Influence of mycorrhiza on nutrient physiology of trees in mixed and mono-specific stands along climatic and land use gradients
von Likulunga Emmanuel Likulunga
Datum der mündl. Prüfung:2022-05-16
Betreuer:Prof. Dr. Andrea Polle
Gutachter:Prof. Dr. Andrea Polle
Gutachter:Prof. Dr. Christian Ammer
EnglischGlobal change drivers such as climate change, land-use change, anthropogenic N deposition, deteriorating P nutrition and other calamities jeopardize temperate forest ecosystem functions. Essential components for the maintenance of ecosystem functions are fine roots and soil microbial communities because they are important for nutrient provision and water uptake of trees. Fungi contribute substantially to nutrient cycling since saprotrophic fungi drive the decomposition of organic material, while symbiotrophic fungi, mainly ectomycorrhizal fungi (EMF) facilitate nutrient uptake through symbiotic associations with tree roots. Understanding factors that influence fine root biomass and soil- or root-associated microbes is highly relevant for sustainable forest management under global change. To enhance knowledge on belowground functions, the responses of roots and fungi to seasonal changes in nutrient input by litter, nutrient input by deposition and different climatic conditions have to be investigated in different forest types. The main forest tree species in Germany, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies, Karst.) are susceptible to drought. Since drought periods are expected to increase in a future climate, the cultivation of more tolerant, non-native species such as Douglas-fir (Pseudotsuga menziesii) may be an option for future forest management. Yet, little is known about the influence of Douglas-fir on local soil fungal composition and diversity in pure and beech mixed tree stands. In this study, our first goal was to investigate which factors contribute to seasonal trends of European beech fine root biomass along a climatic and soil P gradient. Field experiments were carried out in study sites characterized by low, medium and high soil P content. The study sites comprise beech dominated plots unfertilized or fertilized with either N, P or combined N and P. These study sites were also used to investigate the impact of nutrient dynamics on EMF composition and diversity as our second research goal. Fine root biomass, soil and root elements were analyzed from soil cores collected in spring and fall. We found an increase in fine root biomass only at the P-low forest site in fall under P fertilization. Higher fine root biomass was observed in fall than in spring only at the P-medium and P-high forest sites contrary to the P-low forest site. We constructed a general model incorporating all measured variables [site (P-low, P-medium, P-high), fertilizer treatment (unfertilized, N, P, P+N), season (spring, fall), climate (temperature, precipitation), soil and root elements (total and soluble P, C, N, C/N ratio, K, Na, Ca, Mg, Mg, Fe, S), soil pH and water content)] and subjected it to stepwise regression. The variables retained by the model with lowest Akaike Information Criterion were used to partition the variance of fine root biomass. Soil chemistry, including soil water content, explained a high fraction of variation of fine root biomass in both organic layer and mineral soil while climate explained high proportion of root biomass variation in the mineral soil. The seasonal patterns in fine root biomass were therefore, attributed to changes in nutrient dynamics and climate but dependent on soil layer and site. Little biomass variation was explained by root resources in the organic layer, suggesting that in the organic layer soil resources dominate because of high biological activity and microbial interactions. We further analyzed the impact of nutrient dynamics on EMF composition and diversity at these study sites by morphotyping and ITS sequencing. Our results revealed that neither EMF composition nor richness and diversity were influenced by fertilization treatment. This was contrary to our expectation and implies that EMF show stability in response to moderate nutrient additions. This finding supports the notion of EMF relative resistance to N addition in beech forests. However, when analyzing fungi by DNA barcoding at the levels of orders, our collaborative study uncovered negative and positive responses of Russulales and Boletales, respectively, following P and P+N addition. Across all sites studied, we found that Russulales were enriched in soils with high N content. These results imply that fungal structures are driven by nutrient availability. To address our third goal, the impact of introduction of Douglas-fir either in pure stands or in mixture with beech on soil fungal structures, field experiments were conducted in a dry and nutrient poor region and in a humid and nutrient rich region. In each region, four sites were used, which contained plots with either pure (beech, spruce, Douglas-fir) or mixed beech-conifer stands (beech-spruce, beech-Douglas-fir). Soil fungal community analyses (0-10 cm depth after removal of non-decomposed litter) were conducted by barcoding of the ITS region and Illumina sequencing. Soil elements (C, N, P, K, Na, Ca, Mg, Mn, Fe, S), C/N ratio, soil pH and soil relative water content were determined and used as explanatory variables for fungal composition. Our results showed distinct differences in fungal assemblages between nutrient-rich and nutrient-poor sites and among the stand types. Fungal communities were separated according tree species composition and explained by soil chemistry. Fungal compositions in spruce and Douglas-fir forests were similar. Intermediate fungal compositions between pure beech and pure spruce (or pure Douglas-fir) stands were observed in mixed beech-conifer tree stands. Forest types did not influence mycorrhizal species richness. However, the relative abundance of symbiotrophs decreased in pure Douglas-fir and beech-Douglas-fir mixtures compared with native species. The relative abundance of the saprotrophic fungal orders Tremellales and Hymenochaetales increased in conifer tree stands while the abundance of other fungal orders was dependent on tree species composition and site properties. The similarity of fungal assemblages observed in non-native Douglas-fir and native Norway spruce implies that Douglas-fir can accommodate native fungi. However, pure Douglas-fir and mixed beech-Douglas-fir favored the relative abundance of saprotrophic fungi. Whether the shifted mycorrhiza/saprotroph ratio affects nutrient turnover, remains to be studied in the future. Overall, our study shows that responses of fungi and root biomass are mainly climate and nutrient dependent. The seasonal cycling of fine root biomass between fall and spring (i.e. higher in spring than fall in organic layer at the P-low site and vice versa at P-high site) indicates plasticity of fine root biomass to nutrient availability. Apparently, beech can adjust its root system to cope with changes in nutrients. Whether the seasonal response of beech fine root biomass observed along climatic and soil P gradient in our study can be extrapolated to beech-conifer tree stands needs investigation. This is because in beech-conifer mixed stands, changes in nutrients attributable to different litter inputs as well as changes in microbial communities are expected. Moreover, we observed differences among fungal communities in pure beech and conifer stands and intermediate pattern in mixed beech-conifer stands in the top layer of the forest floor. How the tree species will influence fungal communities in deeper soil horizons due to variations in nutrient availability and microbial activity is yet unknown. The relative stability responses of EMF to nutrient additions implies that beech forests can still rely on microbial interactions for nutrient uptake but more information is needed how environmental fluctuation and anthropogenic deposition, different types of litter input and different root distribution influence forest nutrition. Our study highlights that root biomass, soil and root fungi are important for tree adaptation to changing environmental conditions, contributing to knowledge on sustainable forest management.
Keywords: Phosphorus and nitrogen; Pure and mixed tree stands; Climate; Tree root biomass; Mycorrhiza