Caroline S. Bledsoe

Contrasting root associated fungi of three common oak-woodland plant species based on molecular identification: host specificity or non-specific amplification?

An increasingly popular approach used to identify arbuscular mycorrhizal (AM) fungi in planta is to amplify a portion of AM fungal small subunit ribosomal DNA (SSU-rDNA) from whole root DNA extractions using the primer pair AM1-NS31, followed by cloning and sequencing. We used this approach to study the AM fungal community composition of three common oak-woodland plant species: a grass (Cynosurus echinatus), blue oak (Quercus douglasii), and a forb (Torilis arvensis). Significant diversity of AM fungi were found in the roots of C. echinatus, which is consistent with previous studies demonstrating a high degree of AM fungal diversity from the roots of various hosts. In contrast, clones from Q. douglasii and T. arvensis were primarily from non-AM fungi of diverse origins within the Ascomycota and Basidiomycota. This work demonstrates that caution must be taken when using this molecular approach to determine in planta AM fungal diversity if non-sequence based methods such as terminal restriction fragment length polymorphisms, denaturing gradient gel electrophoresis, or temperature gradient gel electrophoresis are used.

Application of network theory to potential mycorrhizal networks.

The concept of a common mycorrhizal network implies that the arrangement of plants and mycorrhizal fungi in a community shares properties with other networks. A network is a system of nodes connected by links. Here we apply network theory to mycorrhizas to determine whether the architecture of a potential common mycorrhizal network is random or scale-free. We analyzed mycorrhizal data from an oak woodland from two perspectives: the phytocentric view using trees as nodes and fungi as links and the mycocentric view using fungi as nodes and trees as links. From the phytocentric perspective, the distribution of potential mycorrhizal links, as measured by the number of ectomycorrhizal morphotypes on trees of Quercus garryana, was random with a short tail, implying that all the individuals of this species are more or less equal in linking to fungi in a potential network. From the mycocentric perspective, however, the distribution of plant links to fungi was scale-free, suggesting that certain fungus species may act as hubs with frequent connections to the network. Parallels exist between social networks and mycorrhizas that suggest future lines of study on mycorrhizal networks.

Epiphytic and terrestrial mycorrhizas in a lower montane Costa Rican cloud forest.

The epiphyte community is the most diverse plant community in neotropical cloud forests and its collective biomass can exceed that of the terrestrial shrubs and herbs. However, little is known about the role of mycorrhizas in this community. We assessed the mycorrhizal status of epiphytic (Araceae, Clusiaceae, Ericaceae, and Piperaceae) and terrestrial (Clusiaceae, Ericaceae) plants in a lower montane cloud forest in Costa Rica. Arbuscular mycorrhizas were observed in taxa from Araceae and Clusiaceae; ericoid mycorrhizas were observed in ericaceous plants. This is the first report of intracellular hyphal coils characteristic of ericoid mycorrhizas in roots of Cavendishia melastomoides, Disterigma humboldtii, and Gaultheria erecta. Ericaceous roots were also covered by an intermittent hyphal mantle that penetrated between epidermal cells. Mantles, observed uniquely on ericaceous roots, were more abundant on terrestrial than on epiphytic roots. Mantle abundance was negatively correlated with gravimetric soil water content for epiphytic samples. Dark septate endophytic (DSE) fungi colonized roots of all four families. For the common epiphyte D. humboldtii, DSE structures were most abundant on samples collected from exposed microsites in the canopy. The presence of mycorrhizas in all epiphytes except Peperomia sp. suggests that inoculum levels and environmental conditions in the canopy of tropical cloud forests are generally conducive to the formation of mycorrhizas. These may impact nutrient and water dynamics in arboreal ecosystems.

Multiple species of ectomycorrhizal fungi are frequently detected on individual oak root tips in a tropical cloud forest

The ecological importance of ectomycorrhizal (EM) fungi in tropical ecosystems is increasingly recognized, but few studies have used molecular methods to examine EM fungal communities in tropical forests. The diversity and composition of the EM community on Quercus crassifolia in a tropical montane cloud forest in southern Mexico were characterized using DNA sequencing of single root tips. Individual root tips commonly harbored multiple fungal species that resulted in mixed polymerase chain reaction (PCR) products. By cloning and performing gel extractions on mixed PCR samples, we identified two or more EM fungi on 26% of the root tips. When non-EM fungi were considered, this figure increased to 31% of root tips. A total of 44 EM taxa and nine non-EM taxa were detected on roots from 21 soil cores (104 root tips). Taxa in the families Russulaceae, Cortinariaceae, Inocybaceae, and Thelephoraceae were frequent. This is the first study to characterize the belowground EM community in a tropical montane cloud forest.

Contribution of relative growth rate to root foraging by annual and perennial grasses from California oak woodlands

Plants forage for nutrients by increasing their root length density (RLD) in nutrient-rich soil microsites through root morphological changes resulting in increased root biomass density (RBD), specific root length (SRL), or branching frequency (BF). It is commonly accepted that fast-growing species will forage more than slow-growing species. However, foraging responses may be due solely to differences in relative growth rates (RGR). There is little evidence, after the effects of RGR are removed, that the fast versus slow foraging theory is correct. In a pot study, we evaluated foraging of four grass species that differed in RGR: one fast-growing annual species, Bromus diandrus, two intermediate-growing species, annual Bromus hordeaceus and perennial Elymus glaucus, and one slow-growing perennial species, Nassella pulchra. We harvested plants either at a common time (plants varied in size) or at a common leaf number (plants similar size, surrogate for common biomass). By evaluating species at a common time, RGR influenced foraging. Conversely, by evaluating species at a common leaf number, foraging could be evaluated independent of RGR. When RGR was allowed to contribute to foraging (common time harvest), foraging and RGR were positively correlated. B. diandrus (fast RGR) foraged to a greater extent than did E. glaucus (intermediate RGR) and N. pulchra (slow RGR). E. glaucus (intermediate RGR) foraged to a greater extent than N. pulchra (slow RGR). Root growth within nutrient-rich microsites was due to significant increases in RBD, not to modifications of SRL or BF. However, when RGR was not allowed to influence foraging (common leaf number harvest), none of the four species significantly enhanced RLD in nutrient-rich compared to control microsites. This suggests that RGR strongly influenced the ability of these grass species to forage and also supports the need to evaluate plastic root traits independent of RGR.

Temporal and Spatial Variability as Neglected Ecosystem Properties: Lessons Learned From 12 North American Ecosystems

Evaluating and monitoring the “health” of large-scale systems will require new and innovative approaches. One such approach is to look for ecological signals in the structure of ecological variability observed in space and time. Such variability is sometimes considered something to minimize by clever sampling design, but may in itself contain interesting ecological information (Kratz et al. 1991). In fact, much of ecology can be considered an attempt to understand the patterns of spatial and temporal variability that occur in nature and the processes that lead to these patterns. Despite widespread interest in patterns of variation there have been relatively few attempts to describe comprehensively the temporal and spatial variation exhibited by ecological parameters. As a result, we have no general laws that allow us to predict die relative magnitude of temporal and spatial variability of different types of parameters across the full diversity of ecological systems. Even within single ecosystems, understanding of the interplay between temporal and spatial variability is lacking. For example, Lewis (1978) noted that despite a large literature, the relation between temporal and spatial variability in plankton distribution within a lake is not well understood. Matthews (1990) makes a similar point regarding fish communities in streams.

A molecular survey of ectomycorrhizal hyphae in a California Quercus–Pinus woodland

Ectomycorrhizal (ECM) hyphal communities have not been well characterized. Furthermore, there have been few studies where the ECM hyphal community is compared to fungi detected as sporocarps or ECM-colonized root tips. We investigated fungi present as hyphae in a well-studied California Quercus–Pinus woodland. Hyphal species present were compared to those found as sporocarps and ECM root tips at the same site. Hyphae were extracted from root-restrictive nylon mesh in-growth bags buried in the soil near mature Quercus douglasii, Quercus wislizeni, and Pinus sabiniana. Taxa were identified using PCR, cloning, and DNA sequencing of internal transcribed spacer and 28s rDNA. Among the 33 species detected, rhizomorph-forming ECM fungi dominated the hyphal community, especially species of Thelephoraceae and Boletales. Most fungi in soils near Quercus spp. and P. sabiniana were ECM basidiomycetes, but we detected two ECM ascomycetes and three non-mycorrhizal fungi. Many ECM species present as hyphae were also previously detected at this site as sporocarps (18%) or on ECM root tips (58%). However, the hyphal community was mostly dominated by different taxa than either the sporocarp or ECM root communities.

Morphotyping and Molecular Methods to Characterize Ectomycorrhizal Roots and Hyphae in Soil

At the interface between plants and soils, ectomycorrhizal (ECM) fungi explore soils, acquire resources, transfer resources to plants, and acquire carbon from plants. Mycorrhizas enhance plant survival, nutrition and growth and play key roles in ecosystems processes such as decomposition, nutrient cycling, soil carbon storage, productivity and sustainability. Mycorrhizas are critical for plant colonization of new soils (e.g. mine spoils, volcanic deposits, glacial moraines). ECM diversity ensures plant reestablishment after disturbance and can enhance survival and growth of trees in reforestation. ECM fungi can promote fine root development as well as produce antibiotics, hormones and vitamins. Mycorrhizal associations may help protect roots from pathogens and moderate effects of heavy metals and toxins. Many environmental problems may be alleviated by mycorrhizas – problems such as pollution, erosion, soil degradation, climate change, degradation of natural resources, and poor land use management. Mycorrhizal abilities to carry out important functions are linked to diversity. ECM diversity is large and documented in many ecosystems, particularly coniferous ecosystems (Gehring et al. 1998; Goodman and Trofymow 1998; Kranabetter and Wylie 1998; van der Heijden et al. 1998; Stendell et al. 1999; Bidartondo et al. 2000). This ECM diversity has been based on surveys of fruiting bodies, but is now based on more recent methods – morphotyping (microscopic observations) and phylotyping (molecular characterization). An advantage of fruiting body surveys is ease of collection and identification based on morphology; a disadvantage is the assumption that fungi fruiting in an area also form ectomycorrhizas on nearby roots. Clearly identification of ectomycorrhizas on roots is preferable. However there are difficulties in ECM identification – complex sampling design and

Nitrogen sink strength of ectomycorrhizal morphotypes of Quercus douglasii, Q. garryana, and Q. agrifolia seedlings grown in a northern California oak woodland

Little information is known on what the magnitude of nitrogen (N) processed by ectomycorrhizal (ECM) fungal species in the field. In a common garden experiment performed in a northern California oak woodland, we investigated transfer of nitrogen applied as 15NH4 or 15NO3 from leaves to ectomycorrhizal roots of three oak species, Quercus agrifolia, Q. douglasii, and Q. garryana. Oak seedlings formed five common ectomycorrhizal morphotypes on root tips. Mycorrhizal tips were more enriched in 15N than fine roots. N transfer was greater to the less common morphotypes than to the more common types. 15N transfer from leaves to roots was greater when