Mixed-species forests: ecology and management

In many parts of the world, forests are likely to face novel disturbance regimes as a result of global change processes, and there is concern that the capacity of forest ecosystems to withstand, recover from, or adapt to these novel disturbance regimes may decline. Creation and maintenance of species-diverse forests is seen as an important option to adapt forests to uncertain future disturbances.However, it is not known whether benefits of mixed-species forests consist mainly of risk spreading among tree species that have different susceptibility to various stressors and disturbance agents or whether they also have emergent properties resulting from interactions among species, which increase the resistance and resilience of participating species or the entire ecosystem. Here we review the evidence for the effects of tree diversity on the resistance and resilience of forests in relation to a number of abiotic (drought, wind, fire) and biotic (insect herbivores, pathogens) stress and disturbance factors. For the abiotic disturbances, damage or reduction in ecosystem function can be reduced, compared with monocultures of susceptible or less resilient species, when more resistant or resilient species are mixed with less susceptible and less resilient species. However, storm, fire, or drought damage to individual species may not be reduced in mixtures when compared to monocultures. The stress or disturbance impacts may even be aggravated for one or more species in somemixtures, as is shown for drought. There is more evidence for beneficial diversity effects in relation to biotic disturbance agents. Mixing tree species reduces the impact of insect herbivores on individual susceptible tree species in the majority of cases, where the community is dominated by specialist herbivores. However, the opposite effect may occur with generalist herbivores, which can be promoted by tree diversity. Similarly, tree diversity can reduce the impact of specialist pathogens on host tree species, whereas there is little evidence for positive influences in the case of generalist pathogens. In most cases, tree species diversity dilutes the impact of disturbance agents and, owing to different susceptibility of species to specific disturbances, insures against a complete damage or loss. In addition, mixing tree species can reduce temporal variation in growth and stabilise productivity. However, there is little evidence for true, positive diversity effects, where diversity leads to an increase in the resistance and resilience of component species in mixed-species communities. From an economic point of view, mixing might help to reduce risk for a more vulnerable and valuable species, even if there are no benefits for the admixed species. However, forest managers should be aware that mixtures do not provide universally higher resistance or resilience in relation to disturbances than monocultures. In most cases, it depends to a large extent on the attributes of the species in mixture inrelation to the specific disturbances