Nematode-plant interactions in grasslands under restoration management Bart Christiaan Verschoor Promotor: prof. dr. L. Brussaard, hoogleraar Bodembiologie en Biologische Bodemkwaliteit co-promotor: dr. R.G.M. de Goede universitair docent bij de sectie Bodemkwaliteit samenstelling promotiecommissie: prof. dr. ir. J. Bakker (Wageningen Universiteit) prof. dr. J. van Andel (Rijksuniversiteit Groningen) prof. dr. V.K. Brown (Centre for Agri-Environmental Research, University of Reading, UK) dr. ir. W.H. van der Putten (Nederlands Instituut voor Oecologisch Onderzoek, Centrum voor Terrestrische Oecologie) Bart Christiaan Verschoor Nematode-plant interactions in grasslands under restoration management proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, prof dr. ir. L. Speelman, in het openbaar te verdedigen op woensdag 3 oktober 2001 des namiddags te 13.30 uur in de Aula. ISBN: 90-5808-455-8 Cover design: Bart Verschoor, Nanette Dijkman Cover photo: Bart Verschoor Printed by Ponsen & Looijen bv, Wageningen The research presented in this thesis was carried out at the Sub-department of Soil Quality, Department of Environmental Sciences, Wageningen University, P.O. Box 8005, 6700 EC Wageningen, The Netherlands. Abstract Verschoor, B.C. (2001) Nematode-plant interactions in grasslands under restoration management. Ph.D. Thesis, Wageningen University, Wageningen, The Netherlands. Plant-feeding nematodes may have a considerable impact on the rate and direction of plant succession. In this thesis the interactions between plants and plant-feeding nematodes in grasslands under restoration management were studied. In these grasslands, a management of ceasing fertiliser application and annual hay-making resulted in a succession of high- to low- production plant communities. It was hypothesised that a reduced nutrient availability and the development of species-specific nematode communities under plant species will increase the sensitivity of plant species to nematode herbivory. This may result in the replacement of such plant species by plant species that are better adapted to nutrient-poor conditions. The reduction in nutrient supplies resulted in a gradual succession of plants and plant-feeding nematodes. Alterations in the species composition of the plant community, but particularly qualitative changes within each plant species after the cessation of fertiliser application affected the plant-feeding nematode succession. Indications were found that the nematode numbers were positively related to the root nitrogen concentrations. Estimations of nematode consumption in the field indicated that in local hotspots nematodes may have a considerable impact on plant productivity. In experimental studies, however, the effects of nematodes on plant productivity were in general small. Some evidence was found that plant-feeding nematodes can affect the competition between an early- and late-successional plant species in favour of the latter, but I did not find experimental evidence that plants under nutrient-poor growth conditions were more sensitive to nematode herbivory. Neither did I find clear-cut evidence for species-specific suppression of plant species by nematodes. It was suggested, therefore, that in addition to progressing nutrient stress, plant species-specific differences in tolerance to plant-feeding nematodes, rather than host specificity of nematodes, may determine the plant species replacement during reversed succession in grasslands. It is concluded that plant-feeding nematodes are potentially an important biotic factor in the succession of plant communities, but their impact on the succession in grasslands under restoration management has yet to be further elucidated. So far the results suggest that the succession of the plant-feeding nematode community is probably more affected by changes in the plant community than the other way round. Keywords: competition, fertilisation, food quality, grassland, herbivory, nitrogen, nutrients, plant-feeding nematodes, productivity, restoration management, succession, synergism, vegetation Contents Chapter 1 General introduction 9 Chapter 2 Changes in the composition of the plant-feeding nematode community in grasslands after cessation of fertiliser application 16 Chapter 3 Seasonal dynamics and vertical distribution of plant-feeding nematode communities in grasslands 42 Chapter 4 Carbon and nitrogen budgets of plant-feeding nematodes in grasslands of different productivity 64 Chapter 5 Do plant-feeding nematodes have differential effects on the productivity of a fast- and a slow-growing grass species? 80 Chapter 6 Do plant-feeding nematodes affect the competition between grass species during reversed vegetation succession? 96 Chapter 7 Plant-nematode interactions in a reversed succession of high to low-production grasslands: how plants affect nematodes, but do nematodes affect plants? 114 Chapter 8 Host-specificity of plant-feeding nematode species and its relationship to grassland succession 144 Chapter 9 General discussion 162 References 169 Summary 183 Samenvatting 189 Dankwoord 195 Curriculum vitae 197 General Introduction 1. General introduction HERBIVORY AND SUCCESSION Succession refers to the colonisation and development of ecological communities in new primary habitats (primary succession) or to temporal changes in ecological communities following a disturbance (secondary succession). The process of succession has intrigued many ecologists during the last century and incited them to study the underlying mechanisms (Clements, 1916; Olson, 1958; Connell and Slatyer, 1977; Tilman, 1985). The importance of the various suggested mechanisms, however, strongly depends on the type of disturbance, the uniqueness of species and habitats, as well as various historical factors. Tilman (1985), therefore, proposed a resource-ratio hypothesis that was meant to explain general patterns observed in different kinds of succession. According to this hypothesis, succession results from a gradient through time in the relative availabilities of limiting resources. When each species is considered a superior competitor for a particular combination of limiting resources, the species composition will change whenever the relative availability of limiting resources changes. Tilman (1985) developed his model for plant species, but did not include in it herbivore- related processes. Connell and Slatyer (1977) suggested that, in addition to the competitive interactions between plants, interactions with herbivores and pathogens are of critical importance to the course of succession. In the last two decades a considerable amount of evidence of herbivore/pathogen-affected plant succession has been found (Brown, 1985; Edwards and Gillman, 1987; Gibson et al., 1987; Brown and Gange, 1992; Van der Putten et al., 1993; Bach, 1994; Jefferies et al., 1994; Mortimer et al., 1999; Van der Putten, in press). Following up on Tilman’s resource-ratio hypothesis, herbivores and pathogens may affect plant competition by changing the relativeavailabilities of resources. Firstly, herbivores and pathogens can affect plant growth, thereby reducing the resource uptake capacity of damaged plants. Secondly, they can change the rates of nutrient supply by affecting the nutrient 9 Chapter 2 turnover from plant to soil. Moreover, herbivores can stimulate the colonisation of new species by creating gaps in plant communities and may increase plant dispersal by transporting plant seeds (Edwards and Gillman, 1987). PLANT-FEEDING NEMATODES AND SUCCESSION Nematodes have frequently been studied in successional series of different habitats (Wasilewska, 1970; De Goede et al., 1993a,b; Wasilewska, 1994; Hánel, 1995; Armendáriz et al., 1996; Brzeski, 1996; Brinkman et al., 1998), but just recently their possible importance as a biotic factor in successional processes has been recognised (Mortimer et al., 1999). In sand dunes plant-feeding nematodes were supposed to be involved in the degeneration of dominant plant species, such as Hippophaë rhamnoides (Sea buckthorn) and Ammophila arenaria (Marram grass) (Oremus and Otten, 1981; Maas et al., 1983; Van der Putten et al. 1988; Van der Putten et al., 1990; De Rooij-van der Goes, 1995; Zoon, 1995). Furthermore, plant-feeding nematodes might have contributed to small-scale shifts in the vegetation composition of grasslands (Blomqvist et al., 2000; Olff et al., 2000). Such effects of plant- feeding nematodes on vegetation dynamics might have been caused by the direct inhibition of plant growth as a result of their feeding activity. It is generally supposed, however, that nematodes might indirectly affect plant growth by increasing the susceptibility of plants to pathogenic micro-organisms (De Rooij-Van der Goes, 1995). As components of pathogen communities, therefore, plant-feeding nematodes may contribute to vegetation succession (Van der Putten et al., 1993; Van der Putten and Peters, 1997; Van der Putten and Van der Stoel, 1998). NEMATODE HERBIVORY IN GRASSLANDS Belowground herbivores are considered to be major consumers in (semi)-natural grasslands. Scott et al. (1979) calculated that only 2-7% of the total primary production in grasslands is consumed aboveground, whereas this is 7-26% belowground. Furthermore, the amount of plant biomass that is wasted by herbivores during feeding can be considerable. In total, the proportion of primary production that is eaten and wasted by herbivores was calculated to be 3-10% aboveground and 13-41% belowground.