The Structure of Chalk Grassland Communities and the Role of Arbuscular Mycorrhizal Fungi Roy Anthony Holm 2011 A thesis submitted in partial fulfilment of the requirements of the University of Brighton for the degree of Doctor of Philosophy School of the Environment and Technology University of Brighton Abstract Semi-natural chalk grassland is an internationally important habitat characterised by high species richness at a fine scale. In both the United Kingdom and other European Countries however, significant areas of chalk grassland have been lost to intensive agriculture practices. In the United Kingdom, conservation and expansion of existing chalk grassland sites has become a high priority. Research that leads to a better understanding of the processes that structure chalk grassland communities may aid these objectives. A number of field trials have been conducted to examine the role of grazing in structuring grassland plant communities, but the role of arbuscular mycorrhizal fungi (AMF)/plant symbiosis has received less attention. In this research project the structure (presence and abundance of species) of chalk grassland communities growing on the South Downs in the United Kingdom is defined. This is achieved by detailed analysis of extensive plant survey data collected in 1991. Analysis revealed strong patterns relating to species presence and abundance in the chalk grassland communities. In particular evidence of „nestedness‟ and a frequency abundance relationship was found. From these patterns it was deduced that AMF/ plant symbiosis may have a significant role in structuring chalk grassland communities. In the experimental component of this research project two separate, but connected, trials were conducted. In the first trial chalk grassland turf was sampled for study in the laboratory under controlled environmental conditions. The second trial was conducted on chalk grassland turf, in situ, at the same site from which the laboratory turf had been collected. In both trials the method adopted was to weaken AMF/plant symbiosis by apply increasing doses of the fungicide Iprodione. In the laboratory trial this was conducted over one growing season and in the field trial over two seasons. There was a good level of consistency between the two trials. At low levels of fungicide there was no discernible change in community structure. But at the higher fungicide dose rates (2gm-2 and 4gm-2), changes to community structure (presence and percentage cover were observed). The results from the field trial suggest that approximately 50% of plants i in the community benefited from the presence of AMF, around 25% were unaffected, whilst the remaining 25% benefited from the absence of AMF. The largest changes in cover were in the mycorrhizal grass Brachypodium pinnatum which declined in percentage cover with increased fungicide dose rates. This was associated with a corresponding increase in the cover of the non-mycorrhizal grass Bromus erectus. Examination of the roots of selected forbs and grasses suggests that in species that benefit from AMF/plant symbiosis the levels of root infection in individual plant species may be related to percentage cover in the community. The research suggests that restoration of species rich chalk grassland communities might be better achieved by a process involving several stages. Initially this would involve creating a community consisting of the most abundant grasses and forbs (core species). These would be established in the presence of a full compliment of mycorrhizal fungi species native to chalk grassland. When this matrix community is established less abundant including scarce species would be added in a sequential process. ii Contents Abstract ............................................................................................................................... i Contents ............................................................................................................................ iii List of Tables ..................................................................................................................... x List of Figures ................................................................................................................. ixii List of Appendicies ........................................................................................................ xvii Acknowledgements ....................................................................................................... ixvii Declaration ....................................................................................................................... xx Chapter 1 - Introduction. ................................................................................................. 1 1.1 General Introduction. ........................................................................................ 1 1.2 Research Rationale ............................................................................................ 2 1.3 Research Aim .................................................................................................... 3 1.4 Thesis Structure ................................................................................................. 4 1.4.1 Chapter 1 - Introduction ....................................................................... 4 1.4.2 Chapter 2 - Literature Review .............................................................. 4 1.4.3 Chapter 3 - The Research Site ............................................................. 6 1.4.4 Chapter 4 - The Structure of Chalk Grassland Communities .............. 6 1.4.5 Chapter 5 - Influence of AMF on Chalk Grassland Community Structure: Laboratory Trials ................................................................. 6 1.4.6 Chapter 6 - Influence of AMF on Chalk Grassland Community Structure: Field Trials .......................................................................... 7 1.4.7 Chapter 7 – Discussion ........................................................................ 7 1.4.8 Chapter 8 – Conclusions ...................................................................... 8 Chapter 2 - Literature Review. ........................................................................................ 9 2.1 Introduction. ...................................................................................................... 9 2.2 The History and Defining Characteristics of Calcareous / Chalk Grassland .. 10 2.2.1 Historical perspective......................................................................... 10 2.2.2 Distribution and importance of calcareous/ chalk grassland ............. 11 2.2.3 Abiotic conditions that affect chalk grassland communities ............. 13 2.2.3.1 Soil pH ............................................................................... 13 2.2.3.2 Nutrient levels .................................................................... 14 2.2.3.3 Site characteristics: Soil depth, aspect, slope, insolation and rainfall ................................................................................ 14 iii 2.2.4 Biotic factors that affect chalk grassland communities ..................... 15 2.2.4.1 Grazing ............................................................................... 16 2.2.4.2 Arbuscular mycorrhizal fungi/plant symbiosis .................. 18 2.3 Plant Community Structure ............................................................................. 18 2.3.1 Definition of plant communities ........................................................ 18 2.3.2 Species richness and diversity ........................................................... 19 2.3.3 Grain structure ................................................................................... 21 2.3.4 Mechanisms contributing to plant community structure ................... 21 2.3.4.1 The competitive exclusion principle .................................. 21 2.3.4.2 The niche concept .............................................................. 22 2.3.4.3 The regeneration niche ....................................................... 23 2.3.4.4 The carousel model ............................................................ 24 2.3.4.5 Mechanisms of plant competion with relevance to chalk grassland ............................................................................ 24 2.4 The Role of Arbuscular Mycorrhizal Fungi (AMF)/Plant Symbiosis in Structuring Plant Communities ....................................................................... 25 2.4.1 Introduction ........................................................................................ 25 2.4.2 The types of mycorrhizal fungi .......................................................... 26 2.4.3 The components and functions of AMF ............................................ 27 2.4.3.1 Spores ................................................................................. 27 2.4.3.2 Hyphae ............................................................................... 27 2.4.3.3 Arbuscules ......................................................................... 27 2.4.3.4 Mycelium ......................................................................... 278 2.4.3.5 Vesicles .................................................................................. 2.4.4 The basis of the symbiotic relationship ............................................. 28 2.4.5 Variable characteristics of AMF/plant symbiosis .............................. 29 2.4.6 Competition for resources