Tree mixtures mediate negative effects of introduced tree species on bird taxonomic and functional diversity

Abstract

1. Recent biodiversity loss has emphasized the necessity to critically evaluate the consequences of human alterations of forest ecosystems. Stand diversification via tree species mixtures and the use of non-native trees are two such alterations currently gaining importance as climate change adaptations. However, the effects of local versus regional tree mixing on associated biodiversity and notably the modifying role of tree species growing outside their natural range remain poorly understood.2. We assessed how monocultures and mixtures of native and introduced tree species influence the taxonomic and functional diversity of bird communities at stand and landscape scales in north-west Germany. We focused on the dominant natural tree species (Fagus sylvatica) and economically important conifer species planted outside their natural range (the native Picea abies and non-native Pseudotsuga menziesii).3. We found that bird species richness and functional diversity were generally higher in pure and mixed stands of native F. sylvatica than in pure conifer stands, especially in comparison to non-native P. menziesii. These differences were particularly strong at the landscape scale. Pure conifer stands harboured only a reduced set of functionally similar bird species. Structural diversity based on tree microhabitat availability emerged as a key predictor of bird diversity. Synthesis and applications.Our study suggests that tree species mixtures do not necessarily increase bird diversity compared to pure stands of native trees, but can promote bird diversity relative to pure stands of species planted outside their natural range. Moreover, local mixtures, rather than a mosaic of pure stands, may promote bird diversity also at the landscape scale. By contrast, pure stands of tree species planted outside their natural range can increase biotic homogenization of forest birds. Promoting structural diversity of microhabitats via tree retention and ensuring that non-native trees are planted in mixtures

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

“…This might have negative cascading effects on the abundance and richness of consumer species such as many insectivorous birds (Gossner & Utschick, 2004). In line with this and with our findings, Schuldt et al (2022) found that bird abundance in summer is lower in Douglas fir stands than in native spruce and beech stands, respectively. Partly supporting our fourth hypothesis, another factor driving the observed differences between non‐native Douglas fir and native spruce was the low number of microhabitats in Douglas fir stands in our study.…”

“…Tree species mixtures mitigated the negative effects of non-native tree species, but did not increase arthropod diversity. This is in line with recent findings that tree-species mixtures can buffer negative tree-species identity effects on associated biodiversity Schuldt et al, 2022;Wang et al, 2019).…”

“…We further observed no differences between arthropod diversity in pure beech stands compared with arthropod diversity in traps that were attached to beech trees in mixtures. This is in line with previous studies on canopy arthropods and other forest-associated biota, which reported intermediate or unchanged diversity in mixtures compared with monospecific stands (Kriegel et al, 2021;Leidinger et al, 2019;Schuldt et al, 2022;Wang et al, 2019). However, other studies suggest positive effects of tree diversification on associated arthropods, as the combined communities add up to higher species numbers (Ampoorter et al, 2020;Schuldt et al, 2019;Sobek et al, 2009).…”

“…Tree microhabitats are important drivers of arthropod assemblages (e.g., Cours et al, 2021; Sallé et al, 2021; Thorn et al, 2016). The reason for the low number of microhabitats in Douglas fir in our study remains unclear, but low diversity and abundance of associated canopy biota, such as birds and fungi, might be a reason for the lower numbers of microhabitats compared with native conifers (Schmid et al, 2014; Schuldt et al, 2022).…”

Beneficial effects of native broadleaved forests on canopy beetle diversity are not reduced by admixture of non‐native conifers

“…Moreover, monospecific Douglas fir stands reduced landscape-scale beetle diversity, yet not more severely than Norway spruce. These results partly corroborate past studies reporting negative effects of non-native conifers on forest biodiversity (Schuldt et al, 2022;. However, they also highlight that native conifers planted outside their natural range can have F I G U R E 5 Abundance-based rarefaction (solid lines) and short-range extrapolation (dashed lines) of canopy beetle species richness (q = 0), number of common species (q = 1) and dominant species (q = 2) for (a-c) individual forest stand types across plots and (d-f) monoculture stands and mixed culture stands across stand types.…”