwann cell providing a unique location to observe changes in the distribution of these synapse-specific molecules. The distribution of anti-agrin and AChR staining at abandoned synaptic sites was altered during the process of reinnervation, changing from a dense, wide distribution to a punctate, pale pattern, and finally becoming entirely absent. Agrin and AChRs were removed from abandoned synaptic sites in reinnervated frog neuromuscular junctions, while in contralateral muscles which were permanently denervated, anti-agrin and AChR staining remained at abandoned synaptic sites. Decreasing synaptic activity during reinnervation delayed the removal of agrin and AChRs from abandoned synaptic sites. Altogether, these results support the hypothesis that synaptic activity controls a cellular mechanism that directs the removal of agrin from synaptic basal lamina and the loss of agrin leads to the dispersal of AChRs. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 999–1018, 1997     

黄土丘陵区撂荒群落演替序列种根系对氮素施肥方式和水平的形态响应

采用盆栽试验,研究了黄土丘陵区撂荒群落演替序列种(即,黄土丘陵区摞荒群落演替主要阶段的优势种)根系对氮素施肥方式和水平的形态响应,对了解我国氮沉降增加背景下的群落生态效应及人为施肥干扰促进植被恢复具有较好的理论和实践意义。测试并分析了6个演替序列种在不同施氮方式(匀质和异质施氮)和水平(高、低和无氮对照)条件下植株个体生物量指标(地上及地下生物量和根冠比)、根系形态指标(根长、直径、表面积、比根长和比表面积)的变化及其差异显著性;并且利用根钻法和单样本T检验比较了异质施氮方式下施氮斑块与不施氮斑块根系形态指标的差异。结果表明:1)6种演替序列种地上、地下生物量和根冠比存在种间固有差别,施氮方式和水平整体上对三者无显著影响;施氮方式和植物种类对根冠比存在显著交互作用,说明个别种的根冠比对施氮方式响应明显,其中猪毛蒿根冠比在异质施氮方式下显著高于匀质施氮。2)6种演替序列种根系塑形指标包括比根长、比表面积和直径存在种间差别,并且施氮水平对比根长影响显著,高、低施氮水平下比根长都显著低于不施氮对照。3)狗尾草和铁杆蒿分别在异质高氮和异质低氮条件下施氮斑块根系生物量密度显著高于未施氮斑块;猪毛蒿在异质高氮条件下施氮斑块发生了更多的伸长生长,其根长、根表面积、比根长和比表面积在施氮斑块中的密度显著高于未施氮斑块;猪毛蒿和狗尾草在异质高氮条件下,以及白羊草在异质低氮条件下,其根系直径在施氮斑块显著小于未施氮斑块。从根系形态变化敏感性和施氮对促进植物生长来看,演替过程中演替序列种对施氮响应的敏感性总体上呈降低趋势,前期种对施氮响应更敏感,从施氮获利也更多,因而恢复前期进行人为干扰促进植被恢复效果也会更好。     

采石场废弃地的早期植被与土壤种子库

调查了广州市白云山地区一个弃置 2 a的大型采石场的排土场、采石坑以及原用作破碎加工石料和成品贮运的平台等的土壤种子库和自然植被。这些废弃地的基质毫无结构 ,质地亦极端 ,排土场 60 %的颗粒直径小于 1 mm,碎石平台则主要由散落的碎石、石粉组成 ,粒径大于 5 mm占 5 0 %以上。3种类型废弃地的养分元素含量都很低 ,水溶性氮和磷含量分别为 0 .74～ 1 .43mg· kg- 1和 0 .0 32～ 0 .0 5 3mg· kg- 1。 3种废弃地在植被自然发生的程度及土壤种子库种子含量上也差异很大。排土场表面极不稳定 ,大大小小水蚀沟分布其上 ,水蚀和水土流失严重 ,基本上没有植物定居 ,土壤种子库接近零 (所有样品只检测到两种植物各 1株幼苗 ,换算成种子库的大小约当 33粒· m- 2 ) ;采石坑废弃地的自然植被较好 ,发展成以湿生植物为主的郁闭草地 ,边坡较为干旱而以禾草类为主 ,盖度亦达 1 0 0 % ,水环境的多样性促进了植物种类的多样性 ,计有 1 3科 30属 31种植物 ,土壤种子库的种子含量为 1 93± 46粒·m- 2 ,有 6个种 ;碎石贮运平台废弃地由于经车辆的反复辗压 ,表面异常坚实 ,废弃后早期只有在周边较疏松处及偶然出现的裂缝中有植物生长 ,总植被盖度小于 1 0 % ,种类也较少 ,只有 4科 9属 9种 ,然而其土壤种子库却是最