Mycorrhizal mediation of nutrient distribution and stress responses in beech and conifer mixtures
by Michela Audisio
Date of Examination:2024-06-10
Date of issue:2024-10-14
Advisor:Prof. Dr. Andrea Polle
Referee:Prof. Dr. Andrea Polle
Referee:Prof. Dr. Christian Ammer
Referee:Dr. Nathaly Guerrero-Ramirez
Persistent Address:
http://dx.doi.org/10.53846/goediss-10807
Files in this item
Name:phd_thesis_noCV.pdf
Size:5.15Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
With global warming, drought is a rising concern for forests worldwide. In Central Europe, drought has caused severe diebacks of Norway spruce (Picea abies), the most common and economically relevant tree species. Non-native tree species are increasingly introduced in managed forests as a form of adaptive forest management to encounter the challenges imposed by climate change, such as drought. In Central Europe, the North American conifer Douglas-fir (Pseudotsuga menziesii) is considered a valid substitute of Norway spruce. Another recommended forest management strategy to face future climate uncertainties, is to establish mixed forests of broadleaf and coniferous species. Therefore, in Germany, as well as in other parts of Central Europe, Douglas-fir is increasingly planted in combination with the native broadleaf species European beech (Fagus sylvatica). Due to high degree of anthropogenic atmospheric emissions, many parts of Central Europe, in particular Germany, are characterized by nitrogen (N) pollution. Elevated N depositions have been linked to negative impacts on the ectomycorrhizal symbiosis. In temperate forests, the symbiosis with ectomycorrhizal (ECM) fungi is fundamental for the nutrition and growth of trees. In addition to playing key roles in soil nutrient mobilization and carbon cycling, ECM fungi mediate the interactions between trees. For instance, by simultaneously colonizing multiple host trees and forming a common mycorrhizal network (CMN), they might redistribute carbon and other nutrients belowground. However, the ecological consequences of planting Douglas-fir on the ECM symbiosis, such as its mediation of the nutrient distribution in mixed forests with beech, are still largely unknown. Further, to investigate the response of trees and the ectomycorrhizal symbiosis to interacting stress factors, in particular water scarcity and N excess in different tree species combinations is urgently needed to predict which forest type will cope better with future climate change in Central Europe. In order to close these knowledge gaps, I addressed the following research questions: • Q1: Is newly assimilated carbon redistributed in CMNs and transferred belowground from beech to Douglas-fir? • Q2: Are nitrogen (N) and phosphorus (P) translocated from beech to Douglas-fir? • Q3: How do mycorrhizal colonization and root tip vitality vary under the concomitant effect of abiotic factors and species interactions? To examine my first question, I applied 13CO2 pulse-labeling to pairs of beech and Douglas-fir saplings to compare the transfer of newly assimilated carbon from beech (donor) to either beech or Douglas-fir (recipient). After labeling, I measured the 13C enrichment in soil, plant tissues, and fungal-containing tissue and plant transport tissues of the ectomycorrhizal roots. I also identified the ectomycorrhizal fungal species colonizing the roots of all saplings. The main finding was that in recipients, only the fungus-colonized tissue of ectomycorrhizas was significantly enriched in 13C while the plant tissues were not. Douglas-fir recipients shared on average one out of four ectomycorrhizal species with donor beech saplings, and showed lower 13C enrichment than beech recipients, which shared on average three out of six species with donors. This supports the transfer of carbon belowground only among ectomycorrhizal fungi, but not among trees. Moreover, we can speculate that in mixed forests with beech and Douglas-fir, the links for carbon movement might be hampered due to a low degree of mycorrhizal overlap. To investigate whether N and P can be shared by ECM fungi of neighboring tree saplings (Q2), I studied the uptake, allocation and translocation of newly assimilated N and P of beech saplings growing with belowground contact with another beech or a Douglas-fir sapling. Here I applied a single-root labeling method with the stable isotope 15N and the radioactive isotope 33P in artificial soil solution and then traced the two compounds from beech saplings (donors) to either beech or Douglas-fir saplings (recipients). As a result, I found that, after 5 days since labeling, most of the labeled compounds taken up remained in the mycorrhizal roots of donor beech saplings, with only a small fraction of N and P being transported upwards to aboveground plant parts or released into the soil. Yet, these results might be influenced by the experimental approach, which entails severing of the mycorrhizal connections of the labeled root. Therefore, more research is needed to clarify the extent of N and P belowground distribution by mycorrhizal hyphae and translocation between trees. To answer Q3, I investigated mycorrhizal colonization, root tip vitality and biomass of juvenile beech, spruce and Douglas-fir growing in a common garden experiment in monospecific and mixed beech-conifer pots, under four treatments: water reduction, N addition, water reduction + N addition and control. In detail, I predicted the changes of each tree species and their mycorrhizal roots under the different treatments and species combinations in comparison to the control with a Bayesian statistical model. Here I found that mycorrhizal colonization rate was only marginally impacted by any of the treatments. Under water reduction and N addition combined, root tip vitality of beech and spruce showed a decline. Moreover, the negative effects of water reduction and N addition on biomass were mitigated in mixed combinations for beech but not for conifers. These results suggest the presence of interspecific responses of trees to species mixing and water + N availability. Thus, the selection of suitable tree species for future forests should consider the interactive responses of tree species to abiotic factors. The findings of this thesis shed light on the role of ectomycorrhizas as mediators of resource distribution in temperate forest soils. From the results of the common garden experiment, we can speculate that this mediation could be compromised under the effect of environmental changes, specifically with water limitation and N excess in the soil. Overall, this work contributes to improve our understanding of how forests trees and their root symbionts might respond to multiple stress factors likely to affect them in the near future due to climate change and human activities.
Keywords: ectomycorrhizas; climate change; non-native tree species; temperate forests; stable isotopes
