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Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.Additional co-authors: Damien Bonal, Olivier Bouriaud, Helge Bruelheide, Filippo Bussotti, Monique Carnol, Bastien Castagneyrol, Yohan Charbonnier, David Anthony Coomes, Andrea Coppi, Cristina C. Bastias, Seid Muhie Dawud, Hans De Wandeler, Timo Domisch, Leena Finér, Arthur Gessler, André Granier, Charlotte Grossiord, Virginie Guyot, Stephan Hättenschwiler, Hervé Jactel, Bogdan Jaroszewicz, Tommaso Jucker, Julia Koricheva, Harriet Milligan, Sandra Mueller, Bart Muys, Diem Nguyen, Martina Pollastrini, Sophia Ratcliffe, Karsten Raulund-Rasmussen, Federico Selvi, Jan Stenlid, Fernando Valladares, Lars Vesterdal, Dawid Zielínski, and Markus Fische

There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodiversity effects on multifunctionality into three mechanisms and apply this to European forest data. We show that throughout Europe, tree diversity is positively related with multifunctionality when moderate levels of functioning are required, but negatively when very high function levels are desired. For two well-known mechanisms, ‘complementarity' and ‘selection', we detect only minor effects on multifunctionality. Instead a third, so far overlooked mechanism, the ‘jack-of-all-trades' effect, caused by the averaging of individual species effects on function, drives observed patterns. Simulations demonstrate that jack-of-all-trades effects occur whenever species effects on different functions are not perfectly correlated, meaning they may contribute to diversity–multifunctionality relationships in many of the world's ecosystems.

1. Promoting mixed-species forests is an important strategy for adaptation and risk reduction in the face of global change. Concurrently, a main challenge in ecology is to quantify the effects of species diversity on ecosystem functioning. In forests, the effects of individual tree species on ecosystem functions depend largely on their dimensions, which are commonly predicted using allometric equations. However, little is known about how diversity influences allometry or how to incorporate this effect into allometric equations. Ignoring the effects of interspecific interactions on allometric relationships may result in severely biased predictions.2. This study examined the effects of tree-species diversity, competition and tree social status on crown-projection area (cpa), height (h) and live-crown length (lcl) of trees using a European-wide data set containing 17 target species and 12 939 trees. The cpa, h and lcl were predicted as functions of stem diameter at 1Á3 m, tree-species diversity, tree height relative to the stand mean height (rh) and a competition index (CI) that accounted for stand density and interspecific differences in competitive ability based on species-specific wood density or shade tolerance. 3. Averaged across species, diameter had the greatest effect on cpa and lcl, followed by the competition index, while rh had the greatest effect on lcl. Tree-species diversity had the smallest effect on cpa, h and lcl. Interspecific variability in cpa, h or lcl responses to diversity, CI, or rh was sometimes related to wood density or shade tolerance. 4. Synthesis. This study shows the strong influence of stand structure and species composition on allometric relationships. These influences can be quantified using measures of competition, tree-species diversity and relative tree height so that general equations can be developed for a given species to be applied to a wide range of species compositions and stand structures. This new approach will greatly improve predictions of biomass and carbon stocks in structurally and compositionally diverse forests.

Humans require multiple services from ecosystems, but it is largely unknown whether trade-offs between ecosystem functions prevent the realisation of high ecosystem multifunctionality across spatial scales. Here, we combined a comprehensive dataset (28 ecosystem functions measured on 209 forest plots) with a forest inventory dataset (105,316 plots) to extrapolate and map relationships between various ecosystem multifunctionality measures across Europe. These multifunctionality measures reflected different management objectives, related to timber production, climate regulation and biodiversity conservation/recreation. We found that trade-offs among them were rare across Europe, at both local and continental scales. This suggests a high potential for 'winwin' forest management strategies, where overall multifunctionality is maximised. However, across sites, multifunctionality was on average 45.8-49.8% below maximum levels and not necessarily highest in protected areas. Therefore, using one of the most comprehensive assessments so far, our study suggests a high but largely unrealised potential for management to promote multifunctional forests.