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Mixed-species forests are sometimes more productive than monocultures. However, this is not always the case. There are many examples where mixtures are not more
productive. Whether or not mixtures are more productive depends on the combined effects of different types of interactions, and these are dynamic, changing through space and time. An understanding of these spatial and temporal dynamics is essential to predict when and where different types of processes are can result in small versus large increases in ecosystem functions in mixtures compared with monocultures.In relatively harsh ecosystems these dynamics are often described using the stress-gradient hypothesis, which suggests that facilitation increases as conditions become harsher (Bertness and Callaway, 1994; He et al., 2013). However, the stress-gradient hypothesis is difficult and often problematic for describing these dynamics in forests due to methodological differences in the quantification of facilitation in forests compared with the relatively harsh ecosystems where the stress-gradient hypothesis was developed (see 5). This is also illustrated by the many studies in forests have shown that complementarity can INCREASE as growing conditions improve (5).
We developed a framework to describe these spatial and temporal dynamics based on a literature review (5) and several studies we did along spatial gradients in resource availability or climatic conditions (2,4,6,7), as well as some long-term studies to examine temporal dynamics (2,3) or pot trials (1).The framework is described in (5) and also in Figure 2 of (9, which is open access). In general, complementarity for a given species will increase as the availability of resource “x” declines (or climatic condition x becomes harsher) if the species interactions improve the availability, uptake, or use efficiency of resource x (or interactions improve climatic condition x).As indicated in Figure 2b (see 9), there are typically multiple types of interactions occurring simultaneously in any given mixture. The framework is therefore too simple when considering simultaneous changes in multiple resources or climatic conditions (5). Similarly, the framework is of very limited value for foresters, who are more likely to require specific information about how mixtures a given group of species will develop compared with monocultures under a specific set of site, climatic and silvicultural conditions. Therefore, in another project, we worked with a process-based forest growth model, 3-PGmix, which is able to reproduce the spatial and temporal dynamics of species interactions (8,10 and this page).
A mixture of F. sylvatica and Pinus sylvestris
A Fagus sylvatica monoculture
A P. sylvestris monoculture
These photos of F. sylvatica and P. sylvestris forests are examples of those used to examine how the complementarity effect between these species changed along a site and climatic gradient across Europe (7,10). The study used a total of 32 of these "triplets" (32 sites), which were distributed from Sweden to Bulgaria and from Spain to the Ukraine.
Journal articles related to this project:
1. Forrester, D.I., Cowie, A.L., Bauhus, J., Wood, J.T. and Forrester, R.I. (2006). Effects of changing the supply of nitrogen and phosphorus on growth and interactions between Eucalyptus globulus and Acacia mearnsii in a pot trial. Plant and Soil 280, 2677-277. doi:10.1007/s11104-005-3228-x
2. Forrester, D.I., Vanclay, J.K., Forrester, R.I. (2011). The balance between facilitation and competition in mixtures of Eucalyptus and Acacia changes as stands develop. Oecologia 166, 265-272. doi:10.1007/s00442-011-1937-9
3. Forrester, D.I., Smith, R.G.B. (2012). Faster growth of Eucalyptus grandis and Eucalyptus pilularis in mixed-species stands than monocultures. Forest Ecology and Management 286, 81-86. doi:10.1016/j.foreco.2012.08.037
4. Forrester, D.I., Kohnle, U., Albrecht, A.T., Bauhus, J. (2013). Complementarity in mixed-species stands of Abies alba and Picea abies varies with climate, site quality and stand density. Forest Ecology and Management. 304, 233-242. doi:10.1016/j.foreco.2013.04.038
5. Forrester, D.I. (2014). The spatial and temporal dynamics of species interactions in mixed-species forests: From pattern to process. Forest Ecology and Management 312, 282-292. doi:10.1016/j.foreco.2013.10.003
6. Forrester, D.I., Albrecht, A.T. (2014). Light absorption and light-use efficiency in mixtures of Abies alba and Picea abies along a productivity gradient. Forest Ecology and Management. 328, 94-102. doi:10.1016/j.foreco.2014.05.026
7. Pretzsch, H., del Río, M., Ammer, C., Avdagic, A., Barbeito, I., Bielak, K., Brazaitis, G., Coll, L., Dirnberger, G., Drössler, L., Fabrika, M., Forrester, D.I., Godvod, K., Heym, M., Hurt, V., Kurylyak, V., Löf, M., Lombardi, F., Matović, B., Mohren, F., Motta, R., den Ouden, J., Pach, M., Ponette, Q., Schütze, G., Schweig, J., Skrzyszewski, J., Sramek, V., Sterba, H., Stojanović, D., Svoboda, M., Vanhellemont, M., Verheyen, K., Wellhausen, K., Zlatanov, T., Bravo-Oviedo, A., 2015. Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient through Europe. European Journal of Forest Research 134, 927-947. doi:10.1007/s10342-015-0900-4.
8. Forrester, D.I., Tang, X. (2016). Analysing the spatial and temporal dynamics of species interactions in mixed-species forests and the effects of stand density using the 3-PG model. Ecological Modelling 313, 276-292. doi:10.1016/j.ecolmodel.2015.07.010
9. Forrester, D.I., Bauhus, J. (2016). A review of processes behind diversity - productivity relationships in forests. Current Forestry Reports 2, 45-61. doi: 10.1007/s40725-016-0031-2
10. Forrester, D.I., Ammer, C., Annighöfer, P.J., Avdagic, A., Barbeito, I., Bielak, K., Brazaitis, G., Coll, L., Río, M.d., Drössler, L., Heym, M., Hurt, V., Löf, M., Matović, B., Meloni, F., Ouden, J.d., Pach, M., Pereira, M.G., Ponette, Q., Pretzsch, H., Skrzyszewski, J., Stojanović, D., Svoboda, M., Ruiz-Peinado, R., Vacchiano, G., Verheyen, K., Zlatanov, T., Bravo-Oviedo, A., (2017) Using the 3-PGmix model to predict the spatial and temporal dynamics of species interactions in Fagus sylvatica and Pinus sylvestris forests across Europe. Forest Ecology and Management 405, 112-133. doi:10.1016/j.foreco.2017.09.029
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