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Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24061
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Sporophore abundance of putatively ectomycorrhizal fungi was compared in a mature mixed hardwood/conifer forest inside of (+) versus outside of (-) Rhododendron maximum thickets (RmT). Experimental blocks (1/4 ha) were established inside of (3) and outside of (3) RmT at the Coweeta Hydrologic Laboratory in Macon County, North Carolina, USA. Litter and organic layer substrates were removed, composited and redistributed among 90 2 x 2m plots within the blocks. Plots received either +RmT or -RmT litter, and either +RmT or -RmT organic layers, or were unmanipulated for controls. Sporophores of 67 putatively ectomycorrhizal species were collected from the blocks. Species diversity and overall community structure were similar inside of and outside of RmTs, and no grouping was detected by substrate type. Differences within the ectomycorrhizal fungus community were associated only weakly with environmental parameters, as indicated by ordination. In light of these results, recent observations of ectomycorrhizal suppression and strong shifts in the proportions of morphotypes on tree seedlings inside of RmT do not appear to be related to differences in sporophore distributions. The changes in seedling mycobiont dominance in relation to RmT and the influence this has on seedling health should be examined directly from root tips.
Key words: ericoid mycorrhiza, fungal community, indicator species, ordination
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Particularly, interest in this topic stems from severe suppression of tree seedling regeneration inside of RmT (e.g., Minkler 1941
, Wahlenberg 1950
, Phillips and Murdy 1985
, Clinton et al 1994
). A previous study found that total ectomycorrhizal colonization of hemlock (Tsuga canadensis L.) seedlings was reduced inside of RmT, and that this depression of mycorrhization was correlated with decreased productivity of the seedlings (Walker et al 1999
). Furthermore, there was a shift in the proportion of ectomycorrhizal morphotypes toward Cenococcum geophilum Fr., a generalized, unspecialized, disturbance tolerant ectomycobiont in RmTs. These observations beg the following question: Is the ectomycorrhizal fungus community negatively affected by the presence of RmT? The hypotheses addressed in this study were: (i) Is the assemblage of ectomycorrhizal fungi different when examined inside of versus outside of RmT? (ii) Are certain ectomycorrhizal fungus taxa indicators with regard to RmT? (iii) Are there differences in the ability to re-colonizing manipulated soil substrates from inside of versus outside of RmT among ectomycorrhizal fungus species? and (iv) Is there a strong relationship between sporophore distributions and abiotic environmental parameters?
Sporophore sampling and ectomycorrhizal community analyses –
Numerous studies have successfully used sporophore abundance (and or biomass) to assess ectomycorrhizal fungus community composition (e.g., Miller 1982a
, Bills et al 1986
, Villeneuve et al 1989
, Nantel and Neumann 1992
, Palmer et al 1994
). The results produced by these studies appear to correspond well with plausible explanations. For example, Nantel and Neumann (1992)
found that the strongest niche dimension of ectomycorrhizal communities was stand composition, but within the range of a stand, fungal assemblages differed in relation to edaphic characteristics. However, we know of no previous studies comparing sporophore assemblages in relation to an ericoid subcanopy shrub in the southeastern Appalachian Mountains.
The relationship between sporophore abundance (or biomass) and the amount of mycelium below ground is dependent upon species-specific differences in life history characteristics. Furthermore, sporophore production in basidiomycetes can be influenced strongly by environmental factors such as light (Miller 1967
). However, an allometric relationship between sporophore and mycelial biomass does appear to hold for some species (Laiho 1970
, Newell 1984
, Cotter and Bills 1985
) and for even a complete community (Menge and Grand 1978
). When a sporophore is produced, we know that part of the individual mycelial unit producing the sporophore is definitely present at that location. Sampling sporophore distributions therefore provides a valuable resource when evaluating ectomycorrhizal fungus communities.
It is known that sporophore abundance of a species is not tightly correlated with the abundance of mycorrhizae in a given area (Gardes and Bruns 1996
, Dahlberg et al 1997
). However, identifying ectomycorrhizae from root cores, like sampling sporophores, provides only a point estimate of the presence of an individual unit of mycelium below ground. In addition, the process of mycorrhization, like sporulation, is influenced by environmental and biotic factors. Finally, root colonization can be variable seasonally. For these reasons, mycorrhizae present in the root zone should still be compared with sporophore samples for the purpose of identifying the pool of fungi potentially available to seedlings. With this in mind, an additional study is being conducted to examine distributions of ectomycorrhizae from root samples in relation to RmT gradients.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plot layout and site preparation –
Three 
ha blocks were randomly located inside of dense RmTs (+RmT) and three blocks were randomly positioned outside of RmTs (-RmT). Fifteen 2 x 2 m plots were systematically located within each block (90 plots total) and randomly assigned one of five treatments (3 plots/block x 6 blocks; 18 plots per treatment). The treatments applied were (i) unmanipulated controls, (ii) -RmT litter with -RmT organic layer (-L/-O), (iii) +RmT litter with +RmT organic layer (+L/+O), iv) +L/-O, and v) -L/+O. The litter (undecomposed leaf material on the soil surface) was removed from all +RmT plots other than controls, composited, and redistributed equally to all plots receiving the +L treatment. The -L, +O (organic layer: the layer between the litter and the mineral horizon) and -O substrates were manipulated in the same fashion. More details on site characteristics and the experimental design can be found in Walker et al (1999)
.
Sporophore sampling –
All sporophores of putative ectomycorrhizal fungi (Miller 1983 and 1982b
, Bills 1986
, Molina et al 1992
) were collected from the substrate manipulation blocks throughout the fruiting season during 1996 and 1997. Collections were gathered once a week during all peak fruiting periods. During periods of sparse sporophore production, the plots were checked at least once every two weeks, and deteriorated, unidentifiable sporophores were rarely observed. All sporophores were identified in the field or dried and examined microscopically in the laboratory in cases when identification was ambiguous based solely on macromorphology. Voucher collections, color photos of fresh specimens, and detailed fresh descriptions were utilized for taxonomically difficult fungi.
Analytical methods – The abundance of sporophores produced on a plot was estimated using the following categories: (i) solitary, 1–2 sporophores, (ii) low abundance, 3–5 sporophores, (iii) medium abundance, 6–10 sporophores, iv) abundant, >10 sporophores. For all analyses, the plots of the manipulation experiment were treated as sample units, and the abundance class for each species/plot pair was entered as a categorical variable (coded as 0 = absent 1 = solitary, 2 = low abundance, 3 = medium abundance, 4 = abundant). Abundance categories for all collection dates in the same year were summed prior to classifying the abundance. For example, a taxon that produced two sporophores one week and another three the next week on the same plot would be coded as medium abundance (category 1+ category 2 = category 3). Species that produced sporophores on an individual plot during both years of the study were placed in the higher abundance category plus one (e.g., 1996—category 2, 1997—category 2; coded as category 3).
Categories were employed for abundance because the types of analyses performed were designed for vegetation and depend on the abundance of individuals, not reproductive structures. Therefore use of absolute counts of sporophores for abundance would over-represent the importance of taxa that produce copious numbers of small sporophores. Because abundance was treated categorically, it is necessary to point out that the relative abundance values presented in Table II are not based on absolute numbers.
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recommended this weighting for use with MRPP, and most recent applications of MRPP have followed suit (McCune and Medfford 1997
). R values approaching 1 indicate groups with more similar samples, and negative R values are possible when groups are less similar than expected by chance. Statistical significance is based on a test of no difference between groups, and P values represent the chance of a more extreme R value originating randomly (based on a calculated mean within group homogeneity for all possible groupings of the data).
Cluster analysis is a method for grouping similar items based on two or more characteristics. Typically a distance measure between items is calculated by methods similar to those used in numerical taxonomy or phenetics. Cluster analyses were calculated with the aid of Numerical Taxonomy and Multivariate Analysis System version 1.8 (Rohlf 1994
) using only fungi collected from a plot. The cluster analysis produced a dendrogram, which was defined by the unweighted pair-group method and arithmetic average (UPGMA), and employed the Bray-Curtis coefficient (Rohlf 1994
). Cluster analyses generated by PC-ORD Multivariate Analysis of Ecological Data version 3.0 for windows (McCune and Medfford 1997
) using Sorensen's distance and UPGMA or nearest neighbor joining gave similar results. Subsampling (with 500 repetitions) was used to generate the species to area curve.
Indicator species analysis was performed using Dufrene and Legendre's (1997)
method, which is based both on the abundance and frequency of species in a priori groups. The indicator species analysis uses a Monte Carlo technique to test statistical significance based on repeated randomizations (1000 in this study) of the dataset, such that P values represent the probability of a higher maximum indicator value arising randomly. Relative abundance is the abundance of a certain taxon in proportion to the abundance of the taxon in all groups. The relative frequency is the percentage of sample units in each group containing a given taxon. The indicator value is a measure of both the relative abundance and reliability of occurrence of the taxon in the group, and ranges from 0–100 (100 representing perfect indication). The relative abundance is relative to the classes (four) used to record sporophore abundance in the field and therefore are not absolute numbers.
Ordination techniques including CCA (Ter Braak 1986
) and RA are used to describe multidimensional data such as species composition on a reduced number of axes while retaining as much of the original information as possible. These are known as eigenvalue type analyses. For RA, the methods involve reciprocation between weighting rows (plots in this study) and columns (species in this study) to obtain a unique solution. Sample positions in RA ordination space are defined by Chi-squared distances. CCA employs the same analytical method as RA, except that CCA is constrained by multiple regression of the species—plot matrix against a second matrix containing environmental parameters. Similar samples are positioned close to one another in both RA and CCA ordination space, thus grouping is evident when the samples appear clustered on the ordination diagram.
The sporophore abundances for 44 putatively ectomycorrhizal species from 43 plots (20 -RmT, 23 +RmT) were used to conduct RA and CCA. CCA included five environmental parameters: soil pH, Ca, P, moisture, and weighted canopy openness. Plots with fewer than two ectomycobiont taxa were excluded from both RA and CCA analyses. Down-weighting rare species gave similar results. CCA axes were scaled to optimize representation of plots. The species-area curve, indicator species, MRPP, and CCA analyses were generated by PC-ORD Multivariate Analysis of Ecological Data version 3.0 for windows (McCune and Medfford 1997
).
Soil cation concentrations and pH were determined by the Soil Testing Laboratory at Virginia Tech by means of inductively coupled plasma mass spectrometry. Canopy openness was determined by means of canopy hemispherical photographs taken during the maximum seedling growth period (July). The images were processed using FEW 4.0 (M. Ishizuka, pers comm) to derive weighted canopy openness, or the ratio of unobstructed sky to the whole hemisphere. Values for soil moisture were collected in July 1996 by Time Domain Reflectometry (Tektronix model 1502C TDR cable tester, Heerenveen, The Netherlands), at a depth of 0–15 cm, in the center of each plot. Methods for characterization of the environmental parameters are described completely in Nilsen et al (2001)
.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plots from in versus out of RmT were not obviously grouped by RA (Fig. 3 ). The absence of any detectable clumping in the RA ordination indicates that the sporophore community was similar inside versus outside of RmT. Ordination by RA (Fig. 3 ), which is not constrained by environmental parameters, lacked the slight shift in + versus -RmT plots evident in CCA (Fig. 4 ).
Canonical correspondence axes 1, 2, and 3 (not shown by convention) captured 4.5%, 3.8%, and 2.9%, respectively, of the variation in abundance data from 44 putatively ectomycorrhizal taxa (Fig. 4 ). Plots from in versus out of RmT were not obviously grouped by CCA. The limited variation explained by CCA was associated with environmental parameters. Axis scores derived from species versus scores that are linear combinations of the environmental variables were highly correlated (Pearson correlation coefficients for axis 1 = 0.943, axis 2 = 0.919, and axis 3 = 0.858). Standardized canonical coefficients versus axis 1 were R = 0.37 for openness, R = -0.32 for pH, R = -0.33 for H2O, R = -0.30 for P, and R = 0.42 for Ca. Standardized canonical coefficients versus axis 2 were R = 0.07 for openness, R = 0.42 for pH, R = 0.03 for H2O, R = 0.10 for P, and R = 0.45 for Ca.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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suggested 100 contiguous 2 x 2 m plots as a guideline for subsampling a limited area, and Nantel and Neumann (1992)
followed suit. However, the species–area curve continued to climb even at this size (Bills et al 1986
). For this study, plots were separated by walkways to isolate the effects of the substrate treatments.
Ectomycorrhizal communities vary in relation to stand age (Marks and Foster 1967
, Mason et al 1982
, Miller 1983
, Dighton et al 1986
), the host tree composition of a stand (e.g., Villeneuve 1989
, Bills et al 1996), and the edaphic characteristics within the range of a particular assemblage (Nantel and Neumann 1992
). Throughout the range of habitats where RmTs are present at Coweeta, we have recorded approximately 250 species of ectomycorrhizal fungi. The results of this study were derived solely from fungi fruiting at the substrate manipulation sites and therefore are not necessarily indicative of other ectomycorrhizal communities where RmTs occur.
Ectomycorrhizal fungi in response to treatments –
Treatment effects on the distribution of ectomycorrhizal fungi were species specific (Table II
). However, treatment effects did not relate to overall ectomycorrhizal community structure (Fig. 2
). There were few differences in mycorrhizal colonization levels by treatment type (Walker et al 1999
), so the lack of grouping of ectomycorrhizal sporophores by treatment type is not incongruent. The homogenization of the substrates was a fairly severe disturbance to the mycelium in the substrates, which meant that the fungi fruiting within the plots had to re-grow from severed hyphal fragments or other propagules if they originated in the substrates. In addition, there is no way to estimate how frequently fungi fruiting in the plots originated from the underlying mineral horizon (which was left intact in all treatments), or how frequently they originated from undisturbed mycelium in the walkways between the plots. Nonetheless, the ability of ectomycorrhizal fungi to re-colonize the substrates from +RmT versus -RmT blocks was not generally different.
Several species of ectomycorrhizal fungi, including Amanita flavoconia Atk., Russula variata, and Strobilomyces floccopus (Vahl. ex Fr.) Karst, were indicative of the +L/-O substrate combination (Table II ). The reasons for this are not clear. Other fungi such as Austroboletus betula and Hydnellum ferrugineum (Fr. : Fr.) Karst were most abundant on control plots (Table II ). This observation may indicate lack of tolerance to disturbance.
The difference between the number of species on the plots versus the total number on the blocks (18 species) could easily be explained by increased sample area. This also tends to indicate that diversity of ectomycorrhizal fungi on the plots was not severely depressed by the disturbance to the plots during the substrate manipulation. Brundrett and Abbott (1995)
found that some bait plants (Mirbelia dilatata R. but not Eucalyptus calophylla Lindley) formed somewhat fewer mycorrhizae in treatments where hyphal networks in soil cores were disrupted by breaking up the core. However, they also reported highly variable mycorrhizal colonization levels related to the amount of organic matter and hyphae in the soil.
Ectomycorrhizal fungi inside versus outside of Rhododendron maximum thickets – Because the R and P values for MRPP (R = 0.006; P < 0.02, probability of a more extreme R arising randomly) are so low in this study, it is highly unlikely that a difference in ectomycorrhizal fungus communities sampled inside versus outside of RmT would be documented through additional sporophore sampling. Lack of grouping in versus out of RmT (Fig. 3 ) could potentially be related to any combination of the following factors: (i) confounding effects from disturbance of the plots by treatment application, (ii) confounding effects from the treatments themselves, (iii) the limited area sampled, (iv) the variable nature of the effect of RmT on individual ectomycobiont species, (v) the failure to detect potentially dominant ectomycorrhizal fungi which fruit rarely or never above ground, and (vi) strong relationships between sporophore distributions and abiotic environmental parameters that are uncorrelated with RmTs. However, with the absence of detectable grouping at the treatment level (Fig. 2 ), including unmanipulated controls, the first two points seem unlikely.
As mentioned above, strong relationships between abiotic environmental parameters and species composition could uncouple or mask any relationship between sporophore distributions and RmTs. Sporophore distributions were not explained by either abiotic factors or RmTs, however. CCA, which uses environmental variables to constrain the ordination of plots and taxa, captured only a small portion (11.2%) of the variation in sporophore composition in this study (Fig. 4 ). This result indicates that abiotic factors did not strongly confound our analyses of the influence of RmT on sporophore distribution, implying that sporophore distributions were indeed similar inside of and outside of RmT (Fig. 3 ). It should be noted that in community analyses such as this, high levels of variation are frequently encountered because of the multitude of potentially important factors affecting fungal distribution and sporulation. Therefore, although the level of species variation captured by CCA was rather low, relationships between sporophore distributions and soil pH, Ca, P, moisture, and weighted canopy openness should probably be considered in future studies (Fig. 4 ).
It is known that the total percent mycorrhizal colonization of some tree seedling taxa is lower, while the level of colonization by Cenococcum geophilum is higher, in RmT at the study site (Walker et al 1999
). If the ectomycorrhizal community is not different in and out of RmT, then the failure of the seedlings to form as many mycorrhizae in RmT is probably due to factors affecting the process of mycorrhization, and not due to a dearth of potential fungal associates. The seedlings are most likely cut off from potential ectomycorrhizal inoculum by the presence of the dense rhododendron root mats in the thickets. The rhododendron root mat, which is symbiotic with ericoid mycorrhizal fungi, occupies the organic layer and may spatially separate the seedlings from potential ectomycorrhizal colonization sources deeper in the soil profile. In addition, competition for nutrients in the mycorrhizosphere may or may not be important. A less parsimonious possibility is that the effect of RmT on total colonization may be due to inhibition of only that portion of the ectomycorrhizal community compatible with the two species of seedlings previously assayed, i.e., hemlock and red oak (Walker et al 1999
). To date, we have no evidence to distinguish among these hypotheses. A study designed to resolve this issue by testing for differences among ectomycobiont communities on seedling roots across undisturbed transects with varying ericaceous shrub densities (including RmT) is underway.
Although there was no apparent change in overall community composition, indicator values suggest that individual ectomycorrhizal fungi (Austroboletus betula, Lactarius speciosus, and Russula krombholzii) were correlated more strongly with either forest type (+RmT or -RmT) (Table I ). However, because of the limited area sampled in this study, additional observations of these taxa will be necessary if they are to be informative for indicator analyses.
Summary and conclusions –
We have found that: (i) In general, the diversity and community structure of ectomycorrhizal fungi is not apparently different inside versus outside of RmT, (ii) Individual ectomycorrhizal fungus taxa are potentially distributed differentially with regard to RmT, (iii) The portion of the ectomycorrhizal fungus community capable of re-colonizing manipulated soil substrates is not different for substrates from inside versus outside of RmT, and (iv) The observed relationships between sporophore distributions and abiotic environmental parameters is weak. The differences in mycorrhizal colonization found previously on seedlings within versus outside of RmT (Walker et al 1999
) were not reflected in the distribution of ectomycorrhizal fungus sporophores. Because the mycorrhizal colonization of seedlings is depressed, but sporophore dominance and diversity are similar inside versus outside RmTs, it is thought that RmTs affect the process of seedling mycorrhization. Factors such as the availability of light and the relationship between light quantity and mycorrhizal colonization should be the focus of further investigations of the inhibition of seedlings by RmT. Most importantly, the types of mycorrhizae formed by seedling root systems should be directly compared across environmental gradients associated with changes in the density of ericoid shrubs.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Accepted for publication August 13, 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Brundrett MC, Abbott LK., 1995 Mycorrhizal fungus propagules in the jarrah forest New Phytol 131:461-469
Clinton BD, Boring LR, Swank WT., 1994 Regeneration patterns in canopy gaps of mixed-oak forests of the southern Appalachians: influences of topographic position and evergreen understory Amer Midl Nat 32:308-320
Cotter HVT, Bills GF., 1985 Comparison of spatial patterns of sexual and vegetative states of Boletinellus merulioides Trans Brit Mycol Soc 85:520-524
Dahlberg A, Jonsson L, Nylund JE., 1997 Species diversity and distribution of biomass above and below ground among ectomycorrhizal fungi in an old-growth Norway spruce forest in south Sweden Can J Bot 75:1323-1335
Dighton J, Poskitt JM, Howard DM., 1986 Changes in the occurrence of basidiomycete fruit bodies during forest stand development: with specific reference to mycorrhizal species Trans Brit Mycol Soc 87:163-171
Dufrene M, Legendre P., 1997 Species assemblages and indicator species: the need for a flexible asymmetrical approach Ecol Monogr 67:345-366
Gardes M, Bruns TD., 1996 Community structure of ectomycorrhizal fungi in a Pinus muricata forest: above- and below-ground views Can J Bot 74:1572-1583
Laiho O., 1970 Paxillus involutus as a mycorrhizal symbiont of forest trees Acta For Fenn 106:1-72
Marks GC, Foster RC., 1967 Succession of mycorrhizal associations on individual roots of radiata pines Aust For 31:193-201
Mason PA, Last FT, Pelham J, Ingleby K., 1982 Ecology of some fungi associated with an aging stand of birches (Betula pendula and B. pubescens) For Ecol and Mgt 4:19-40
McCune B, Medfford MJ., 1997 Multivariate analysis of ecological data. Version 3 Gleneden Beach, Oregon: MJM Software
Menge JA, Grand LF., 1978 Effect of fertilization on production of epigeous basidiocarps by mycorrhizal fungi in loblolly pine plantations Can J Bot 56:2357-2362
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Miller OK Jr., 1967 The role of light in the fruiting of Panus fragilis Can J Bot 45:1939-1943
———. 1982a Mycorrhizae, mycorrhizal fungi and fungal biomass in subalpine tundra at Eagle Summit, Alaska Holarctic Ecology 5:124-134
———. 1982b Taxonomy of ecto- and ectendomycorrhizal fungi In: Schenck NC, ed. Methods and principles of mycorrhizal research. St. Paul, Minnesota: The American Phytopathological Society. p 91–101
———. 1983 Ectomycorrhizae in the agaricales and gasteromycetes Can J Bot 61:909-916
Minkler L., 1941 Planting white pine in laurel and rhododendron slicks J For 39:1036.
Molina R, Massicotte H, Trappe JM., 1992 Specificity phenomena in mycorrhizal symbioses: Community-ecology consequences and practical implications In: Allen MF, ed. Mycorrhizal functioning—an integrative plant-fungal process. New York, New York: Chapman and Hall. p 357–421
Nantel P, Neumann P., 1992 Ecology of ectomycorrhizal-basidiomycete communities on a local vegetation gradient Ecol 73:99-117
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Walker JF, Lei T, Semones S, Nilsen ET, Clinton BD, Miller OK Jr., 1999 Suppression of ectomycorrhizae on canopy tree seedlings in Rhododendron maximum L. (Ericaceae) thickets in the southern Appalachians Mycorrhiza 9:49-56
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