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Keywords:

  • mycorrhiza;
  • orchid;
  • rarity;
  • recruitment limitation;
  • specificity

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

The distribution and abundance of orchid populations depend on a suite of biological and ecological factors, including seed production and dispersal, availability of mycorrhizal fungi and appropriate environmental conditions, with the weighting of these factors depending on the spatial scale considered. Disentangling the factors determining successful orchid establishment represents a major challenge, involving seed germination experiments, molecular techniques and assessment of environmental conditions. Identification of fungi from large-scale surveys of mycorrhizal associations in a range of orchid species has shown that mycorrhizal fungi may be widespread and occur in varied habitats. Further, a meta-analysis of seed introduction experiments revealed similar seed germination in occupied and unoccupied habitat patches. Orchid rarity was also unrelated to mycorrhizal specificity. Nonetheless, seed germination within sites appears to depend on both biotic and abiotic conditions. In the few cases that have been examined, coexisting orchids have distinct mycorrhizal communities and show strong spatial segregation, suggesting that mycorrhizal fungi are important drivers of niche partitioning and contribute to orchid coexistence. A broader investigation of orchid mycorrhizal fungus distribution in the soil, coupled with fungus and recruitment mapping, is needed to translate fungal abundance to orchid population dynamics and may lead to better orchid conservation.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

Recruitment from seeds is a primary determinant of the distribution of nearly all plant populations (Clark et al., 2007). The two major determinants of where plants recruit are seed and microsite limitation (Eriksson & Ehrlén, 1992). When suitable habitats remain uncolonized because seeds do not reach them, plants are considered, depending on the scale, dispersal or seed limited. When seeds are widely dispersed but fail to recruit because habitats are unsuitable, plants are considered habitat- or microsite-limited (Münzbergová & Herben, 2005; Johansson & Eriksson, 2013). While most plants are now thought to be at least somewhat seed-limited (Eriksson & Ehrlén, 1992), the extent to which seed and microsite limitation dominate in determining plant distribution depends to some extent on the types of seeds produced by a plant. In nature there is a general tradeoff between the number of seeds produced and seed mass (Leishman et al., 2000). With a given amount of resources available for seed production, a plant will be able to produce more small seeds than larger seeds (Henery & Westoby, 2001). On the other hand, seedlings from small-seeded species generally have a lower survival rate than seedlings from large-seeded species (Moles & Westoby, 2004). As a result, small-seeded species may be more likely to disperse long distances to colonize vacant sites and therefore be less dispersal-limited, but more habitat-limited than large-seeded species (Venable & Brown, 1988; Greene & Johnson, 1993). In the most extreme case, seeds may be so small that they contain almost no resources and have to rely on external sources for resource acquisition, often mycorrhizal fungi (Eriksson & Kainulainen, 2011). In this case, habitat or microsite limitation may result from the absence of suitable mycorrhizal fungi.

Seeds that are extremely small have been called dust seeds and have evolved independently in at least 12 plant families (Eriksson & Kainulainen, 2011), including the Orchidaceae. Despite the vast numbers of dust-like seeds that can be easily dispersed across large distances (reviewed in Arditti & Ghani, 2000), the exceedingly low rates of orchid recruitment have astounded researchers for centuries. Charles Darwin, being among the first to count the number of seeds within a single orchid fruit, found that a capsule of Orchis (Dactylorhiza) maculata contained c. 6200 seeds. Given that an individual plant can produce > 30 capsules within 1 yr, Darwin calculated that a single plant would produce 186 300 seeds (Darwin, 1862). To put these figures in perspective, he also calculated possible rates of increase (Darwin, 1862):

An acre of land would hold 172 240 plants, each having a space of six inches square, and this would be just sufficient for their growth; so that, making the fair allowance of 400 bad seeds in each capsule, an acre would be thickly clothed by the progeny of a single plant. At the same rate of increase, the grandchildren would cover a space slightly exceeding the Island of Anglesea; and the great grand children of a single plant would nearly clothe with one uniform green carpet the entire surface of the land throughout the globe.

These calculations are in sharp contrast with the general observation that many orchid species tend to have small population sizes and that populations occur scattered across the landscape, suggesting that important factors are limiting the settlement of orchid populations. Darwin himself was puzzled by this observation when he wrote: ‘What checks this unlimited multiplication cannot be told.’ It was not until the pioneering works of Noël Bernard on orchid seed germination (Boullard, 1985; Selosse et al., 2011) that the idea arose that mycorrhizal fungi may be involved in determining the abundance and distribution of orchid populations. Because of their minute size, orchid seeds lack an endosperm and rely on fungal colonization for germination and growth into an underground heterotrophic achlorophyllous stage called a protocorm (Rasmussen, 1995; Smith & Read, 2008). Because the presence of fungi is indispensable, at least for germination and early growth, this suggests that mycorrhizal fungi can be a strong factor limiting the distribution of orchid populations.

However, compelling evidence for the availability of mycorrhizal fungi alone affecting orchid population dynamics is still lacking and it is likely that other factors are of comparable or greater importance in determining orchid establishment success. Seed dispersal and microsite characteristics such as soil moisture content or pH can also be expected to have a profound impact on establishment success at both the landscape and local scales, and the balance between the factors affecting orchid distribution may depend on the scale considered. When multiple orchid species co-occur at a site, local competition for resources, possibly mediated by the availability of or competition between different fungi, may further contribute to the abundance and spatial distribution of orchids in natural environments.

The aim of this paper is to provide an overview of the potential edaphic and mycorrhizal factors affecting the abundance and spatial distribution of orchid species at different scales in natural environments. Given that many orchids are currently threatened or endangered, a better understanding of the factors governing orchid population dynamics may be critical to their long-term conservation. We first give a brief overview of the main fungal species associating with orchids and highlight what is known of their ecological characteristics. We then investigate the roles of seed and recruitment limitation and distribution of mycorrhizal fungi in determining orchid abundance at the landscape and local scales. Finally, we highlight avenues for future research.

Orchid mycorrhizal fungi

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

Orchid mycorrhizas have been studied for more than a century (Rasmussen, 2002), yet identification of associated fungi was severely limited until DNA sequencing was applied to the task. The majority of fungal taxa found associated with both terrestrial and epiphytic green orchids are Basidiomycetes in the Ceratobasidiaceae, Sebacinales, and Tulasnellaceae and association with fungi in these taxa is thought to be the ancestral state for the Orchidaceae (Rasmussen, 2002; Taylor et al., 2002; Dearnaley, 2007). Most of these are resupinate fungi that produce morphologically depauperate fruiting structures, which has hindered their taxonomic resolution. Orchid fungal associates in these groups include saprotrophs, ectomycorrhizal fungi, and some parasites and plant pathogens (Dearnaley et al., 2012). There have also been a few examples of orchids associating with the Ascomycetes Tuber, Wilcoxina, Tricharina, Peziza, and Phialophora (Bidartondo et al., 2004; Selosse et al., 2004; Dearnaley, 2007) and, in the tropics, with Basidiomycetes in the Atractiellomycetes (Kottke et al., 2010; Dearnaley et al., 2012).

Most fully and some partially mycoheterotrophic orchids have primarily been found to associate with ectomycorrhizal Agaricomycetes. These include many members of the Russulales (Taylor et al., 2004), Thelephorales (McCormick et al., 2009), and Sebacinales (Roy et al., 2009), and some associate with ectomycorrhizal Ascomycetes (Selosse et al., 2004). In addition to these ectomycorrhizal fungi, recent studies have also identified a diverse group of saprotrophic fungi forming mycorrhizas with mycoheterotrophic orchids in tropical forests (Martos et al., 2009; Ogura-Tsujita et al., 2009).

Fungi associated with orchids have diverse ecological strategies and nutritional needs, but it is not currently thought that they need to associate with orchids. Although Cameron et al. (2006, 2008) found that Goodyera repens contributed carbon to its mycorrhizal fungus, suggesting that association with the orchid might benefit the fungus, even in this case, the fungi grow well without orchids and are probably distributed independently.

Landscape-scale dynamics

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

Despite the production of many seeds with the potential to disperse across long distances (Arditti & Ghani, 2000) and, for some species, the potential to survive for many years in the soil (Whigham et al., 2006), many orchid species show highly scattered distribution patterns at the landscape scale. This suggests that many sites are not suitable for germination and establishment of seedlings (habitat limitation sensu Münzbergová & Herben, 2005). One might suppose that this is because orchid mycorrhizal fungi, like most ectomycorrhizal fungi, are uncommon even within suitable habitats and are restricted to small areas (Villeneuve et al., 1989; Horton & Bruns, 2001; Taylor et al., 2002). Yet, identification of fungi from surveys of mycorrhizal associations in a wide range of orchid species and populations across large scales has shown that many orchid mycorrhizal fungi are widespread and occur in a wide variety of habitats. For example, the most widespread fungal lineage associating with species from the genus Orchis was found from the Mediterranean up to northern Belgium and was retrieved from dry calcareous grasslands, mesic grasslands, and both pine and temperate deciduous forests (Jacquemyn et al., 2011). Similarly, the mycorrhizal fungi associating with several species of Australian orchids also have very wide distribution ranges (Swarts et al., 2010; Phillips et al., 2011). In Cypripedium, closely related fungal lineages were even found across different continents (Shefferson et al., 2007). An alternative hypothesis for the scattered distribution of orchid populations is that orchid populations are absent from seemingly suitable habitat patches because they are sufficiently far from existing orchid populations that seeds rarely encounter them (dispersal limitation sensu Münzbergová & Herben, 2005). Seed and fungal limitation may also act in concert, such that if the local distribution of fungi is patchy then correspondingly more seeds would need to arrive in an uncolonized habitat to have a high probability of encountering locations with appropriate fungi.

The most straightforward way to unravel the importance of habitat vs dispersal limitation is to perform seed introduction experiments, in which germination of seeds is compared between sites where the species is absent and present (Turnbull et al., 2000). Because of the minute size of orchid seeds, most seed introduction experiments are conducted using seed packets (Rasmussen & Whigham, 1993). These are small packets that consist mostly of fine nylon netting, such as is used in phytoplankton research, either imposed within a 35 mm slide mount or heat-sealed into discrete mesh packets (Brundrett et al., 2003). The mesh size of the phytoplankton netting is small enough to retain the seeds in the packets, but large enough to allow hyphae to reach the seeds and promote germination.

The few studies that have compared seed germination between occupied and unoccupied habitat patches have shown that in all investigated species at least some seed packets that were deployed in unoccupied habitat patches produced protocorms (Table 1). Comparing the proportion of seed packets with and without protocorms showed no significant difference in seed germination between occupied and unoccupied habitat patches (Table 1). However, nine of the 12 (germinating) species germinated more at the occupied sites, so the results may be biased by the low number of studies available. It should also be noted that four of the five studies deployed seed packets only in sites that were potentially suitable for sustaining orchid growth (but see Těšitelová et al., 2012). A different picture would emerge if less suitable habitats were also considered. Nonetheless, these results suggest that dispersal limitation is an important factor determining the distribution of orchids at the landscape scale. These results are also supported by the fact that many studies have found little relationship between orchid specificity for mycorrhizal fungi and rarity (Table 2). It has been hypothesized that low specificity evolved to counteract limitations resulting from a rare fungal associate. However, an orchid that required a specific but widespread fungus could even encounter greater opportunities for germination than an orchid that associated with many fungi, each of which was very narrowly distributed. A comparison of very rare, rare, and common orchids (note that the rarity designation is also positively correlated with the degree of habitat specificity) revealed no statistical difference in the number of fungal operational taxonomic units (OTUs) they associated with (Table 2), regardless of how rarity was partitioned (i.e. whether uncommon was counted as rare or common, whether rare and restricted were counted as very rare or only rare). These results challenge the general assumption that orchid seeds are easily capable of traveling long distances and so are able to colonize all suitable habitat patches. However, it is worth noting that none of these studies monitored subsequent growth to the adult stage, so it remains unclear whether some habitat patches may be only temporarily suitable or whether introduced seeds germinating in seed packets would establish and form self-sustaining populations.

Table 1. The proportion of seed packets showing germination (protocorm formation) of several orchid species at sites with and without the orchid species present
StudySpeciesOccupiedUnoccupied
  1. A paired t-test with subsequent bootstrapping showed no significant difference (t12 = 0.40; > 0.05) in seed germination between occupied and unoccupied habitat patches.

De hert et al. (2013) Dactylorhiza fuchsii 0.430.25
Dactylorhiza praetermissa 0.220.35
Herminium monorchis 0.060.09
Phillips et al. (2011) Drakaea elastica 0.100.08
Drakaea glyptodont 0.170.04
Drakaea livida 0.080.04
Drakaea micrantha 0.080.05
McKendrick et al. (2000) Neottia nidus-avis 0.710.01
Těšitelová et al. (2012) Epipactis albensis 0.000.00
Epipactis atrorubens 0.080.47
Epipactis helleborine 0.550.40
Epipactis purpurata 0.090.00
McKendrick et al. (2000) Corallorhiza trifida 0.580.61
Mean0.240.18
Table 2. Studies comparing the mycorrhizal specificity of rare and common orchids
 Orchid (no. of populations, no. of plants)RarityNo. of fungal OTUs
  1. Orchid rarity is classified as very rare (VR), rare and geographically restricted (R, R), rare (R), uncommon (U), common but geographically restricted (C, R), or common (C), in order of decreasing rarity. Numbers in parentheses after each orchid name are (number of populations sampled, number of individual plants sampled). A linear mixed-effects model with study as a random effect and rarity as a fixed effect revealed no significant relationship between rarity and mycorrhizal specificity (df = 1, 23; = 2.99; > 0.05; Systat 12 for Windows, Systat Software Inc., San Jose, CA, USA). OTUs, operational taxonomic units.

Jacquemyn et al. (2012c)Dactylorhiza fuchsii (5, 16)R5
Dactylorhiza incarnata (5, 20)VR6
Dactylorhiza maculata (6, 23)C6
Dactylorhiza majalis (6, 25)C9
Dactylorhiza praetermissa (3, 7)U6
Phillips et al. (2011)Drakea elastica (3, 6)VR1
Drakea glyptodon (6, 14)C1
Drakea livida (2, 13)C1
Drakea micrantha (2, 6)VR1
Drakea thynniphila (1, 9)C1
Swarts et al. (2010)Caladenia huegelii (9, 9)R, R1
Caladenia arenicola (1, 3)C, R1
Caladenia longicauda (1, 3)C, R2
Caladenia discoidea (1, 3)C3
Caladenia flava (1, 3)C5
Wright et al. (2010)Caladenia audasii (1, 5)VR1
Caladenia amoena (1, 1)VR1
Caladenia sp. aff. fragrantissima (1, 2)VR1
Caladenia sp. aff. patersonii (1, 2)VR1
Caladenia rosella (1, 1)VR1
Caladenia orientalis (1, 1)VR1
Caladenia tentacula (6, 36)C6
Shefferson et al. (2005)Cypripedium calceolus (2, 2)C2
Cypripedium californicum (5, 10)R, R5 (3 genera)
Cypripedium candidum (3, 6)C, R1
Cypripedium fasciculatum (7, 16)C, R1
Cypripedium guttatum (2, 2)C1
Cypripedium montanum (8, 12)C, R5
Cypripedium parviflorum (7, 11)C3

Local-scale dynamics

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

One interesting observation from the seed introduction experiments is that, in most species, a substantial proportion of the seed packets deployed, even within existing orchid populations, failed to develop protocorms (Jacquemyn et al., 2007, 2012a,b), suggesting that suitable microsites are not evenly distributed (i.e. microsite limitation sensu Münzbergová & Herben, 2005). Nonrandom distribution of mycorrhizal fungi, spatial restrictions in the availability of suitable environmental conditions, or a combination of the two can be expected to translate into nonrandom above-ground distributions of orchid individuals. Such a nonrandom distribution was evident in a comparison of the spatial distribution of seedlings and adults in two populations of the Lady orchid (Orchis purpurea) in Belgium. This study showed that seedling establishment was mainly restricted to areas where adults occurred (Jacquemyn et al., 2007) (Fig. 1), suggesting that germination and seedling establishment were strongly dependent on biotic and abiotic environmental conditions reflected by the presence of adult plants. Similar results have also been obtained for other species (e.g. Caladenia arenicola (Batty et al., 2001), Goodyera pubescens (Diez, 2007), Anacamptis morio, Orchis mascula and Gymnadenia conopsea (Jacquemyn et al., 2012a)).

image

Figure 1. Visualization of the spatial clustering of adult and seedling plants in two populations of the Lady orchid (Orchis purpurea) in Belgium (from Jacquemyn et al., 2007). Different colors distinguish areas within the populations with high and low abundances of orchids. Circles represent the area where c. 86% of adults (a, c) or seedlings (b, d) are expected to be located.

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What defines suitable microsites for different orchid species will likely reflect a combination of how specific orchid fungus requirements are and the distribution of abiotic conditions, factors we have just begun to tease apart. Appropriate mycorrhizal fungi are unquestionably a component of any suitable microsite, and many researchers hypothesized that an orchid that required specific fungi at any life stage would be more likely to be restricted by a mycorrhizal fungus than one that could associate with many different fungi (Brundrett et al., 2003; Bonnardeaux et al., 2007; Dearnaley, 2007; Ogura-Tsujita et al., 2009). Molecular tools have been used to document varying degrees of specificity in orchid–fungal associations (reviewed in Taylor et al., 2002). Photosynthetic (McCormick et al., 2004; Phillips et al., 2011) and mycoheterotrophic orchids (Taylor & Bruns, 1999; Roy et al., 2009), and both terrestrial and epiphytic (Suárez et al., 2006) orchids may associate with a very narrow range of mycorrhizal fungi (e.g. Liparis liliifolia; McCormick et al., 2004) or a very broad range (e.g. Microtis media; Bonnardeaux et al., 2007; De Long et al., 2013). Another possibly widespread but rarely assessed specificity pattern is associating with a wide range of fungi as mature, photosynthetic plants, yet requiring a very narrow range of fungi to support germination and protocorm development (e.g. Tipularia discolor (McCormick et al., 2004)).

Some rare orchids do require specific fungi and may be limited by their availability (e.g. Swarts et al., 2010; Graham & Dearnaley, 2012), yet others associate with diverse fungi (Pecoraro et al., 2012; Pandey et al., 2013) and may be limited by other factors. However, separating the components of a suitable microsite requires independent assessment of abiotic conditions, mycorrhizal fungus distribution, and ability to support orchids. Batty et al. (2001) and Diez (2007) were the first to directly assess germination and environmental conditions independently by combining seed packet experiments and environmental measurements. They found that, beyond spatial clustering near conspecific plants, seed germination was also affected by soil moisture, pH, and organic content. However, whether these factors affected orchids directly or whether seed germination was affected indirectly through mycorrhizal fungi was not examined.

Additional advances have been made by researchers using molecular techniques to assess the distribution of fungi in the soil independent of seed germination. McCormick et al. (2012) combined seed packets, manipulations of environmental conditions, and direct quantification of mycorrhizal fungi in the soil using specific primers and quantitative real-time PCR (qPCR) to demonstrate that organic amendments affected the germination of orchid seeds by altering the abundance of mycorrhizal fungi. This suggested that areas with existing orchids might be ‘hot spots’ of abundant mycorrhizal fungi (Otero & Flanagan, 2006; Waterman & Bidartondo, 2008; McCormick et al., 2009; M. K. McCormick et al. unpublished). Combined, these studies suggest that the abundance of orchid mycorrhizal fungi, and hence their ability to support orchid germination and growth, may depend on edaphic conditions. However, this has only been investigated in a few orchids, and edaphic conditions may also affect orchids directly. Additional experimental manipulation of environments and mycorrhizal fungi will provide further insight into the mechanisms behind observed patterns of orchid and fungus distribution.

The distribution of mycorrhizal fungi associated with epiphytic orchids is less well studied, perhaps because early seed packet techniques using slide mounts were less successful on branches than in the ground (Zettler et al., 2011). Otero et al. (2002) suggested that epiphytic orchids may also be affected by the distribution and abundance of their mycorrhizal fungi, but research is only beginning on mycorrhizal fungus distribution in epiphytes (Kartzinel et al., 2013). Assessment of how often orchids are limited by mycorrhizal fungus abundance, particularly in epiphytic orchids, and clear separation of environmental effects on mycorrhizal fungi from those on orchids will be a fruitful topic for future research.

The need for mycorrhizal fungi to be abundant to either provide sufficient nutrients or a high probability of the fungus encountering seeds suggests that locations where orchids can reach maturity may be sites with persistently abundant fungi. Just as at larger scales, fungi or environmental conditions in microsites without existing orchids may be ephemeral (Wright et al., 2010), or may represent distinct, possibly less favorable, species or genotypes of fungi than those at occupied microsites (McCormick et al., 2009). For example, McCormick et al. (2006) found that Goodyera pubescens in sites that supported diverse mycorrhizal fungi recovered after a drought by switching to associations with new mycorrhizal fungi and McCormick et al. (2009) suggested that the dynamics of a population of Corallorhiza odontorhiza might be driven by the distribution of differently drought-resistant fungi.

Assessing survival to maturity awaits future research, but understanding how edaphic conditions affect the distribution and abundance of orchid mycorrhizal fungi may improve orchid conservation and restoration success by identifying conditions that favor persistent growth of mycorrhizal fungi (Gale et al., 2010). The first step in this direction has been taken by Nurfadilah et al. (2013), who examined the nutrient acquisition abilities of fungi associated with several species of Australian orchids. They found that fungi that associated with rare, habitat-specialist orchids were able to utilize a narrower range of nutrient sources than those that associated with more widespread orchids, suggesting a possible mechanism for the connection between mycorrhizal fungus distribution and abundance and orchid rarity. Additional research into the ecology of orchid mycorrhizal fungi will no doubt improve our understanding of fungal habitat requirements.

Heterogeneous environments and coexistence of orchid species

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

Factors affecting the distribution of orchids at regional and local scales also impact which orchid species co-occur (Waterman & Bidartondo, 2008). When two or more orchid species co-occur at a site, they are likely to compete for the same set of resources. Classical ecological theory predicts that two species competing for the same resources cannot coexist stably. This raises the question of why there are numerous examples of natural systems where a large number of orchid species co-occur. Spatially explicit models have shown that localized dispersal and interactions may lead to stable coexistence of species because of strong interspecific spatial segregation (Pacala & Levin, 1997). In this case, one would expect coexisting orchid species to show strong spatial clustering and segregation. Recent research investigating the fine-scale spatial distribution of seven different orchid species in Mediterranean grasslands in southern Italy has indeed shown that at small neighborhood sizes the proportion of conspecifics was high in all species (Fig. 2, Jacquemyn et al., 2013). This high local dominance was mainly caused by the strong clustering of individual species and little overlap among species.

image

Figure 2. Spatial distribution of seven orchid species in a 25 × 25 m plot in southern Italy (from Jacquemyn et al., 2013). Colors indicate different species.

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However, similar patterns of spatial segregation could also emerge from small-scale habitat heterogeneity. In this case, co-occurring species segregate and coexist, not because of finite dispersal and local interactions alone, but because each species has a competitive advantage in, or is specifically adapted to, a different habitat type (Pacala & Levin, 1997). It is plausible that competitive rankings among orchids depend on the availability and abundance of suitable mycorrhizal fungi. Coexistence may then depend on differences in mycorrhizal fungi that allow each orchid species to germinate and establish, making fungal diversity crucial to coexistence. Recent research has shown that coexisting orchid species of the subtribe Coryciinae associated with different mycorrhizal fungi (Waterman et al., 2011). Similarly, coexistence of diploid and tetraploid cytotypes of the terrestrial orchid Gymnadenia conopsea (Těšitelová et al., 2013) and pure and hybrid plants in three species of the genus Orchis (Jacquemyn et al., 2012b) could be explained by association with distinct mycorrhizal communities. In the latter case, both the dominant fungal lineage and the total fungal community differed significantly between pure and hybrid species (Jacquemyn et al., 2012b). Further evidence that mycorrhizal fungi were involved in determining the spatial distribution of co-occurring orchid species was provided by Jacquemyn et al. (2012a). Using seed germination experiments in a site where three orchid species that associated with different mycorrhizal fungi co-occurred, they showed that spatial variation in seed germination of Anacamptis morio, Gymnadenia conopsea and Orchis mascula was the limiting factor determining the above-ground spatial distribution of the orchids. Altogether these results suggest that mycorrhizal fungi are important factors driving niche partitioning in terrestrial orchids and may therefore contribute to orchid coexistence. However, this has only been investigated in a few terrestrial orchids, so how broadly applicable these results are remains to be seen. Determining whether species segregation is explicitly attributable to competition between or for fungi or whether it can be attributed to orchid or fungal adaptation to specific microhabitats will require experiments specifically designed to discriminate between these options. One possibility could be to add seeds of different orchid species in seed packets and to compare seed germination at different locations in the population.

Future directions

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

In addition to the increased use of experimental manipulation of environmental conditions and fungal availability and explicit tests of the competition hypothesis that were called for in the text, several tracks of future research are poised to transform our understanding of orchid mycorrhizas dramatically. First, a broader investigation of the distribution of orchid mycorrhizal fungi in the soil and on host phorophytes through the use of qPCR and next-generation sequencing will better define the link between mycorrhizal fungus distribution and abundance and successful association with orchids. The distribution of mycorrhizal fungi has only been investigated for a few temperate terrestrial orchids (M. K. McCormick et al., unpublished) and even less is currently known about the distribution of epiphytic orchid mycorrhizal fungi and their distribution on host trees. Kartzinel et al. (2013) have begun to investigate mycorrhizal fungus distribution on different phorophyte species and at a range of distances from existing orchids. However, they focused on using seed packets to define fungus distribution and did not examine fungus distribution independent of orchid germination. Coupling such fungus mapping with assessment of edaphic or phorophyte conditions and fungal nutrient utilization abilities will allow translation of fungal characteristics to fungus and orchid distribution.

Secondly, Nurfadilah et al. (2013) investigated the nutrient access abilities of several Australian orchids and found that fungi associated with habitat specialist orchids were less able to access diverse nutrient sources than those associated with widespread orchids, suggesting that mycorrhizal fungus ability to access nutrients may be an important driver of orchid distribution. The ability to leverage such research on a few fungi to predict conditions needed by many others will rely on understanding how fungal nutrient access abilities are constrained by evolution and whether they can be predicted based on their relatedness to other fungi that have been studied. However, this will depend critically on development of an accurate phylogeny for orchid mycorrhizal fungi. Phylogenetic relationships of fungi in general are in a state of flux and those for the fungi that most commonly form mycorrhizas with orchids are especially poorly understood. However, Linde et al. (2013) recently identified new nuclear loci that are useful for delimiting species in these difficult groups of fungi.

Thirdly, application of transcriptomics to elucidate how the relationship between fungi and orchids is established and affected by environmental conditions promises to provide an unprecedented window into the functioning of this symbiosis. This will be enabled by annotating and comparing the genomes of many mycorrhizal fungi, including some orchid mycorrhizal fungi, being sequenced as part of the 1000 Fungal Genomes Project (Marmeisse et al., 2013). Zhao et al. (2013) have begun research in this direction by comparing genes expressed in germinating symbiotic and asymbiotic orchid seeds. However, additional research will be required to identify the functions of fungal gene products and to separate genes involved in growth from those involved in establishment of the symbiosis. When combined with extremely high-resolution microscopy, which can reveal the timing and locations of nutrient transfers (as per Yukari Kuga, pers. comm.), such transcriptomic studies promise to reveal the dynamics of how orchid mycorrhizal associations function.

Many of the unique characteristics of orchid mycorrhizal associations, such as their wide range of specificities and degree of dependence on fungi, make this a particularly powerful system in which to study the evolution and functioning of symbiotic associations. Yet, our understanding of these mycorrhizal associations is still strongly limited by our restricted knowledge of the ecology and spatial distribution of fungi. Many of the advances in understanding the correspondence between orchids and their mycorrhizal fungi have not yet been addressed in arbuscular mycorrhizas or ectomycorrhizas, although a similar study has been carried out with one species of Pyrola (Hashimoto et al., 2012). Yet other aspects of the functioning of mycorrhizas, such as the dynamics and mechanisms of nutrient exchange, are much better studied in those systems. We feel that similar studies conducted in orchid mycorrhizas will be extremely powerful ways to advance understanding of the evolution and functioning of symbiotic associations. Such studies will also transform orchid conservation from mere land preservation to more targeted, active conservation and restoration.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References

The authors would like to thank the New Phytologist Trust for inviting them to the 31st New Phytologist Symposium, Orchid symbioses: models for evolutionary ecology, where the idea for this paper was generated and some of the new directions for future research were presented. Three anonymous reviewers and Marc-André Selosse provided helpful comments on an earlier draft. We would like to thank Ryan Phillips and Tamara Těšitelová for providing data on seed germination. This project was supported by the European Research Council (ERC starting grant 260601 – MYCASOR) and the US National Park Service (PMIS #144281).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Orchid mycorrhizal fungi
  5. Landscape-scale dynamics
  6. Local-scale dynamics
  7. Heterogeneous environments and coexistence of orchid species
  8. Future directions
  9. Acknowledgements
  10. References