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154

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Increasing atmospheric CO

2

and stomata

Practically all of the CO

2

fixed by terrestrial plants, and mostof the water evaporated from them, passes through stomatalpores in leaves and other surfaces. The way in which stomatarespond to the environment and control photosynthesis andtranspiration is, therefore, a key determinant of plant growthand water status. Moreover, these gas exchange processes affectthe global carbon and hydrological cycles, and thereforefeedback on climate, so they are central to the concerns of globalwarming and climate change. It is widely stated that increasedatmospheric CO

2

concentration ( [CO

2

] ) will cause reducedstomatal conductance ( g

s

), and there have been many attemptsto derive a ‘typical value’ for the reduction of stomatal conductance(

∆

g

s

), but this value has varied widely, if not wildly, asdifferent reviews have been published. The latest review, byMedlyn

et al

. (2001) (see pp. 247–264 in this issue), suggestsa mean

∆

g

s

of 21% (95% CI of 5–33%) for tree species.

‘The exact reduction of stomatal conductance is

actually very important, as shown by several recent

global climate modelling studies’

Bringing stomatal conductance measurements up to date

The many early and usually short-term studies on agricultural/horticultural crops and other herbaceous species suggestedthat a doubling of [CO

2

] typically reduced g

s

by 40% (Morison,1987), although with substantial variation. In 1989, Eamus& Jarvis (1989) reviewed literature for studies on tree species,and came up with a

∆

g

s

value of 30 – 40%, but with a rangeof 0 –70%. Some years later, Field

et al

. (1995) derived asmaller

∆

g

s

value of 27% in a synthesis of studies with 20species of woody plants. Drake

et al

. (1997) then summarized

∆

g

s

as 20%, using data from 41 studies, with some 28herbaceous and tree species. Most recently, Saxe

et al

. (1998)suggested that the main factor causing variation in

∆

g

s

wasdifferences between less responsive woody and more respons-ive herbaceous species, and Curtis & Wang (1998) derived

for woody species alone only an 11% reduction, which wasnot even a statistically significant effect. While the differencesbetween these older estimates and the latest figures fromMedlyn

et al

. (2001) might seem small, given the major roleof vegetation cover in exchanges of water, energy and CO

2

between the land surface and the atmosphere, the exact valueof

∆

g

s

is actually very important, as shown by several recentglobal climate modelling studies (e.g. Sellers

et al.

, 1996).Medlyn

et al

. (2001) use a dataset drawn from a range ofmultiyear experiments on trees exposed with a variety of tech-niques collected in two European programmes. The paperhighlights several of the key questions about how stomata willchange with increased atmospheric [CO

2

], addressing whetherthere are differences in

∆

g

s

between the following:• plant functional groups (e.g. conifers vs broadleaves);• long-term experiments vs short;• mature trees and saplings or seedlings.In addition, they looked for evidence of stomatal ‘acclimation’,

that is, physiological changes after growth in high [CO

2

](Drake

et al.

, 1997). They tackle this by using both a meta-analysis, and by using the dataset to parameterize predictivemodels of stomatal response to the environment.

Meta-analysis in global environmental change impact experiments

Though used for a long period of time in social and medicalstudies, meta-analysis in the synthesis of information fromdifferent datasets has only recently been applied to ecophysio-logical experiments, and in particular Peter Curtis (one ofthe coauthors of the paper) was instrumental in raising itsprofile with an analysis of data on photosynthetic rates fromexperiments on plants grown in high [CO

2

] (Curtis, 1996).Meta-analysis can offer a more impartial way to synthesizedata than an essentially qualitative review, and the observationsfrom individual studies are weighted in the overall assessmentof effects by the sample size. The use of such an approachreflects the need for quantification and synthesis that is centralto the philosophy of the International Geosphere–BiosphereProgramme – Global Change and Terrestrial Ecosystems(IGBP – GCTE), and the requirement for robust conclusionsto parameterize models and inform policy makers workingwith the impacts of global change. An additional benefit ofthis approach is that it highlights the need for proper replicationin experiments (i.e., replicating treatment units and not plantsor samples within units (Hurlbert, 1984) ). This was certainlya problem in many earlier experiments on plant responses toglobal change. However, even now, more rigorous experimentaldesign and consideration of replication is required as Filion

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et al

. (2000) have recently pointed out the low statistical‘power’ of many experiments in detecting the effect of CO

2

enrichment, experiments that are often very resource intensive.

Stomatal response to CO

2

in trees

One key conclusion of the meta-analysis reported here wasthat taking the longer-term experiments only (more than1 yr) led to a larger

∆

g

s

and there was more variation inshorter-term (< 1 yr) experiments, which reduced any overallestimates of

∆

g

s

. Second, by examining the few results whereplants grown in both high and low [CO

2

] were also measuredin both [CO

2

]s they did not detect any stomatal acclimation.This is particularly interesting as few studies have actuallycompared g

s

values in this way (Morison, 1998) so that wecan separate the short-term

response

(i.e. an indicator ofstomatal sensitivity to CO

2

) from the long-term

effect of

growth in high [CO

2

]. In some of the species examined(Medlyn

et al.

, 2001, Table 8) there was little or no short-termresponse just as found with the long-term effect, but in somespecies the short-term response was substantially larger thanthe long-term effect (Medlyn

et al.

, 2001, Table 3). Our ownrecent data from a scrub oak in Florida (

Quercus myrtifolia

;Lodge

et al

., 2001) is relevant here: we found

∆

g

s

wasapproximately 40%, but there was reduced sensitivity to ashort-term change in [CO

2

] in the high CO

2

grown plants. Akey point is that in the Florida study we measured the stomatalresponses on excised shoots, to try to isolate differences insensitivity from differences in conditions that might haveoccurred between treatments. It may be that the short-termsensitivity of some of the species in the Medlyn

et al

. (2001)study was in part mitigated in the long-term by, for example,improved water status in the high CO

2

grown plants.The variation in

∆

g

s

is a typical feature over the season(Medlyn

et al.

, 2001, Table 3) and even day to day (Bunce,1998). This should not be surprising because of all the inter-actions with other variables such as temperature, water status,humidity, light and leaf age on stomata (Morison, 1998). Asan example of the complexity of the possible effects, Bunce(1998) attributed daily variations in

∆

g

s

to humidity condi-tions, and suggested that differences in transpiration ratecaused variation in delivery of ABA to the guard cells, therebyaffecting CO

2

sensitivity. It could also be that such differencesin transpiration rate affect the apoplastic sucrose concentra-tion adjacent to the guard cells and, therefore, guard cell waterrelations and metabolism (Lu

et al.

, 1997). Clearly, as plantsare grown for long periods in different [CO

2

], then differencesin soil water and nutrient status, morphology, phenologyanatomy and physiology may occur. We should really ques-tion whether it is useful to make such ‘point’ comparisons ofplants grown in different [CO

2

]. What is much more import-ant is to ascertain whether the responses to the key environ-mental variables will be different after long-term growth inhigh [CO

2

].

Modelling stomatal responses to increased CO

2

: the future

This is why the second part of the analysis by Medlyn

et al

.(2001) is particularly useful, in that it examines whether ornot for their European tree dataset the parameters of twowidely used modelling approaches for predicting g

s

werealtered by growth in high CO

2

concentration. That they didnot seem to be might give some confidence in using suchmodels to predict g

s

within overall ecosystem and forest growthand water use models under scenarios of future atmosphericconditions. However, in the Florida study and others (Bunce,1993; Heath & Kerstiens, 1997), it appears that stomata inhigh CO

2

grown plants did show physiological acclimation,and became less sensitive to CO

2

and other variables, there-fore this needs further careful examination. Interestingly, anystomatal acclimation seems to have been unrelated to the degreeof photosynthetic acclimation (Lodge

et al.

, 2001; Medlyn

et al.

, 2001), which may limit the use of modelling approachesthat rely upon the apparent linkage of photosynthesis andstomata. Furthermore, while in some studies stomata may haveshown a linear decline of g

s

with increased [CO

2

], in othersthe response is very nonlinear, with peak sensitivity aroundpresent atmospheric [CO

2

] (Lodge

et al.

, 2001; Morison, 1998,1987), therefore, the sort of stomatal sensitivity models usedwill be very important. We still await a less empirical approachto describing stomatal response to the environment.

James I. L. Morison

Dept of Biological Sciences, University of Essex, WivenhoePark, Colchester CO4 3SQ, UK

(tel +44 1206873327; fax +44 1206873416;email morisj@essex.ac.uk)

References

Bunce JA. 1993.

Effects of doubled atmospheric carbon dioxide concentration on the responses of assimilation and conductance to humidity.

Plant, Cell & Environment

16

: 189 –197.

Bunce JA. 1998.

Effects of humidity on short-term responses of stomatal conductance to an increase in carbon dioxide concentration.

Plant, Cell & Environment

21

: 115–120.

Curtis PS. 1996.

A meta-analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide.

Plant, Cell & Environment

19

: 127–137.

Curtis PS, Wang X. 1998.

A meta-analysis of elevated CO

2

effects on woody plant mass, form and physiology.

Oecologia

113

: 299–313.

Eamus D, Jarvis. PJ. 1989.

The direct effects of increase in the global atmospheric CO

2

concentration on natural and commercial temperate trees and forests.

Advances in Ecological Research

19

: 1–55.

Drake BG, Gonzàlez-Meler MA, Long SP. 1997.

More efficient plants: a consequence of rising atmospheric CO

2

?

Annual Reviews of Plant Physiology and Plant Molecular Biology

.

48

: 609 –639.

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Field CB, Jackson RB, Mooney HA. 1995.

Stomatal responses to increased CO

2

: implications from the plant to the global scale.

Plant, Cell & Environment

18

: 1214–1225.

Filion M, Dutilleul P, Potvin C. 2000.

Optimum experimental design for Free-Air Carbon dioxide Enrichment (FACE) studies.

Global Change Biology

6

: 843– 854.

Hurlbert SH. 1984.

Pseudoreplication and the design of ecological field experiments.

Ecological Monographs

54

: 187–211.

Heath J, Kerstiens G. 1997.

Effects of elevated CO

2

on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech.

Plant, Cell

&

Environment

20

: 57 – 67.

Lodge RJ, Dijkstra P, Drake BG, Morison JIL. 2001.

Stomatal acclimation to increased CO

2

concentration in a Florida scrub oak species

Quercus myrtifolia

Willd.:

Plant, Cell & Environment

. (In press.)

Lu P, Outlaw WH, Smith BG, Freed GA. 1997.

A new mechanism for the regulation of stomatal aperture size in intact leaves – Accumulation of mesophyll-derived sucrose in the guard-cell wall of.

Vicia faba. Plant Physiology

114

: 109 –118.

Medlyn BE, Barton CVM, Broadmeadow MSJ, Ceulemans R, De Angelis P, Forstreuter M, Freeman M, Jackson SB, Kellomaki S, Laitat E, Rey A, Robertnz P, Sigurdsson BD, Strassemeyer J, Wang K, Curtis PS, Jarvis PJ. 2001.

Stomatal conductance of European forest species after long-term exposure to elevated [CO

2

]: a synthesis of experimental data.

New Phytologist

149

: 247–264.

Morison JIL. 1987.

Intercellular CO

2

concentration and stomatal response to CO

2

. In: Zeiger E, Cowan IR, Farquhar GD, eds.

Stomatal Function

. Stanford, CA, USA: Stanford University Press, 229 –251.

Morison JIL. 1998.

Stomatal response to increased atmospheric CO

2

.

Journal of Experimental Botany

49

: 443 – 452.

Saxe H, Ellsworth DS, Heath J. 1998.

Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395– 436.

Sellers PJ, Bounoua L, Collatz GJ, Randall DA, Dazlich DA, Los SO, Berry JA, Fung I, Tucker CJ, Field CB, Jensen TG. 1996. Comparison of radiative and physiological effects of doubled atmospheric CO2 on climate. Science 271: 1402–1406.

Key words: Elevated CO2 concentration, Atmospheric CO2, Stomal conductance, Climate change, Gas exchange, Global carbon, Transpiration, Water status.149no issue no.2001046000000Graphicraft Limited, Hong Kong

Nitrogen and ectomycorrhizal fungal communities: what we know, what we need to know

Understanding the effect of changing nitrogen availabilityon biodiversity is critical, not only to address basic questions

about the factors that structure communities, but also becauseatmospheric nitrogen deposition has been increasing in recentdecades (Galloway et al., 1995). One hypothesized effect ofexcess N deposition is the loss of diversity of ectomycorrhizalfungi (Arnolds, 1991). Sporocarp production is one way toassess ectomycorrhizal fungal diversity, but it is not reliable –for that, it is necessary to look belowground, now a realisticproposition with the development of PCR-based molecularidentification methods. The study by Peter and co-workers(2001) in this issue (see pp. 311– 325), is the first in-depthmolecular study to track the changes in belowground ecto-mycorrhizal communities from the initiation of fertilizationonwards.

Monitoring loss of diversity

Given that N is often a limiting plant nutrient, one mightthink that its addition would be a welcome form of freefertilization. However, there is always the possibility of toomuch of a good thing, and excess N availability is knownto have negative effects on plant biodiversity and ecosystemfunction (Vitousek et al., 1997). Analysing temporal trendsin sporocarp production, Arnolds (1991) summarized theevidence for a dramatic decline of diversity in ectomycorrhizalfungi in Europe. He proposed that N deposition was a likelycontributor to this decline. Subsequent fertilization experi-ments and deposition gradient studies demonstrated thatadditions of N can lead to changes in sporocarp productionthat paralleled those seen over time in Europe (Wallenda &Kottke, 1998; Lilleskov et al., 2001b).

However, sporocarp production is unlikely to reflect below-ground communities: first, fungi may reduce allocation tosporocarps in response to N fertilization, without any changein community structure; second, even in the absence of fert-ilization, ectomycorrhizal fungal species are not equally rep-resented as sporocarps and on roots. Not all ectomycorrhizalfungi produce conspicuous epigeous sporocarps. Some pro-duce thin crusts on logs or leaf litter (e.g. Thelephoraceaeand Corticiaceae), and others have no known sexual stage(e.g. Cenococcum geophilum). Of those fungi that do produceconspicuous sporocarps, a species’ sporocarp production doesnot necessarily reflect its below-ground abundance (Gardes &Bruns, 1996).

Thus, it is necessary to look below-ground to know what isreally happening. Unfortunately, this is a much more difficultproposition, because of the methodological challenges involvedin fungal identification on roots, combined with high diversityand spatial variability. Previously, most studies were limitedto morphological typing. Unfortunately, morphotypes werenot easily comparable among studies, and often lead to falselumping and splitting of taxa (Mehmann et al., 1995). Theseproblems have recently been overcome by the development ofPCR-based molecular identification methods. Several studieshave applied these methods to the question of below-ground

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community composition in response to N fertilization or overN deposition gradients, but the study by Peter et al. (2001)covers new territory.

Tracking changes below-ground

The authors used a carefully designed sampling regime toenable them to determine relative frequencies of differentectomycorrhizal fungi with minimal disturbance of the site.They also utilized rapidly expanding sequence databases forfungal identification, and an innovative approach for precisequantification of DNA restriction fragment sizes. As a result,they identified a large proportion of fungi present on root tipsto the family level or lower. By examining ectomycorrhizalcommunities before and for 2 yr after fertilization in N-treated and control plots, they have been able to track short-termcommunity changes, and compare them with the trends insporocarp production in the same plots. What they found isthat there is a change in ectomycorrhizal fungal communitiesas seen both above- and below-ground, but that the below-ground response is less pronounced. Whereas in the sporocarpsthere was a significant decline in species richness and diversity,below-ground there was no loss of diversity in response to thetreatment. However, certain taxa declined in frequency inresponse to N inputs (e.g. Russula spp.), whereas others didnot (e.g. Tylospora asterophora).

This study provides the best evidence of short-term below-ground community response to N inputs. Previous short-termstudies have shown little or no change below-ground (Saunderset al., 1996; Kårén & Nylund, 1997; Brandrud & Timmermann,1998; Jonsson et al., 2000). The ability of Peter et al. (2001)to detect a change may be due to two factors. First, theiradditions of N were larger than those in previous studies,perhaps leading to more rapid change. Second, the combinationof molecular identification methods with the above-mentionedexperimental design may have given the authors more sensit-ivity to detect small changes than in previous studies.

The smaller change that Peter et al. (2001) report in theectomycorrhizal fungal community below-ground relative tosporocarps can be interpreted in two ways: as a system oper-ating with no lags, in which the community shows littleresponse because N does not affect the below-ground speciescomposition, but rather only affects allocation to sporocarpproduction; or as a system which exhibits lags, in which thebelow-ground community shows less short-term responsethan the sporocarp community, but significant long-termresponse. Two molecular studies of the effects of long-terminputs on below-ground communities suggest that the lattermodel is more likely. In both a fertilization study (Kårén,1997), and an N deposition gradient study (Lilleskov et al.,2001a), high N inputs are associated with strikingly similarchanges in ectomycorrhizal fungal communities, resulting inthe loss of many ectomycorrhizal fungal taxa and a shift in thedominants.

Generality and mechanisms of community change

It is important to know how much we can generalize theseresults. Arnolds (1991) found that diversity of sporocarps haddeclined more for conifer-associated than for deciduous-associatedectomycorrhizal fungal taxa. Along a north–south transectin Europe Taylor et al. (2000) found a negative relationshipbetween morphotype richness and soil inorganic N in sprucestands, and a weaker positive relationship between these variablesin beech stands. Given that all other below-ground studies ofN effects have been carried out in conifer forests, the work byTaylor et al. (2000) underscores the need for more conifer–deciduous tree comparisons.

Although evidence is accumulating that N inputs can causechanges in ectomycorrhizal fungi communities, the proximalmechanisms for this change are unclear. Some possibilitiesinclude the large carbon cost of assimilation of inorganic Ninto amino acids under high N conditions (Wallander, 1995);changes in host-plant nutrition and subsequent shifts in host-plant carbon allocation and receptivity to ectomycorrhizal fungi;and N-mediated declines in soil pH, base cation availabilityand toxic metal availability. Given the difficulties involved indefining individuals of ectomycorrhizal fungi at the root level,we also cannot say whether observed changes in species abund-ance occur via clonal expansion or spore colonization.

The N-induced change in sporocarp production couldaffect below-ground community composition, if certain spe-cies require spore inputs for local maintenance. For example,one might expect that species colonizing mature forest standswould be effective at vegetative spread, but it appears thatsome Russula species from mature forests actually have numeroussmall genets, suggesting either that genets expand very slowly,or that continued colonization by spores may be important(Redecker et al., 2001). Given that Russula sporocarp produc-tion declines with increasing N inputs (Peter et al., 2001;Wallenda & Kottke, 1998; Mehmann et al., 1995; Lilleskovet al., 2001b), this raises the possibility that reduced sporeinoculum may be one mechanism leading to below-grounddecline of Russula.

Functional effects

Perhaps the biggest question is, what functional effect doesectomycorrhizal fungal community change have on forestecosystems? N appears to limit plant growth in manytemperate and boreal forests dominated by ectomycorrhizaltrees, and its availability varies over a broad range of temporaland spatial scales. So it is not surprising that the organismsthat supply the majority of N to these trees – theectomycorrhizal fungi – possess a broad physiologicalpotential for N uptake, utilizing a range of inorganic andorganic N sources, and supplying this N to plant hosts (Smith& Read, 1997). Recent work suggests that there may be a

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decline in the importance of taxa utilizing organic N withincreasing soil N availability (Lilleskov et al., 1998; Tayloret al., 2000). There are many other ways that ectomycorrhizalfungi community function might change as community structurechanges. It seems likely that carbon supply and nutrient avail-ability interact to select functionally distinct fungi. As nutrientsother than N become limiting, do we see functional shiftstowards species that specialize on the new limiting resources?As carbon supply to roots declines, do we see a shift towardtaxa that are more carbon efficient, or toward taxa that arecarbon parasites? Can we disentangle the effects of changingresources and changing ectomycorrhizal fungal communitieson plant growth? How do shifts in community compositionand changing carbon allocation below ground affect mycorrhizos-phere organisms, and below-ground food webs? These questionsremain largely unanswered. The study by Peter et al. (2001)reinforces our belief that they are worth asking.

Erik A. Lilleskov* and Thomas D. Bruns

Department of Plant and Microbial Biology,University of California, Berkeley, CA, 94720 USA

*Author Correspondence(tel +1510 643–5483; fax +1510 642–4995;

email lillesko@uclink4.berkeley.edu)

References

Arnolds E. 1991. Decline of ectomycorrhizal fungi in Europe. Agric Ecosyst Environ 35: 209–244.

Brandrud TE, Timmermann V. 1998. Ectomycorrhizal fungi in the NITREX site at Gardsjon, Sweden: Below and above-ground responses to experimentally-changed nitrogen inputs 1990–95. Forest Ecology and Management 101: 207–214.

Galloway JN, Schlesinger WH, Levy HI, Michaels A, Schnoor JL. 1995. Nitrogen fixation: Anthropogenic enhancement-environmental response. Global Biogeochemical Cycles 9: 235 –252.

Gardes M, Bruns TD. 1996. Community structure of ectomycorrhizal fungi in a Pinus muricata forest: Above- and below-ground views. Canadian Journal of Botany 74: 1572 –1583.

Jonsson L, Dahlberg A, Brandrud TE. 2000. Spatiotemporal distribution of an ectomycorrhizal community in an oligotrophic Swedish Picea abies forest subjected to experimental nitrogen addition: Above- and below-ground views. Forest Ecology and Management 132: 143 –156.

Kårén O. 1997. Effects of air pollution and forest regeneration methods on the community structure of ectomycorrhizal fungi. PhD thesis, Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, Uppsala, Sweden.

Kårén O, Nylund J-E. 1997. Effects 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.

Lilleskov EA, Fahey TJ, Horton TR, Lovett GM. 2001a. Belowground ectomycorrhizal fungal community change over a nitrogen deposition gradient in Alaska. Ecology. (In press.)

Lilleskov EA, Fahey TJ, Lovett GM. 2001b. Ectomycorrhizal fungal aboveground community change over an atmospheric nitrogen deposition gradient. Ecological Applications. (In press.)

Lilleskov EA, Hobbie EA, Fahey TJ. 1998. Does atmospheric nitrogen deposition cause a functional shift in ectomycorrhizal fungal communities? Abstracts of the Ecological Society of America Annual Meeting. Baltimore, Maryland, USA.

Mehmann B, Egli S, Braus GH, Brunner I. 1995. Coincidence between molecularly and morphologically classified ectomycorrhizal morphotypes and fruitbodies in a spruce forest. In: Stocchi V, Bonfante P, Nuti M, eds. Biotechnology of ectomycorrhizae: molecular approaches. New York, USA: Plenum Press, 41–52.

Peter M, Ayer F, Egli S. 2001. Nitrogen addition in a Norway spruce stand altered macromycete sporocarp production and belowground ectomycorrhizal species composition. New Phytologist 149: 311–325.

Redecker D, Szaro TM, Bowman RJ, Bruns TD. 2001. Small genets of Lactarius xanthogalactus Russula cremoricolor and Amanita francheti in late stage ectomycorrhizal successions. Molecular Ecology. (In press.)

Saunders E, Taylor AFS, Read DJ. 1996. Ectomycorrhizal community response to simulated pollutant nitrogen deposition in a Sitka spruce stand, North Wales. In: Szaro TM, Bruns TD, eds. Program and abstracts of the first international conference on mycorrhizae. University of California, Berkeley, California, USA.

Smith SE, Read DJ. 1997. Mycorrhizal symbiosis, 2nd edn. San Diego, CA, USA: Academic Press.

Taylor AFS, Martin F, Read DJ. 2000. Fungal diversity in ectomycorrhizal communities of Norway spruce (Picea abies (L.) Karst.) and Beech (Fagus sylvatica L.) in forests along north–south transects in Europe. In: Schulze E-D, ed. Carbon and nitrogen cycling in European forest ecosystems. Ecological studies, vol. 142. Heidelberg, Germany: Springer-Verlag, 343 –365.

Vitousek PM, Aber JD, Howarth RH, Likens GE, Matson PA, Schindler DW, Schlesinger WH, Tilman DG. 1997. Human alteration of the global nitrogen cycle: Source and consequences. Ecological Applications 7: 737–750.

Wallander H. 1995. A new hypothesis to explain allocation of dry matter between mycorrhizal fungi and pine seedlings in relation to nutrient supply. Plant and Soil 169: 243 –248.

Wallenda T, Kottke I. 1998. Nitrogen deposition and ectomycorrhizas. New Phytologist 139: 169–187.

Key words: Nitrogen, ectomycorrhizal fungi, fungal diversity, community structure, sporocarp production, molecular identification, PCR identification, nutrient availability.

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