Water requirements for germination and early seedling establishment in four African savanna woody plant species   总被引：1，自引：0，他引：1  

T.B. Wilson  E.T.F. Witkowski 《Journal of Arid Environments》1998,38(4):541-550

The aim of this study was to determine the water requirements for germination and early seedling establishment of four African savanna tree species, namelyAcacia karroo, A. nilotica, A. tortilisandMundulea sericea. The acacias are characteristic of nutrient-rich, andM. sericeaof nutrient-poor savannas. Imbibition times of scarified seeds ofA. karroo(4 h),M. sericea(6 h) andA. tortilis(8 h) were rapid relative toA. nilotica(28 h) and were inversely correlated with seed size. Imbibed water is lost in about 2 h after drying at 25°C for all species exceptA. nilotica(only 70% moisture loss). Seeds ofA. karrooandM. sericeakept at 40°C dried to below their original moisture contents within 2 h. Seeds ofA. niloticaandA. tortilisdried at 40°C lost viability relative to (undried) control seeds, while there was no significant loss of viability forA. karrooandM. sericea. For fully imbibed seeds to germinate in sandy savanna soils, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungerminated seeds were still viable despite drying-out from a fully imbibed state. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedlings ofA. niloticaandM. sericeato continue growth for a further 5 weeks under greenhouse conditions, althoughA. niloticamostly survived 25% FC with one seedling surviving 12·5% FC. Root penetration was rapid in sandy soils, withA. niloticaandM. sericeaattaining a depth of 40 cm within 15 days. Frequent, but not necessarily high, rainfall appears to be essential for germination and seedling survival over the first 7 weeks.  相似文献   

Rainfall-driven Episodic Flood Events: are they a major factor in moulding New South Wales arid land vegetation patterns?     

Martin E. Westbrooke 《The Australian geographer》2005,36(2):171-181

Episodic high rainfall has been proposed as an important factor in perennial species recruitment but flooding based on rainfall at a distance from the site has received little attention. Although such flood events in arid Australia are rare, studies of the ephemeral Olary Creek indicate that occasional floods can also have a high impact on the vegetation and landscape. During February 1997, a high-rainfall event caused flooding in the Olary Creek. One branch of the creek created a terminal lake within mallee vegetation on Nagaela Station in far-western New South Wales. The flood path of Olary Creek and this terminal lake allow study of the importance of rainfall-driven flood events in shaping vegetation in arid environments. This paper reports (i) the response of arid land plant species to high-rainfall-driven episodic flood events and (ii) how grazing pressure from native and introduced herbivores can impact on native species response.

A systematic study was conducted to understand the botanical composition in flooded and control areas based on 25 m² fenced and unfenced plots subjected to flooding and non-flooding. For 6 years following flooding, species richness in the flooded area was twice that of unflooded areas. In particular, 27 native species from 13 families were recorded both in the enclosed and open plots located in the flooded area. Over the study period nine species: Brachyscome ciliaris, Helichrysum leucopsidium, Vittadinia cuneata, Casuarina pauper, Maireana sedifolia, Salsola kali, Sclerolaena obliquicuspis, Eremophila sturtii and Eucalyptus foecunda germinated only in the flooded enclosed plots. Further, 11 exotic species from five families were recorded in the flooded (both enclosed and open) plots over the study period. Knowledge gained from this study will contribute to management strategies for arid land vegetation.  相似文献   

Morphological evolution of gravel and sand extraction pits, Tromper Wiek, Baltic Sea     

Adam Kubicki  Faustino Manso  Markus Diesing 《Estuarine, Coastal and Shelf Science》2007,71(3-4):647-656

The growing demand for marine mineral resources introduces anthropogenic impacts in the coastal zone, among others also through aggregate dredging. Pits created by anchor hopper dredging may affect local sediment budgets, local hydrodynamics and biological habitats. In this study we investigate the processes and time-scales of pits refilling at two extraction sites over 6 years following cessation of dredging. We focused on the evolution of a single pit at a gravel extraction site and the development of a group of three pits located at a sand extraction site. In the case of the gravel pit, a series of six sonograph and two multibeam surveys were evaluated. We observed a spatial expansion of the edge of the pit, decreasing availability of screened sand in the neighbourhood of the pit (a possible source of refilling), and a slowing down of the refilling process with time. At the sandy pit site a series of seven sonographs and two multibeam surveys were available. We observed a smoothing of the edges and larger mean refilling rates than in the gravel pit case. We conclude that the most effective method for monitoring of pit evolution is to make measurements every six months by simultaneously deploying sidescan sonar and multibeam devices.  相似文献   

东海磷酸盐的分布与再生     

祝陈坚  石晓勇  李铁 《中国海洋大学学报(自然科学版)》1996,(1)

根据1994年海上观测资料讨论东海磷酸盐的分布。指出黑潮水与沿岸水有一明显界面，而黑潮次表层水在此西北部沿陆架涌升并使用平流一扩散模型探讨200～600m层磷酸盐的再生速率的范围和平均逗留时间。  相似文献   

郊区大型居住区邻里关系与社会空间再生——以北京回龙观为例   总被引：2，自引：1，他引：1  

冯健  吴芳芳  周佩玲 《地理科学进展》2017,36(3):367-377

从社会空间的视角研究邻里关系是城市社会地理学的重要课题。本文利用针对北京回龙观居民的问卷调查数据,并结合质性研究方法,探讨了郊区大型居住区的邻里关系及其与郊区社会空间再生的有机联系。作为典型的郊区大型居住区,回龙观的邻里交往较为浅层化但居民具有较高的交往意愿,邻里互助情况较少但具有很强的互助意愿,居民整体责任意识、社区参与和维权意识强烈。入住小区初期的集体采购、日常生活中的维权和文体活动、“童子军”外交和依托社区网的互动方式使居民相互之间维持了较好的邻里关系。邻里关系与居民受教育程度、入住小区时间长度及有无互助经历等因素相关。对社区形成起到更直接作用的是居民在社会生活方面的互动,这种互动基于实体物理空间和虚拟空间提供的平台,以及通过固化居民间的联系而萌生社区文化。回龙观居民在脱离原居住地社会关系网络背景下,通过邻里关系在郊区大型居住区构建了新的社会关系,改善了郊区社会空间的破碎化状况,促使社会空间再生。  相似文献   

Laboratory study of nitrogen and phosphorus remineralization during the decomposition of coastal plankton and seston   总被引：1，自引：0，他引：1  

Jonathan H. Garber 《Estuarine, Coastal and Shelf Science》1984,18(6):685-702

The decomposition of cultured marine phytoplankton (Skeletonema costatum) and natural estuarine seston from Narragansett Bay, RI, was studied at two temperatures (8°C and 18°C) in bottles containing sterile bay-water (30‰) and in bay-water with micro-organisms small enough to pass through a glass fibre filter (nominally < 1μ). About 50% of the particulate organic nitrogen (PON) and particulate phosphorus (PP) was immediately released to the water in dissolved organic forms from both types of organic matter. Comparison of changes in the dissolved organic nitrogen (DON) fraction in the sterile and non-sterile systems indicated that nearly all of the DON initially released was subsequently remineralized. Ammonification proceeded only in non-sterile bay-water. 20–25% of the PP was converted to dissolved inorganic-P (DIP) fraction after only 7 h in both sterile and non-sterile bay-water. Following autolytic releases of DON, DOP and DIP the initial rates of N and P remineralization were temperature dependent: Q₁₀ values for PON and PP decay during first phase of microbially mediated decomposition ranged from 1·3 to 6·4. Rates of remineralization then slowed so that about equal amounts of nutrients were remineralized (45–50% of the N and 57–60% of the P in the phytoplankton and 60–63% of the N and 36–60% of the P in the natural seston) after 30 days storage at either temperature. During 30 days of decomposition in non-sterile seawater the N/P ratios in the dissolved inorganic fractions converged on the ratios of total-N/total-P initially present in the bottles. Kinetic analysis of the decay of total organic-N (TON) and total organic-P (TOP) in the non-sterile systems and analysis of similar sets found in the literature showed that the initial stages of the decomposition of N and P from planktonic POM in vitro could be modelled as the sequential decay, at first-order rates, of two particulate fractions. The first, more labile, fraction comprised about 60% of the particulate N and P. First-order rate constants (−k, base e) for decomposition during the 1st and 2nd phases were 0·02 to 0·2 day⁻¹ and 0·003 to 0·02 day⁻¹, respectively. The decay rates are far too slow to account for the ‘rapid in situ recycling’ of nutrients needed to support phytoplankton production when other means of nutrient resupply (by advection, fixation, rainfall, etc.) are very low.  相似文献   

How Episodic Coral Breakage Can Determine Community Structure: A South African Coral Reef Example     

Bernhard  Riegl Andrea  Riegl 《Marine Ecology》1996,17(1-3):399-410

Abstract. Africa's southernmost coral reefs are situated in Natal Province, South Africa. The Natal coast is exposed to open Ocean swells and episodic storm swell conditions. Benthic communities on these reefs differentiated into three community types: shallow reefs (8–18 m) were dominated by alcyonacean corals and low-growing, massive Scleractinia; intermediate reefs (18–25 m) were dominated chiefly by branching and tabular Scleractinia of the genus Acropora (A. austera, A. clathrata); deep reefs were not dominated by corals but by sponges. Breakage and recovery experiments indicated that the difference in Acropora dominance between shallow and intermediate sites was caused by breakage in high swell conditions. Survival of experimentally produced A. austera fragments was significantly higher in intermediate than in shallow sites, where higher surge made re-attachment and regeneration unlikely. Also, colony morphology was adapted to differential surge conditions: colonies on the shallow reefs were smaller with shorter branches, while on intermediate reefs they were much bigger with long, widely spreading branches. Episodic breakage and low fragment survival due to high water-motion thus excluded branching corals from shallow reef sites.  相似文献   

Decoding dispossession: Eviction and urban regeneration in Johannesburg's dark buildings          下载免费PDF全文

Matthew Wilhelm‐Solomon 《Singapore journal of tropical geography》2016,37(3):378-395

In January 2012 the residents of an inner‐city tenement building in Doornfontein, Johannesburg, were evicted on a court order. The building was situated in a post‐industrial neighbourhood in which thousands of South Africans and foreign nationals, many blind or disabled, live in unlawfully occupied buildings without access to water, basic sanitation, electricity and waste management services. Such buildings are known in policy discourse as ‘bad buildings’, and informally as ‘dark buildings’, invoking both a sense of developmental failure and spiritual insecurity. In this paper I analyse how urban renewal policies created social divisions and alliances not only among the residents of Chambers, which were channelled along nationalist lines, but also between the able‐bodied and disabled, and produced new social alliances. In particular, I document how a group of blind Zimbabweans experienced threats of violence and accusations of betrayal, as they were offered alternate accommodation by the evicting company because of their disability. I argue here that the pressures of private‐sector housing developments intersected with the insecurities and divisions of inner‐city social spaces and also fostered new alliances. Following the work of Deleuze and Guattari, I invoke the concept of ‘decoding dispossession’, proposing that ongoing evictions and dispossessions are characterized by simultaneous movements of ‘decoding and deterritorialization’ and ‘overcoding‐reterritorialization’.  相似文献   

历史街区更新驱动下的旅游绅士化研究 ——以佛山岭南天地为例     

常江  谢涤湘  陈宏胜  陈惠琪 《热带地理》2018,38(4):586-597

综合采用质性和定量方法,从社会阶层、物质环境和消费方式等层面研究佛山岭南天地的旅游绅士化现象,并分析原住民在绅士化过程中的情感特征。结果发现,物质环境改造加速了地方社会阶层的更替,原住民大多被游客、中产阶级和房产投资者所替代,空间利用方式呈现出高度商业化特征。旅游绅士化虽有力地推动了地方的环境改造和经济发展,却衍生出阶层更替、社会排斥和空间隔离等副产品,使传统的本地化生活空间转变为“资本空间”与“消费空间”。城市历史街区改造不同于一般城市更新,街区更新应以地方性为落脚点,尊重原住民的情感与历史记忆,不应简单通过商业化手段推进历史街区更新。  相似文献   

Politics of urban regeneration in Turkey: Possibilities and limits of municipal regeneration initiatives in a highly centralized country     

Tuna Kuyucu 《Urban geography》2018,39(8):1152-1176

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