e. a lower abiotic stress ( Bertness and Callaway, 1994). This driver is expected to be different in TAE with a higher vegetation density consecutive to

the absence of durable snowbeds and Cabozantinib ic50 a continuous vegetative period. In two studies along TAE gradients, decreasing vegetation cover was correlated with amplified facilitative interactions among plants ( Smith, 1984 and Anthelme et al., 2012). While disentangling the respective influences on plant–plant interactions of all these abiotic parameters would require future studies, two hypotheses related to stress and disturbance sound particularly relevant to be tested in TAE. First, according to the SGH, increased aridity is expected to generate a higher frequency of facilitative interactions among plants, up to a threshold where Ixazomib chemical structure competitive effects predominate in the interactions (Michalet et al., 2006 and Maestre et al., 2009). Accordingly, a hypothesis to be tested is whether the stress–interaction

relationship is similar to that found along aridity gradients (with a reduction of facilitative interactions at the extremity of the gradient of stress) or whether the stress–interaction relationship rather resembles to that along altitudinal gradients in alpine environments, with an increase or at least a stabilization of the frequency of positive interactions with increasing stress (Choler et al., 2001 and le Roux and McGeoch, 2008). Second, variations Casein kinase 1 in frost-heaving regime is likely to alter the outcome of plant–plant interactions, with more facilitative effects observed either with stronger frost-heaving amplitudes (Venn et al., 2009) or with higher frost heaving frequency (Smith, 1981 and Pérez, 1987a). From this viewpoint, determining to what extent the amplitude of frost heaving (stronger in seasonal environments; Francou et al., 2001) or their frequency (higher in aseasonal environments) drive the outcome of plant–plant interactions

in TAE is a stimulating, unresolved challenge. Niche differentiation-related drivers of plant–plant interactions include at least four factors that are expected to vary between extratropical alpine environments and TAE (Fig. 1). First, facilitator’s size, which is expected to increase the frequency of facilitative interactions (Callaway and Walker, 1997), is larger in TAE than in extratropical alpine environments and associated with a higher ratio of aboveground to belowground biomass (Smith, 1994). A striking example of giant growth forms in TAE (Rundel, 1994) is giant rosettes such as Puya raimondii, which reach up to 4–6(12) m high ( Sgorbati et al., 2004). This “facilitator size” hypothesis seems to be corroborated by frequent observations of positive effects of giant rosettes on other plants ( Table 1: Smith, 1981, Smith, 1984, Pérez, 1987a, Young and Peacock, 1992 and Smith and Young, 1994).