Arbuscular mycorrhizal fungi in changing mountain grassland ecosystems

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MOUNTAIN VEGETATION UNDER GLOBAL CHANGE

Considerable attention is hence given to determine and predict climate change effects for mountain vegetation. For the time being, it can be considered that increasing temperatures and concomitant altered precipitation induce an upward movement of plant species resulting in richer but unbalanced vegetation communities: initially dominant species are submitted to suboptimal growing conditions while facing increased competition for important resources (Kullman 2010; Grabherr et al. 2010; Pauli et al. 2012). Increased seed production and better viability of most tree species leads to an upward movement of the treeline and therefore a shrinking alpine life zone situated above (Kullman 2010; Gottfried et al. 2012). Higher up the altitudinal gradient, a retreat and a transformation of snow bed communities to rich grasslands can be observed (Kullman 2010; Gottfried et al. 2012; Pauli et al. 2012). Enhancing the low CO2 availability in alpine ecosystems can lead to positive effects on photosynthesis and to some plant species’ specific effects on plant tissue quality or growth rate, but not to the expected extent (Körner 2003). Nitrogen (N) and phosphorus (P) enrichment in cool ecosystems such as mountain grasslands leads to an accumulation of N (respectively P) and chlorophyll in plant tissue, but does not necessarily increase plant biomass in all plant species. Graminoid species in particular tend to react to increased nutrient availability with increased growth and increased specific leaf areas. In addition to that, a general increase of graminoid proportion in grassland communities is observed, especially of sedges. The proportion of legumes and herbaceous species is reported to decline with increased nutrient availability, which results in a general change in the dominant plant functional groups (Michelsen et al. 1996; Bassin et al. 2007; Bassin et al. 2009; Liu et al. 2012).
Plant community composition is structured by the competition for resources in the majority of ecosystems, and community composition in grassland ecosystems by belowground competition for nutrients in particular (Tilman 1985; Casper and Jackson 1997). A change of plant community composition is expected whenever the availability of limiting resources is changed. Specialised root functional traits as well as symbiotic relationships with mutualistic soil microbes such as arbuscular mycorrhizal fungi (AMF) to enhance nutrient acquisition become therefore a crucial factor for the persistence of a plant species in a grassland ecosystem and also play an important role when environmental parameters are changed (Tilman 1985; Casper and Jackson 1997).

AMF AND THEIR RELATIONSHIP TO PLANTS

AMF from the fungal phylum Glomeromycota colonise the roots of about 70% of the world’s vascular plant species and thus provide the plant with important nutrients, mainly N and P, as well as water and protect it from pathogens (Smith and Read 2008). Thereby they can also alter plant responses to different global change parameters (Kivlin et al. 2013). In exchange, AMF receive photosynthetic carbon compounds which they invest in intra- and extra-radical fungal structures such as hyphae, spores and vesicles (Smith and Read 2008). Via their extra-radical hyphae AMF also positively affect soil aggregation and structure (Piotrowski et al. 2004; Wilson et al. 2009; Leifheit et al. 2014). While AMF are obligate symbionts, plant species can be either dependent on AMF, optional in a relationship with AMF, or completely non-mycorrhizal (Klironomos 2003; Bidartondo 2005; Smith and Read 2008). The AMF–plant relationship is generally defined as mutualistic. However, in about 25% of studies it is described as parasitic (Johnson et al. 1997; Klironomos 2003; van der Heijden and Horton 2009; Johnson and Graham 2013). The functioning of the plant–AMF relationship has been conceptualised in different ways over the last few years (Smith and Smith 2015), for example as a mutualism–parasitism continuum (Johnson et al. 1997; Johnson and Graham 2013).
Individual plant species or groups of plant species display a varying responsiveness to different AMF species (Werner and Kiers 2015). The growth of subordinate plant species is for example promoted by AMF and the competitiveness of dominant species reduced (Urcelay and Diaz 2003; Mariotte et al. 2013a). In that way, AMF community composition seems to determine plant community composition (van der Heijden 2002). Altogether, AMF biodiversity and plant biodiversity are correlated (van der Heijden et al. 1998). Two possibilities to explain the drivers of interactions between AMF and plant communities are presented in the Driver/Passenger hypothesis by Hart et al. (2001): if AMF were Drivers, they would orchestrate the changes in the plant community. If they were Passengers, changes in their community composition would be a by-product of changes within the plant community. Zobel and Öpik (2014) add the Habitat hypothesis as a third possibility to explain concurrent changes in plant and AMF communities by abiotic conditions. First and last, the role of abiotic drivers and which community drives which remains to be determined (Hart et al. 2001; Zobel and Öpik 2014).
The advanced use of molecular tools in AMF research during the last few decades has illustrated that individual AMF species or whole species communities differ in their characteristics and needs. Knowledge on different traits of AMF species and their specific interaction with plants contributes to the development of new trait-based frameworks to understand symbiotic partner selection, specific adaptations to particular environmental conditions and eventually whole-ecosystem processes (van der Heijden and Scheublin 2007; Reinhart et al. 2012; Chagnon et al. 2013; Behm and Kiers 2014; Mccormack et al. 2014; Aguilar-Trigueros et al. 2015; Chagnon et al. 2015; Werner and Kiers 2015). Advances in research on their traits and functioning also serve to quantify and eventually predict soil aggregation and ecosystem productivity (van der Heijden and Scheublin 2007; Aguilar-Trigueros et al. 2015; Rillig et al. 2015).

AMF RESPONSE TO GLOBAL CHANGE

The effects of global change can affect AMF both directly and indirectly. For example, warming induces mineralisation of N compounds, which leads to an increase of N and P availability in the soil (Sierra et al. 2015), which in turn affects AMF community composition and properties as well as their relationship to plants (e.g. Lesica and Antibus 1986; Mullen and Schmidt 1993; Kivlin et al. 2013; Ren et al. 2013). Warming-induced shifts of host–plant community composition additionally affect AMF via the identity of their individual host plant and its (new) neighbours (van der Heijden et al. 2003; Hausmann and Hawkes 2009; Compant et al. 2010; Mohan et al. 2014). Warming also affects water availability: drought stress can have negative, positive or no effect at all on AMF. This variety of results might be due to AMF species-dependent reactions (Fitter et al. 2004; Compant et al. 2010; Sun et al. 2013; Mohan et al. 2014). Plants under drought stress are known to benefit from AMF symbiosis in many different aspects, such as greater leaf water content or an increased photosynthetic rate (Augé 2001; Mohan et al. 2014).
Direct effects of warming on AMF are mainly demonstrated by increased root colonization rates, but opposite or neutral effects of warming on AMF or their relationship to plants can also be found. Like other soil micro-organisms, AMF normally achieve their optimal performance at higher average temperatures than are found in mountain ecosystems. They might therefore increase their performance and biomass with increasing temperatures in previously cooler ecosystems (Tibbett and Cairney 2007; Margesin et al. 2009).
Furthermore, warming enhances carbon allocation to AMF (Compant et al. 2010; Kivlin et al. 2013; Mohan et al. 2014), an aspect that becomes even more interesting with regard to globally rising CO2 levels. These can increase plant photosynthesis and thus the availability of carbohydrates for allocation to AMF. A positive effect of increased CO2 on both AMF abundance and plant performance has been shown by several studies. In return, however, AMF showed no direct beneficial effects on plant performance under elevated CO2 (Treseder 2004; Alberton et al. 2005; Kivlin et al. 2013). N and P enrichment in ecosystems is also known to directly affect AMF community composition, AMF abundance, the AMF colonisation rate and allocation depending on the degree of nutrient limitation of host plants in the ecosystem concerned (Treseder and Allen 2002; Johnson et al. 2003; Treseder 2004; Egerton-Warburton et al. 2007; Johnson 2010). The beneficial effect of AMF on plant performance commonly decreases with nutrient enrichment (Kivlin et al. 2013). Changes in air chemistry – mainly increased presence of nitrogen oxides, sulphur dioxide and carbon dioxide – lead to acidification of soils. However, the effects of changes in air chemistry on soil organisms like AMF can be delayed to a certain extent because most soils feature a certain buffering capacity (Bellgard and Williams 2011). As a result, AMF performance and community composition in mountain ecosystems may at first be indirectly impacted by altered air chemistry through the changes in plant life forms and functional groups (Davison et al. 2011; López-García et al. 2014) induced by warming and increased nutrient availability. Different land use practices alter nutrient conditions in the soil as well. Besides that, they also affect AMF in a direct way through soil disturbance: AMF sporulation cycles, root colonisation rate, abundance and diversity differ between organic and conventional farming practices, between arable land and grassland (Oehl et al. 2009; Oehl et al. 2010) as well as between meadows and pastures (Morris et al. 2013). AMF abundance, root colonisation rate and diversity generally decrease as land use intensity increases (Oehl et al. 2009; Oehl et al. 2010; Morris et al. 2013).

TIME TO REASSESS

Due to their profound long-term effects on both vegetation and soil, research on AMF community composition, their traits and ecology under the conditions of future global change in mountain grasslands is important for predicting shifts in vegetation composition, carbon sequestration and soil stability in mountain ecosystems (Entry et al. 2002; Rillig 2004; Treseder and Turner 2007; Hawkes et al. 2008; Wilson et al. 2009). Twenty years ago, a review on mycorrhizal fungi in arctic and alpine tundra (Gardes and Dahlberg 1996) was published, followed several years later by another paper assessing the functioning of the symbiosis in cooler ecosystems including mountain ecosystems (Tibbett and Cairney 2007). Since then research on AMF in mountain ecosystems has progressed, also due to new laboratory methods and techniques. Here we summarise research on AMF in grasslands along altitudinal gradients. We present a working hypothesis on the functioning of the AMF–plant interaction along altitudinal gradients: we expect this interaction to change along the mutualism–parasitism continuum following changes in environmental stress. Based on this hypothesis we propose future research directions. In the second part we present the state of the art of the research on AMF in mountain grasslands under global change, attempt to predict the functioning of the AMF–plant interaction along the mutualism–parasitism continuum at different altitudes and highlight the considerable potential for future research on mountain AMF and global change.

AMF IN MOUNTAIN ECOSYSTEMS

AMF PERFORMANCE AND DISTRIBUTION ALONG ALTITUDINAL GRADIENTS

Research on AMF distribution and root colonisation along altitudinal gradients has gained increased attention over the last 10 years (App. 1). We note a concentration of studies in the European Alps, the Rocky Mountains and the Chinese Tibetan Plateau, but no studies from an African mountain range or mountains in Australia were found (Fig. 2.1).
Fig. 2.1: Worldwide distribution of studies on AMF in mountain ecosystems: studies focus on A – altitudinal gradients, C – climate change, F – fertilisation and nutrient enrichment, H – habitat patchiness and environmental gradients, I – host identity, L – land use change, S – succession and V – season
Published studies show that AMF commonly colonise plant roots from mountain foothill zones up to as high as 5391 m above sea level (asl) in the Peruvian Andes (Haselwandter and Read 1980; Read and Haselwandter 1981; Cripps and Eddington 2005; Schmidt et al. 2008; Ranelli et al. 2015). The colonisation rate varies widely between plant hosts and among studies from an approximately 13% average colonisation rate (e.g. Lugo et al. 2012; Rudgers et al. 2014) to more than 70% (e.g. Ruotsalainen et al. 2004; Kagawa et al. 2006) of root lengths colonised by AMF. Inter-annual AMF colonisation rates are consistent (Ranelli et al. 2015) but can vary seasonally (Lugo et al. 2003; Sun et al. 2013). Spore density also displays seasonal patterns (Lugo and Cabello 2002).
The intensity of AMF root colonisation, spore density as well as the abundance of phospholipid fatty acid (PLFA) biomarkers for AMF in the soil microbial community generally declines with increasing altitude (Read and Haselwandter 1981; Haselwandter 1987; Väre et al. 1997; Ruotsalainen et al. 2004; Cripps and Eddington 2005; Lugo et al. 2008; Budge et al. 2011; Gai et al. 2012; Li et al. 2015). This is in line with an observed vegetation cover increase of non-mycorrhizal plants as well as the increase in the proportion of plants in symbiosis with other mycorrhiza than AMF with increasing altitude (Väre et al. 1997). The percentage of total root length colonised by vesicles or arbuscules is not affected and does not decrease with rising altitude in most studies (Schmidt et al. 2008; Lugo et al. 2012; Li et al. 2014). Vesicles are regarded as AMF storage organs (Smith and Read 2008), and therefore their proportion of intra-radical colonisation is expected to be higher in extreme environments where storage organs are necessary to survive times of adverse conditions. Oehl and Körner (2014), for example, report the abundance of multiple vesicles in plant roots from an ice-free crest at 4545m asl in the Swiss Alps, the most extreme place showing evidence of a well-established AMF community, which is comparable to AMF communities in alpine and mountain grasslands (Oehl and Körner 2014).
Different colonisation rates and abundance levels of AMF structures in soil and root can also be the expression of different AM fungal traits in a changing AMF community along the altitudinal gradient, depending on host plant species and on changing environmental conditions according to different seasons (e.g. Bever et al. 1996; Egerton-Warburton and Allen 2000; Oehl et al. 2009). The AMF community composition along altitudinal gradients is documented in many studies by identification of colonisation type and spores (Crush 1973; Haselwandter and Read 1980; Lugo et al. 2008; Gai et al. 2009; Zubek et al. 2009; Gai et al. 2012; Shi et al. 2014; Coutinho et al. 2015) as well as PCR-based genetic analysis (Li et al. 2014; Li et al. 2015). It differs between root and soil (Liu et al. 2012; Yang et al. 2013), in accordance with other observations (Hempel et al. 2007; Varela-Cervero et al. 2015).
Different AMF species are active or inactive (dormant as spores) under varying environmental conditions. Some AMF species are known for example to be active under cooler or wetter environmental conditions (seasons) and dormant during warmer conditions (Pringle and Bever 2002; Oehl et al. 2006; Oehl et al. 2012b), while other are active under exactly opposite conditions and still others do not display seasonal patterns at all (Oehl et al. 2009). It is no surprise therefore that AMF diversity in mountain ecosystems also changes with varying conditions of temperature and drought depending on the season of the year (Lugo and Cabello 2002), which is displayed in a seasonal variation of different mycorrhizal traits such as sporulation or root colonisation (Lugo et al. 2003; Sun et al. 2013). However, diversity indices are not always affected by altitude. While AMF species richness decreases with increasing altitude (Lugo et al. 2008; Gai et al. 2012; Shi et al. 2014) other diversity indices do not show consistent patterns across studies and mountain ranges. α-diversity calculated by Shannon-Wiener index either decreases with increasing altitude (Shi et al. 2014) or is not affected by it (Gai et al. 2012) and β-diversity assessed by Whittaker’s index indicates that the heterogeneity is increased with increasing altitude (Shi et al. 2014).
All in all, up to more than 60 different AMF species are found in mountain ecosystems (Oehl and Sieverding 2004; Shi et al. 2014). Acaulosporaceae and above all Glomeraceae are the dominant AMF families reported from altitudinal studies (Sýkorová et al. 2007; Gai et al. 2012; Li et al. 2014; Zheng et al. 2014; Coutinho et al. 2015). The abundance of Glomeraceae decreases with altitude, whereas species of other genera tend to increase (Li et al. 2015). Altitude favours the intra-radical colonisation of fine root endophytes (Crush 1973; Haselwandter and Read 1980; Read and Haselwandter 1981; Ruotsalainen et al. 2004; Zubek et al. 2009), also described as Glomus tenuis (Hall 1977), which again shows that different fungal traits are displayed at higher altitudes what could also be caused by a change of AMF community composition.
Taxonomic studies lead to the conclusion that both generalist AMF and unique specialised AMF species are common in mountain ecosystems (Liu et al. 2011; Oehl and Körner 2014; Liu et al. 2015a) and several new AMF species from mountain ecosystems are described (Oehl and Sieverding 2004; Oehl et al. 2006; Spain et al. 2006; Oehl et al. 2011a; Oehl et al. 2012a; Oehl et al. 2012b; Palenzuela et al. 2013) and distinct AMF communities reported (Liu et al. 2015a). Species such as Acaulospora alpina, Acaulospora nivalis and Ambispora reticula, which have been extracted exclusively from mountain grasslands, might be characteristic of these ecosystems and may have as yet unidentified adapted mycorrhizal traits (Oehl et al. 2006; Oehl et al. 2012a; Oehl et al. 2012b). So far, research cannot account for the underlying mechanisms of species diversity and distribution along altitudinal gradients, as different studies and methods are difficult to compare. When considering growth-limiting factors, plant-derived carbon is the main factor for AMF (Chagnon et al. 2013). This is reflected in a model for optimal AMF colonisation and advantage for the plant along altitudinal gradients, which suggests that the host plant’s photosynthetic nutrient use efficiency determines AMF colonisation along altitudinal gradients when nutrient concentration remains constant (Ruotsalainen et al. 2002). More research evaluating the Driver/Passenger and Habitat hypotheses (Hart et al. 2001; Zobel and Öpik 2014) in mountain ecosystems will provide answers.
As shown, altitude is only a weak predictor for AMF distribution and colonisation (Ranelli et al. 2015). AMF distribution does not necessarily have an altitudinal limit but is determined by multiple other co-occurring variables such as land use intensity, water availability, soil structure and composition and AMF propagule availability with decreasing vegetation cover (Haselwandter and Read 1980; Lesica and Antibus 1986; Cázares et al. 2005; Schmidt et al. 2008; Gai et al. 2009; Casanova-Katny et al. 2011; Oehl et al. 2011b; Coutinho et al. 2015; Liu et al. 2015a), which can blur the altitudinal effect. In the following we discuss factors other than altitude that determine AMF presence and performance in mountain ecosystems.

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THE ROLE OF HABITAT PATCHINESS AND ENVIRONMENTAL GRADIENTS

Mountain ecosystems with their environmental changes over short distances lead to a visible mosaic of plant communities. Both abiotic and biotic factors are known to condition AMF distribution and performance (Entry et al. 2002; Kivlin et al. 2011; Soudzilovskaia et al. 2015). Research on whether the belowground AMF mosaic mirrors the aboveground patchiness helps to disentangle environmental factors from the host plant impact (Gardes and Dahlberg 1996; Barnola and Montilla 1997; Cripps and Eddington 2005; Becklin and Galen 2009; Ranelli et al. 2015) and eventually understand if AMF are Drivers and assist plants in defining their ecological niche (Hart et al. 2001; de Carvalho et al. 2012; Zobel and Öpik 2014; Li et al. 2015). We found few studies that directly address habitat patchiness and localised spatial environmental gradients in mountain ecosystems (App. 1).
Water availability is found to be a determining factor for the colonisation rate in several studies conducted in mountain grassland settings: wetter patches display lower AMF root colonisation (Barnola and Montilla 1997; Rudgers et al. 2014) and higher abundance of extra-radical hyphae (Rudgers et al. 2014) than dryer patches. Soil texture, a property directly linked to retaining and soaking water as well as drainage, also determines AMF richness (De Carvalho et al. 2012).
In another study, the variation of light, nutrient availability and host-specific life strategies leads to a mosaic of mycorrhizal patterns that mirrors plant patterns above (Becklin and Galen 2009). Eventually, all of these studies partly confirm the Habitat hypothesis (Zobel and Öpik
2014) and suggest that numerous environmental parameters determine AMF community composition and AMF performance (Barnola and Montilla 1997; Becklin and Galen 2009; de Carvalho et al. 2012; Rudgers et al. 2014; Zhang et al. 2014; Ranelli et al. 2015). Changing AMF parameters along different environmental gradients can also be interpreted as expression of different fungal traits, which could also indicate a change in AMF community composition along these gradients. The development of AMF community composition, their dispersal capacities as well as the drivers of plant–soil–AMF interactions can be studied especially well along successional sequences.

AMF ALONG SUCCESSION GRADIENTS

Successional sequences ranging from 0 to 135 years in the forefront of glaciers, after volcanic eruptions and mechanical anthropogenic disturbances have allowed research along developmental gradients in mountain ecosystems in the USA, the European Alps, the Brazilian Altiplano and Japan (Allen et al. 1984; Allen et al. 1987; Allen et al. 1992; Titus et al. 1998; Titus and Del Moral 1998; Titus and Tsuyuzaki 2002; Wu et al. 2004; Fujiyoshi et al. 2005; Cázares et al. 2005; Wu et al. 2007; Liu et al. 2011; Oehl et al. 2011b; Duchicela et al. 2013; Welc et al. 2014).
Plants in their initial successional stages, emerging after glacier retreat or on volcanic substrates, are not in relationship with AMF (Allen et al. 1984; Allen et al. 1992; Titus and Tsuyuzaki 2002; Fujiyoshi et al. 2005; Cázares et al. 2005; Oehl et al. 2011b). Heat sterilisation and subsequent gas emissions after volcanic eruptions can lead to an inhibited and slow reestablishment of fungal propagules. Moreover, soil-borne AMF spores together with extra-radical hyphae comprise low-mobility fungal propagules. This is illustrated on the pumice plain of Mount St. Helens where AMF symbiosis was established only after the arrival of ants and rodents (Allen et al. 1984; Allen et al. 1992). Distribution by birds, as described by other studies from lowland ecosystems (Nielsen et al. 2016), is also conceivable but has not yet been reported from mountain ecosystems. Spores of some smaller species are also transported by wind, but bigger spores and extra-radical hyphae might arrive in bare soils mainly through animals, erosion or events such as landslides (Allen et al. 1984; Allen et al. 1987; Allen et al. 1992; Cázares et al. 2005; Oehl et al. 2011b; Oehl and Körner 2014). Moreover, since AMF are obligate symbionts, it may be concluded that, as stated in the Passenger hypothesis (Hart et al. 2001), AMF follow the development of the vegetation community rather than precede it during primary succession (Titus et al. 1998; Hart et al. 2001; Cázares et al. 2005; Oehl et al. 2011b; Zobel and Öpik 2014). The development of AMF–plant interaction is, however, not only dependent on propagule availability or successional age. Microhabitat factors such as soil moisture, nutrient availability, plant seed trapping ability and light conditions affect AMF distribution across different successional sequences in mountain ecosystems as well (Allen et al. 1987; Titus et al. 1998; Titus and Tsuyuzaki 2002). A linear improvement of soil aggregation due to the fungal community and in particular mycorrhiza during secondary succession (Duchicela et al. 2013) is not always observed (Liu et al. 2011).
Changing AMF parameters along successional sequences in mountain ecosystems with advancing soil successional stage, namely increased AMF propagule abundance and increased species richness in the soil as well as a decreased intra-radical colonisation rate (Allen et al. 1987; Fujiyoshi et al. 2005; Cázares et al. 2005; Liu et al. 2011; Oehl et al. 2011b; Welc et al. 2014), suggest a concurrent underlying change in AMF community composition with progressing succession. At first view this seems in accordance with a trait-based C-S-R (competitor, stress tolerator, ruderal) framework (Chagnon et al. 2013), which suggests that early successional stages are dominated by ruderal AMF, fast-growing and early-sporulating species, followed by a dominance of competitor AMF, species investing mainly in resource acquisition by growing extra-radical mycelium, which are finally replaced by stress-tolerant AMF, species that cope well with low carbon input. Whether AMF species distribution along successional sequences indeed follows the proposed C-S-R framework remains to be examined. Additionally a transition of the dominance in mycorrhizal types from AMF to other mycorrhizal types such as ecto-mycorrhizae has been observed and fits the shift of the vegetation community to a dominance of woody plants with increasing successional stages (Welc et al. 2014).
Based on studies on habitat patchiness and succession in mountain ecosystems, we conclude that abiotic soil properties determine AMF community composition and performance only to a certain extent and that other factors such as plant species identity, vegetation cover and neighbouring plants have to be taken into account (Cázares et al. 2005; Liu et al. 2011; Oehl et al. 2011b; Welc et al. 2014).

THE ROLE OF HOST IDENTITY AND ITS NEIGHBOURING PLANTS

Several studies in mountain ecosystems suggest that host identity is more important for the colonisation rate and AMF community than environmental parameters (Lugo et al. 2008; Becklin et al. 2012; Rudgers et al. 2014; Zubek et al. 2014; Welc et al. 2014; Ranelli et al. 2015). In some studies, the intra-radical colonisation rate, for example, is strongly determined by host plant identity and its evolutionary history in mountain ecosystems (Rudgers et al. 2014; Ranelli et al. 2015). However, it was not possible to regroup specific AMF colonisation patterns by plant functional groups or metabolic type in the available studies set in mountain ecosystems (Lugo et al. 2008; Ranelli et al. 2015). Different plants growing in the same habitat patch can host different AMF communities. This is visibly expressed in different colonisation rates and clearly suggests host specificity of AMF communities in mountain ecosystems (Sýkorová et al. 2007; Becklin et al. 2012; Li et al. 2014). In the study reported by Sýkorová et al. (2007), a strong host preference of AMF species was visible. Only Glomus intraradices sequences are present in the closely related target species Gentiana verna and Gentiana acaulis at 1800 and 2000 m asl in the Swiss Alps. However, not all studies support this (Li et al. 2015).
Alien plant species or weeds in mountain ecosystems were less dependent on AMF symbiosis (Becklin and Galen 2009; Casanova-Katny et al. 2011). However, the use of local AMF inoculum promotes the native plant community after the removal of alien plant species (Rowe et al. 2007; Rowe and Brown 2008), indicating that a specific local AMF community might nevertheless provide a certain barrier to plant invasion. The interaction of invasive plant species with AMF in mountain ecosystems was considered in only a few studies and leaves room for more research. All in all, the role of abiotic drivers (Zobel and Öpik 2014) and whether AMF are Drivers or Passengers (Hart et al. 2001) in mountain ecosystems remains to be determined.

AMF INTERACTION WITH OTHER SOIL MICROBES AND ABOVE-GROUND SYMBIONTS IN MOUNTAIN ECOSYSTEMS

AMF are also studied in context with other soil organisms such as bacteria or other fungi (e.g. Haselwandter and Read 1980; Väre et al. 1997; Cripps and Eddington 2005; Budge et al. 2011; Zubek et al. 2011; Vanesa et al. 2013; Ranelli et al. 2015) as well as above-ground endophytes (Ranelli et al. 2015) in mountain ecosystems. Factors such as host plant species (Haselwandter and Read 1980; Cripps and Eddington 2005; Ranelli et al. 2015), successional stage (Welc et al. 2014) and pH (Ruotsalainen and Eskelinen 2011) appear to have more direct influence on other soil microbes and above-ground endophytes than their interaction with AMF and vice versa. However, not a lot of evidence exists because all but few studies in mountain ecosystems ignore that multiple symbiotic interactions can have different effects on the host plant than pair-wise interactions alone. In multiple symbiotic interactions, as they exist for example when both foliar endophytes and root endophytes colonise a plant, it is likely that those symbionts also affect each other, especially when they receive similar benefits from the host plant (Mack and Rudgers 2008). Ranelli et al. (2015), to our knowledge the only study that considered the interaction between above-ground plant endophytes, DSE and AMF in mountain grassland, see no interaction of above-ground and below-ground endophytes and report that DSE and AMF were positively correlated. Including other endophytes and their effects into future studies will help to get a clearer picture of the actual effect mechanisms driving AMF presence in mountain ecosystems. Interaction of AMF with non-fungal soil organisms is rarely studied, because it is necessary to manipulate both underground microbes and AMF. This is difficult in the field, hence the lack of realistic field studies (van der Heijden et al. 2003).

Table of contents :

1 Introduction
1.1 Global change
1.2 Mountain grassland
1.3 Plant-soil interactions
1.4 Thesis outline
1.5 References
2 Arbuscular mycorrhizal fungi in changing mountain grassland ecosystems – a challenge for research
2.1 Abstract
2.2 Résumé
2.3 Introduction
2.3.1 Mountain vegetation under global change
2.3.2 AMF and their relationship to plants
2.3.3 AMF response to global change
2.3.4 Time to reassess
2.4 AMF in mountain ecosystems
2.4.1 AMF performance and distribution along altitudinal gradients
2.4.2 The role of habitat patchiness and environmental gradients
2.4.3 AMF along succession gradients
2.4.4 The role of host identity and its neighbouring plants
2.4.5 AMF interaction with other soil microbes and above-ground symbionts in mountain ecosystems
2.4.6 Applying the present knowledge
2.4.7 Outlining the functioning of AMF–plant interaction along an altitudinal gradient
2.4.8 Future research on AMF along altitudinal gradients
2.5 AMF in changing mountain ecosystems
2.5.1 Climate change: the effects of warming and altered precipitation on AMF in mountain ecosystems
2.5.2 Mountain AMF and altered atmospheric chemistry and nutrient enrichment
2.5.3 Land use change
2.5.4 Interaction of global change factors
2.5.5 Outlining the functioning of the AMF–plant relationship along an altitudinal gradient under future global change conditions
2.5.6 Future research on AMF in changing mountain ecosystems
2.6 Conclusion and perspectives
2.7 Acknowledgements
2.8 References
3 Arbuscular mycorrhizal fungi along an altitudinal gradient in the Central European Alps
3.1 Abstract
3.2 Introduction
3.3 Study site
3.4 Material and methods
3.5 Results
3.5.1 Soil properties
3.5.2 AMF abundance in the soil
3.5.3 AMF colonisation rate
3.6 Discussion
3.7 Conclusion
3.8 References
4 Arbuscular mycorrhizal fungi along altitudinal gradients under climate change conditions
4.1 Abstract
4.2 Introduction
4.3 Material and Methods
4.3.1 Study site
4.3.2 Transplantation process
4.3.3 Soil sampling
4.3.4 Determining AMF abunance in the soil
4.3.5 Root sampling
4.3.6 Analysis of fungal colonisation
4.3.7 Data analysis
4.4 Results
4.4.1 Soil properties
4.4.2 Abundance of AMF in the soil microbial community
4.4.3 AMF root colonisation rate
4.5 Discussion
4.6 Conclusion
4.7 Acknowledgements
4.8 References
5 Effects of AMF, climate change and plant diversity on plant productivity
5.1 Introduction
5.2 Material and Methods
5.2.1 Seed and soil collection
5.2.2 Experimental design
5.2.3 Soil sterilisation and microbial wash
5.2.4 Seed germination
5.2.5 Temperature and air humidity
5.2.6 Irrigation
5.2.7 Plant sampling
5.2.7 Soil sampling
5.2.8 DNA extraction and quantification
5.2.9 PCR amplification of target regions
5.2.10 Control of sequence quality, OTU clustering, and taxonomic assignment
5.2.11 Competitive effect
5.2.12 Statistical analysis
5.3 Results
5.3.1 Glomeromycota and microbial community
5.3.2 Effects of AMF presence on plant biomass production
5.3.3 Effect of future climate factors on plant biomass production
5.3.4 Effects of AMF presence on plant competitiveness
5.3.5 Effects of future climate factors on plant competitiveness
5.4 Discussion
5.5 Conclusions
5.6 Acknowledgements
5.7 References
6 Synthesis
6.1 A novel framework to assess the AMF-plant relationship along altitudinal gradients
6.2 AMF in mountain grassland ecosystems
6.3 Climate change effects on AMF in mountain grassland ecosystems
6.4 Outlook
6.5 Conclusion
6.6 References
Appendices
Appendix 1: Supplementary material for Chapter 2
Appendix 2: Supplementary materials for Chapter 4
Curriculum Vitae
Publications and Presentations
Résumé
Abstract

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