How can you get the most wood out of a forest? For commercial use, you might expect that monocultures, so you’re getting the same wood from each tree, are good. In a new study, Pretzsch and Schütze examine the alternative, using a mix of species.
There are some obvious benefits for mixed species in a forest. For example, insect attack is reduced if there’s a mix of species, as often insects specialise in attacking one type of tree. There’s also some evidence that tree growth can be denser, more resilient to drought and more efficient in resource use.
For a commercial forest, there’s a trade-off in how you grow the trees. Space them further apart, and you get bigger stem diameters. Plant them closer together, and you get more trees per unit area, but the trees are less likely to be high quality. For mixed stands, it appears that you can plant trees more densely without impacting stem diameter so much. This would seem to give you more and bigger trees in the same area. However, as Pretzsch and Schütze point out, the evidence is patchy.
The authors analysed 63 plots covering the species combinations of (1) Norway spruce/European beech, (2) Norway spruce/silver fir/European beech, (3) Norway spruce/Scots pine, (4) Scots pine/European beech, (5) sessile oak/European beech and (6) European ash/sycamore maple. They compared these with monoculture stands. From these results, the scientists were then able to create models of the relationship between height, stem diameter, and age.
The authors found stand productivity gains of 7–53 % of mixed versus mono-specific stands continuing over the entire rotation. However, different mixes had different improvements over monoculture stands.
The authors found that the increased productivity was mainly based on a density increase in the case of Norway spruce/silver fir/European beech and sessile oak/European beech, and it was based on a more efficient resource use given the same stand density in the case of Scots pine/European beech and European ash/sycamore maple.
Pretzsch and Schütze argue that the density effect depends not on the site but the structural species complementarity. For the common mixtures examined in this study, the results show that thinning for the acceleration of stem growth requires less density reduction and causes less stand growth losses than in monocultures.
Pretzsch and Schütze conclude by stating: “The strong beneficial effects of species mixture and stand structure on tree and stand growth suggest potential research for the future. This study provides various new starting points for better understanding, design, and silvicultural steering, and exploits the benefits of mixed compared with mono-specific stands. The essential 3-D structure may be better and less costly, as measured by T-LiDAR in existing and newly established experiments. The species-specific behaviour suggests the avoidance of premature species-overarching generalization. The differentiation between density and efficiency effects offers promising starting points for further causal analyses and modelling mixing effects depending on site conditions.”
Pretzsch, H., Schütze, G., 2021. Tree species mixing can increase stand productivity, density and growth efficiency and attenuate the trade-off between density and growth throughout the whole rotation. Annals of Botany. https://doi.org/10.1093/aob/mcab077