Tamm Review: On the strength of evidence when comparing ecosystem functions of mixtures with monocultures

•We review methodological issues for quantification of mixture effects in forests.•Potential stand density effects are often critical to account for.•Upscaling using information from monocultures for mixture distorts mixing effects.•Correlations between ecosystem functions and processes do not mean causality.•Tree- and neighbourhood-level analyses can greatly expand treatment ranges. The diversity of dominant tree species in a forest might strongly influence ecosystem functions and services, but the current evidence is not strong enough to provide general insights on when and where these diversity effects will be large or small for a given combination of species. With this goal in mind, the aim of this study is to discuss some of the factors that may need to be considered when designing studies or judging the strength of evidence provided in studies about tree-species mixing effects in forests. While the focus is on productivity, other ecosystem functions relating to light, water and nutrients are also considered. Firstly we consider the implications of stand-level spatial replication, the effects of stand density and tracking mixing effects through time in the same stand or by using chronosequences. Mixing effects at single sites (or ages) can represent significant increases in productivity while the mean mixing effect for the same mixture across a wide range of sites (or a whole rotation) can be much smaller and insignificant. The use of tree- and neighbourhood-level analyses to expand the range of treatments compared with stand-level analyses is then discussed before examining upscaling issues relating to inter- and intra-specific variability in morphology, allometry, physiology and phenology. Ignoring intra-specific variability between individuals in monocultures and mixed-species stands when upscaling to the stand level can strongly distort mixing effects, resulting in very misleading conclusions. The difference between correlations and causality is then discussed using the production ecology equation and mass balance approaches. We also discuss some of the methodological considerations when calculating mixing effects. All of these factors can have significant implications for the calculation and interpretation of mixing effects in forests.