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

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

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

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

Leaf damage per tree species.

<p>All six investigated deciduous tree species are ordered from top to bottom by increasing mean values of log(x+1)-transformed leaf damage. 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