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Arbuscular mycorrhizal fungi (AMF) are soil fungi that form symbiotic associations with the roots of many plant species and are important in plant nutrition. Individual fungal species are able to interact with many plant species (Allen, 1991; Smith & Read, 1997) and fungal communities are perceived as being depauperate in comparison to most plant communities (Allen et al., 1995). Only c. 250 species of AMF have been described, and until recently these fungi were assumed to be functionally redundant (Bever et al., 2001). However, recent research has shown that some AMF species are ecologically distinct in their phenology (Gemma et al., 1989; Lee & Koske, 1994; Schultz et al., 1999; Pringle & Bever, 2002), their distribution with respect to edaphic factors (Wetzel & van der Valk, 1996) including soil pH (Abbott & Robson, 1977; Porter et al., 1987), moisture (Anderson et al., 1984; Rickerl et al., 1994; Miller & Bever, 1999; Miller, 2000; He et al., 2002) and nutrients (Anderson et al., 1984; Johnson et al., 1991, 1992; He et al., 2002), and their effect on plant species (Streitwolf-Engel et al., 1997, 2001; van der Heijden et al., 1998a).
The composition of AMF communities and their relationship to environmental gradients has been investigated in old-field (Johnson et al., 1991; Barni & Siniscalco, 2000), desert (Stahl & Christensen, 1982; Bethlenfalvay et al., 1984; Stutz et al., 2000), grassland (Gibson & Hetrick, 1988; Eom et al., 2001; Eriksson, 2001; Landis et al., 2004), tropical (Brundrett et al., 1996; Johnson & Wedin, 1997; Lovelock et al., 2003), urban (Stabler et al., 2001; Cousins et al., 2003), and wetland (Anderson et al., 1984; Turner & Friese, 1998; Miller & Bever, 1999; Turner et al., 2000) environments; however, little is known about the composition or distribution of the AMF community associated with riparian ecosystems in arid or semiarid regions (Kennedy et al., 2002). In the south-western United States, riparian ecosystems dominated by Populus and Salix form ecotones between aquatic habitats associated with lakes and rivers, and dry upland deserts (Gregory et al., 1991; Naiman & Decamps, 1997). These oases comprise only 1% of the land area in the south-west but are vital as wildlife habitat and migration corridors (Gregory et al., 1991; Briggs, 1996). These areas contain a unique assemblage of plant species not found in permanently flooded wetlands, nor in the arid deserts (Szaro, 1990; Minckley & Brown, 1994), and may also contain unique AMF species.
Wetlands usually have strong gradients of water depth, soil moisture and nutrient availability (Mitsch & Gosselink, 2000) and, although AMF colonization does occur in flooded environments, most measures of AMF activity and abundance, including root colonization, spore numbers and species richness, are negatively correlated with soil moisture or water depth (Anderson et al., 1984; van Duin et al., 1990; Rickerl et al., 1994; Stevens & Peterson, 1996; Turner & Friese, 1998). Studies in deserts show the opposite trend, with AMF activity and species richness positively correlated with soil moisture (Zak et al., 1995; Jacobson, 1997; He et al., 2002). Environmental gradients in riparian ecosystems include decreases in soil moisture with increasing distance from and elevation above the active channel (Boggs & Weaver, 1994). Investigations in riparian Sporobolus wrightii Munro ex Scribn (big sacaton) grasslands along the San Pedro River in south-eastern Arizona found a positive correlation between AMF colonization and soil moisture (Kennedy et al., 2002), mimicking patterns of the desert upland. It is unknown if AMF communities in Populus–Salix-dominated floodplains exhibit activity and distribution patterns similar to those of wetlands or the arid desert. Other environmental gradients in riparian systems that could influence the activity, richness and distribution of AMF include increases in the percentage of organic matter, the concentrations of phosphorus, nitrogen and potassium (Boggs & Weaver, 1994) and herbaceous richness (Menges, 1986; Lite et al., 2005) as stands age and elevation above and distance from the channel increase.
Understanding how the AMF community is structured with respect to environmental gradients in riparian ecosystems is an important step towards understanding the function of AMF within this ecosystem and the ecological requirements of AMF species. Additionally, information about the species composition and distribution of the AMF community in Populus–Salix forests may enhance the success of efforts to restore degraded riparian areas. Populus and Salix are capable of forming relationships with both AMF and ectomycorrhizal fungi (Lodge, 1989; van der Heijden & Vosatka, 1999) and both types of mycorrhizal fungi are important and little-studied components of riparian areas. However, in this study we chose to focus only on the AMF associated with Populus–Salix stands, as recent work by Jacobson (2004) has begun to examine the ectomycorrhizal fungi present in these systems but no other researchers, to our knowledge, are investigating AMF.
The objectives of this study were: to identify AMF species associated with south-western riparian Populus–Salix forests; to identify environmental variables related to AMF species richness and community composition in a Sonoran Desert riparian area; and to examine AMF community patterns along complex environmental gradients in river floodplains. We expected that AMF activity and richness would change along a successional gradient of Populus–Salix stands and that these changes would be related to changes in soil or vegetation that occur as the stands age. We anticipated that the AMF activity and composition of the youngest Populus–Salix stands would be most different from those of the older age classes, either because AMF activity would be restricted in young stands by the anoxic conditions created by flooding (Anderson et al., 1984; Rickerl et al., 1994) or because it would be stimulated by the high levels of soil moisture maintained by stream flow and shallow groundwater levels (Zak et al., 1995; Jacobson, 1997; He et al., 2002).
In this study, we used AMF spore wall characteristics to identify the AMF encountered in our surveys of Populus–Salix stands. Molecular identification methods are common in studies of ectomycorrhizal fungi (Dahlberg, 2001; Horton & Bruns, 2001) and are being increasingly used in the study of AMF (Clapp et al., 1995; Vandenkoornhuyse et al., 2002, 2003; Douhan et al., 2005; Stukenbrock & Rosendahl, 2005). These methods have the potential to revolutionize studies of AMF community structure, but their use is hampered by several obstacles including the polymorphic, multigenomic nature of AMF, the lack of a single primer to amplify AMF DNA and the difficulty in matching sequence groups to morphospecies (Redecker et al., 2003). Additionally, the potentially large numbers of AMF species that may be encountered in ecological studies across broad environmental gradients make the use of molecular methods prohibitively time consuming and expensive (Landis et al., 2004). While extensive molecular analysis was outside the scope of this study, the identification and isolation of AMF morphospecies from semiarid riparian areas should aid in the use of molecular identification techniques in future studies.
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- Materials and Methods
The flora of Populus–Salix stands sampled for this study consisted of 102 plant species and was dominated, in terms of both cover and richness, by annual species and by members of typically mycorrhizal plant families (Table 1).
Table 1. Richness and cover of plant species grouped by life history and family-level mycorrhizal affinity
|Plant type||Average plot richness||Average plot cover (%)||% of total richness||% of total cover|
|Annual||5.8 ± 0.5||17.6 ± 2.8||67||76|
|Perennial||2.1 ± 0.3|| 5.4 ± 1.3||33||24|
|Mycorrhizal||6.0 ± 0.5||16.3 ± 2.4||75||71|
|Nonmycorrhizal||1.9 ± 0.2|| 6.7 ± 1.5||25||29|
AMF colonization levels of plant roots collected from Populus–Salix stands ranged from 0 to 59% (average 22.8 ± 2.1%). Total colonization and arbuscule and vesicle colonization were highest in sapling stands when compared with other age classes (Table 2) but did not show a significant relationship to any of the other environmental variables measured.
Table 2. Comparison of arbuscular mycorrhizal fungal (AMF) colonization and species richness among Populus–Salix stand age classes on the Verde River (Arizona, USA)
|Age class||Total colonization||Hyphae||Arbuscules||Vesicles||AMF richness|
|Sapling||29.0 ± 3.7 a||13.2 ± 2.2 a||11.8 ± 1.9 a||2.3 ± 0.6 a||8.6 ± 0.5 a|
|Mature||20.3 ± 3.7 ab||11.0 ± 2.1 a|| 6.8 ± 2.2 b||1.9 ± 0.8 ab||5.8 ± 0.4 b|
|Old-growth||17.1 ± 3.7 b||10.1 ± 2.8 a|| 6.4 ± 2.1 b||0.5 ± 0.5 b||5.0 ± 0.5 b|
Thirty species (or morphospecies) of AMF were detected in trap cultures of soil collected from Populus–Salix stands along the Verde River. Fourteen of these species belonged to the genus Glomus, 11 to Acaulospora, three to Entrophospora and one each to Paraglomus and Archaeospora. Nine of these species (one Glomus, two Entrophospora and six Acaulospora) are undescribed. Common species (detected in approximately half the plots sampled) included Glomus intraradices Schenck & Smith, Glomus microaggregatum Koske, Gemma & Olexia, Glomus spurcum Pfeiffer, Walker & Bloss, Glomus deserticola Trappe & Menge, Glomus eburneum Kennedy, Stutz & Morton, and Glomus mosseae (Nicol & Gerd.) Gerdemann & Trappe. Six species were encountered in three or fewer plots. AMF were encountered in samples from every plot and plot species richness ranged from 2 to 13 AMF species.
AMF community composition was similar among age classes, with Sorenson similarity scores between age classes ranging from 0.79 to 0.88 (Table 3). Of the 30 AMF species detected, 18 were found in all three age classes and five were found in two age classes. Six of the AMF species encountered were unique to sapling stands and one was unique to mature stands (Fig. 2). AMF species richness was highest in sapling stands (Table 2) and decreased significantly with increasing plot age and distance from and elevation above the active channel. AMF species richness increased with increasing perennial species richness and cover, but not with increasing total species richness and cover or increasing annual species richness and cover (Table 4). AMF species richness was not significantly correlated with any of the soil variables measured.
Table 3. Sorenson index of similarity for arbuscular mycorrhizal fungal (AMF) species composition of plots on the Verde River (Arizona, USA) by Populus–Salix forest age class
|Mature|| || ||0.88|
Figure 2. Arbuscular mycorrhizal fungal (AMF) species frequency of occurrence in pot cultures of soil collected from Populus–Salix stands on the Verde River (Arizona, USA). Genera: Ac., Acaulospora; Ar., Archaeospora; E., Entrophospora; G., Glomus; P., Paraglomus. AZ112 and AZ123 are previously discovered but undescribed Glomus species. All other numbered species are additional undescribed species encountered in this study.
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Table 4. Spearman rank correlations (rs) of arbuscular mycorrhizal fungal (AMF) species richness with environmental variables on the Verde River (Arizona, USA)
|Cover of typically mycorrhizal species||–|
|Richness of typically mycorrhizal species||–|
|Cover of typically nonmycorrhizal species||–|
|Richness of typically nonmycorrhizal species||–|
Stepwise multiple regression on all independent variables also identified perennial species cover (positive influence) and plot elevation (negative influence) as significant explanatory variables of AMF species richness. Stepwise regression restricted to soil variables identified moisture (positive influence) as the most significant explanatory variable of AMF species richness. The model based on all variables explained substantially more of the variation in AMF species richness than did the model using only soil variables (Table 5).
Table 5. Multiple regression analysis of environmental variables contributing to arbuscular mycorrhizal fungal (AMF) species richness on the Verde River (Arizona, USA)
|All independent variables|
|Perennial cover|| 0.057||0.038|| 3.05|| 0.0859|
|Intercept|| 7.888|| || || |
|Moisture|| 0.269||0.149||10.32|| 0.0021|
|Intercept|| 5.460|| || || |
Nonmetric multidimensional scaling (NMDS) identified a two-dimensional solution for the AMF species presence/absence data which explained 51 and 34% (cumulative = 85%) of the variation in species presence/absence data (final stress = 17.105; instability = 1.0 × 10−5). Annual species cover, perennial species richness, distance from and elevation above the active channel, plot age and exchangeable potassium concentration showed significant correlations (r2 > 0.20) with sample axis scores, indicating that these variables likely play a role in structuring AMF composition in this riparian area (Fig. 3).
Figure 3. Nonmetric multidimensional scaling (NMS) ordination diagram for arbuscular mycorrhizal fungal (AMF) species from Populus–Salix stands. Bold font indicates undescribed ‘riparian affiliate’ species encountered in this study. Arrows indicate the direction and magnitude of environmental variables significantly correlated (r2 > 0.20) with ordination axes. Genera: Ac., Acaulospora; Ar., Archaeospora; E., Entrophospora; G., Glomus; P., Paraglomus. AZ112 and AZ123 are previously discovered but undescribed Glomus species. All other numbered species are additional undescribed species encountered in this study.
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No AMF species encountered appeared to be restricted to wet or dry plots, although some showed a tendency to occur more frequently in wet plots and a few others tended to occur more frequently in dry plots (Fig. 4). The majority of the undescribed species encountered in the study (five of nine) had 60% or more of their occurrences in wet plots.
Figure 4. Hydrologic affinity for arbuscular mycorrhizal fungal (AMF) species encountered in pot cultures from Populus–Salix stands. Each column shows the frequency of encounters for a particular species in dry (solid bars), intermediate (shaded bars) and wet (open bars) plots. The numbers at the top of the graph are the total number of detections for each species. Genera: Ac., Acaulospora; Ar., Archaeospora; E., Entrophospora; G., Glomus; P., Paraglomus. AZ112 and AZ123 are previously discovered but undescribed Glomus species. All other numbered species are additional undescribed species encountered in this study.
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- Materials and Methods
The AMF community structure of Populus–Salix stands on the Verde River has some unique characteristics but also has many similarities to that of the surrounding Sonoran Desert. Some of the AMF species most frequently encountered in this study (G. intraradices, G. microaggregatum, G. spurcum, G. eburneum and G. mosseae) are also common in the desert uplands (Stutz & Morton, 1996; Stutz et al., 2000). Additionally, all of the species we encountered were small-spored, a characteristic typical of desert environments (Bethlenfalvay et al., 1984; Jacobson, 1997; Stutz et al., 2000) which may be an adaptation to limit moisture loss in arid environments. The two AMF genera not encountered in this study, Gigaspora and Scutellospora, contain species with much larger spore sizes, and are infrequent in desert ecosystems (Bethlenfalvay et al., 1984; Jacobson, 1997; Stutz et al., 2000). While many of the species encountered in the riparian area are also common to the desert uplands, this study has also identified a suite of AMF species that appear to be riparian affiliates. We detected eight undescribed Acaulospora and Entrophospora morphotypes, two of which were quite common, that have not been found in extensive surveys of the Sonoran Desert (Stutz & Morton, 1996; Stutz et al., 2000). We also detected an additional set of species including Glomus luteum Kennedy, Stutz & Morton, Acaulospora morrowiae Spain & Schenck, and Acaulospora delicata Walker, Pfeiffer & Bloss that have been commonly detected in riparian areas of the San Pedro River in south-eastern Arizona (Kennedy et al., 2002), but infrequently in the Sonoran desert.
Because spore densities in desert soils can be low (Stutz & Morton, 1996; Kennedy et al., 2002), trap cultures were used amplify AMF collected in this study and yielded 30 AMF species associated with riparian Populus–Salix stands. Other work with trap cultures has shown that the season, the geographic location of the glasshouse, the host plant, and other factors can influence sporulation of AMF species (Stutz & Morton, 1996; Bever et al., 2001). Subsequent generations of trap cultures can reveal additional cryptic AMF species (Stutz & Morton, 1996; Bever et al., 2001), so it is likely that some AMF species were not detected and are missing from this analysis.
Several interrelated environmental variables appear to be important influences on AMF activity and species richness in this desert riparian ecosystem, including soil moisture, distance from and elevation above the active channel, stand age and perennial plant species richness. Many studies have examined the relationship of AMF activity and richness to soil moisture and have found that, in wetland environments, AMF richness and activity respond negatively to increasing levels of soil moisture (Anderson et al., 1984; van Duin et al., 1990; Rickerl et al., 1994; Stevens & Peterson, 1996; Turner & Friese, 1998) while in deserts or dune systems AMF presence and activity are positively correlated with soil moisture (Zak et al., 1995; Jacobson, 1997; He et al., 2002). Our study, as well as a desert riparian study by Kennedy et al. (2002), found AMF activity and species richness to decrease with increasing distance from or elevation above the active channel, meaning that the sites with the highest AMF activity and richness tended to be in the lower and wetter floodplain areas. Our study also found a direct positive relationship of AMF species richness with soil moisture and found that AMF colonization was highest in the youngest (and typically wettest) Populus–Salix stands. In this respect, AMF activity and richness in desert riparian ecosystems more closely resemble patterns found in arid deserts than in inundated wetlands. Differences in the hydrology of wetlands and arid region floodplains may explain these differences in AMF activity and species richness patterns. In wetlands, surface soils are permanently or seasonally saturated, creating anaerobic conditions that are thought to restrict AMF activity (Khan, 1974; Mosse et al., 1981; LeTacon et al., 1983). In arid region floodplains, localized areas of permanently saturated soil can develop from short-duration floods or rainfall events, but surface soils throughout the floodplain are dry for much of the year. Groundwater in the south-western desert tends to be oxic, and the fast-flowing flood waters may be well aerated, thus allowing more aerobic microbial activity (Turner et al., 2000). Alternating periods of wetting and drying, which are common in riparian zones, can also stimulate AMF community development (Braunberger et al., 1996; Brown & Bledsoe, 1996; Miller, 2000). AMF colonization and community composition can vary temporally (van der Heijden & Vosatka, 1999; Kennedy et al., 2002; Bohrer et al., 2004) as well as spatially, but seasonal changes in AMF activity and AMF community composition within Populus–Salix stands were not investigated in this study.
The above-ground plant community can also have significant effects on the richness of the AMF community, with positive correlations between plant species richness and AMF species richness found in glasshouse studies (Grime et al., 1987; van der Heijden et al., 1998b; van der Heijden, 2004) and field studies (Landis et al., 2004). However, in this study, plot-level measures of total herbaceous cover, richness or diversity were not correlated with AMF species richness. We did find that, when the flora was divided into annual and perennial species, AMF richness was positively correlated with cover and richness of perennial herbaceous species. This differentiation is likely attributable to differences in dominance and mycorrhizal dependence of herbaceous perennials and annual or short-lived ephermeral plants. As in most desert environments, a substantial portion of the riparian flora in arid and semiarid riparian areas has an annual life-span and can have a rapid response to rainfall events (Bagstad et al., 2005); these species can dominate the flora in moderate- to high-rainfall years or wet seasons, such as the year and season of this study. Unlike the herbaceous perennials, these precipitation-reactive species, which are mostly ephermeral or short-lived annual species, are unlikely to be highly dependent on mycorrhizal fungi (Allen, 1991; Collier et al., 2003).
While we did find a relationship between AMF richness and the life history characteristics of the above-ground vegetation, we were unable to demonstrate any relationship between AMF colonization or richness and the cover or richness of plant species from typically mycorrhizal or nonmycorrhizal families. Classification of plant families on the tendency of members to form mycorrhizas (Gerdemann, 1968; Newman & Reddell, 1987) may be useful for general discussions on relationships between plants and mycorrhizal fungi; however, these classifications are not absolute. Some members of typically nonmycorrhizal plant families can form mycorrhizae at some point in their life history or under certain environmental conditions (Muthukumar et al., 2004; Plenchette & Duponnois, 2005). Moreover, some plants from typically mycorrhizal plant families, such as many of the annual species encountered in this study, may not benefit from associating with mycorrhizal fungi because of their short life-span and ruderal nature (Collier et al., 2003). In this semiarid riparian setting, where annual species are a major component of the flora, the life history characteristics of the above-ground vegetation appear to be a better predictor of AMF richness than the membership of species of typically mycorrhizal or nonmycorrhizal families.
Findings from this study indicate that environmental gradients also influenced species composition of AMF communities found in riparian Populus–Salix stands. Many AMF species appear to have wide ranges of environmental tolerance (Koske, 1987; Stahl & Christensen, 1991), but the presence of some species appears to be restricted by environmental variables. Ordination identified several variables related to floodplain topography, including plot elevation above and distance from the active channel and stand age, which may have some influence on the composition of the AMF community. Some species showed a clear affinity for young, moist fluvial surfaces while others were more likely to occur in drier sites higher on the floodplain. Most AMF species identified as riparian affiliates (60% or more of their occurrences in wet sites) have not been detected in other studies of the surrounding Sonoran desert (Stutz & Morton, 1996; Stutz et al., 2000). These results are similar to a study of South Carolina wetlands where different AMF dominated the dry, intermediate and wet portions of the moisture gradient (Miller & Bever, 1999).
Some studies of AMF in relation to environmental gradients have found decreasing AMF activity and richness with increasing soil phosphorus concentration (Tanner & Clayton, 1985; Johnson et al., 1991; Amijee et al., 1993; Wetzel & van der Valk, 1996; Kennedy et al., 2002), and another found that AMF richness was positively correlated with soil nitrogen concentration (Landis et al., 2004). Our study identified exchangeable potassium as having a relationship to AMF community composition but did not find a relationship between soil phosphorus or nitrogen concentration and AMF species richness, degree of colonization, or community composition.
The AMF community composition of Populus–Salix stands along the Verde River is similar to that of the surrounding desert, but also contains a subset of unique species. Several interrelated successional, positional, edaphic and vegetation variables were important in structuring the AMF community. AMF richness decreased with increasing Populus–Salix stand age and plot elevation above and distance from the active channel. AMF richness and degree of colonization were highest in young Populus–Salix stands occupying low, moist areas on the floodplain. AMF richness showed no relationship with cover and richness of all herbaceous species considered together, but was positively related to the cover and richness of perennial plant species. Some AMF species showed a clear affinity for young, moist fluvial surfaces and many of these ‘riparian affiliate’ species have not been encountered in previous extensive studies of the surrounding Sonoran Desert.