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The Ecology and Nitrogen-Fixing Ability of Selected Aspalathus Spp

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The Ecology and Nitrogen-Fixing Ability of Selected Aspalathus Spp - I - The ecology and nitrogen-fixing ability of ; - selected Aspalathus spp. in fynbos ecosystems. Martin Cocks UniversityThesis presented in fulfillmentof Cape of the Town requirements for the degree of MASTER OF SCIENCE (MSc) in the Department of Botany UNIVERSITY OF CAPE TOWN. March 1994 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town 1 Contents. Abstract. 1 Chapter 1 Introduction. 3 Chapter 2 Literature review. 5 1. Introduction. 5 2. Nutrient cycling in Mediterranean ecosystems with specific reference to nitrogen and phosphorus. 5 2.1. The importance of nitrogen and phosphorus in ecosystem functioning. 5 2.2. Litter production and decomposition. 6 2.3. The role of fire in nutrient cycling in Mediterranean ecosystems. 9 3. Losses of nitrogen and phosphorus due to r.re. 10 4. Input of nitrogen and phosphorus into the ecosystem. 12 4.1. Atmospheric deposition. 1L 4.2. Nitrogen fixation. 13 4.2.1. Non-symbiotic nitrogen fixation in Mediterranean Ecosystems. 13 4.2.2. Symbiotic fixation of nitrogen in Mediterranean ecosystems. 14 5. Methods for measuring nitrogen fiXation. 16 5.1. The acetylene reduction method. 16 5.2. The oiSN natural abundance method. 17 11 6. Legumes in fynbos ecosystems. 22 6.1. The importance of legumes in particular the genus Aspalathus in the Cape Flora. 22 6.2. Legumes and fynbos fire ecology. 23 6.2.1. Fire survival strategies of Aspalathus and 23 other fynbos legumes. 6.2.2. Legumes and pyric succession in fynbos ecosystems. 25 7. Conclusion. 28 Chapter 3 Study sites. 29 1. Introduction. 29 2. Bainskloof. 30 3. Cape Point Nature Reserve. 31 3.1. Cape Point I. 31 3.2. Cape Point II. 32 4. DuToitskloof. 32 5. Groot Hagelkraal. 33 6. J onkershoek. 34 6.1. Jonkershoek I. 34 6.2. Jonkershoek II. 34 7. Orangekloof. 35 8. Pella. 36 9. Scarborough. 36 10. Silvermine. 37 11. Climate. 38 lll Chapter 4. Nodulation in various species of Aspalathus. 40 1. Introduction. 40 2. Methods. 42 2.1. Field data collection. 42 2.1.1. Study sites. 42 2.1.2. Data collection. 42 2.2. Laboratory analysis. 43 2.2.1. Sorting, drying and weighing procedures. 43 2.2.3. Chemical analysis. 44 2.2.3.1. Soil total nitrogen. 44 2.2.3.2. Soil total phosphorus. 45 2.2.3.3. Total soil organic matter content. 46 2.2.3.4. Soil pH. 47 3. Results. 48 3.1 Aspalathus species nodule production. 48 3.1.1. Nodule production of different species in various environments. 48 3.1.2. Seasonal nodule production. 49 3.2. Total soil phosphorus and nitrogen. 51 3.3. Regression analysis of nodulation and six environmental factors. 55 4. Discussion. 59 Chapter 5 Nitrogen fixation in Aspalathus as measured by the o15N natural abundance method. 65 1. Introduction. 65 2. Methods. 67 2.1. Collection and growth of Aspalathus plant samples needed for o15N natural abundance technique of determining of nitrogen fixation. 67 lV 2.1.1. Field grown Aspalathus plants (o 15Nt). 67 2.1.2. Aspalathus plants grown on native soil (ol5No). 68 2.1.3. Aspalathus plants grown in acid-washed sand on nitrogen-free growth medium (ol5Na). 68 2.2. Stable isotope analysis of nitrogen. 69 3. Results. 71 3.1. Average nodule mass to shoot mass ratios. 71 3.2. The otSN values of individual plants. 71 4. Discussion. 76 Chapter 6 Aspalathus species growth, senescence and interaction with vegetation composition. 79 1. Introduction. 79 2. Methods. 81 2.1. Study sites. 81 2.2. Data collection. 81 2.3. Data analysis. 82 2.3.1. Species composition. 82 3. Results. 84 3.1. Contribution of major fynbos families to vegetation of different ages. 84 3.2 The phytomass and viability of stands of Aspalathus plants over one year. 86 3.3. Correlations between total Aspalathus phytomass and various species diversity parameters. 94 4. Discussion. 97 v Chapter 7 Heat triggered germination in Aspalathus seed. 103 1. Introduction. 103 2. Methods. 106 2.1. The collection of seed. 106 2.2 Heat exposure, time duration germination trials. 106 2.3 Seed, seed coat and embryo mass. 108 3. Results. 109 4. Discussion. 117 Chapter 8 General discussion. 121 1. Synthesis. 2. Scope for further research on Aspalathus spp •• References. 127 Acknowledgements. 140 Appendix. 141 1 Abstract. Aspects of the ecology of selected species of Aspalathus spp. (Fabaceae) were investigated to elucidate the role that this genus may play in the nitrogen economy and ecology of fynbos ecosystems. From the literature it is argued that fire plays a pivotal role in these ecosystems as decomposition is very slow. Decomposition returns nutrients trapped in the vegetation back to the soil. However, nitrogen is lost via volatilisation during fires and as deposition of nitrogen in precipitation is low, symbiotic and non-symbiotic nitrogen-fixation may be of importance in replacing lost nitrogen. The influence of soil total nitrogen and phosphorus on the occurrence of dense stands of Aspalathus plants in vegetation of different ages was investigated. Soil of young vegetation containing stands of Aspalathus plants had the same or higher concentration of phosphorus as nearby older vegetation which lacked stands of Aspalathus. Total nitrogen however showed no trends. Nodulation of 15 Aspalathus spp. at 11 study sites showed varying amounts of nodulation although all these species nodulated well under pot culture. Nodulation of three Aspalathus spp. at three seasonal study sites was followed over the course of one year. A. carnosa showed peak nodulation in the late winter months but A. retroflexa and A. abietina displayed no similar trends. Across the three seasonal sites, which had very similar soil, nodulation was positively correlated with soil total phosphorus and negatively correlated with soil total nitrogen in a multiple regression analysis. The ratio of soil total nitrogen to phosphorus was negatively correlated with nodulation. The opposite correlations for soil total nitrogen and phosphorus were found across all 11 study sites. Differences in soil types and organic content may be responsible for this apparent contradiction. Other soil factors such as pH, temperature and moisture were not correlated with nodulation. Total organic content, however, was negatively correlated with nodulation across the geographic survey study sites. The Nitrogen fixation in 13 species of Aspalathus at 10 study sites was determined using the o15N natural abundance technique for determining nitrogen fixation. Instead of using nearby non-fixing species as reference plants, plants of the same species were used. These were grown on their native soil and supplied with oxygen to prevent 2 nodulation. There appeared to be some trend between nitrogen fixation determined in this way and nodulation in the Aspalathus spp. at the various study sites. This method shows promise for future ecological studies as it removes the problem of finding a suitable reference plant. Various vegetational aspects of Aspalathus spp. and Fabaceae were investigated. The Fabaceae where found to be associated with young vegetation as there was a great reduction in the cover of this family when it was compared with nearby older vegetation. This fact was further investigated in the growth and senescence of three Aspalathus species over the course of one year. Two of these species, A. retroflexa and A. abietina, appeared to be very early successional species which die out 1-3 years post-fire while the third, A. camosa appeared to be prominent in later succession. Circumstantial evidence suggested that summer drought may be an important immediate cause of death and senescence in all three species. The dense stands of Aspalathus were also found to affect alpha species diversity. This appeared to be a spatial rather than an overtopping effect which has been observed in previously Proteaceae. An investigation into the effect of heat exposure in stimulating germination in 13 species of Aspalathus and three other Fabaceae species showed temperature between 80 °C and 100 °C to be most effective. Temperatures above 120 °C were lethal in most cases. The thickness of the seed coat relative to the embryo was shown as important in determining the most effective heat exposure for germination. The ecology of Aspalathus spp. and their role in the fynbos is discussed in the light of the results obtained in these studies. 3 Chapter 1 Introduction. Fire plays an important role in cycling nutrients in fynbos ecosystems (Stock & Allsopp 1992). These fires may lead to nitrogen, a scarce element in these environments, being lost via volatilisation. The role of nitrogen-fixing organisms in these environments is therefore of interest, as the replacement of lost nitrogen depends on atmospheric inputs, which are very low in this case, and symbiotic and non-symbiotic nitrogen fixation. It has been suggested that Aspalathus spp. may be of particular importance in replacing lost nitrogen because these plants form large dense stands in fynbos environments after fires. It is hypothesised that they have a competitive advantage over other plants because they are able to fix nitrogen and therefore do not have to compete for the limited supplies present in the soil aftert fire. The argument continues by suggesting that as the vegetation ages, changes in the nutrient status of the soil, specifically in phosphorus, cause the plants to loose this advantage and rapidly give way to other species (Rundel 1983). The aim of this thesis was to investigate aspects of the ecology of Aspalathus spp.
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