Tiemo Kahl

The influence of climate and fructification on the inter-annual variability of stem growth and net primary productivity in an old-growth, mixed beech forest

The periodic production of large seed crops by trees (masting) and its interaction with stem growth has long been the objective of tree physiology research. However, very little is known about the effects of masting on stem growth and total net primary productivity (NPP) at the stand scale. This study was conducted in an old-growth, mixed deciduous forest dominated by Fagus sylvatica (L.) and covers the period from 2003 to 2007, which comprised wet, dry and regular years as well as two masts of Fagus and one mast of the co-dominant tree species Fraxinus excelsior (L.) and Acer pseudoplatanus (L.). We combined analyses of weather conditions and stem growth at the tree level (inter- and intra-annual) with fruit, stem and leaf production, and estimates of total NPP at the stand level. Finally, we compared the annual demand of carbon for biomass production with net canopy assimilation (NCA), derived from eddy covariance flux measurements, chamber measurements and modelling. Annual stem growth of Fagus was most favoured by warm periods in spring and that of Fraxinus by high precipitation in June. For stem growth of Acer and for fruit production, no significant relationships with mean weather conditions were found. Intra-annual stem growth of all species was strongly reduced when the relative plant-available water in soil dropped below a threshold of about 60% between May and July. The inter-annual variations of NCA, total NPP and leaf NPP at the stand level were low (mean values 1313, 662 and 168 g C m(-2) year(-1), respectively), while wood and fruit production varied more and contrarily (wood: 169-241 g C m(-2) year(-1); fruits: 21-142 g C m(-2) year(-1)). In all years, an annual surplus of newly assimilated carbon was calculated (on average 100 g C m(-2) year(-1)). The results suggest that stem growth is generally not limited by insufficient carbon resources; only in mast years a short-term carbon shortage may occur in spring. In contrast to common assumption, stem growth alone is not a sufficient proxy for total biomass production or the control of carbon sequestration by weather extremes.

Land-use intensification causes multitrophic homogenization of grassland communities.

Land-use intensification is a major driver of biodiversity loss. Alongside reductions in local species diversity, biotic homogenization at larger spatial scales is of great concern for conservation. Biotic homogenization means a decrease in β-diversity (the compositional dissimilarity between sites). Most studies have investigated losses in local (α)-diversity and neglected biodiversity loss at larger spatial scales. Studies addressing β-diversity have focused on single or a few organism groups (for example, ref. 4), and it is thus unknown whether land-use intensification homogenizes communities at different trophic levels, above- and belowground. Here we show that even moderate increases in local land-use intensity (LUI) cause biotic homogenization across microbial, plant and animal groups, both above- and belowground, and that this is largely independent of changes in α-diversity. We analysed a unique grassland biodiversity dataset, with abundances of more than 4,000 species belonging to 12 trophic groups. LUI, and, in particular, high mowing intensity, had consistent effects on β-diversity across groups, causing a homogenization of soil microbial, fungal pathogen, plant and arthropod communities. These effects were nonlinear and the strongest declines in β-diversity occurred in the transition from extensively managed to intermediate intensity grassland. LUI tended to reduce local α-diversity in aboveground groups, whereas the α-diversity increased in belowground groups. Correlations between the β-diversity of different groups, particularly between plants and their consumers, became weaker at high LUI. This suggests a loss of specialist species and is further evidence for biotic homogenization. The consistently negative effects of LUI on landscape-scale biodiversity underscore the high value of extensively managed grasslands for conserving multitrophic biodiversity and ecosystem service provision. Indeed, biotic homogenization rather than local diversity loss could prove to be the most substantial consequence of land-use intensification.

Site adapted admixed tree species reduce drought susceptibility of mature European beech

Some forest-related studies on possible effects of climate change conclude that growth potential of European beech (Fagus sylvatica L.) might be impaired by the predicted increase in future serious drought events during the growing season. Other recent research suggests that not only multiyear increment rates but also growth resistance and recovery of beech during, respectively, after dry years may differ between pure and mixed stands. Thus, we combined dendrochronological investigations and wood stable isotope measurements to further investigate the impact of neighborhood diversity on long-term performance, short-term drought response and soil water availability of European beech in three major geographic regions of Germany. During the last four decades, target trees whose competitive neighborhood consisted of co-occurring species exhibited a superior growth performance compared to beeches in pure stands of the same investigation area. This general pattern was also found in exceptional dry years. Although the summer droughts of 1976 and 2003 predominantly caused stronger relative growth declines if target trees were exposed to interspecific competition, with few exceptions they still formed wider annual rings than beeches growing in close-by monocultures. Within the same study region, recovery of standardized beech target tree radial growth was consistently slower in monospecific stands than in the neighborhood of other competitor species. These findings suggest an improved water availability of beech in mixtures what is in line with the results of the stable isotope analysis. Apparently, the magnitude of competitive complementarity determines the growth response of target beech trees in mixtures. Our investigation strongly suggest that the sensitivity of European beech to environmental constrains depends on neighborhood identity. Therefore, the systematic formation of mixed stands tends to be an appropriate silvicultural measure to mitigate the effects of global warming and droughts on growth patterns of Fagus sylvatica.

Network analysis reveals ecological links between N-fixing bacteria and wood-decaying fungi.

Nitrogen availability in dead wood is highly restricted and associations with N-fixing bacteria are thought to enable wood-decaying fungi to meet their nitrogen requirements for vegetative and generative growth. We assessed the diversity of nifH (dinitrogenase reductase) genes in dead wood of the common temperate tree species Fagus sylvatica and Picea abies from differently managed forest plots in Germany using molecular tools. By incorporating these genes into a large compilation of published nifH sequences and subsequent phylogenetic analyses of deduced proteins we verified the presence of diverse pools corresponding to functional nifH, almost all of which are new to science. The distribution of nifH genes strongly correlated with tree species and decay class, but not with forest management, while higher fungal fructification was correlated with decreasing nitrogen content of the dead wood and positively correlated with nifH diversity, especially during the intermediate stage of wood decay. Network analyses based on non-random species co-occurrence patterns revealed interactions among fungi and N-fixing bacteria in the dead wood and strongly indicate the occurrence of at least commensal relationships between these taxa.

Linking molecular deadwood-inhabiting fungal diversity and community dynamics to ecosystem functions and processes in Central European forests

Fungi play vital roles in the decomposition of deadwood due to their secretion of various enzymes that break down plant cell-wall complexes. The compositions of wood-inhabiting fungal (WIF) communities change over the course of the decomposition process as the remaining mass of wood decreases and both abiotic and biotic conditions of the wood significantly change. It is currently not resolved which substrate-related factors govern these changes in WIF communities and whether such changes influence the deadwood decomposition rate. Here we report a study on fungal richness and community structure in deadwood of Norway spruce and European beech in temperate forest ecosystems using 454 pyrosequencing. Our aims were to disentangle the factors that correspond to WIF community composition and to investigate the links between fungal richness, taxonomically-resolved fungal identity, and microbial-mediated ecosystem functions and processes by analyzing physico-chemical wood properties, lignin-modifying enzyme activities and wood decomposition rates. Unlike fungal richness, we found significant differences in community structure between deadwood of different tree species. The composition of WIF communities was related to the physico-chemical properties of the deadwood substrates. Decomposition rates and the activities of lignin-modifying enzymes were controlled by the succession of the fungal communities and competition scenarios rather than fungal OTU richness. Our results provide further insights into links between fungal community structure and microbial-mediated ecosystem functions and processes.

A pyrosequencing insight into sprawling bacterial diversity and community dynamics in decaying deadwood logs of Fagus sylvatica and Picea abies.

Deadwood is an important biodiversity hotspot in forest ecosystems. While saproxylic insects and wood-inhabiting fungi have been studied extensively, little is known about deadwood-inhabiting bacteria. The study we present is among the first to compare bacterial diversity and community structure of deadwood under field conditions. We therefore compared deadwood logs of two temperate forest tree species Fagus sylvatica and Picea abies using 16S rDNA pyrosequencing to identify changes in bacterial diversity and community structure at different stages of decay in forest plots under different management regimes. Alphaproteobacteria, Acidobacteria and Actinobacteria were the dominant taxonomic groups in both tree species. There were no differences in bacterial OTU richness between deadwood of Fagus sylvatica and Picea abies. Bacteria from the order Rhizobiales became more abundant during the intermediate and advanced stages of decay, accounting for up to 25% of the entire bacterial community in such logs. The most dominant OTU was taxonomically assigned to the genus Methylovirgula, which was recently described in a woodblock experiment of Fagus sylvatica. Besides tree species we were able to demonstrate that deadwood physico-chemical properties, in particular remaining mass, relative wood moisture, pH, and C/N ratio serve as drivers of community composition of deadwood-inhabiting bacteria.

Changes within a single land-use category alter microbial diversity and community structure: Molecular evidence from wood-inhabiting fungi in forest ecosystems

The impact of changes within a single land-use category or land-use intensity on microbial communities is poorly understood, especially with respect to fungi. Here we assessed how forest management regimes and a change in forest type affect the richness and community structure of wood-inhabiting fungi across Germany. We used molecular methods based on the length polymorphism of the internal transcribed spacers and the 5.8S rRNA gene to assess fungal operational taxonomic units (OTUs). A cloning/sequencing approach was used to identify taxonomic affinities of the fungal OTUs. Overall, 20-24% and 25-27% of native fungal OTUs from forest reserves and semi-natural forests became undetectable or were lost in managed and converted forests, respectively. Fungal richness was significantly reduced during a regeneration phase in age-class beech forests with a high level of wood extraction (P = 0.017), whereas fungal community structures were not significantly affected. Conversion of forests from native, deciduous to coniferous species caused significant changes in the fungal community structure (R = 0.64-0.66, P = 0.0001) and could reduce fungal richness (P < 0.05) which may depend on which coniferous species was introduced. Our results showed that Ascocoryne cylichnium, Armillaria sp., Exophiala moniliae, Hyphodontia subalutacea and Fomes fomentarius, all known for wood-decaying abilities were strongly reduced in their abundances when forests were converted from beech to coniferous. We conclude that changes within a single land-use category can be regarded as a major threat to fungal diversity in temperate forest ecosystems.

Dissolved Organic Carbon from European Beech Logs: Patterns of Input to and Retention by Surface Soil

Abstract: The flux of dissolved organic carbon (DOC) from aboveground litter into the soil is generally considered an important pathway for carbon transport. However, the extent to which dead wood, a highly concentrated source of carbon (C), may contribute not only to this flux but also to the accumulation of soil organic carbon (SOC) is still unknown. Here, concentrations and fluxes of DOC in solution beneath 5 logs of Fagus sylvatica were quantified using tension lysimeters. Soil samples beneath and adjacent to an additional 18 logs were analyzed for SOC. Concentrations of stable C isotopes were determined in wood of logs, DOC, and SOC to follow the fate of C from logs to the soil. Mean DOC concentrations in soil solution beneath logs were highly variable and ranged between 11.6 ± 5.8 mg·L-1 (± SD) and 696 ± 654 mg·L-1, while beneath litter without logs the DOC concentrations had an average value of 10 ± 3 mg·L-1. Peak DOC concentrations beneath logs reached 4317 mg·L-1. At 0–20 cm soil depth, SOC concentrations and SOC pools beneath logs were not higher than for control soils. The difference in the composition of stable C isotopes between wood (-25.5 ± 1.0‰) and litter (-28.4 ± 0.2‰) was maintained in DOC and SOC beneath respective substrates. A calculated amount of 20.5 ± 13.6% of the original SOC within 0–20 cm mineral soil was exchanged over a period of 17 ± 8 y by C from logs. However, despite the increased DOC fluxes, SOC pools beneath logs did not increase.

Comparing fungal richness and community composition in coarse woody debris in Central European beech forests under three types of management

Managing forests by selection cutting is a promising silvicultural technique for maintaining forest biodiversity. Despite the importance of fungi in decomposition and nutrient cycling in forest ecosystems, no study to date has investigated the effects of selection cutting on fungal communities, especially using a culture-independent molecular technique to assess more than just the species that are fruiting at the time of sampling. Based on operational taxonomic units (OTUs) found in coarse woody debris, we compared the richness and community composition of wood-inhabiting fungi from selection cutting, age-class, and unmanaged European beech-dominated forests. We found that fungal OTU richness in selection cutting and unmanaged forests was not significantly different (P > 0.05), but it was higher, in both cases, than that in the age-class forest (P = 0.0002). Fungal community composition was not significantly different among the three forest types (P > 0.05). Abundances of common, wood-inhabiting fungal OTUs in different forest types were significantly correlated: the highest and lowest correlations were found between unmanaged forests and selection cutting (ρ = 0.52, P < 0.0001, n = 94), and between unmanaged and age-class forests (ρ = 0.30, P = 0.0080, n = 79), respectively.

First insight into dead wood protistan diversity: a molecular sampling of bright-spored Myxomycetes (Amoebozoa, slime-moulds) in decaying beech logs

Decaying wood hosts a large diversity of seldom investigated protists. Environmental sequencing offers novel insights into communities, but has rarely been applied to saproxylic protists. We investigated the diversity of bright-spored wood-inhabiting Myxomycetes by environmental sequencing. Myxomycetes have a complex life cycle culminating in the formation of mainly macroscopic fruiting bodies, highly variable in shape and colour that are often found on decaying logs. Our hypothesis was that diversity of bright-spored Myxomycetes would increase with decay. DNA was extracted from wood chips collected from 17 beech logs of varying decay stages from the Hainich-Dün region in Central Germany. We obtained 260 partial small subunit ribosomal RNA gene sequences of bright-spored Myxomycetes that were assembled into 29 OTUs, of which 65% were less than 98% similar to those in the existing database. The OTU richness revealed by molecular analysis surpassed that of a parallel inventory of fruiting bodies. We tested several environmental variables and identified pH, rather than decay stage, as the main structuring factor of myxomycete distribution.