Decline of acid-sensitive plant species in heathland can be attributed to ammonium toxicity in combination with low pH

The effects of increasing ammonium concentrations in combination with different pH levels were studied on five heathland plant species to determine whether their occurrence and decline could be attributed to ammonium toxicity and/or pH levels. Plants were grown in growth media amended with four different ammonium concentrations (10, 100, 500 and 1000 micromol l(-1)) and two pH levels resembling acidified (pH 3.5 or 4) and weakly buffered (pH 5 or 5.5) situations. Survival of Antennaria dioica and Succisa pratensis was reduced by low pH in combination with high ammonium concentrations. Biomass decreased with increased ammonium concentrations and decreasing pH levels. Internal pH of the plants decreased with increasing ammonium concentrations. Survival of Calluna vulgaris, Deschampsia flexuosa and Gentiana pneumonanthe was not affected by ammonium. Moreover, biomass increased with increasing ammonium concentrations. Biomass production of G. pneumonanthe reduced at low pH levels. A decline of acid-sensitive species in heathlands was attributed to ammonium toxicity effects in combination with a low pH.     

Recovery of South African fynbos vegetation following alien woody plant clearing and fire: implications for restoration

Abstract The recovery of fynbos vegetation after invasion by dense stands of alien trees, and clearing by either ‘burn standing’, ‘fell and burn’, or ‘fell, remove and burn’ treatments, was investigated in two watersheds in the Western Cape Province, South Africa. Native plant density, cover, functional and biological guilds and species richness were compared with matched control sites that were not invaded, but were burnt in the same fires. Species richness was lower for invaded sites compared to controls, at all scales measured (up to 2000m²). Species area curves for invaded sites did not converge with those of controls, indicating that lower richness at smaller scales was not compensated by increased species survival at a larger scale. Indigenous plant density and cover were lower for invaded sites compared to controls. Overall, treatment differences were non‐significant, but the ‘burn standing’ treatment caused the least change to vegetation variables, and the ‘fell, remove and burn’ and ‘fell and burn’ treatments caused greater, similar changes. Changes to the guild structure of the recovering fynbos stands differed among treatments, and indicated that the ‘fell and burn’ treatment had the greatest negative effect on guild survival. In the ‘fell and burn’ treatment, which resulted in an exceptionally intense fire, only non‐mycorrhizal graminoids (predominantly myrmecochores) persisted relatively well. Because of practical problems associated with the ‘burn standing’ and ‘fell, remove and burn’ treatments, managers often have little option but to apply the ‘fell and burn’ treatment. Our results illustrate the dangers of this, and highlight the need for intervention before areas become densely invaded. They also highlight the need for effective biological control agents to reduce rates of spread of aggressively invasive species.     

土壤原生动物对川滇高山栎恢复时间的响应及生长季动态

青藏高原东缘生态环境脆弱, 森林频繁遭到砍伐, 生物多样性受到严重威胁, 森林砍伐后的生态恢复成为研究热点。原生动物在生态恢复中作为指示生物起着重要作用。本文就未砍伐、砍伐后不同恢复时期(20年、10年和1年)生的川滇高山栎(Quercus aquifolioides)林的土壤理化性质和原生动物在生长季的变化进行比较研究, 以探讨生长季不同月份、不同恢复期的原生动物数量变化规律, 分析土壤理化性质与其相关性。结果表明: (1)恢复10年和1年的林地的鞭毛虫数量(193个/g干土, 164个/g干土)显著高于原始林地(22个/g干土), 肉足虫在恢复1年的林地中数量最多(600个/g干土), 纤毛虫数量则随次生演替进程逐渐增多。(2)在生长季不同月份原生动物的数量呈先增加后减少的趋势。鞭毛虫和肉足虫的峰值分别出现在7月和8月, 而纤毛虫的数量在7、8、9月明显高于6月。(3)原生动物数量与土壤理化性质密切相关。鞭毛虫数量与pH值呈显著正相关(P = 0.019), 纤毛虫数量与铵态氮(P = 0.002)和碳氮比呈显著正相关(P = 0.022), 肉足虫数量与硝态氮(P = 0.008)和碳氮比(P = 0.016)呈显著负相关。结果显示, 三种原生动物数量在生长季不同月份表现出较大的波动性, 其数量变化受土壤理化性质等多种因素调控。纤毛虫数量对川滇高山栎林砍伐恢复有正响应, 而鞭毛虫、肉足虫数量有负响应。     

Nutrient limitations in wet, drained and rewetted fen meadows: evaluation of methods and results

Restoration of wet grassland communities on peat soils involves management of nutrient supply and hydrology. The concept of nutrient limitation was discussed as well as its interaction with drainage and rewetting of severely drained peat soils. Different methods of assessing nutrient limitation were compared and the type and extent of nutrient limitation were determined for several wet grassland communities. It was concluded that a full-factorial field fertilisation experiment is the most preferable method. Plant tissue analyses and soil chemical analyses were considered less suitable, although they may provide helpful additional information. Fertilisation experiments in the laboratory using sods or using test plants appear to be the proper means to study mechanisms or processes, but have a restricted predictive value for field situations. Generalising the results, it seems that many relativily undisturbed grassland plant communities on peaty soils are characterised by N limitation. Phosphate limitation for vegetation on peat soils is mainly observed in specific circumstances such as extreme calcium richness, high concentrations of Fe or as a result of drainage or long-term hay cropping. The latter two may also cause K limitation. Rewetting is regarded as a prerequisite in restoring wet grassland communities. Further restoration measures to influence nutrient availability depend on aims of the management and the individual site conditions. This revised version was published online in June 2006 with corrections to the Cover Date.