Mixed species forests tend to be more productive, so whey are we still planting monocultures?
A lot of recent research indicates a positive relationship between tree species diversity and productivity. So why aren’t monoculture plantations being replaced with more diverse mixtures of species to gain a boost in productivity?
Overyielding vs. transgressive overyielding
Part of the reason is the conventions used in reporting productivity gains. The productivity of mixed species stands is typically compared to the weighted average productivity of each species grown in monoculture. In most cases, a mixture of species outperforms this average but is less productive than the best monoculture. When mixtures perform better than the weighted average of the species in the mixture, it is called overyielding and when mixtures perform better than the most productive monoculture it is called transgressive overyielding. Most plantation managers are only interested in transgressive overyielding since they are usually planting the most productive species for a region.
>>READ MORE: The growth before death: a better understanding of tree mortality using tree ring data
In some cases, transgressive overyielding has been demonstrated in plantations. For example, a mixture of eucalyptus grown with acacia can be more productive than monoculture plantations of either species. The acacia trees fix nitrogen and fertilize the eucalyptus. This is more the exception than the rule however, and intensive plantation practices like adding chemical fertilizer, controlling weeds and density management may reduce interaction-both beneficial and competitive- between species.
For much more on mixed-species forests check out this book: Mixed-Species Forests: Ecology and Management
Productivity isn’t everything
Furthermore, productivity isn’t the most important thing for plantation managers, they are usually most concerned about the value of the trees. For mixed species plantations to work, all species must have compatible growth rates, rotation lengths and market value. Productivity also needs to be allocated to merchantable stems. Even if a mixed species plantation demonstrates transgressive overyielding, in terms of net productivity, it would be a bad choice for managers if the gains are concentrated in low value trees or product recovery is dramatically reduced because the average stem size is small.
>>READ MORE: What factors determine whether tree species compete or complement each other?
A boost to productivity isn’t the only reason to consider managing plantations for a mixture of tree species. Having multiple tree species in a plantation can protect managers from devastating losses as the result of species-specific pests and disease just like having a diversified investment portfolio reduces the risk of losing your entire retirement if a single stock crashes. Greater tree species diversity is also linked to greater ecosystem stability which is particularly attractive in the face of global change.
Species diversity and ecosystem services
More species diverse forests tend to support higher levels of multiple ecosystem functions, not just productivity. Multiple ecosystem functions, in turn are hypothesized to increase the potential for more ecosystem services. Recent research from the Pacific Northwest, U.S. suggests that species diverse plantations do provide higher levels of more ecosystem services but also cautions that there are trade-offs. For instance, in the study production values (timber revenue and wood volume) are maximized in a monoculture and increasing the species diversity comes at a cost to these ecosystem services. However, a diverse mixture of species minimized total trade-offs between nine different ecosystem services.
As long as plantations management prioritized timber production and profitability, it is likely that monoculture will dominate. Planting, managing, harvesting, and marketing timber from monocultures is just easier and often has the potential for greater financial returns. However, as biological risks from climate change and invasive species increase there may be more incentive to forgo some potential profit and diversity plantations to avoid catastrophic losses. Further, if plantation managers are incentivized to increase the amount of ecosystem services provided by plantations—either through direct payments for those services, sustainable forestry certification requirements, or corporate sustainability initiatives—mixed species plantations may become more common.
Austin Himes, Ph.D., is an assistant professor in the Department of Forestry at Mississippi State University where he teaches and conducts research. His research is focused on silviculture, forest ecology, ecosystem services, and environmental values. He is interested in understanding interactions between forests and people through an interdisciplinary lens and finding ways silviculture can be used to improve the many ways forests benefit people. He and Adam Polinko, Ph.D., run the Silviculture at MS State lab, which you can follow on Twitter @silviculture.