Litter quality in¯uences on decomposition, ectomycorrhizal
community structure and mycorrhizal root surface acid
phosphatase activity
Christine Conn, John Dighton*
Rutgers University Pinelands Field Station, P.O. Box 206, New Lisbon, NJ, 08064, USA
Accepted 24 September 1999
Abstract
The in¯uence of litter quality on root growth, ectomycorrhizal communities and decay processes was investigated through a litter bag experiment. Litter bags containing either pine needles, oak leaves or oak+pine mix were placed within the O horizon of a lowland pitch pine (Pinus rigida) forest in the New Jersey Pinelands. Upon retrieval, ingrown pine roots were removed and quanti®ed for total length and percent ectomycorrhizal colonization by morphotype. Phosphatase activity was determined for dominant morphotypes. In addition, litter decay rates and N and P litter content were measured. Mixed litter (oak+pine) had highest total pine root ingrowth. Dominant ectomycorrhizal morphotypes diered in response to litter type. A tuberculate form dominated (35%) in pine litters while distinctly dierent nontuberculate morphotypes dominated in oak and mixed litters. High phosphatase activity of morphotypes was correlated with high phosphorus immobilization during oak leaf decay. Results indicate that a mix of forest litters (oak and pine) optimizes retention of scarce nutrients such as nitrogen and phosphorus. The diverse chemical environment of these dierent litter types induces dierent ectomycorrhizal community development which show functional dierences in the way phosphorus is likely to be cycled. The in¯uence of litter type on diversity and function of ectomycorhizae is an important step in identifying linkages between biodiversity of this group and ecosystem functions.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Litter decay; Oak+pine forest; Phosphorus cycling; Mycorrhiza
1. Introduction
The development of mycorrhizae increases the abil-ity of roots to absorb nutrients and water from the soil environment. The degree to which this ability is enhanced depends both on the extent of mycorrhizal colonization and the unique functional attributes of the mycorrhizal fungal species involved. Community structure of ectomycorrhizas on a root system may be related to nutrient uptake eciency if dierent mycor-rhizal species have diering abilities to sequester
nutri-ents from the soil solution (Dighton, 1995). The structure of the ectomycorrhizal community is in¯u-enced by competitive interactions between the coloniz-ing mycorrhizal fungi and, possibly, between mycorrhizal and saprotrophic fungal species (Shaw et al., 1995). Edaphic factors, such as anthropogenic pol-lution, also have been shown to in¯uence mycorrhizal abundance and community structure. Dighton and Skengton (1985) showed that acidifying pollutants altered ectomycorrhizal community structure by redu-cing abundance of species produredu-cing large amounts of extramatrical mycelium. Other eects of acidifying pol-lutants on mycorrhizal communities are reported in Jansen et al. (1988) and are modeled in Dighton and Jansen (1991). Nitrogen amendment in a spruce forest produced dierent ectomycorrhizal species composition
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* Corresponding author. Tel.: 8849; fax: +1-609-894-0472.
and decreased root tip number in relation to an una-mended community as observed by mycorrhizal mor-phology (Alexander and Fairley, 1983) and molecular taxonomy (KaÊreÂn and Nylund, 1997). Increased soil nitrogen and phosphorus can also reduce ectomycor-rhizal development (Brun et al., 1995) and increase root growth (Pregitzer et al., 1993; Weber and Day, 1996).
Heterogeneity of resource units in soil as an in¯uen-cing factor has been less well documented. Pregitzer et al. (1993) have shown that patchiness of nutrient ad-dition to soil altered root growth patterns and Baar et al. (1994) showed that removal of litter and humus from the forest ¯oor altered the pattern of distribution
of Laccaria bicolor sporophore production. This eect
could be manifest through altered nutrient availability or as a result of the in¯uence of leaf litter extracts on fungal growth. Soluble litter extracts have been shown to stimulate or inhibit mycorrhizal fungal activity (Baar et al., 1994; Michelsen et al., 1995; Koide et al., 1998).
Dierences in ectomycorrhizal eciency have been suspected, but not clearly demonstrated in the ®eld. Dighton et al. (1990) found dierent phosphorus uptake rates from soil pools to the tree canopy among various ectomycorrhizal communities. In addition, mycorrhizal enzymes may be able to directly in¯uence nutrient cycling by acquiring both nitrogen and phos-phorus from complex organic forms (Dighton, 1991; Read, 1991).
Plant communities of the New Jersey pine barrens have a nearly total reliance on mycorrhizal associ-ations to obtain and conserve limiting nutrients and water. We have speci®cally addressed the role litter quality has on root growth and ectomycorrhizal devel-opment and how these factors integrate to aect eco-system nutrient cycling. Speci®c null hypotheses were (i) leaf litter decomposition rate is the same between leaf litter species, (ii) leaf litter type does not aect root growth into that litter and (iii) leaf litter type does not in¯uence ectomycorrhizal species composition and their enzyme expression.
2. Material and methods
2.1. Site description
The New Jersey Pinelands are located on the south-eastern coastal plain of New Jersey, USA. Soils are sandy and oligotrophic (total N: 0.5±3 g kgÿ1, Olson extractable P: 70±140mg kgÿ1); see Tedrow (1979) and Markley (1979). Pinelands communities consist of upland forests dominated by pitch pine (Pinus rigida) and various oak species (Quercus alba, Q. velutina, Q.
ilicifolia, Q. marilandica, etc.) with lowland stands
dominated by pitch pine forests, red maple swamps (Acer rubrum) or Atlantic white cedar swamps
(Cha-maecyparis thyoides). Understory ¯ora is primarily
eri-caceous, represented by various huckleberry (Gaylussacia) and blueberry (Vaccinium) species (McCormick, 1979). Mycorrhizal regulation of nutrient cycles is considered very important in this nutrient poor ecosystem, where interception of mineralized nutrients in the organic horizons is of great import-ance. The study site was located in a lowland pitch pine forest. Soil pH ranged from 3.4 to 3.8 with a 5± 10 cm deep organic horizon over sand.
2.2. Litter decomposition experiments and root ingrowth
Oak and pine litter was gathered from the forest ¯oor in January 1995, dried at 708C for 48 h and placed in 1 mm mesh nylon litter bags. Bags contained a total of 5 g of leaf material comprised of either pine needles (pure pine), oak leaves (pure oak) or a mix of oak and pine (2.5 g oak, 2.5 g pine). The mixed species bags were included because these two litters do co-occur extensively and it was hypothesized that import-ant interactions may be present that could not be quanti®ed by the single species approach. Seven 2.25 m2 plots were located in close proximity to mature pine trees in order to maximize encounters between pine roots and litter bags. Equal numbers of bags for each litter treatment were randomly located in each plot. Half of the bags were placed on the forest ¯oor and the remaining half were buried at the interface between the organic and mineral horizon on 23 Febru-ary 1995. Subsets of bags were recovered on 25 May, 17 August and 9 November (13, 27 and 41 weeks, re-spectively). A total of seven replicate bags were har-vested on each sampling occasion. For each sampling event, we only retrieved the number of bags on a daily basis that could be processed that day in order to reduce any experimental error due to root aging in the phosphatase enzyme assay. The entire sampling eort lasted for less than 1 week.
Extracted litter bags were brie¯y rinsed with tap water to remove any adhering sand. Roots that had grown into bags were removed by careful dissection of the litter after opening the bags. Litter was dried at 708C for 48 h and weighed to determine percent mass loss. Phosphorus and nitrogen content of initial litter and decayed litter were determined by the Kjeldahl (N) and molybdate blue (P) methods (Allen, 1989). Oak and pine fractions from mixed species bags were analyzed separately in order to determine changes in chemistry (nutrient loss or gain) in each leaf species within the mixed litter bags.
was determined by morphotype identi®cation of all root tips and expressed as a percentage of the total number of root tips. Classi®cation was based on color and gross sheath morphology, following the methods of Agerer (1987) and Ingleby et al. (1990).
2.3. Acid phosphatase activity of ectomycorrhizas
Immediately following ectomycorrhizal classi®cation, acid phosphatase activity of root tips of each of the dominant ectomycorrhizal morphotypes were assayed following the methods used by Dighton and Coleman (1992). The phosphatase assay was used as an indi-cator for linkages between functional attributes of mycorrhizae and their chemical environment. The phosphatase assay measures the amount of phosphate cleaved from an organophosphate (p-nitrophenol phosphate). Root tip volume (diameter and mycorrhi-zal unit length measured using an eyepiece graticule in a steromicroscope) was measured in order to express enzyme activity as mg nitrophenol per mm2 root tip produced.
2.4. Litter decay measurements and fungal development
The decay dynamics of litter in bags placed above-ground were compared against bags of litter placed below-ground under the assumption that fungal in¯u-ences on decay and nutrient ¯ux from litter would be greater below-ground, especially within the mycorrhi-zal group. Subsamples of the decomposed litter were analyzed for hyphal length. Fungal hyphal lengths were measured by homogenizing a 1-g sample of leaf material in a blender in 1 l of sterile water. Five 1-ml aliquots were ®ltered through membrane ®lters (0.25 mm pore size), stained with Loe¯er's methylene blue, mounted in immersion oil. Fungal hyphal length was measured using a grid-line intersect procedure (Frank-land et al., 1990). Litter hyphal length may suggest how desirable these substrates were for fungal utiliz-ation.
2.5. Statistics
All data were analyzed using three-way ANOVA or GLM sequences of SAS (SAS, 1990). Pair-wise mean separation was determined using Tukey's Honestly Sig-ni®cant Dierence Test. Data pertaining to leaf litter mass loss and litter chemistry were separated into com-ponent litter species (i.e. oak, pine, oak in oak/pine mixture and pine in oak/pine mixture) yielding a total of 168 samples. Data regarding root length were ana-lyzed on a per litter bag basis, yielding 126 samples and phosphatase activity based on the number (vari-able) of root tips of each morphotype available within any one litter bag.
3. Results
Analysis of variance (three-way ANOVA) showed that leaf litter decomposition was signi®cantly greater in leaf litter placed below-ground (P < 0.001; F
71:09 (Fig. 1). Overall, there was a signi®cant loss of mass over time (P< 0.01; F62:2). Although there were no signi®cant dierences between decomposition rates of litter types overall (P> 0.05; F1:52). The presence of pine needles aboveground increased the rate of oak leaf decomposition (mixed oak). The pre-sence of oak leaves had no eect on the decay of pine needles. However, the opposite pattern occurred below-ground. Oak leaves increased pine needle decay, while the presence of pine needles had no eect on the decay of oak leaves. These dierences in response account for the signi®cant (P< 0.05;F4:5depth time interaction of the ANOVA.
Initial nitrogen content was higher (P< 0.01 by t-test) in oak leaves (1.6120.22 mg gÿ1
) than pine nee-dles (0.2920.03 mg gÿ1). When changing nutrient con-tent is coupled with decay rate, patterns of nutrient immobilization or mineralization can be detected. Nitrogen immobilization refers to a net increase in the absolute amounts of nitrogen (>100% initial nitrogen remaining). This generally occurs in litters that are
nitrogen poor and can re¯ect nitrogen limited decay. Mineralization refers to the net release of a nutrient (<100% initial nitrogen remaining) and generally occurs once that nutrient is available in amounts over that needed by decomposers. Pine needles initially im-mobilized signi®cantly more nitrogen (Fig. 2) than oak leaves in both aboveground and below-ground lo-cations (P< 0.01 by three-way ANOVA; F37:33). Pair-wise comparison of means show that pine alone or pine in the oak/pine mix immobilize more N than oak alone or oak in the oak/pine mix. There is a sig-ni®cant reduction in immobilized N over time (P< 0.001;F16:92 but all other factors and interactions are not signi®cant. By November, the pine needles still exhibit net N immobilization, while oak leaves demon-strate nitrogen mineralization. Placement eects are also signi®cant in pine needle N immobilization. Aboveground, mixed pine needles show less nitrogen mineralization than pure pine needles while below-ground, the presence of oak leaves has no eect on percent nitrogen remaining in pine needles.
Initial phosphorus content in pine needles (71211 mg gÿ1) was greater (P< 0.05 by t-test) than in oak leaves (3625 mg gÿ1). Oak leaves show a pattern of net phosphorus immobilization throughout the ®rst two sampling periods and appear to enter a phase of phosphorus mineralization at the ®nal sampling
inter-val. The pattern of percent P remaining is signi®cantly dierent between pine and oak as revealed by ANOVA (P< 0.01; F20:45). Pair-wise comparison of means show that oak alone or in the oak/pine mix immobilize more P than pine alone or pine in the oak/ pine mix. Percent phosphorus remaining showed a sig-ni®cant decrease over time for all litter types (Fig. 3;P < 0.001;F63:85 by three-way ANOVA).
Signi®cantly greater hyphal colonization was observed on litter decaying below-ground (P < 0.01; F6:8 by three-way ANOVA) suggesting greater fun-gal utilization of leaf substrate resources below-ground than aboveground (Fig. 4). This trend continued over time among oak leaves decaying below-ground vs. aboveground. There were no signi®cant dierences between litter types.
Three-way ANOVA showed no overall dierences in fungal colonization between leaf litters (P> 0.05; F2:18), but two-way analysis within the below-ground litter bags showed that fungal hyphal coloniza-tion of pine litter alone was signi®cantly less than oak or oak in oak/pine mix (P< 0.01; F4:34). There was signi®cantly greater fungal colonization of litter decaying below-ground (P< 0.01; F6:6), suggesting greater fungal utilization of leaf substrate resources below-ground than aboveground (Fig. 4). There were
Fig. 3. Phosphorus content of single and mixed pine and oak litter in litter bags placed on the soil surface or buried in the organic horizon of New Jersey pine barrens soil. Data points are means of seven replicates2standard error.
signi®cantly more hyphae in the ®nal harvest than either of the ®rst or second (P< 0.001;F83:98).
Discussion of pine root colonization and mycorrhi-zal classi®cation is necessarily con®ned to litter bags placed below-ground since root ingrowth did not occur aboveground. The root ingrowth data presented is limited to the third sampling interval since root ingrowth was universally absent during the ®rst sampling date and absent from many litter bags for the second sampling date. More root ingrowth occurred in mixed species litter bags (425.02155.5 mm per bag) than in single species litter bags (oak 218.62
59.0; pine 230.6268.6 mm per bag), but this was not statistically signi®cant P0:31; F1:24). Of the ectomycorrhizal morphotypes identi®ed, the ®ve most abundant (90% of all ectomycorrhizae) are described in Table 1. The other mycorrhizae were lumped together in a ``miscellaneous'' category. Each litter type was dominated by a dierent mycorrhizal mor-photype (Fig. 5). Types 2 and 3 dominated root tips invading pure oak leaves while Type 10 dominated root tips invading pure pine needles. Roots in mixed oak and pine litter supported a mycorrhizal ¯ora dominated by type 4.
Phosphatase activity measures the potential to cleave inorganic phosphate from an organic molecule. Table 2 shows the phosphatase activity of the main mycorrhi-zal morphotypes. The highest rates of phosphatase ac-tivity were associated with the morphotypes (types 2, 3 and 4) most common on root tips in¯uenced by oak leaves and, consequently, to litter that immobilized phosphate in organic forms. Root tips associated with pure pine needles, which show rapid losses of phos-phorus, were dominated by type 10 mycorrhizae which had signi®cantly lower phosphatase activity than type 4 and slightly less phosphatase activity than types 2 and 3. Type 1 was non-mycorrhizal and had the lowest phosphatase activity. The in¯uence of rapid P mineral-ization from pine leaves on the phosphatase enzyme production by roots is suggested by the signi®cant re-duction in phosphatase prore-duction in oak/pine mixed
Fig. 4. Fungal hyphal length associated with single and mixed pine and oak litter in litter bags placed on the soil surface or buried in the organic horizon of New Jersey pine barrens soil. Data points are means of seven replicates2standard error.
Table 1
Mycorrhizal morphotype classi®cation
Morphotype Description
Type 1 non-mycorrhizal
Type 2 cream mantle, white granular hyphae with clamp connections, outer sheath loosely woven
Type 3 cream mantle, cream rhizomorphs, slightly granular hyphae with clamp connections, hyphal dia > type 2
Type 6 Black mantle, black wirey hyphae, club shaped tip
Type 4 rusty brown mantle, no hyphae, sheath loose prosenchyma
Type 10 tuberculate, frosty white mantle, black non-granular hyphae
litter compared to oak alone (2262125, 3012165 ng
p-nitrophenol mmÿ2 root surface, respectively, P <
0.05) and a lower, though not signi®cant, activity in pine alone (2712194 ng p-nitrophenol mmÿ2root sur-face). From the analysis of variance, there was a sig-ni®cant litter mycorrhizal interaction for phosphatase activity. This could be explained by the diering response of the same mycorrhizal type to dierent leaf litters. For example, type 1 showed simi-lar enzyme activity between leaf litter types (125252, 121256, 121277 ngp-nitrophenol mmÿ2root surface in oak, oak±pine and pine, respectively). Type 6 is a mycorrhizal morphotype that occurred in low abun-dance in all leaf litter types. Type 6 showed highest levels of activity when in the presence of oak leaves alone (3002160, 135249, 153240 ng p-nitrophenol mmÿ2root surface in oak, oak±pine and pine, respect-ively). Type 4 exhibited lowest activity on oak±pine mixed litter (4712115, 196236, 5642296 ng p-nitro-phenol mmÿ2
root surface in oak, oak±pine and pine, respectively).
4. Discussion
Our study has shown that, aboveground, decompo-sition of oak leaves is slower than that of pine needles or of oak in combination with pine needles. However, there is no dierence below-ground in the decompo-sition rates. Pine needles represent a source of easily mobilized phosphorus while oak leaves serve as a phosphorus sink. Both litter types immobilize nitrogen, but this immobilization is signi®cantly greater in pine than oak. Litter quality dierences in¯uence the turn-over of organic matter in the Pinelands ecosystem. Pure and mixed litters decompose at dierent rates and these patterns dier in response to placement above or below ground. It was hypothesized that faster decay would be related to greater hyphal length. Indeed there was a signi®cantly higher fungal biomass on litter placed below ground, where decomposition was accelerated, but there were no dierences between leaf litter types. Resources of diering nutrient content
have been shown to in¯uence fungal colonization (Rayner, 1991; Ritz, 1995) by the in¯uence of dierent hyphal growth rates and branching patterns.
Although root growth into litter bags containing a mixture of oak and pine was greater than single litter species, the dierence was not statistically signi®cant. This enhanced root growth suggests that patches of high resource diversity encourage root production. Dierential rates of root growth into patches of soil with dierent nutrient contents has been shown by Pregitzer et al. (1993), Eissenstat and Van Rees (1994) and Barber (1995). In the case of dierences in root growth into litter bags, a number of interacting factors may be present, including availability of nutrients, water relations, physical constraints on root growth and allelopathic chemical leaching from the litter (Michelsen et al., 1995).
We found that ectomycorrhizal community structure also responds to patches of resource diversity. Ectomy-corrhizal species composition was dierent between leaf litter types. J. Baar (unpublished Ph.D. thesis, University of Wageningen, 1995) and Baar et al. (1994) showed signi®cant eects of leaf litter and humus on the ectomycorrhizal community structure of Scots pine. Koide et al. (1998) has shown dierential response of ectomycorrhizal fungal hyphal growth to leaf litter extracts, with growth of some species being suppressed whilst being promoted in other species. Michelsen et al. (1995) showed that leaf litter extracts aected nutrient acquisition by mycorrhizal plants.
Ectomycorrhizal root surface acid phosphatase ac-tivity has been shown to dier between fungal species. In this study, we have shown that our morphotype 4 produced signi®cantly more phosphatase than types 1, 6 and 10. When combined with the distribution of morphotypes within the dierent litter types, a pattern emerges of the presence of greater abundance of phos-phatase producing morphotypes (especially type 4 in oak litter and types 2 and 3 in the oak/pine litter mix-ture) where P immobilization is greatest. It is suggested, therefore, that the greater amounts of orga-nically-bound phosphorus in decaying oak leaves may be a factor selecting for phosphatase producing
ecto-Table 2
Acid phosphatase activity (ngp-nitrophenol mmÿ2root surface area) of the main ectomycorrhizal morphotypes from litter bags and from litter types. Means within columns sharing identical superscripted letters are not signi®cantly dierent from each otherp0:05: (Tukey's Honestly Signi®cant Dierences test)
Mycorrhizal morphotype Phosphatase mean2S.E. Litter type Phosphatase mean2S.E.
1 100211a oak 301225a
2 270215b oak/pine 226219bc
3 370238bc pine 194230ac
4 420254c
6 204243ab
mycorrhizal species. The dierent litters of this exper-iment demonstrated very dierent nutrient dynamics. Pine needles are low in nitrogen and high in phos-phorus. Phosphorus is rapidly lost, presumably as lea-chate, while nitrogen is conserved and accumulated via immobilization. Oak leaves, initially higher in nitrogen and lower in phosphorus, did not exhibit a net loss of nitrogen and accumulated phosphorus. It may be that decaying oak leaves represent a biological sink for phosphorus leached from pine leaves. These varying chemical environments are likely to select for dierent ectomycorrhizal species. Mycorrhizae with high phos-phatase activity are better suited to utilize immobilized phosphorus and were, therefore, associated more strongly with oak litters.
In mixed species forest, the distribution of leaf litter on the forest ¯oor is heterogeneous. The nature of lit-ter patches may have eects on the soil biota below it. In an unpublished study, we (Dighton et al.) have found that the distribution of litter patches in the New Jersey upland oak±pine habitats of the pine barrens is related to litter dams created by stems of the ericac-eous understory community. Litter patch size (area) was positively correlated r20
:908 with the density of ericaceous stems. Small litter patches contained a higher percentage contribution (by weight) of pine nee-dles and large patches contained a higher percentage contribution of oak leaves. Large litter patches were also found to have a higher Simpson's diversity index of ectomycorrhizal species than small patches. Our results suggest one mechanism (selective chemical en-vironment) by which litter patches of varying compo-sition could in¯uence mycorrhizal diversity.
Forming links between biodiversity and function was one of the main areas of perceived future research in soil microbiology (Coleman et al., 1994). In this paper we have shown that environmental factors can signi®cantly aect ectomycorrhizal community struc-ture. These micro-spatial changes in communities are related to changes in physical and chemical properties of the environment. We have also provided prelimi-nary evidence of diering functional attributes of the resultant ectomycorrhizal community developing within leaf litter patches of diering resource quality. This function has been expressed as root surface phos-phatase enzyme production, but represents possible wider overall physiological competence of dierent ectomycorrhizal communities (see also Dighton et al., 1990).
Acknowledgements
We would like to thank The Victoria Foundation for supporting Christine Conn's postdoctoral fellow-ship and Dennis Gray for his assistance with the
chemical analyses. We would also like to thank two anonymous reviewers for their constructive comments on the original draft of this paper.
References
Agerer, R., 1987. Colour Atlas of Ectomycorrhizae. Einhorn-Verlag, Schwabish GmuÈnd.
Alexander, I.J., Fairley, R.I., 1983. Eects of N fertilization on populations of ®ne roots and mycorrhizae in spruce humus. Plant and Soil 71, 49±53.
Allen, S.E. (Ed.), 1989. Chemical Analysis of Ecological Materials. Blackwell, Oxford.
Baar, J., Ozinger, W.A., Sweers, I.L., Kuyper, Th.W., 1994. Stimulatory and inhibitory eects of needle and grass extracts on the growth of some ectomycorrhizal fungi. Soil Biology & Biochemistry 26, 1073±1079.
Barber, S.A., 1995. Soil Nutrient Bioavailability. Wiley, New York. Brun, A., Chalot, M., Finlay, R.D., Soderstrom, B., 1995. Structure
and function of the ectomycorrhizal association betweenPaxillus involutus (Batsch) and Betula pendula (Roth). I. Dynamics of mycorrhizal formation. New Phytologist 129, 487±493.
Coleman, D.C., Ritz, K., Dighton, J., Giller, K.E., 1994. Perspectives on the compositional and functional analysis of soil communities. In: Ritz, K., Dighton, J., Giller, K.E. (Eds.), Beyond the Biomass: Compositional and Functional Analysis of Soil Microbial Communities. J. Wiley, Chichester, pp. 261±271. Dighton, J., Skengton, R., 1985. Eects of arti®cial acid
precipi-tation on the mycorrhizas of Scots pine seedlings. New Phytologist 107, 191±202.
Dighton, J., Mason, P.A., Poskitt, J.M., 1990. Field use of32P tracer to measure phosphate uptake by birch mycorrhizas. New Phytologist 116, 655±661.
Dighton, J., 1991. Acquisition of nutrients from organic resources by mycorrhizal autotrophic plants. Experientia 47, 362±369. Dighton, J., Jansen, A.E., 1991. Atmospheric pollutants and
ectomy-corrhizas: more questions than answers? Environmental Pollution 73, 179±204.
Dighton, J., Coleman, D.C., 1992. Phosphorus relations of roots and mycorrhizas of Rhododendron maximum L. in the southern Appalachians, NC. Mycorrhiza 1, 175±184.
Dighton, J., 1995. Nutrient cycling in dierent terrestrial ecosystems in relation to fungi. Canadian Journal of Botany 73 (Supp.), S1349±S1360.
Eissenstat, D.M., Van Rees, K.C.J., 1994. The growth and function of pine roots. Ecological Bulletins 43, 76±91.
Frankland, J.C., Dighton, J., Boddy, S., 1990. Methods for studying fungi in soil and forest letter. In: Grigorova, J., Norrid, J.R. (Eds.), Methods in (over) Microbiology, vol. 22, pp. 343±404. Ingleby, K., Mason, P.A., Last, F.T., Fleming, L.V., 1990.
Identi®cation of Ectomycorrhizas. ITE Research Publication No. 5, HMSO, London.
Jansen, A.E., Dighton, J., Bresser, T., 1988. Ectomycorrhiza and Acid Rain. CEC Air Pollution Research Report 12, Brussels. KaÊreÂn, O., Nylund, J.-E., 1997. Eects of ammonium sulphate on
the community structure and biomass of ectomycorrhizal fungi in a Norway spruce stand in southwestern Sweden. Canadian Journal of Botany 75, 1628±1642.
Koide, R., Suomi, L., Berghage, R., 1998. Tree±fungus interactions in ectomycorrhizal symbiosis. In: Romeo, J.T., Downum, K.S., Verpoorte, R. (Eds.), Phytochemical Signals and Plant±Microbe Interactions. Plenum Press, New York, pp. 57±70.
nutri-ent uptake of arctic graminoids by leaf extracts: allelopathy or resource competition between plants and microbes. Oecologia. Markley, M.L., 1979. Soil series of the Pine Barrens. In: Forman,
R.T.T. (Ed.), Pine Barrens: Ecosystem and Landscape. Academic Press, New York, pp. 81±93.
McCormick, J., 1979. The vegetation of the New Jersey Pine Barrens. In: Forman, R.T.T. (Ed.), Pine Barrens: Ecosystem and Landscape. Academic Press, New York, pp. 229±243.
Pregitzer, K.S., Hendrick, R.L., Fogel, R., 1993. The demography of ®ne roots in response to patches of water and nitrogen. New Phytologist 125, 575±580.
Rayner, A.D.M., 1991. The challenge of the individualistic my-celium. Mycologia 83, 48±71.
Read, D.J., 1991. Mycorrhizas in ecosystems. Experientia 47, 376± 389.
Ritz, K., 1995. Growth responses of some soil fungi to spatially het-erogenous nutrients. FEMS Microbial Ecology 16, 269±280. SAS, 1990. SAS Institute Inc., Cary, NC.
Shaw, T.M., Dighton, J., Sanders, F.E., 1995. Interactions between ectomycorrhizal and saprotrphic fungi on agar and in association with seedlings of lodgepole pine (Pinus contorta). Mycorrhizal Research 99, 785±791.
Tedrow, J.C.F., 1979. Development of Pine Barrens soils. In: Forman, R.T.T. (Ed.), Pine Barrens: Ecosystem and Landscape. Academic Press, New York, pp. 61±79.
Tennant, D., 1975. A test of a modi®ed line intersect method of esti-mating root length. Journal of Ecology 63, 995±1001.
