The Importance Of Soil Organic Matter
Nurse plants are also reported to indirectly affect the survival and fitness of plants growing under its canopy through an intermediary species (nurse–intermediary–protégé) (Callaway, 2007). For instance, the decomposition and conversion of plant-derived organic matter into nutrients that can be readily used by the plant depends on the below-ground microbial community and activity (Rodríguez-Echeverría, Lozano, & Bardgett 2016). Numerous studies have shown soil communities have a substantial influence on the diversity and composition of plant communities, in addition to individual plant performance (van der Heijden et al. , 1998; Lozano, Hortal, & Armas 2014).
However, the role of below-ground communities beneath nurse plants has received little attention. Within harsh environments, the soil under nurse plants are characterised by having higher decomposition and nutrient mineralisation rates, and are considered microbial hotspots (Rodríguez-Echeverría, Lozano, & Bardgett 2016). Recent experiments conducted in microcosms have shown that the associated soil microbial communities enhance establishment and growth of beneficiary species, independent of nutrients within the soil (Rodríguez-Echeverría, Armas, & Pistón 2013).
The development of nitrogen-rich microsites beneath nodulating nurse plants, such as legumes and alders, are shown to increase N availability for plants, biomass turnover, and microbial rates of N mineralisation (Flores & Jurado 2003). Legumes commonly act as nurse plants in stressful nutrient-limiting environments (Ren, Yang, & Liu 2008). In fact, leguminous plants represent approximately one-third of all nurse plants documented in arid and semi-arid regions (Flores & Jurado 2003; Rodríguez-Echeverría, Lozano, & Bardgett 2016). Numerous studies have shown that nurse plants typically increase microbial abundance and activity as well as the abundance of mycorrhizal fungi (spores and mycelium) (Hortal et al. , 2013; Navarro-Cano et al. , 2014). It is speculated that higher soil moisture and nutrient content underneath these nurse plants has lead to the increased microbial abundance in the environment (Rodríguez-Echeverría, Lozano, & Bardgett 2016). It is clearly demonstrated that mycorrhizal fungi associated with nurse plants can have a positive effect on the establishment, growth and performance of other plant species in arid and semi‐arid areas (Azcon-Aguilar et al. , 2003; Carrillo-Garcia, La Luz, & León, 1999), and in alpine communities (Casanova-Katny, Torres-Mellado, & Palfner 2011).
These findings highlighted the importance of soil microbial communities on individual plant performance. Although it is becoming increasingly more recognised within the literature, there is still very few studies that have assessed the importance of the soil microbiota as mediators of facilitation by nurse plants. Some of the most clearly documented instances of nurse plant facilitation are mediated indirectly through herbivores. Groups of plants can interact in ways to reduce herbivory of one or more species (Smit, Vandenberghe, & Den Ouden 2007). The establishment of vulnerable species in heavily grazed systems can be facilitated by nurse plants in the form of chemical (toxins) and mechanical (thorns, spines, waxy cuticles) defensive species, which protects vulnerable life stages of plant species against herbivores (Smit, Vandenberghe, & Den Ouden 2007).
For example, Quercus robur seedlings and saplings are highly palatable and suffer higher mortality from ungulate grazers when not associated with other unpalatable plant species (Bakker et al. , 2004). Field transplantation experiments have shown that Q. robu seedling mortality rates significantly decline when grown amongst unpalatable spiny Prunus spinosa shrubs through associational resistance (Bakker et al. , 2004). A study from northern Sweden found that birch (Betula pubescens) suffered high herbivory when associated with highly palatable plant species, such as Sorbus aucuparia and Populus tremuloides, and low herbivory when associated with unpalatable species, such as Alnus incana (Hjalten, Danell, & Lundberg 1993). Similar examples of associational defences have been reported by García & Ramón Obeso (2003), Smit, den Ouden, & Müller-Schärer (2006), and Rousset & Lepart (2001). This process is also referred to as defence guilds, associational avoidance, associational resistance or associational plant refuges (Milchunas & Noy-Meir 2002; Atsatt & Odowd 1976; Root, 1972; Pfister & Hay, 1988). It is suggested that facilitation becomes increasingly more important as grazing pressure increases (Holmgren & Scheffer, 2010). Indirect nurse-plant mechanisms are less extensively studied compared to direct mechanisms. It is mentioned that nurse plants can increase seed arrival of target species either by directly preventing movement of seeds past a physical barrier, or indirectly via animal assisted transport by providing birds perching sites (Vander Wall & Joyner, 1998; Giladi, Segoli, & Ungar 2013).
Although nurse plants can positively affect seed arrival of dispersing plant species, seed trapping is a mechanism that is not usually associated with nurse-plant interactions and is often overlooked in the literature. Additionally, nurse plants are observed to act as a magnet species for plants that are less attractive to pollinators, consequently increasing pollination of those target species (Laverty, 1992; Callaway, 1995). Due to the sheer complexity of plant-pollinator interaction networks, recent attempts to explore the indirect facilitation of pollinator visits by nurse plants (‘magnet species’) have been met with challenges (Feldman, Morris, & Wilson 2004; Ghazoul, 2006). As direct and indirect mechanisms may interact non-linearly, future research should aim to decouple the different pathways of facilitation and assess whether these mechanisms act independently of function in concertOccurrence of nurse plant syndromePlant interactions appear to be highly dynamic, whereby both competition and facilitation change along gradients of environmental stress (Pugnaire & Luque, 2001).
The importance of facilitation in arid environments, and therefore the strength of the SGH, was highly debated in 2005 (Maestre, Valladares, & Reynolds 2005; Maestre, Callaway, & Valladares 2009; Michalet et al. , 2006). Subsequent publications followed this controversy proposing two alternative theoretical models to the SGH, predicting a decline in facilitation intensity in extreme conditions and a switch from facilitation to competition in water-stressed ecosystems (Michalet, Le Bagousse‐Pinguet, & Maalouf 2014; Holmgren & Scheffer 2010; Verwijmeren, Rietkerk, & Wassen 2013). Since then, the field has rapidly expanded with recent developments furthering current knowledge on the evolutionary consequences of facilitation (Schöb, Prieto, & Armas 2014), beneficiary feedback effects on benefactors (Schöb et al. , 2014), implications of facilitation on the whole ecosystem (Danet, Kéfi, & Meneses 2017; Ren, Yang, & Liu 2008), and interactions with host-associated microbiota (Duponnois et al. , 2011; Rodríguez-Echeverría, Lozano, & Bardgett 2016; Hortal et al. , 2013). Numerous meta-analyses and theoretical research have since evaluated the validity of the SGH, which has lead to contrasting conclusions (de Toledo Castanho & Prado, 2014; Soliveres & Maestre 2014; Catorci, Malatesta, & Velasquez 2015). Some authors argue that the facilitative effects of nurse plants become more important in plant communities as abiotic or herbivory pressure increases (Antonsson, Björk, & Molau, 2009; Holmgren & Scheffer, 2010). However, experimental evidence supports a switch from facilitation to competition along gradients of environmental resource availability some plant communities water‐stressed ecosystems (Tielbörger & Kadmon, 2000).
Other studies have found that the facilitative effects weakened as soil fertility improves rather than by a change in competition (Pugnaire et al. , 2001). By contrast, de Toledo Castanho & Prado (2014) found that the benefit of shading by nurse plants did not change along a stress gradient. Furthermore, studies have shown that the soil under nurse plants was always richer than on barren areas irrespective of altitude (Mihoč et al. , 2016). It is now recognised that the occurrence or disappearance of facilitation across environmental gradients cannot be explained by a single theoretical model (Michalet & Pugnaire, 2016).
The presence of nurses and the strength of positive interactions with other plant species is determined by a suit of factors, such as the size and age of target species, plant traits and strategies, and abiotic conditions of the environment (Ren, Yang, & Liu 2008). For instance, nurses have stronger positive influence when the target species are young, whereas older or bigger target plants have a stronger competitive interaction (Ren, Yang, & Liu 2008). In many cases the beneficiary species will outcompete their nurses when nursing services are no longer required, causing its demise (Ren, Yang, & Liu 2008). This is commonly observed for columnar saguaro cactus species and their nurses (Fleming & Valiente-Banuet, 2002). The general focus within the literature has been on the role of nurse plants under harsh and highly variable climates. Very little attention has been paid to the role of positive interactions under fertile and moist ecosystems. There has been documented cases of facilitation under ‘seemingly benign’ conditions.
For instance, isolated studies have shown that shade provided by nurse plants is critical for the survival of seedlings, even in regions with relatively high soil moisture and fertility, such as rainforests in Amazonia (Vieira, Uhl, & Nepstad 1994), and New Caledonia (Rigg, Enright, & Perry 2002), temperate deciduous forests (Simard & Vyse, 2006), as well as in riparian forests in Tanzania (Sharam, Sinclair, & Turkington 2009). Organisms present in any given ecosystem is adapted to local abiotic and biotic conditions. Even slight fluctuations in optimal conditions may inflict a high degree of stress. For instance, rainforest tree species have evolved large leaf area ratio and specific leaf area to capture sunlight, making them vulnerable to water-deficit stress and high temperatures experienced with higher irradiance (Holmgren & Scheffer, 2010).
These shade-tolerant forest species may need facultative associations with nurse plants as they are unable to survive and grow in large treefall gaps; a pattern that is observed in many tropical and temperate forests (Hoffmann 2000; Ganade & Brown 2002). As temperate ecosystems provide the majority of liveable environments for the human population, and tropical ecosystems are the most heavily degraded on the planet, the role that nurse plants play in these environments can provide valuable information. Future research should be interested in evaluating the application of nurse plants in ecological restoration.
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