DOI: 10.5061/dryad.j6q573nhc
title: Tree mixtures increase bird taxonomic and functional diversity over pure stands of tree species planted outside their natural range—but not over pure native stands

description: 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.
description: 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.
description: Files are: 1. bird and environmental
 data.txt 2. readme.xlsx: explanation/description of
 variables