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 tree species 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.

We assessed how monocultures and mixtures of native and introduced tree species influence the taxonomic and functional diversity of northwest German bird communities at stand and landscape scales. 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).

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 harbored 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 the biotic homogenization of forest birds. Promoting structural diversity of microhabitats via tree retention and ensuring that non-native trees are planted in mixtures with native trees may alleviate potential limitations of climate change-oriented management for biodiversity.

The study was conducted across six study sites in the federal state of Lower Saxony, Northwest Germany. Each of the six sites comprised five rectangular study plots with a size of 2500 m², representing five different stand types: 1) pure European beech, 2) pure Norway spruce, 3) pure Douglas fir, 4) mixtures of European beech and Norway spruce, and 5) mixtures of European beech and Douglas fir. This amounted to 30 study plots, of which one pure beech plot was excluded, so all analyses are based on 29 study plots. 

Birds were surveyed with standardized ten-minute point counts on five sampling dates (late March, late April, early May, late May, and early June) for each plot in 2020. Point counts were conducted from the center of each study plot by always the same observer. All bird species seen and heard during the point counts as well as their abundances were recorded (excluding overflying individuals) within a 50 m radius from the plot center. Counts were conducted in the morning hours between sunrise and 11:00 am at the latest and in adequate weather conditions (avoiding strong winds and rain). The order of plots visited on consecutive sampling dates was varied to account for the potential effects of survey time on detection probability.

We selected key functional traits of the recorded bird species to quantify bird functional diversity with respect to habitat use and resource requirements.

Environmental variables included latitude and elevation above sea level, we compiled data on mean annual temperature and mean annual precipitation in the period from 1980-2019 (Deutscher Wetterdienst, DWD).  We measured canopy openness, leaf area index (LAI), mean tree diameter, variability in diameter, number of tree microhabitats, richness of microhabitat types, and total deadwood volume in each plot. Canopy openness and LAI were measured at 12 sampling points (3 x 4 sampling grid with 10 m distance between sampling points) per plot with a Solariscope (SOL300, Behling) in July 2019, and we used averaged values per plot for our analyses. Tree diameter at breast height (DBH) was measured for all trees ≥ 7 cm DBH during plot establishment in 2017/2018. Variability in DBH was calculated as the coefficient of variation (CV) by dividing the standard deviation of DBH values by the mean DBH per plot. Tree microhabitats and their abundance were recorded for all trees with DBH ≥ 15 cm on all plots following the general classification of Larrieu et al. (2018), but modified to contain a set of 13 microhabitat types (woodpecker cavities, rot holes, insect galleries, and boreholes, other cavities, bark and wood injuries, bark shelter, crown deadwood, burrs and cankers, fungal fruiting bodies, mosses and lichens, ivy, nests, microsoil, and fork splits). Richness of tree microhabitats was calculated as the number of different types per plot; number of microhabitats were the summed occurrences per plot. Total deadwood volume was calculated from length/height and diameter measures of all lying and standing (tree heights estimated based on diameter-based height curves) deadwood with a minimum diameter at the thicker end of 7 cm in the above-mentioned 3 x 4 sampling grid, covering the central 1200 m² of each plot.

Files are:

1. bird and environmental data.txt

2. readme.xlsx: explanation/description of variables