Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment

<div><p>Tree diversity in forests is an important driver of ecological processes including herbivory. Empirical evidence suggests both negative and positive effects of tree diversity on herbivory, which can be, respectively, attributed to associational resistance or associational susceptibility. Tree diversity experiments allow testing for associational effects, but evidence regarding which pattern predominates is mixed. Furthermore, it is unknown if herbivory on tree species of native vs. exotic origin is influenced by changing tree diversity in a similar way, or if exotic tree species escape natural enemies, resulting in lower damage that is unrelated to tree diversity. To address these questions, we established a young tree diversity experiment in temperate southwestern Germany that uses high planting density (49 trees per plot; plot size 13 m²). The species pool consists of six congeneric species pairs of European and North American origin (12 species in total) planted in monocultures and mixtures (1, 2, 4, 6 species). We assessed leaf damage by leaf-chewing insects on more than 5,000 saplings of six broadleaved tree species. Plot-level tree species richness increased leaf damage, which more than doubled from monocultures to six-species mixtures, strongly supporting associational susceptibility. However, leaf damage among congeneric native and exotic species pairs was similar. There were marked differences in patterns of leaf damage across tree genera, and only the genera likely having a predominately generalist herbivore community showed associational susceptibility, irrespective of the geographical origin of a tree species. In conclusion, an increase in tree species richness in young temperate forests may result in associational susceptibility to feeding by generalist herbivores.</p></div

Leaf damage by chewing insects.

<p>(A) Relationship between leaf damage and tree species richness. The solid line illustrates the prediction of a linear model (significant at <i>P</i> < 0.05). (B) Comparison of leaf damage on native European (EU) and exotic North American (NA) trees. The data are log(x+1)-transformed mean damage values per tree individual and presented as violin plots that combine a boxplot with a kernel density plot; medians and means are, respectively, indicated by white dots and horizontal black lines.</p

Network Analysis Reveals Ecological Links between N-Fixing Bacteria and Wood-Decaying Fungi

<div><p>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 <i>nifH</i> (dinitrogenase reductase) genes in dead wood of the common temperate tree species <i>Fagus sylvatica</i> and <i>Picea abies</i> from differently managed forest plots in Germany using molecular tools. By incorporating these genes into a large compilation of published <i>nifH</i> sequences and subsequent phylogenetic analyses of deduced proteins we verified the presence of diverse pools corresponding to functional <i>nifH</i>, almost all of which are new to science. The distribution of <i>nifH</i> 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 <i>nifH</i> 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.</p></div

Mean leaf damage by chewing insects per tree species richness level for the six studied tree species.

<p>Mean leaf damage by chewing insects per tree species richness level for the six studied tree species.</p

Results of the species-specific linear mixed-effect models.

<p>Results of the species-specific linear mixed-effect models.</p

Relationships between tree species richness and mean leaf damage per tree species.

<p>Left and right panels show native European and exotic North American tree species for each genus pair, respectively. Shown are log(x+1) transformed values of mean leaf damage. Regression lines indicate significant relationships at <i>P</i> < 0.05. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0168751#pone.0168751.g002" target="_blank">Fig 2</a> for further explanations.</p

Rank abundance chart displaying the distribution of the 12 most abundant <i>nifH</i> MOTUs derived from the dead wood dataset within the compiled <i>nifH</i> dataset comprising 26,383 sequences.

<p>Only the 200 largest MOTUs are shown due to space limitations. Colored bars indicate dead wood tree species (green, <i>Fagus sylvatica</i>; red, <i>Picea abies</i>). The inserted table lists the best BLASTn hit reference sequences in NCBI Genbank for the same 12 most abundant wood-derived MOTUs from our study.</p

Distribution of the 7,730 <i>nifH</i> MOTUs according to the environments where they have been detected, and whether described as originating from an isolate in GenBank.

<p>168 of the 176 MOTUs derived from this dead wood study have been exclusively identified in wood samples. The integrated heatmap displays proportions of rare sequence types (singletons).</p

Results of the best-performing linear mixed-effect for leaf damage by chewers.

<p>Results of the best-performing linear mixed-effect for leaf damage by chewers.</p

Network organized around 23 <i>nifH</i> MOTUs and 27 fungal species (abbreviations according to the legend).

<p>Fungi (sporocarps) and <i>nifH</i> MOTUs serve as connected nodes, solid lines display co-occurrence patterns (Z-Score < -1.96) and dotted lines avoidance patterns. Edge widths display significance levels from thinnest  =  0.049 to thickest  =  0.0017. Differently shaped and colored nodes/ hubs display taxonomic differences on phylum level and their ecological role in wood decay. Subnetworks are grouped by tree species, and colored background circles indicate affiliations of included taxa to substrate dead wood species (green  =  <i>Fagus</i> affiliated, red  =  <i>Picea</i> affiliated, blue  =  unaffiliated “Generalists”).</p