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Dynamics in vegetation composition: Plant - Pathogen Interactions

Vries, S. de (2007) Dynamics in vegetation composition: Plant - Pathogen Interactions. Master's Thesis / Essay, Biology.

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Plant community composition is known to be dynamic due to abiotic and biotic factors. However little is known about the interactive effect of above- and belowground herbivores on plant community composition and dynamics, but recently it has become apparent that the interaction of both spatially separated herbivores may be an important factor in determining plant growth and consequently also dynamics in plant community composition. Aboveground herbivores such as cattle can create regeneration niches by trampling and via their dung. Smaller herbivores, such as rabbits and large rodents, commonly burrow, creating other kinds of recruitment opportunities. However belowground herbivores can play an important role in community dynamics as well. Soil-borne organisms can have both positive and negative effects on plant growth. A positive effect occurs when the presence of soil biota increases plant growth or survival. A well-known example of a positive effect of soil biota on plant growth is the arbuscular mycorrhizal fungi growing on roots of plants. A negative effect occurs when the presence of soil-borne pathogens decrease plant growth or plant survival. The most well known example of negative effects comes from agricultural systems. The growth of the same crop on the same field year after year is known to decrease productivity. In natural systems plant growth can also be regulated by the negative feedback of plant specific pathogens. Van der Putten et al. (1997) have shown the importance of soil-borne pathogens during succession in coastal sand dunes. They found that species that were grown in soils from previous and later successional species were reduced most in soils from the later successional species, indicating that species disappear during succession from sites where the soil has become colonised with specific growth-reducing pathogens on the one hand. But, on the other hand, indicating that later successional species are able to grow in sites from which previous successional plants have disappeared. Bever (1994) and Bever et al. (1997) found evidence that the soil community can contribute to the maintenance of diversity within plant communities. Soil biota can be involved in the creation of micro sites that are favoured by different plant species and thus creating certain patchiness in the vegetation, often observed in grasslands. In a long-term exclosure experiment in the Junner Koeland in the Netherlands temporal dynamics and spatial heterogeneity in the plant community were observed as well. In this area temporal plant community dynamics and spatial plant community heterogeneity seem to be increasing at sites primarily grazed by voles, while the community composition in the areas grazed by large herbivores is more constant. These dynamics in the vole grazed parts do not appear to be solely dependent on changes in the abiotic conditions, for when plant species are sensitive to such changes, it is expected that all species show a similar pattern over time and space, which is not the case. In this study I will try to answer the following questions regarding plant population dynamics in grazed and ungrazed grasslands. (1) Can the presence of soil-borne pathogens contribute to temporal and spatial variation in plant species composition in a grassland? (2) Does the presence of cattle inhibit this effect, i.e. are plant community dynamics controlled by soil organisms in the absence of cattle only? Sub questions will be: (la) is the growth of specific plant species negatively related to the cover of that same plant species in the field and (2a) is the variance in response of the plant species less variable in cattle grazed soils compared to vole grazed soils? (3a) Are abiotic circumstances more important for fluctuations in plant growth and differences in plant growth between treatments than soil borne pathogens? To answer these questions I will set up a greenhouse experiment, in which I will study the growth responses of four different dominant plant species (H. lanatus, F. rubra, A. capillaris and R. acetosa). The plants will be grown on 'cattle grazed" and 'vole grazed" soils from a long-term experiment in the Junner Koeland (the Netherlands). Total biomass was not significantly different between cattle and vole-grazed soils, nor were shoot and root biomass. However in general total biomass and root biomass tend to be higher on cattle-grazed soils and shoot biomass tends to be higher on vole-grazed soils. When total biomass derived in the experiment was compared to the abundance of that same species in the field, it showed a positive effect for H. lanatus, F. rubra and A. capillaris, but a negative effect for R. acetosa independent of grazing treatment. For the abiotics, pH was significantly more alkaline in cattle-grazed areas and regression analysis with total biomass mostly showed positive trends (more total biomass in more alkaline soils). The Soil Organic Matter (SOM) content was also higher in cattle-grazed areas although not significantly. Regression analysis showed a negative trend for all species (less total biomass with higher SOM content), except for F. rubra which showed no trend at all. Potential mineralization and nutrient content of nitrate was higher in vole-grazed sites, while ammonium was higher in cattle-grazed sites. Only F. rubra showed a positive trend in total biomass versus potential mineralization, all other species were constant. Finally, soil texture was found to have a large impact on some analysis. Blocks A and B were found to be more sandy, while blocks C, D and E had higher clay content. The block effects were consistent with the partition in texture. Growth of a specific plant species is negatively related to the cover of that same plant species in the field. And also the variance in growth with which the plants respond is less variable in cattle-grazed soils compared to vole-grazed soils. This response in plant growth appears not to be consistent with the variance of nitrate content and potential mineralization, but the nutrient contents and pH are probably very important factors for the dispersal and living conditions for soil-borne bacteria. Finally, the coefficient of variation points in the direction of presence of soil-borne pathogens determining plant growth. Therefore they can contribute to temporal and spatial variation in plant species composition. And since the cattle-grazed soils contain more nutrients and also show a higher biomass without actual grazing during the experiment, it is possible that the presence of cattle inhibits the effect of soilborne pathogens.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Biology
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 07:31
Last Modified: 15 Feb 2018 07:31

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