Báo cáo khoa học: " Humus form development and succession of dwarf shrub vegetation in grass dominated primary Pinus sylvestris forests"

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article Original Humus form development and succession of dwarf shrub vegetation in grass dominated primary Pinus sylvestris forests IM Emmer Laboratory of Physical Geography and Soil Science, University of Amsterdam, Netherlands Centre for Geo-Ecological Research (ICG), Nieuwe Prinsengracht 130, 1018 VZAmsterdam, The Netherlands (Received 2 January 1994; accepted 31 January 1995) Summary — Because of its role in nutrient and water supply to plants, the humus form is strongly inter- related with the vegetation and is likely to at least partly control vegetation succession. Causal relations between vegetation succession and soil development are generally difficult to study because of the mul- tifactorial nature of ecology. Primary succession of Pinus sylvestris forests on recent inland dunes provides interesting information on succession related humus form development. Humus form char- acteristics and vegetation composition in these drift sand areas were studied along an age series of Scots pine stands, ranging from 15 to 124 years old, all situated on similar parent materials and lacking antecedent soil formation. From the information thus obtained, successional processes were deduced and the following conclusions were drawn: 1) Chemical variability in the ectorganic profiles under the Scots pine forests concerns a downward decrease of plant available elements as well as allelochem- icals, which both may have implications for reproduction and competitive ability of undergrowth species involved in the succession. 2) The concentration of Deschampsia flexuosa roots in the F horizon indi- cates a pronounced niche differentiation, which enables this species to maintain dominance in the herb layer for a long period of time during succession in pine forests. 3) Facilitation in the succession of the undergrowth of the pine forests seems to emanate from horizon differentiation and associated Dwarf shrubs take advantage of the development of an H horizon, leading to a botan- chemical variability. variable undergrowth vegetation in older pine forests. ically more humus form / vegetation succession / Pinus sylvestris Résumé — Développement des profils humiques et succession des arbustes nains dans des forêts de Pinus sylvestris à graminées. Une succession primaire de forêts à base de Pinus syl- vestris sur dunes continentales récentes fournit des informations intéressantes sur les interrelations entre successions végétales et évolution des types d’humus. La caractérisation des types d’humus et la composition de la végétation sur ces sables éoliens ont été étudiées le long d’une séquence de pins sylvestres, allant de 15 à 24 ans, sur roches mères semblables, sans pédogenèse antérieure à l’ins-
tallation du peuplement. À partir des informations mesurées, les étapes de la succession ont été reconstruites et les conclusions suivantes obtenues : i) sous les forêts de Pinus sylvestris, les profils ectorganiques montrent la variabilité chimique qui se traduit par une diminution des éléments dispo- nibles pour les plantes et des composés allélochimiques, qui tous 2 peuvent avoir des conséquences pour la régénération et le pouvoir compétitif des espèces herbacées qui participent à la succession végé- tale ; ii) la concentration dans l’horizon F des racines des Deschampsia flexuosa indique une diffé- renciation prononcée en niches, qui permet à cette espèce de prolonger sa dominance pour une période assez longue pendant la succession végétale ; iii) la différenciation en horizons et la variabi- lité chimique qui en résulte semblent favoriser la succession de la couche herbacée. Les arbustes nains profitent du développement d’un horizon H, ce qui mène à une couche herbacée plus variée dans les forêts de pins âgés. type d’humus et succession végétale / pin sylvestre INTRODUCTION terised by the partial replacement of Deschampsia flexuosa (L) Trin by Empetrum nigrum L, Vaccinium myrtillus L and forest In the past decades, various models have mosses, which coincides with the develop- been developed describing general path- ment of an H horizon. Scots pine forests in ways of primary and secondary succession western Europe commonly have developed (eg West et al, 1981).Environmental fac- as secondary successions on former heath- tors, such as light intensity, grazing, nutrient lands or have been strongly influenced by availability and allelopathy, have been found forestry and episodic soil management. The to play a role in species competition, and primary succession of Scots pine forests in they may act as driving forces behind suc- the Hulshorsterzand area has provided an cession (Muller, 1969; Ahlgren and Ahlgren, exceptional opportunity to study the tem- 1981; Miles, 1985; Hester et al, 1991 a, b; poral and vertical variability of humus form Leuschner, 1993). There is, however, still characteristics evolving from the vegetation little knowledge about causal relations development, without interference of between soil development and concomitant antecedent soil formation. The study of tem- changes in vegetation composition. poral and vertical variability of the ector- In coniferous forest ecosystems, the ganic profile in a chronosequence of these ectorganic profile is generally the major root- forests has been reported by Emmer and ing environment for the undergrowth vege- Sevink (1994). tation. Furthermore, it determines the con- The objective of this paper is to describe ditions under which germination of how the humus form development may con- propagules takes place. Under undisturbed trol the succession of the undergrowth veg- conditions, succession related changes in etation in the pine forests. soils particularly concern the autogenic development of humus forms, as they are controlled by the actual species composi- MATERIALS AND METHODS tion. Humus forms therefore deserve special attention in studies on vegetation succes- sion (Emmer and Sevink, 1993). The Pinus sylvestris forests are situated on recent inland dunes in the Hulshorsterzand area in the Fanta (1986) described a primary suc- Veluwe (5°44’E, 52°20’N, 10-15 m above sea cession of Pinus sylvestris L forests on poor level), the Netherlands. The age series studied sandy soils in the Netherlands. In later tran- spans a period of about 120 years, representing sient stages, this succession is charac- a primary successional sere, thus lacking
percentages of the herb and moss species antecedent soil formation pretreatment. The cover or and vertical distribution of herb and pine roots in vegetation has been studied by Fanta (1986) and the humus form profile. For the description of the Prach (1989). The initial tree-less stages are dom- rooting profile in the stands mentioned previously, inated by grasses (Corynephorus canescens [L] the soils were sampled to a depth of 40 cm using Beauv and Festuca ovina L) and moss (Poly- a soil monolith sampler (surface 42 cm Warde- ; 2 trichum piliferum Hedw). In young pine stands naar, 1987). These root distributions were (less than about 20 years old and with a tree den- obtained using a semiquantitative method for pro- sity exceeding 1 500 trees ha ground vege- ), -1 file descriptions such as outlined by Klinka et al tation is almost absent. Later on, in 40-50-year- (1981). This method distinguishes 6 classes of old stands, Deschampsia flexuosa appears and root abundance, relating to the number of roots soon becomes dominant. The species association counted in a 6.25 cm area for roots smaller than 2 is described as Leucobryo-Pinetum sub associ- 5 mm in diameter. For the root estimations, soils ation Deschampsietosum (Van der Werf, 1991). were sampled in places where the particular herb D flexuosa is considered to have increased its species had a cover percentage of 80-100%. surface cover in the past decades due to increased atmospheric N deposition (cf Heij and In stands 15 and 30, at 12 randomly selected Schneider, 1991).In the following stages of suc- points, the organic layer was sampled by horizon cession (80-100-year-old stands), D flexuosa with a 100 cm metal frame. The 12 samples of 2 slowly declines and mosses (such as Pleuroz- each horizon bulked afterwards. In the other were ium schreberi Hedw, Hypnum cupressiforme profiles were sampled along a nested 20 stands, Hedw, Dicranum scoparium Hedw and Dicranum triangular grid, with minimum distances of 2.3 m polysetum Hedw) become more abundant. In between 2 neighbouring points and maximum dis- 120-year-old stands, described as Empetro-Pine- tances of 63 m. For these samplings, a 25 x 25 cm tum (Van der Werf, 1991), the herb layer has a metal frame was used. The organic horizons were much more varied composition, with a co-domi- sampled and subdivided into L, F F Hr and Hd ,, 12 nance of dwarf shrubs (in particular, Empetrum horizons (Klinka et al, 1981), conforming to L, Fr, nigrum and Vaccinium myrtillus), D flexuosa and Fm, Hr and Hf horizons (Babel, 1971) and approx- forest mosses. The declining vigour of D flexu- imately to Oi, Oei, Oe, Oea and Oa horizons (Soil osa has been established from decreasing flow- Survey Staff, 1981).Samples were analysed sep- ering intensity and increasing dead/living shoot arately for organic matter content and elemental ratios in the 2 oldest stands (Moszynska, 1991). composition of the organic matter and of water The Empetro-Pinetum is considered to be a discli- extracts. Details about sampling and analytical max vegetation, because the possible develop- procedures are given by Emmer and Sevink ment towards the regional climax vegetation (1994), and Emmer and Verstraten (1993). (Betulo-Quercetum robori or Fago-Quercetum; In a subsequent sampling, bulked samples Van der Werf, 1991) is impeded by animal brows- (n = 7-10) of organic horizons (L+ F F Hr and ,, 12 ing (Fanta, 1986). Hd) were collected in all the dune sites for the The pine stands have been gradually thinned analysis of water extractable phenols. For this to about 650 trees ha during the first 4 decades. -1 purpose, in the 3 oldest stands, samples were The structure of the stands has remained taken in patches of D flexuosa only, to prevent unchanged during subsequent development. effects of different herb species. An estimation of total water soluble phenols in 1:25 (w/v) extracts The soils (Haplic or Cambic Arenosols, accord- was done using the Folin-Ciocalteus reagent (Box, ing to FAO-Unesco, 1988) are well drained and 1983). Phenol concentrations are expressed as exhibit the development of mor-type humus forms tannic acid equivalents in organic matter (mg TAE (classified according to Klinka et al, 1981),show- -1 100 g OM). ing a strong horizon differentiation. Climate is humid with a mean annual rainfall of temperate about 800 mm, rather evenly distributed over the year, and with a potential precipitation surplus of RESULTS 325 mm. Vegetation descriptions and soil samplings presented by Emmer and Sevink The data have been carried out along a chronosequence, (1994) concern organic matter accumula- comprising 15, 30, 59, 95 and 124-year-old stands tion during succession and chemical and on sand dunes. Vegetation descriptions concern
physical properties of organic horizons. Dur- constant (Emmer and Sevink, 1994). There- ing soil development, a subsequent devel- fore, it is sufficient only to present data for opment of L, F F Hr and Hd horizons ,, 12 the stand aged 124 years (figs 2a-d). Fig- could be observed. These horizons attain ures 2a and b indicate that, except for N a more or less constant amount of organic and P, the F Hr and Hd horizons have sim- , 2 matter within the time span of the chronose- ilar concentrations of labile (organic bound, quence studied (fig 1). adsorbed plus water-soluble) cations, while the F and L horizons have markedly higher 1 During stand development, the relative values. Thus, the major vertical change contribution of the various horizons to the occurs in the upper organic horizons (L and total amount of ectorganic matter changes F rather than between the F and H hori- ) 1 markedly in favour of the humus horizons. A zons. It furthermore appears that the ratio survey of ectorganic profiles in the study K/Ca (fig 2a) is below unity and decreases area revealed that the H horizon becomes downward. Water-extractable element con- visible as pockets after about 30 years of forest development. Distinct vertical gradi- centrations (figs 2c, d) also show a strong ents in the ectorganic profile can be decline from L to H horizons. Water- observed for the elemental composition of extractable K exceeds Ca, in part related the organic matter and the water extracts. It to a higher mineralisation rate of the former, has been shown that during soil develop- but also due to the stronger desorption of ment the pH and elemental concentrations K in aqueous extracts. The K ratio 2+ /Ca + in the various horizons remain more or less also decreases downward.
horizons. As for the extractable elements, The interpretation of the data on water- the phenol concentrations show a marked extractable phenols (table I) is less straight- decrease from the LF to F The individ- 12 . forward. The tanning action of phenolic com- ual compounds are not identified, but Kuiters pounds is particularly related to the slightly and Denneman (1987) reported that Scots condensed phenols in the cell protoplasm pine litter contains relatively high concen- (inherited phenols). The water-soluble phe- trations of ferulic acid. Along the chronose- nols in the more humidified layers, formed as quence, the phenol concentrations in the the result of biodegradation of lignin and LF and F horizons first show an increase 1 2 microbial neoformation, are complexed and and then a decrease. However, this trend insolubilised rapidly (Duchaufour, 1982). cannot be statistically tested. Moreover, the analytical procedure employed overestimates phenolic acids in all The average rooting profiles of the dom- organic horizons, while for the deeper inant species are presented in figure 3. P organic horizons, the method is likely to give sylvestris makes use of the F and H hori- much higher overestimations due to the zons throughout the succession, but in later interference of higher concentrations of ful- stages rooting of the H horizons prevails. vic and humic acids (Box, 1983). For this The density of pine root in the upper min- reason, the results for the Hr and Hd hori- eral horizon remains approximately con- zons in table I should be considered as too stant throughout the succession. D flexu- high in relation to those for the L + F and F 1 2 osa is present in the 3 oldest stands and
shows strong preference to the F horizon, substrate may have significant implications the scores being much higher than for P for the dimension and character of the habi- sylvestris. E nigrum has a very dense root- tat for forest plants and soil fauna. The lat- ing system in the F and H horizons. This ter has been demonstrated for Collembola species exhibits a radial spreading, over- by Faber (1992) and for soil nematodes by growing D flexuosa. V myrtillus has a dense De Goede et al (1993a, b). These authors rooting system in the F and H horizons as advanced the view that for soil animals the well. E nigrum alike, this species propagates occupation of habitats is, to a large extent, itself in a concentric fashion, but making defined by marked vertical gradients in the use of rhizomes which are predominantly environmental conditions and substrate located in the H horizon. At the edge of V availability in mor- and moder-type humus myrtillus patches, the rooting profile there- forms. Such gradients may as well give rise fore consists of D flexuosa roots mainly in to a functional niche differentiation of plant the F horizon and V myrtillus roots and rhi- species (Parrish and Bazzazz, 1976; Van zomes in the H horizon. Nabuurs (1991) den Bergh and Braakhekke, 1978), similar to found that, at very low surface covers of the niche formation in a lateral sense under tree plant (1-10%), V myrtillus roots were mainly canopies consisting of different species located in the H horizon. In addition, during (Lohdi and Johnson, 1989). Differences in vegetation descriptions and soil sampling it resource availability may be partly caused by observed that young individuals of the was inhibitive organic compounds in the profile dwarf shrubs had their roots concentrated in (Rice, 1974). Therefore, the vertical differ- the H horizons. The lateral spreading of the entiation of the ectorganic profile may be dwarf shrubs can be inferred from the pivotal for succession of undergrowth veg- macroscopic composition of the organic hori- etation in the pine forests studied. zons (ie dwarf shrub litter overlies grass lit- From studies by, for example, Hackett ter near the edge of dwarf shrub patches). (1964, 1965), Heil (1984), Hester et al (1991 a, b), Kuiters (1987b) and Kuiters et al (1987), growth, germination and competi- DISCUSSION tive vigour of various undergrowth species under specific experimental conditions have development of a well-stratified ector- The been established. Studies concerning the ganic profile in pine forests on poor sandy effects of nutrient availability and allelo-
chemicals species competition involve Dominance of Deschampsia flexuosa on pot experiments or, in general, consider the soil as 1 compartment. In these experi- early stages of succession, the ector- In the ments, the importance of the vertical distri- ganic profile consists of relatively large bution of allelochemicals and plant avail- amounts of fresh and slightly decomposed able nutrients in the major rooting needle litter, with relatively high concentra- environment remain unknown. tions of allelochemicals. Pine-induced Upon decomposition, needle litter allelopathy (Lohdi and Killingbeck, 1982) releases phenolic acids much slower than seems to affect germination of D flexuosa most leaf litters, resulting in higher concen- less than dicotyledonous species (Kuiters trations in the organic layers under conifer- et al, 1986; Kuiters, 1987b). The former ous forests (Evans, 1980; Kuiters and Den- establishes probably due to large numbers 1987). During long-term of seeds in seed rain (Hester et al, 1991c). neman, decomposition experiments, extractable phe- For its growth, D flexuosa can take advan- nolic acids were found to steadily decrease tage of the high mineralisation rates of major after a large initial decline (Hayes, 1965; cations in the L and upper F horizons. More- Berg et al, 1982; Kuiters, 1987a). As a result, over, in this part of the ectorganic profile, the H horizon produces much lower amounts the pH (Hackett, 1964, 1965) and the K/Ca of these acids than the overlying horizons, ratio in the soil solution are favourable to evidently giving rise to vertical gradients, growth of D flexuosa. Given the fact that D such found for bioelements. Tam et al flexuosa mainly roots in the F and F hori- 1 2 as (1991) and Van Praag et al (1991) reported zons, table I suggests that the establish- a strong downward decline of water-soluble ment of this species causes an increase of phenolic acids in the ectorganic profile under water-soluble phenol concentrations in these pine and spruce, respectively. horizons. Jarvis (1964) observed that D flex- uosa roots produce inhibiting substances D flexuosa is noted for its high competi- in such amounts that they are detrimental tive vigour. Jarvis (1964) related this vigour to birch growth. These compounds were, to its strong competition for elements and however, not identified. The lower concen- to the inhibitive action of its humus and root trations in stands 95 and 124 may relate to exudates on the growth of other plants. Hes- the declining vitality, and therefore lower ter et al (1991 b) showed that under various organic matter production of D flexuosa in shading conditions, the competitive vigour of these stands. D flexuosa exceeds that of V myrtillus. This finding agreed with the observation that D flexuosa outcompetes V myrtillus during Replacement by dwarf shrubs succession under birch on previous heather moorland. However, under the P sylvestris forests in the central Netherlands in later About 40 to 60 years are required for H hori- stages of succession, a consistent process zons to play a role as rooting environment of D flexuosa replacement by dwarf shrubs (fig 1).The H horizon is a compartment that has been observed. With the results of the meets the germination requirements of dwarf experiments by Hester probably still valid, shrubs (ie relatively low concentrations of the vertical differentiation in nutrient avail- allelochemicals and little competition for ability and concentrations of allelochemi- nutrients by D flexuosa). The theoretical cals observed in this study might explain considerations of Berendse (1979), con- the observed succession. This will be elu-. cerning competition between plants with dif- cidated later. ferent rooting depths, may very well apply to
root mat in the F horizon, intraspecific com- the undergrowth vegetation in the pine petition may play an important role in the forests. This theory predicts that the devel- opment of an H horizon under the major decrease of its vitality, under circumstances rooting compartment of D flexuosa increases where input of bioelements through litterfall the competitive ability of deeper rooting decreases as the stand ages (Cole and species, such as the dwarf shrubs. Rapp, 1981).Furthermore, interspecific competition with forest mosses is likely to Once established, V myrtillus can prop- increase considerably, as the latter become agate itself through rhizomes in the H hori- a co-dominant species in the ground layer, Flower-Ellis (1971) measured annual zon. sharing their habitat with D flexuosa. rates of 7 cm. E nigrum increases growth its cover by migrating over the grass carpet Complex interactions of ecological fac- and overshading it. In this sense, the com- tors during succession make it difficult to petition between E nigrum and D flexuosa produce conclusive scientific proof, probably seems similar to that between Calluna vul- even when experimental data are available. garis (L) Hull and D flexuosa in dry heath- Furthermore, implications of soil physical lands. From an extensive study by Heil and changes due to a gradual development from Bobbink (1993), it was concluded that, LF to LFH profiles were not considered in although perennial grasses are found to be this study. Nonetheless, the successional able to replace dwarf shrubs at high levels of processes apparent from the inventory of available N, closed canopies of C vulgaris the undergrowth vegetation are likely to will prevent this species from being crowded evolve from a tight soil-vegetation interac- out by D flexuosa, even at extreme N depo- tion. Although mainly based on circum- sition rates of 75 kg ha y -1 -1. stantial evidence, the study of reciprocal soil-vegetation relations in the Hul- The development of a well-stratified shorsterzand area has provided a strong ectorganic profile, which implies a marked indication for facilitative succession. Aspects continuous change in the rooting environ- of inhibition, due to D flexuosa dominance ment, provides E nigrum and V myrtillus the during a prolonged phase in the succes- opportunity to compete against D flexuosa, sion, indicate that both facilitation and inhi- and succession in these pine forests will bition may act at the same time, which has continue to proceed in their advantage. Ele- also been recognised by Finegan (1984). vated N depositions of about 20 kg ha y -1 -1 at the research site may, however, increase Facilitation due to the development of the time period of D flexuosa dominance. ectorganic profile in the pine forests also the Whereas Berendse (1990) suggested that a seems to relate to the presumed develop- positive effect of N deposition on organic ment towards a Betulo-Quercetum roboris or matter accumulation rate would explain the Fago-Quercetum in the area (Fanta, 1986). expansion of perennial grasses in heath- In places along roads in the pine forests, lands, organic matter accumulation in the where animal browsing is likely to be much pine forest thus seems to have the oppo- less intensive, oaks, birches and beeches site effect. dominate the understorey tree layer. Rode et al (1993) suggested that the accumulation The decreasing vitality of D flexuosa in of plant available nutrients in the ectorganic the oldest stands is of special interest in profile in pine forests (intermediate succes- view of the changing composition of the sional stage after Calluna heather), would undergrowth vegetation and the prospect explain the development of oak-beech for forest managers, facing a prolific growth forests. In this primary Scots pine succes- of this grass in younger pine stands in the sion, the broad-leaved species induce a mor Netherlands. As D flexuosa forms a dense
to moder transformation of the humus form Emmer IM (1994) Humus form characteristics in rela- tion to undergrowth vegetation in a Pinus sylvestris (Emmer, 1994). A more detailed study of forest. Acta Oecologica 15, 677-687 this soil development is needed to deter- Emmer IM, Sevink J (1993) Horizon differentiation in mine as to how this may affect the succes- humus forms and its relevance for the study of suc- sion of the undergrowth vegetation. cession related changes in soil element stocks in primary forests of Pinus sylvestris. Scripta Geo- botanica 21, 129-134 Emmer IM, Sevink J (1994) Temporal and vertical ACKNOWLEDGMENTS changes in the humus form profile during a primary succession of Pinus sylvestris. Plant Soil 167, 281 - 295 The author appreciated the comments and sug- gestions by Prof J Sevink and Dr AT Kuiters. The Emmer IM, Verstraten JM (1993) Applicability of HNO - 3 "Vereniging tot Behoud van Natuurmonumenten" H digestion for elemental analysis of organic mat- O 2 gave permission to carry out the research in the ter in mor humus forms. Z Pflanzenernähr Bodenk 156, 507-514 Hulshorsterzand area. Evans LJ (1980) Podzol development north of Lake Huron in relation to geology and vegetation. Can J Soil Sci 60, 527-539 REFERENCES Faber JH (1992) Soil fauna stratification and decompo- sition of pine litter. PhD Thesis, Free University, IF (1981) Some effects of differ- Amsterdam Ahlgren CE, Ahlgren ent forest litters on seed germination and growth. Fanta J (1986) Primary succession on blow-out areas in Can J For Res 11, 710-714 the Dutch drift sands. In: Forest dynamics research Babel U (1971) Gliederung und Beschreibung des in Western and Central Europe. Proceedings IUFRO Humusprofils in mitteleuropäische Wäldern. Geo- workshop ecosystems, Wageningen 1985 (J Fanta, derma 5, 297-324 ed), Pudoc Sci Publ, Wageningen, 164-169 Berendse F (1979) Competition between plant popula- FAO-Unesco (1988) Soil map of the world (revised leg- tions with different rooting depths. I. Theoretical con- end). Technical Paper 20, ISRIC, Wageningen siderations. Oecologia 43, 19-26 B (1984) Forest succession. Nature 312, 109- Finegan Berendse F (1990) Organic matter accumulation and 114 nitrogen mineralisation during secondary succes- Flower-Ellis JGK (1971) Age structure and dynamics in sion in heathland ecosystems. J Ecol 78, 413-427 stands of bilberry (Vaccinium myrtillus L). Avdelnin- Berg B, Hannus K, Popoff T, Theander O (1982) gen för Skogsekologi Stockholm, Rapporter och Changes in organic chemical components of nee- Uppsater 9, 1-108 dle litter during decomposition. Long-term decom- Hackett C (1964) Ecological aspects of the nutrition of position in a Scots pine forest I. Can J Bot 60, 1310- Deschampsia flexuosa (L) Trin I. The effect of alu- 1319 minium, manganese and pH on germination. J Ecol Box JD (1983) Investigation of the Folin-Ciocalteau phe- 52, 159-167 nol reagent for the determination of polyphenolic sub- Hackett C (1965) Ecological aspects of the nutrition of stances in natural waters. Water Res 17, 511-525 Deschampsia flexuosa (L) Trin II. The effects of Al, Cole DW, M in forest Elemental Rapp (1981) cycling Ca, Fe, K, Mn, N, P and pH on the growth of seedlings ecosystems. In: Dynamic properties of forest ecosys- and established plants. J Ecol53, 315-333 tems. IBP 23 (DE Reichle, ed), Cambridge Univ Hayes AJ (1965) Studies on the decomposition of conif- Press, UK, 341-410 erous leaf litter. I. Physical and chemical changes.J De Goede R, Georgieva SS, Verschoor BC, Kamerman Soil Sci 16, 121-140 JW (1993a) Changes in nematode community struc- T (1991) Acidification research in Heij GJ, Schneider ture in a primary succession of blown-out areas in a the Netherlands. Studies Environmental Science 46. drift sand landscape. Fundam Appl Nematol 16, 501- Elsevier, Amsterdam 513 Heil GW (1984) Nutrients and the species composition De Goede R, Verschoor BC, Georgieva SS (1993b) of heathiands. PhD Thesis, University of Utrecht Nematode distribution, trophic structure and biomass in a primary succession of blown-out areas in a drift Heil GW, Bobbink R (1993) "Calluna", a simulation model sand landscape. Fundam Appl Nematol 16, 525-538 for evaluation of impacts of atmospheric nitrogen Duchaufour P (1982) Pedology. Pedogenesis and clas- deposition on dry heathlands. Ecol Modelling 68, sification. Allen & Unwin, London, UK 161-182
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