Functional diversity of mycorrhiza in relation to land-use changes and ecosystem functions

Managing forest ecosystems has a long tradition in Germany. However, the influence of this intervention on the most important symbiosis partners of the trees, mycorrhizal fungi, is poorly understood. The fungal partner profits from the supply of plant carbohydrates. The plant partner receives nutrients, for example nitrogen (N). In temperate forests, ectomycorrhizas (EM) are the dominant type of this symbiosis on trees. The “plant-economic theory” predicts that trees invest fewer carbohydrates in their EM partners when nutrients are readily available. In addition to EM, fungi with other lifestyles including endophytes, saprophytes or pathogens grew in and on the tree roots, forming a community of root-associated fungi. As information has been very rare until now, it is an important issue to understand how forest management impacts the root-associated fungal community, with special regard on its different fungal lifestyles. The overarching goal of this thesis was to explore the community structures of root-associated fungi with different lifestyles, and specifically to investigate the effects of environmental variables and forest management on these communities. Root disturbance caused by tree harvesting was simulated by cutting roots; the resulting degradation of root litter and recolonization by living roots and associated EM community were observed. Taxonomic and functional diversity, represented by EM hyphal exploration types, were addressed. The Biodiversity Exploratories (Swabian Alb, Hainich Dun and Schorfheide Chorin) with differences in management intensity, tree species composition, climate conditions and soil properties provide an excellent opportunity to address the following hypothesis and research aims: (I) To characterise the root-associated fungal community in temperate forests in Germany. The root-associated fungal community was observed by 454 pyrosequencing on tree roots from forest plots of the Biodiversity Exploratories. The lifestyles of specific fungi were assigned by an intensive literature search. Dominant tree species of the plot was found to have a greater effect on the overall root-associated fungal community than the study region. These differences of the whole fungal community were primarily driven by EMs, as they accounted for more than 60% of the fungi to which a lifestyle could be annotated. In contrast, the saprophytic community, with 20% abundance of all fungi to which a lifestyles could be annotated, was less influenced by dominant tree species than by regional origin. The sequence numbers of other lifestyles such as endophytes or pathogens were underrepresented within the pyrosequencing dataset and could therefore not be used to identify drivers for their community structure. (II) To investigate the “plant-economic theory” on a large scale. Therefore, it was hypothesized that there is a relationship between forest management, root nitrogen and carbohydrate concentrations in roots as well as EM richness, diversity and community structure. (III) Furthermore, it was hypothesized that different abiotic and biotic environmental variables influence the richness, diversity and community of EM and of saprophytes to differing degrees, based on their different lifestyles. Root carbon (C), N, glucose and fructose concentrations were measured. Information on soil properties, like pH or soil C and N concentration, were provided by other working groups from the Biodiversity Exploratories for the same soil samples. Additionally, indices describing the intensity of forest management were available for the same plots. Generalized linear and adaptive models suggest a dynamic interaction between the EM diversity and community to forest management, root N and carbohydrate concentrations. The present study indicates that under low root N concentrations and higher root glucose concentrations EM fungal diversity was enhanced. Root glucose, rather than fructose concentration, was found to be important for EM richness and community structure. The richness of EM was also positively associated with forest management intensity. In intensively utilized forests a lower N concentration in the roots was detected, probably caused by nutrient export via tree harvesting. Furthermore, root glucose concentration increased with forest management intensity perhaps the result of higher light availability for remaining trees. This large scale study had borne out indications from laboratories studies, that glucose appears to be specifically important for EM fungi. In contrast to EM, the diversity of the saprophytic fungi was negatively correlated with the intensity of forest management. Their diversity and richness mainly relied on forest management, on root C and both carobhydrate concentration without specification, as well as on some rare elements. Community differences of saprophtic fungi were fewer dependent on dominant tree species than that of the EM community. (IV) To test whether root litter, resulting for example from tree harvesting, affects EM communities locally. Small scale root disturbance was simulated by severing roots in soil of beech plots in the Hainich Exploratory by a cutting devise. This treatment resulted in patches free from living roots. The degradation of this root litter and the recolonization of those patches were monitored for one year and a half year. For comparison, undisturbed EM communities were also monitored. After a year and a half, the EM community of roots within disturbed patches reached a climax state that was not significantly different from that of undisturbed EM communities anymore. Tis demonstrated on a small scale the high resilience of the EM fungal community against disturbance. Fungi communly present in the undisturbed control soil cores were also those fungi, which were most able to recolonized disturbed patches. Furthermore differences were identified in functionality, represented by exploration types of the EMs during recolonization. Short distance exploration type was identified to preferably recolonize cut patches during the first year, possibly caused by the release of soluble N from the decomposing roots. In conclusion, this study demonstrated that forest management influences root-associated fungal community on a large scale, whereas different lifestyles react in different ways. Forest management had a negative influence on the diversity of saprophytic fungi and a positive on the diversity and richness of EM fungi. This negative influence on the saprophytes could be caused by fewer deadwood in intensively managed forests. This positive influence of forest management on EM fungal community may be due to two main drivers: disturbances by tree harvesting may cause short term changes. In the long term, environmental factors such as removal of nutrients due to biomass removal or higher carbohydrate production, due higher light availability, may have stronger impact on EM community structures than root litter or niche occupation. Our small scale root cutting experiment revealed a high resilience of the EM community to disturbance.