A Review of Processes Behind Diversity—Productivity Relationships in Forests

Abstract

Through complementarity interactions, mixedspecies forests can be more productive than monocultures, and this effect can increase with tree-species richness. However, this is not always the case. This review examines the processes and stand structural attributes that can influence diversity-productivity relationships (DPRs); how they influence resource availability, resource uptake, and resource-use efficiency; and also describes some important differences between tree-diversity versus grassland-diversity experiments. The size of the complementarity effects caused by these processes and stand structures varies along spatial and temporal gradients in resource availability and climate. These spatial and temporal dynamics have now been examined in many studies, and the general patterns are summarized using a simple framework; complementarity is predicted to increase as the availability of resource BX^declines (or climatic condition X becomes harsher) if the species interactions improve the availability, uptake, or use efficiency of resource X (or interactions improve climatic condition X). Importantly, this framework differs from the stress-gradient hypothesis to account for a wider range of inter-specific plant interactions (not only facilitation) by considering contrasting methods used to quantify species interactions while accounting for stand structure. In addition, complementarity (as opposed to facilitation) for a given species combination can increase as growing conditions improve in forests, contrary to predictions of the stressgradient hypothesis with regards to facilitation. This review indicates that while the effects of tree-species diversity on growth and other forest functions are now receiving a lot of attention, far less is known about the effects of structural diversity on growth or forest functioning. Direct measurements of the processes, as opposed to focusing mainly on growth responses, could greatly contribute to our understanding of structural diversity effects.

Mixing of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) enhances structural heterogeneity, and the effect increases with water availability

“…1), not only of monocultures but especially of mixed-species stands (Forrester and Pretzsch, 2015;Forrester and Bauhus, 2016;Pretzsch, 2014). A deeper insight into stand structure and its dependency on site conditions is also important for the further development of silvicultural guidelines for the management of mixed stands which may address multiple services (Río et al, 2016).…”

“…Alternatively, species mixing could be used to enrich stand structure and heterogeneity Schütze, 2014, 2015) but strong competitive superiority of one species may also cause structural homogenization (Wiedemann, 1951, p. 134). The mixing of species with differing ecological traits may enhance structural complexity above and below ground (Bauhus, 2009;Pretzsch, 2014) and this can increase stand productivity compared with monocultures (Forrester and Bauhus, 2016).…”

“…Multiplicative mixing effects on structure and the resulting outcomes such as productivity, stability, and resistance emerge from the species interactions and cannot be predicted by only studying the species in their monocultures. Many of the species interactions that occur in mixtures are at least partly the result of structural differences between mixtures and monocultures (Forrester and Bauhus, 2016;Río et al, 2014) and are likely to modify resource use and forest functioning in terms of stand productivity (Fig. 1).…”

Forest biodiversity, ecosystem functioning and the provision of ecosystem services

“…Facilitation between plant species growing together has often been found to lead to enhanced growth of certain tree mixtures (Thompson et al 2014). For example, at nitrogen-limited sites, tree species that are nitrogen-fixers may enhance the growth of other tree species in mixed stands (e.g., Binkley 2003;Forrester and Bauhus 2016). Resistance to disturbance is facilitated by forest and tree diversity, leading to a reduction or dilution of resources (e.g., for herbivores), diversion or disruption, and multi-trophic interactions (e.g., enhanced abundance and action of natural enemies) .…”

“…For example, mixed forests tend to be more effective in delivering a range of provisioning services (e.g., Gamfeldt et al 2013;Forrester and Bauhus 2016), and are more resistant to various disturbances than single-species planted forests . These relationships between forest type, biodiversity and ecosystem services are highly relevant for informing forest policy and management.…”