Water use and drought susceptibility of mixtures

Mixed-species forests are often promoted because they are sometimes more productive than monocultures. They have become a main element of strategies to reduce risks and adapt forests to climate change. However, increases in growth are often associated with increased tree water use (3), which could result in a reduction in ground water recharge and an increased level of stress between competing trees, in particular during drought. Despite a long history of research into mixing effects, very little was known about water fluxes and water-use efficiency of tree species in mixtures.

We used a Eucalyptus globulus and Acacia mearnsii experiment to examine the relationships between growth, transpiration and water-use efficiency, which were all greater in mixtures than both of the monocultures (1). Several years later, other studies had been done and we reviewed these studies to identify any general patterns (3). The review showed that if the growth of a given species increases in mixture, then it will also use more water and/or be more water-use efficient; when growth increases there will not be a reduction in transpiration or water-use efficiency. When growth does not change for a given species in mixture, there is generally no change in its water use or water-use efficiency. A more detailed analysis of the Eucalyptus globulus and Acacia mearnsii experiment showed that within a stand, trees of a given species can show contrasting growth and transpiration responses depending on their size (diameter) and the density of their neighbourhood. That is, the stand-level patterns are the average of several different tree-level responses that are not evident at the stand level.

When growth and transpiration increase in mixtures, the trees may dry the soil more rapidly and become more susceptible to droughts. This conflicts with strategies where mixtures are assumed to reduce risks and adapt forests to climate change. The drought exposure of trees growing in mixtures, compared with monocultures, was examined in six forest types across Europe using the carbon isotope composition of latewood. In the two forests types that were in the most drought-prone environments, the species interactions appeared to reduce the exposure to droughts. There was no change in drought exposition in the other three forest types and in the boreal forest type drought stress increased with increasing diversity (2,4). However, in the forests where these significant community-level relationships occurred (three of six regions), there was

only one species within the given community that showed a significant relationship (three of 25 species–region combinations). This shows that that information about a single species can be a poor indicator of the response of other species or the whole community. Interestingly, there were many two-species mixtures in which both species were less water-stressed compared with their monocultures, but also many mixtures where both species were more stressed compared with their monocultures (4).

All of these studies illustrate the trade-off between using species interactions to increase the growth of mixtures, but that this may need to be done cautiously in some regions to avoid increasing growth so much that transpiration or other water fluxes lead to drier soils and greater susceptibility to droughts (3).

Journal articles related to this project:

1. Forrester, D.I., Theiveyanathan, S., Collopy, J.J., Marcar, N.E., (2010). Enhanced water use efficiency in a mixed Eucalyptus globulus and Acacia mearnsii plantation. Forest Ecology and Management 259, 1761-1770. doi:10.1016/j.foreco.2009.07.036

2. Grossiord, C., Granier, A., Ratcliffe, S., Bouriaud, O., Bruelheide, H., Chećko, E., Forrester, D.I., Dawud, S.M., Finér, L., Pollastrini, M., Scherer-Lorenzen, M., Valladares, F., Bonal, D., Gessler, A., (2014). Diversity does not always improve adaptation of forest ecosystems to extreme drought. Proceedings of the National Academy of Sciences. 111, 14812-14815 doi:10.1073/pnas.1411970111

3. Forrester, D.I. (2015). Transpiration and water-use efficiency in mixed-species forests versus monocultures: effects of tree size, stand density and season. Tree Physiology 35, 289-304. doi:10.1093/treephys/tpv011

4. Forrester, D.I., Bonal, D., Dawud, S., Gessler, A., Granier, A., Pollastrini, M., Grossiord, C., 2016. Drought responses by individual tree species are not often correlated with tree species diversity in European forests. Journal of Applied Ecology 53, 1725-1734. doi:10.1111/1365-2664.12745

5. de Streel, G., Collet, C., Barbeito, I., Bielak, K., Bravo‑Oviedo, A., Brazaitis, G., Coll, L., Drössler, L., Forrester, D., Heym, M., Löf, M., Pach, M., Pretzsch, H., Ruiz‑Peinado, R., Skrzyszewski, J., Stankevičiūtė, J., Svoboda, M., Verheyen, K., Zlatanov, T., Bonal, D., Ponette, Q., (2020). Contrasting patterns of tree species mixture effects on wood δ¹³C along an environmental gradient. European Journal of Forest Research, 139, 229-245. doi:10.1007/s10342-019-01224-z

6. Bottero, A., Forrester, D.I., Cailleret, M., Kohnle, U., Gessler, A., Michel, D., Bose, A.K., Bauhus, J., Bugmann, H., Cuntz, M., Gillerot, L., Hanewinkel, M., Lévesque, M., Ryder, J., Sainte-Marie, J., Schwarz, J., Yousefpour, R., Zamora-Pereira, J.C., Rigling, A., (2021). Growth resistance and resilience of mixed silver fir and Norway spruce forests in central Europe: Contrasting responses to mild and severe droughts. Global Change Biology 27, 4403-4419 doi:10.1111/gcb.15737