Fungal and Bacterial Diversity Patterns of Two Diversity Levels Retrieved From a Late Decaying Fagus sylvatica Under Two Temperature Regimes

Abstract

Environmental fluctuations are a common occurrence in an ecosystem, which have an impact on organismic diversity and associated ecosystem services. The aim of this study was to investigate how a natural and a species richness-reduced wood decaying community diversity were capable of decomposing Fagus sylvatica dead wood under a constant and a fluctuating temperature regime. Therefore, microcosms with both diversity levels (natural and species richness-reduced) were prepared and incubated for 8 weeks under both temperature regimes. Relative wood mass loss, wood pH, carbon dioxide, and methane emissions, as well as fungal and bacterial community compositions in terms of Simpson's diversity, richness and evenness were investigated. Community interaction patterns and co-occurrence networks were calculated. Community composition was affected by temperature regime and natural diversity caused significantly higher mass loss than richness-reduced diversity. In contrast, richness-reduced diversity increased wood pH. The bacterial community composition was less affected by richness reduction and temperature regimes than the fungal community composition. Microbial interaction patterns showed more mutual exclusions in richness-reduced compared to natural diversity as the reduction mainly reduced abundant fungal species and disintegrated previous interaction patterns. Microbial communities reassembled in richness-reduced diversity with a focus on nitrate reducing and dinitrogen-fixing bacteria as connectors in the network, indicating their high relevance to reestablish ecosystem functions. Therefore, a stochastic richness reduction was followed by functional trait based reassembly to recover previous ecosystem productivity.

Keywords: Fagus sylvatica; bacterial and fungal community composition; dead wood decomposition; fluctuating temperature regime; insurance hypothesis; microbial network analysis.

FIGURE 1

Relative wood mass loss (A) and wood pH (B) from Fagus sylvatica dead wood chips under two temperature regimes (fluctuating, f and constant, c) and two diversity levels (natural, N and richness-reduced, R) over 8 weeks of incubation. Relative wood mass loss was defined as percentage change from the initial wood mass (N₀ and R₀). Correlation coefficient was calculated for each diversity level (N, R) and temperature regime (f, c) and was indicated by a linear regression (see table to the figure). Significant correlation (p ≤ 0.05) is highlighted in bold. See figure legend for symbol of each diversity level and temperature regime.

FIGURE 2

Simpson’s diversity, species richness and evenness for bacterial (A,C,E) and fungal (B,D,F) community structures of Fagus sylvatica dead wood chips under two temperature regimes (fluctuating, f and constant, c) and two diversity levels (natural, N and richness-reduced, R) over 8 weeks of incubation. Correlation coefficient of Simpsons diversity, species richness and evenness of two diversity levels and temperature regimes was indicated by a linear regression (see table to the figure). Significant correlation (p ≤ 0.05) is highlighted in bold.

FIGURE 3

Non-metric multidimensional scaling (NMDS) of the bacterial natural (A), bacterial richness-reduced (B), fungal natural (C), and fungal richness-reduced (D) community composition incubated in two temperature regimes (fluctuating, f ▲ and constant c ). Overview of the NMDS sections (A–D) is comprised separately (E). Community compositions were calculated of the relative OTU abundances after each week over an incubation period of 8 weeks (see color code of the symbols). Initial bacterial or fungal community composition is indicated as 0 (+). NMDS is based on a Bray-Curtis dissimilarity matrix. Further details can be found in the Supplementary Figure S4.

FIGURE 4

Network roles of each OTU of a microbial network analyses for natural (blue, N) and richness-reduced (gray, R) community composition after 8 weeks of incubation. Bacterial OTUs (), fungal OTUs (▲), and environmental parameters (◆) are denoted. Each OTU was categorized into network hubs, module hubs, connectors or peripherals according to Olesen et al. (2007) (see also Table 3 for additional information). Environmental parameters (see Table 2) were included and highlighted by arrows. Module hubs represent strong interactions inside a module, while connectors denote interactions outside a module. Network hubs have strong connections to both, inside and outside a module, while peripherals have neither of both interaction types.

FIGURE 5

Network of natural (left) and richness-reduced (right) diversity after 8 weeks of incubation. Interaction types were represented as mutual exclusion (red lines) and co-presence (green lines) and symbol size indicate the number of interactions (see legend in figure). Bacterial OTUs (), fungal OTUs (▲), and environmental parameters (◆) are denoted. Bacterial and fungal OTUs as well as environmental parameters of the same cluster are indicated in the same color. Additional information to the numbers of selected OTUs can be found in Figure 4 and Table 3.