There is an ongoing debate on mechanisms underlying a positive relationship between productivity and plant diversity which has been observed in a number of experimental ecosystems, such as grasslands (Tilman et al., 1996, 2001; Hooper & Vitousek, 1998; Hector et al., 1999; Schläpfer & Schmid, 1999), forests (Lovelock & Ewel, 2005), a natural phytoplankton community (Ptacnik et al., 2008), as well as in a meta-analysis of both aquatic and terrestrial ecosystems (Cardinale et al., 2006). The debate on the mechanisms accounting for such positive relationships between ecosystem productivity and species diversity has focused on three aspects: the ‘sampling effect’ (Huston, 1997), ‘complementarity’ and ‘facilitation’ (Hector et al., 1999). Under the sampling effect (or selection effect) hypothesis, when communities are assembled at random from a pool of species, more diverse mixtures have a higher probability of containing a species with higher productivity which may result in overyielding if it becomes dominant (Huston, 1997). The complementarity hypothesis states that complementary resource use and niche differentiation in space and time, or facilitation leading to increased resource availability, reduce interspecific competition and could lead to greater acquisition of limiting resources (Tilman et al., 2001). Generally, the complementarity effect includes both niche differentiation and facilitation, because distinguishing between them is difficult in practice (Loreau & Hector, 2001). There is some evidence for niche differentiation in space and time, for instance, via different rooting depths or in the time of the year for maximum nutrient acquisition (Hector et al., 2002; Fargione & Tilman, 2005). Direct plant–plant facilitation, as defined by Callaway (1995), is a beneficial effect of one individual on another due to, for example, the amelioration of harsh environmental conditions, including increased availability of a limiting resource. Alternatively, facilitation may act indirectly in other ways, such as elimination of potential competitors, introduction of beneficial organisms (i.e. soil bacteria, mycorrhizal fungi), introduction of pollinators or protection from herbivores.
There is limited direct experimental evidence for mechanisms of interspecific facilitation in ecological biodiversity experiments. A review by Brooker et al. (2008) presented plant–pollinator interactions, positive impacts of N2-fixing legumes on soil nitrogen (N) availability, the capacity for resource sharing through mycorrhizal networks, and classic ‘nurse plant’ effects as the major mechanisms of ecological facilitation in plant communities. However, our review of recent studies of overyielding in agricultural intercropping systems reveals additional mechanisms of interspecific facilitation that may well be highly relevant to natural plant communities. These mechanisms involve belowground processes whereby a plant of one species increases the availability of soil phosphorus (P), iron (Fe), zinc (Zn) or some other non-N nutrient that is limiting in a habitat, and thus, besides benefiting itself, also increases the growth of another species. Ecological studies of semi-natural and natural systems have not investigated such mechanisms (but see Muler et al., 2014).
Intercropping – that is, growing individuals of at least two crop species in close proximity at (about) the same time – is associated with interspecific interactions, including above- and belowground competition, and can lead to facilitation and overyielding in tropical and temperate habitats (Vandermeer, 1989). Research on rhizosphere processes and nutrient utilization in intercropping systems has provided a wealth of physiological evidence for interspecific facilitation between species (Horst & Waschkies, 1987; Ae et al., 1990; Zuo et al., 2000; El Dessougi et al., 2003; Li et al., 2003, 2007). The mechanisms underlying overyielding of intercropping systems, as documented in the last decade, are generally attributed to niche complementarity (Hector et al., 1999) and positive interspecific interactions (facilitation) in resource use (Hauggaard-Nielsen & Jensen, 2005; Li et al., 2007). This paper reviews and synthesizes recent work on interspecific facilitation of nutrient acquisition in intercropping systems, highlights possible mechanisms for increased productivity with increasing plant diversity, and suggests approaches for studying facilitative rhizosphere mechanisms in terrestrial ecosystems.