Quantifying mixing effects in forests

Many of the world’s forests are mixed-species forests and it is often useful to quantify the effects of species interactions on the functioning of those forests. With this goal in mind, we were invited to review some of the factors that are important to consider when designing studies and also when judging the strength of evidence provided in existing studies (3). While none of the ideas we discussed were new, they are sometimes forgotten or ignored, which can result in biased calculations or predictions of mixing effects. The review begins by providing some definitions because the same terms can sometimes have different meanings or vice versa:

- Level of analysis – tree, neighbourhood, species, total stand and community

- Empirical data sources – planted experiments, specific forest plots or inventory

- Types of interactions

- Quantification of mixing effects

- Quantification of mixing proportions using stand density

- (A separate study was used to review definitions of mixed-species forests (2) – there are surprisingly many views about this.)

The review (3) then discusses the implications of stand-level spatial replication, the effects of stand density on mixing effects and tracking mixing effects through time. The complementary use of tree- and neighbourhood-level analyses with stand-level analyses is discussed before examining upscaling issues relating to inter- and intra-specific variability in morphology, allometry, physiology and phenology. Intra-specific variability between individuals in mixtures vs. monocultures is sometimes ignored but this can then result in biased calculations of mixing effects, and misleading conclusions.

The difference between correlations and causality is discussed using the production ecology equation (1) and mass balance approaches. This draws on ideas that are described in more detail here.

Journal articles related to this project:

1. Richards, A.E., Forrester, D.I., Bauhus, J., Scherer-Lorenzen, M., 2010. The influence of mixed tree plantations on the nutrition of individual species: a review. Tree Physiology 30, 1192-1208. doi:10.1093/treephys/tpq035

2. Bravo-Oviedo, A., Pretzsch, H., Ammer, C., Andenmatten, E., Barbati, A., Barreiro, S., Brang, P., Bravo, F., Coll, L., Corona, P., Ouden, J.d., Ducey, M.J., Forrester, D.I., Giergiczny, M., Jacobsen, J.B., Lesinski, J., Löf, M., Mason, B., Matovic, B., Metslaid, M., Morneau, F., Motiejunaite, J., O’Reilly, C., Pach, M., Ponette, Q., Rio, M.d., Short, I., Skovsgaard, J.P., Soliño, M., Spathelf, P., Sterba, H., Stojanovic, D., Strelcova, K., Svoboda, M., Verheyen, K., Lüpke, N.v., Zlatanov, T., 2014. European Mixed Forests: definition and research perspectives. Forest Systems 23, 518-533. doi:10.5424/fs/2014233-06256

3. Forrester, D.I., Pretzsch, H., 2015. Tamm Review: On the strength of evidence when comparing ecosystem functions of mixtures with monocultures. Forest Ecology and Management 356, 41-53. doi:10.1016/j.foreco.2015.08.016