Eucalypt and acacia mixtures: productivity, carbon sequestration, nutrient, water and light dynamics

Monospecific plantations of eucalypts have been managed successfully and sustainably in many countries. The simplicity of these plantations makes them easier to manage, but sometimes there are concerns about their associated costs of fertilisers, biodiversity and productivity losses from pests and diseases. Few silvicultural alternatives to these intensively managed monocultures have been examined.

This project examined whether mixtures of Eucalyptus and nitrogen-fixing species, such as Acacia, can be more productive than monocultures. This would not only be useful from a wood, biomass, and carbon perspective but also because synthetic nitrogen fertiliser, which can be expensive and costs energy to produce and apply, may no longer be required.

A 1:1 mixed species plantation of Eucalyptus globulus and Acacia mearnsii, aged 9 years.

Main findings:

*Mixed-species plantations containing a nitrogen fixing species are often more productive than monocultures, but this effect is site specific, and not surprisingly, works better on sites with low nitrogen availability (4,5,11,14).

*Traditional measures of site quality such as site indices provide very little useful information about whether mixtures will be more productive (14). A much better indicator is the nitrogen availability of the soil. For example, very high mixing effects occur at very productive sites where water and nutrients (except N) are not limiting (14). Very low mixing effects can occur on sites with low site indices because these other resources (e.g. low phosphorus or water availability or droughts (4,8)) restrict growth and prevent any facilitative effect via nitrogen fixation.

*It is not just higher nitrogen availability that causes the mixing effects in these systems; other simultaneously occurring processes are also at work (5). The processes include:

• Symbiotic nitrogen-fixation by acacia (7)

• Accelerated rates of phosphorous (and nitrogen) cycling via litterfall and its decomposition (3,10)

• The improved nutrient availability can shift carbon partitioning more aboveground. In our experiment below-ground carbon allocation was similar in all treatments, but mixtures had greater aboveground, and hence total, productivity (6)

• Soil carbon contents were higher in the mixtures (13)

• Eucalyptus trees grew taller than acacia trees, resulting in a stratified canopy and a reduction in the intensity of light competition experienced by the eucalypts (1). Mixtures also intercepted more light than monocultures (12)

• The greater growth of mixtures was accompanied by greater transpiration than in the monocultures and the eucalypts may have benefited even further because they appeared to acquire more deep soil water than the acacia and hence could have experienced less competition for water in mixtures (9 & 15)

• The improved resource availability also resulted in higher water-use efficiency (9 & 15) and light-use efficiency (12) of mixtures than monocultures (mainly due to the eucalypts). This was related to greater rates of photosynthesis (12) and shifts in carbon partitioning from belowground to aboveground (6)

*A management implication of the increased water use AND water-use efficiency of the mixtures was that based on this particular experiment it would be possible to use mixtures to produce the same amount of wood, but on only about half the land area and with about 2/3 of the water that a eucalypt monoculture would use (9).

*The contribution of these processes vary as the stands develop (11,14) and as stand density changes (15). Silviculturalists could probably design mixed-species systems where either the nitrogen-fixing species is thinned, retained to fix more nitrogen, or both species are thinned. The nitrogen fixer might also shade the eucalypt stems, accelerate the rise of the live crown, and reduce the size of branches and of the knotty core. Alternatively, the nitrogen-fixing species may be a pasture component or used as a bioenergy crop, and grow underneath widely spaced eucalypts that are managed for solid-wood. In contrast, the eucalypt trees may be planted with a high diversity of native species and used as a source of income to pay for the restoration of those native species (16,17,18).

Journal articles related to this project:

1. Forrester D.I, Bauhus J. and Khanna, P.K. (2004). Growth dynamics in a mixed species plantation of Eucalyptus globulus and Acacia mearnsii. Forest Ecology and Management 193, 81-95. doi:10.1016/j.foreco.2004.01.024

2. Forrester D.I., Bauhus J. and Cowie A.L. (2005). On the success and failure of mixed-species tree plantations: lessons learned from a model system of Eucalyptus globulus and Acacia mearnsii. Forest Ecology and Management 209, 147-155. doi:10.1016/j.foreco.2005.01.012

3. Forrester, D. I., Bauhus, J. and Cowie, A.L. (2005). Nutrient cycling in a mixed-species plantation of Eucalyptus globulus and Acacia mearnsii. Canadian Journal of Forest Research 35, 2942-2950. doi:10.1139/x05-214

4. 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

5. Forrester, D.I., Bauhus, J., Cowie, A.L. and Vanclay, J.K. (2006). Mixed-species plantations of Eucalyptus with nitrogen-fixing trees: A review. Forest Ecology and Management 233, 211-230. doi:10.1016/j.foreco.2006.05.012

6. Forrester, D.I., Bauhus, J. and Cowie, A.L. (2006). Carbon allocation in a mixed-species plantation of Eucalyptus globulus and Acacia mearnsii. Forest Ecology and Management 233, 275-284. doi:10.1016/j.foreco.2006.05.018

7. Forrester, D. I., Schortemeyer, M., Stock, W. D., Bauhus, J., Khanna, P. K. and Cowie, A. L. (2007). Assessing nitrogen fixation in a mixed- and single-species plantation of Eucalyptus globulus and Acacia mearnsii. Tree Physiology 27, 1319-1328. doi:10.1093/treephys/27.9.1319

8. Forrester, D.I., Bauhus, J., Cowie, A.L., Mitchell, P.A. and Brockwell, J. (2007). Productivity of Three YoungMixed-Species Plantations Containing N₂-Fixing Acacia and Non-N2-FixingEucalyptus and Pinus trees in Southeastern Australia. Forest Science 53, 426-434.

9. 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

10. 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

11. 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

12. Forrester, D. I., Lancaster, K., Collopy, J. J., Warren, C. R., Tausz, M. (2012). Photosynthetic capacity of Eucalyptus globulus is higher when grown in mixture with Acacia mearnsii. Trees-Structure and Function 26, 1203-1213. doi:10.1007/s00468-012-0696-5

13. Forrester, D. I., Pares, A., O´Hara, C.; Khanna, P. K., Bauhus, J. (2013). Soil organic carbon is increased in mixed-species plantations of Eucalyptus and nitrogen-fixing Acacia. Ecosystems. 16, 123-132. doi:10.1007/s10021-012-9600-9

14. 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

15. 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

16. Amazonas, N.T., Forrester, D.I., Silva, C.C., Almeida, D.R.A.d., Rodrigues, R.R., Brancalion, P.H.S., 2018. High diversity mixed plantations of Eucalyptus and native trees: an interface between production and restoration for the tropics. Forest Ecology and Management 417, 247-256. doi:10.1016/j.foreco.2018.03.015

17. Amazonas, N.T., Forrester, D.I., Oliveira, R.S., Brancalion, P.H.S., 2018. Combining Eucalyptus wood production with the recovery of native tree diversity in mixed plantings: Implications for water use and availability. Forest Ecology and Management 418, 34-40. doi:10.1016/j.foreco.2017.12.006

18. Amazonas, N.T., Forrester, D.I., Silva, C.C., Almeida, D.R.A.d., Oliveira, R.S., Rodrigues, R.R., Brancalion, P.H.S., 2021. Light- and nutrient-related relationships in mixed plantations of Eucalyptus and a high diversity of native tree species. New Forests 52, 807-828. doi:10.1007/s11056-020-09826-x