Tree species identity, canopy structure and prey availability differentially affect canopy spider diversity and trophic composition

Abstract

Forest canopies maintain a high proportion of arthropod diversity. The drivers that structure these communities, however, are poorly understood. Therefore, integrative research connecting tree species identity and environmental stand properties with taxonomic and functional community composition of canopy arthropods is required. In this study, we investigated how the taxonomic, functional and trophic composition of arboreal spider communities is affected by tree species composition and associated differences in canopy structure and prey availability in temperate forests. We sampled canopy spiders as well as their potential prey using insecticidal fogging in monospecific and mixed stands of native European beech, native Norway spruce and non-native Douglas fir. Trophic metrics were obtained from stable isotope analysis and structural canopy properties were assessed with mobile laser scanning. Monospecific native spruce stands promoted local canopy spider abundance and diversity, but native beech and beech–conifer mixtures had the highest diversity at landscape scale. Spider community composition differed between monospecific stands, with broadleaf–conifer mixtures mitigating these differences. Irrespective of tree species identity, spider abundance, taxonomic diversity, functional richness and isotopic richness increased in structurally heterogeneous canopies with high prey abundances, but functional evenness and trophic divergence decreased. Our study shows that canopy spiders are differentially affected by tree species identity, canopy structure and prey availability. Broadleaf–conifer mixtures mitigated negative effects of (non-native) conifers, but positive mixture effects were only evident at the landscape scale. Structurally heterogeneous canopies promoted the dominance of only specific trait clusters. This indicates that intermediate heterogeneity might result in high stability of ecological communities.

Metabarcoding reveals that mixed forests mitigate negative effects of non‐native trees on canopy arthropod diversity

“…This supports the finding of Roques et al (2006) who reported that in Europe mostly polyphagous arthropods feed on nonnative Douglas fir. Notably, arthropod communities in monospecific conifer stands were promoted by canopy openness, corroborating that stand structural properties are major drivers of arthropod communities in coniferous forests (Kriegel et al, 2021;Wildermuth, Dönges, et al, 2023). This positive effect of openness in conifers on taxon richness and the abovereported negative effect of lacking microhabitats in Douglas fir on taxon richness support our fourth hypothesis that structural habitat conditions imposed by the tree species contribute to explaining observed diversity patterns.…”

“…A possible explanation for the lack of such an effect in our study might be that the isolation effects of trees in admixture with phylogenetically distant tree species reduce the number of associated specialist arthropods (Gossner et al, 2009; Seifert et al, 2020; Yguel et al, 2011). We conclude that broadleaf–conifer mixtures do not necessarily promote associated diversity, but can buffer negative tree‐species identity effects of economically attractive (non‐native) conifers (Budde et al, 2011; Leidinger et al, 2021; Wildermuth, Dönges, et al, 2023). A recent study, however, showed that, over a 14 year‐timespan, ground beetle diversity was increasing in temperate broadleaf–conifer mixtures, but decreasing in coniferous monocultures, indicating the positive long‐term effects of such mixtures (Evans et al, 2022).…”

“…In forests as well as in many other ecosystems, arthropods play essential roles in trophic interactions such as herbivory, decomposition, or predation (Wildermuth, Dönges, et al, 2023; Wildermuth, Fardiansah, et al, 2023; Yang & Gratton, 2014). Arthropod composition (taxonomic and functional) therefore strongly influences forest functioning and integrity (Schowalter, 2017).…”

Linking small mammal capture probability with understory structural complexity using a mobile laser scanning‐derived metric: A case study

“…This flexibility also allows the MLS user to approach and circumnavigate areas of dense vegetation or other complex structures, avoiding occlusion by tree stems or large shrubs and thus overcoming a limitation of TLS (Abegg et al, 2017) to obtain highly detailed scans of all objects. Researchers have recently taken advantage of MLS's flexibility to investigate seasonality in forest structural complexity (Neudam et al, 2022), quantify crown defoliation and tree vitality (Heidenreich & Seidel, 2022), and study the effects of canopy structure on canopy spider diversity (Wildermuth et al, 2023).…”