Forests are one of the biomes mostly affected by the recent climatic changes (Change, 2018), expressed in an increase in fire frequency, tree mortality, and slower forest regeneration (Kowaljow et al., 2019). This recent increase in mature tree mortality highlights the role of seedling establishment in guaranteeing the sustainability of forests in the future (Whitmore, 1998). However, seedling establishment is often limited by numerous biotic (Gorchov and Trisel, 2003, De La Cruz et al., 2008) and abiotic (Alvarez-Aquino et al., 2004) environmental factors. Because of their small size, seedlings are often at a competitive disadvantage when competing with mature trees (Dickie et al., 2005) or herbaceous vegetation (Van Der Waal et al., 2009). This disadvantage, which results from their limited access to aboveground (light) and belowground (water and nutrients) resources, can hamper seedling growth and reduce their resilience to droughts (Cavender-Bares and Bazzaz, 2000, Pozner et al., 2022) and competition (Fetene, 2003).
The survival, establishment and growth of many tree species depends on mutualistic interactions. Mycorrhiza, the relationship between plants and root inhabiting fungi, is one of the most widespread mutualistic interactions in nature (Peay, 2016). Ectomycorrhiza is a reciprocal relationship in which the tree supplies carbon to the fungi, which in return makes minerals accessible to the tree (Smith and Read, 2010). In addition, the fungi increase the surface area of the root tip, allowing the roots to absorb minerals and water from a wider surface (Wu et al., 2012). Moreover, for many tree species, the ectomycorrhizal relationship is obligatory (Smith and Read, 2010). Therefore, ectomycorrhiza is a mutual interaction that greatly affects the health of the forest and its persistence (Rudgers et al., 2007).
It has been suggested that the nature of mutualistic interactions depends on environmental conditions (Begon et al., 1986). In the case of ectomycorrhiza, benign conditions, such as high nutrient availability (Bai et al., 2020), have been shown to limit fungal colonization of roots. This is probably the result of independent resource uptake by plant roots, making the interaction redundant from the plant’s perspective. However, natural conditions are often not benign, and mutualism can also be limited by competition over shared resources or by the ability of one or both partners to cope with environmental stress. It is therefore expected that under natural conditions, mutualisms in general and mycorrhiza specifically, would range from being mutually beneficial to being detrimental (Johnson et al., 1997).
Under drought conditions, mycorrhizae were found to transfer water between trees (Egerton-Warburton et al., 2007, Kakouridis et al., 2020). Moreover, ectomycorrhizal mycelium can benefit trees by extracting inaccessible water from micro-crevasses within soil particles (Bornyasz et al., 2005). Wang et al. (2021), show that under water limited conditions, ectomycorrhizal fungi can alleviate hydraulic failure and reduce carbon starvation of plants, which leads to an increase in plant growth (i.e., height and biomass increase), while reducing their mortality rate. However, all these plant benefits are dependent on the EMF ability to survive and perform under stress. Because both the fungi and the plant require water, it is not clear whether under drought conditions the ectomycorrhizal interaction would remain beneficial for the plant, or rather shift to neutrality, or even become detrimental. While drought is a pivotal stressor in many ecological systems, it is often combined and possibly enhanced by inter-plant competition (Kaisermann et al., 2017). Unlike the effects of drought, which did receive some attention in the literature (Garbaye, 2000, Lehto and Zwiazek, 2011, Nickel et al., 2018, Sebastiana et al., 2018, Wang et al., 2021), the influence of EMF on a seedling’s ability to cope with other plant competitors was hardly studied.
Only a few studies tested the effect of EMF on inter-specific plant competition (Pedersen et al., 1999, Shi et al., 2017, Peay, 2018, Van Nuland et al., 2022). EMF inoculation has been shown to affect the outcomes of competition. These effects seem to depend on the nature of the competitor (Shi et al., 2017, Van Nuland et al., 2022), the nutrients available in the soil (Pedersen et al., 1999, Van Nuland et al., 2022) and the timing of the inoculation (Peay, 2018). To the best of our knowledge, no manipulative studies have been conducted on the interaction between drought, inter-specific plant competition and EMF in their effect on seedling establishment.
In this study we tested whether EMF provide an advantage to Allepo pine (Pinus halepensis Miller) seedlings experiencing drought, competition with an annual grass, or both. Mediterranean tree species are prone to relatively frequent environmental changes such as droughts (Petit et al., 2005, García de Jalón et al., 2020) which could be exacerbated by competition with annual vegetation. Aleppo pine is the most common forest tree species around the Mediterranean basin (Ne'eman and Osem, 2021), is well adapted to local conditions (Klein et al., 2011, Voltas et al., 2018, Patsiou et al., 2020) and highly dependent on EMF (Livne-Luzon et al., 2017, Avital et al., 2022, Cahanovitc et al., 2022). We hypothesized that the relative advantage that the EMF can provide to the pine seedlings will increase under drought and/or competitive conditions since both stressors will minimize the ability of the plant to acquire soil resources independently.