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Fig. 2. Frequency and magnitude of disturbance are often inversely related. Events must meet a minimum magnitude to be considered a disturbance at a particular scale for a particu- lar ecosystem. Large, infrequent disturbances have been dis- cussed as a special case of disturbance (TURNER et al. 1998 ex WHITE & JENTSCH 2001).

Figure 2 Frequency and magnitude of disturbance are often inversely related. Events must meet a minimum magnitude to be considered a disturbance at a particular scale for a particu- lar ecosystem. Large, infrequent disturbances have been dis- cussed as a special case of disturbance (TURNER et al. 1998 ex WHITE & JENTSCH 2001).

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Tab. 1. Literature data for soil properties along a successional series (JENTSCH 2001) in dry acidic grassslands. Records are given as range or mean [1 + SE] across a0—10 cm or 0—30 cm soil column. Reference data from FROMM et al. (2002), WEIGELT (2001), FISCHER (1998), STorRM et al. (1998), JECKEL (1984), LACHE (1976). layers of coarse-grained Syrosem, Regosole and Podzol with low amounts of organic matter cover the sandy sub- strate, but soil horizons rarely exceed a few centimeters. Soil development has permanently been interrupted and impeded, due to decades of wind and flooding dyna- mics, 0 grazing and human and use or to military training activities on relatively dry and nutrient-poor ground peated . The natural soil struc materials added to the origina mixing, loosening and compaction, destruction of soil texture, erosion and decrease of infiltration rate typical- ly accompany land use and mi 1983; BECHER 1985; PEARSON 1990; DEMARAIS et al. 1999). ure may have been re- y destroyed and re-arranged or allochthone soil composition. Soil itary activity (LATHROP also result in a low water storage capacity, so that resour- ces are readily washed out. With time for succession, soil humus content increases, thereby ameliorating the water holding capacity (VOLK 1931; JECKEL 1984). Soil water contents in dry grasslands on inland dunes typi- cally range from 2.5% to 9.0% dry weight and exhibit a nearly twofold increase with successional development (JECKEL 1984; ScHmipT 1998; SToRM et al. 1998; JENTSCH 2001). As surveys of annual changes in water content revealed, soil water content in upper soil layers is highly variable. This is due to high lateral and vertical mobility and superimposed by substantial seasonal variation (RITESEMA & DEKKER 1994; SCHEFFER & SCHACHTSCHABEL 1992; STORM et al. 1998). The per- manent wilting point for dry grassland vegetation domi- nated by Corynephorus canescens (pF = 4,2) is esti- mated to be reached at soil water contents between 1.0%-3.1% of total volume (BERGER-LANDEFELDT & Sukopp 1965), between 0.4—0.7% (LACHE 1976), or around 2.8% (JECKEL 1984). With regard to soil surface temperatures, sandy soils show high daily amplitudes as a function of high irradiation. Light-colored sand may reflect 40% (albedo) of total radiation (KRATOCHWIL & SCHWABE 1984). Sandy substrates warm up and cool down rapidly. In summer, warm drought periods may exceed several weeks. In winter, cold air cover with frost may persist over open grasslands for several days.
Tab. 1. Literature data for soil properties along a successional series (JENTSCH 2001) in dry acidic grassslands. Records are given as range or mean [1 + SE] across a0—10 cm or 0—30 cm soil column. Reference data from FROMM et al. (2002), WEIGELT (2001), FISCHER (1998), STorRM et al. (1998), JECKEL (1984), LACHE (1976). layers of coarse-grained Syrosem, Regosole and Podzol with low amounts of organic matter cover the sandy sub- strate, but soil horizons rarely exceed a few centimeters. Soil development has permanently been interrupted and impeded, due to decades of wind and flooding dyna- mics, 0 grazing and human and use or to military training activities on relatively dry and nutrient-poor ground peated . The natural soil struc materials added to the origina mixing, loosening and compaction, destruction of soil texture, erosion and decrease of infiltration rate typical- ly accompany land use and mi 1983; BECHER 1985; PEARSON 1990; DEMARAIS et al. 1999). ure may have been re- y destroyed and re-arranged or allochthone soil composition. Soil itary activity (LATHROP also result in a low water storage capacity, so that resour- ces are readily washed out. With time for succession, soil humus content increases, thereby ameliorating the water holding capacity (VOLK 1931; JECKEL 1984). Soil water contents in dry grasslands on inland dunes typi- cally range from 2.5% to 9.0% dry weight and exhibit a nearly twofold increase with successional development (JECKEL 1984; ScHmipT 1998; SToRM et al. 1998; JENTSCH 2001). As surveys of annual changes in water content revealed, soil water content in upper soil layers is highly variable. This is due to high lateral and vertical mobility and superimposed by substantial seasonal variation (RITESEMA & DEKKER 1994; SCHEFFER & SCHACHTSCHABEL 1992; STORM et al. 1998). The per- manent wilting point for dry grassland vegetation domi- nated by Corynephorus canescens (pF = 4,2) is esti- mated to be reached at soil water contents between 1.0%-3.1% of total volume (BERGER-LANDEFELDT & Sukopp 1965), between 0.4—0.7% (LACHE 1976), or around 2.8% (JECKEL 1984). With regard to soil surface temperatures, sandy soils show high daily amplitudes as a function of high irradiation. Light-colored sand may reflect 40% (albedo) of total radiation (KRATOCHWIL & SCHWABE 1984). Sandy substrates warm up and cool down rapidly. In summer, warm drought periods may exceed several weeks. In winter, cold air cover with frost may persist over open grasslands for several days.
Fig. 1. Total carbon [% dry weight] and total nitrogen [% dry weight] increase along a theoretical chronosequence of succes- sion in dry acidic grasslands (data from JENTSCH 2001). The various successional stages are abbreviated by characteristic plant genera or species composition (ruderal). Soll nitrate and ammonia. Dry acidic grasslands typ1- cally occur on sandy soils of low nutrient availability, which is why nitrogen fixing bacteria are especially important in pioneer communities (TATE 1995). Soil nitrate and ammonia records of inland sand dune areas in Germany vary between 0.01 and 0.05%, (BERGER- LANDEFELDT & SUKOPP 1965; JECKEL 1984; LACHE 1976; ScHmIDT 1998; STorm et al. 1998; JENTSCH 2001). In soils of early successional stages of dry acidic grasslands the following nutrient contents were found: 0.2-0.4 mg NO, kg"! soil; <1 mg NH, kg"! soil mg (ScHMipT 1998), 0.2—0.8 mg NO, kg" soil (StoRM et al. 1998), 1.5-2.0 mg NO, kg"! soil (JECKEL 1984), 0.4—0.8 mg NO, kg" soil and 0.2—0.5 mg NH, kg"! soil (JENTSCH 2001). Still, presenting absolute values for soil nitrate and ammonia content is ambiguous, because these are subject to methodological variation and day- to-day changes in the soil. On inland sand dunes, nitra- te content is typically higher than ammonia content (SCHMIDT 1998 ; STorM et al. 1998). Nitrogen minerali- sation primarily occurs as nitrification (NO) rather than ammonification (NH,), due to activity of heterotrophic nitrifying bacteria (JECKEL 1984). Depending upon the C/N-ratio of the site, most available ammonia in the soil is not taken up by plants, but is oxidized to nitrate during spring and summer (FOCHT & VERSTRAETE 1977; VER- HAGEN & LAANBROEK 1991). In dry acidic grasslands (Spergulo-Corynephoretum), the degree of nitrification is estimated to reach 90% (STorM et al. 1998). Total nitrogen and inorganic carbon. Total nitrogen and soil inorganic carbon exhibit no seasonal variation (VAN RHEENEN et al. 1995). Literature values of total nitrogen in dry acidic grasslands particularly dominated by Corynephorus canescens range from 0.2—4.0 mg g! (SCHEFFER & SCHACHTSCHABEL 1992), and for soils under plant communities of Sedo-Scleranthetea be- ween 0.05-0.5 mg g! (SCHMIDT 1998; SToRM et al. 998 ; JENTSCH 2001). In the course of succession from pioneer vegetation to closed grassland canopy, a 3 to 4-fold accumulation of organic material, soil water con- ent and total nitrogen takes place (Fig. 1) due to soil consolidation and litter production (SToRM et al. 1998; JENTSCH 2001). Soil nitrate content also increases and nitrogen turnover rate drops proportionately. Generally, net mineralisation is high in unconsolidated sandy soils, which does not hint at nitrogen limitation. Mineralisation values or annual nitrogen delivery be- tween 23.5—55 kg N ha"! were recorded for a Spergulo- Corynephoretum typicum (JECKEL 1984; STorM et al. 1998) and 35-40 kg N ha"! for a Xerobrometum on calcareous soils. Mineralisation rates vary between 0.6-1.9 mg N kg! w! (JEcCKEL 1984; STorM et al. 1998). Even so, not all nitrogen is available for plant
Fig. 1. Total carbon [% dry weight] and total nitrogen [% dry weight] increase along a theoretical chronosequence of succes- sion in dry acidic grasslands (data from JENTSCH 2001). The various successional stages are abbreviated by characteristic plant genera or species composition (ruderal). Soll nitrate and ammonia. Dry acidic grasslands typ1- cally occur on sandy soils of low nutrient availability, which is why nitrogen fixing bacteria are especially important in pioneer communities (TATE 1995). Soil nitrate and ammonia records of inland sand dune areas in Germany vary between 0.01 and 0.05%, (BERGER- LANDEFELDT & SUKOPP 1965; JECKEL 1984; LACHE 1976; ScHmIDT 1998; STorm et al. 1998; JENTSCH 2001). In soils of early successional stages of dry acidic grasslands the following nutrient contents were found: 0.2-0.4 mg NO, kg"! soil; <1 mg NH, kg"! soil mg (ScHMipT 1998), 0.2—0.8 mg NO, kg" soil (StoRM et al. 1998), 1.5-2.0 mg NO, kg"! soil (JECKEL 1984), 0.4—0.8 mg NO, kg" soil and 0.2—0.5 mg NH, kg"! soil (JENTSCH 2001). Still, presenting absolute values for soil nitrate and ammonia content is ambiguous, because these are subject to methodological variation and day- to-day changes in the soil. On inland sand dunes, nitra- te content is typically higher than ammonia content (SCHMIDT 1998 ; STorM et al. 1998). Nitrogen minerali- sation primarily occurs as nitrification (NO) rather than ammonification (NH,), due to activity of heterotrophic nitrifying bacteria (JECKEL 1984). Depending upon the C/N-ratio of the site, most available ammonia in the soil is not taken up by plants, but is oxidized to nitrate during spring and summer (FOCHT & VERSTRAETE 1977; VER- HAGEN & LAANBROEK 1991). In dry acidic grasslands (Spergulo-Corynephoretum), the degree of nitrification is estimated to reach 90% (STorM et al. 1998). Total nitrogen and inorganic carbon. Total nitrogen and soil inorganic carbon exhibit no seasonal variation (VAN RHEENEN et al. 1995). Literature values of total nitrogen in dry acidic grasslands particularly dominated by Corynephorus canescens range from 0.2—4.0 mg g! (SCHEFFER & SCHACHTSCHABEL 1992), and for soils under plant communities of Sedo-Scleranthetea be- ween 0.05-0.5 mg g! (SCHMIDT 1998; SToRM et al. 998 ; JENTSCH 2001). In the course of succession from pioneer vegetation to closed grassland canopy, a 3 to 4-fold accumulation of organic material, soil water con- ent and total nitrogen takes place (Fig. 1) due to soil consolidation and litter production (SToRM et al. 1998; JENTSCH 2001). Soil nitrate content also increases and nitrogen turnover rate drops proportionately. Generally, net mineralisation is high in unconsolidated sandy soils, which does not hint at nitrogen limitation. Mineralisation values or annual nitrogen delivery be- tween 23.5—55 kg N ha"! were recorded for a Spergulo- Corynephoretum typicum (JECKEL 1984; STorM et al. 1998) and 35-40 kg N ha"! for a Xerobrometum on calcareous soils. Mineralisation rates vary between 0.6-1.9 mg N kg! w! (JEcCKEL 1984; STorM et al. 1998). Even so, not all nitrogen is available for plant
Fig. 3. Schematic vegetation profile of a pioneer community on sandy soil (Spergulo morisonii — Corynephoretum canes- centis (Tx. 28) Libb. 33). Depicted from left to right: Poly- trichum piliferum, Corynephorus canescens , Cladonia sylva- tica, Cladonia foliacea (from FUKAREK 1961). (Tx. 28) LIBB. 33. equivalent to Corniculario aculea- tae—Corynephoretum canescentis STEFFEN 31. (KRAT- ZERT & DENGLER 1999), of which the Spergulo mori- sonii-Corynephoretum cladonietosum on consolidated ground is strongly dominated by cryptogams. This stage characterizes least productive sites. Given slightly basiphilous conditions, the highly threatened associa- tion Jurineo cyanoidis-Koelerietum glaucae has been recorded (SCHWABE et al. 2000; RUSSEL 2002). Sites of more favourable nutrient supply rapidly develop into later successional stages, such as Genisto pilosae- Callunetum vulgaris OBERD. 38, Echio-Melilotetum Tx. 47 and Artemisio vulgaris-Tanacetetum vulgaris BR.-BL. 31. corr. 49 (NEZADAL & HEIDER 1994). The latter characterizes ruderal acidic grassland. Grazed, long- term stable grassland often results into Armerio elonga-
Fig. 3. Schematic vegetation profile of a pioneer community on sandy soil (Spergulo morisonii — Corynephoretum canes- centis (Tx. 28) Libb. 33). Depicted from left to right: Poly- trichum piliferum, Corynephorus canescens , Cladonia sylva- tica, Cladonia foliacea (from FUKAREK 1961). (Tx. 28) LIBB. 33. equivalent to Corniculario aculea- tae—Corynephoretum canescentis STEFFEN 31. (KRAT- ZERT & DENGLER 1999), of which the Spergulo mori- sonii-Corynephoretum cladonietosum on consolidated ground is strongly dominated by cryptogams. This stage characterizes least productive sites. Given slightly basiphilous conditions, the highly threatened associa- tion Jurineo cyanoidis-Koelerietum glaucae has been recorded (SCHWABE et al. 2000; RUSSEL 2002). Sites of more favourable nutrient supply rapidly develop into later successional stages, such as Genisto pilosae- Callunetum vulgaris OBERD. 38, Echio-Melilotetum Tx. 47 and Artemisio vulgaris-Tanacetetum vulgaris BR.-BL. 31. corr. 49 (NEZADAL & HEIDER 1994). The latter characterizes ruderal acidic grassland. Grazed, long- term stable grassland often results into Armerio elonga-
Fig. 4. Succession in dry acidic grasslands on inland dunes (JENTSCH et al. 2002c, changed after WEIGELT 2001). Initially, bare sandy substrate is loosely covered by pioneer species such as Corynephorus canescens and Rumex acetosella. These first plants reduce surface wind dynamics and contribute to fixing of the sand with a slowly building organic crust (HOHENESTER 1960; PLuis 1994; BUDEL 2001). Then, annuals, such as Spergula morisonii and Teesdalia nudicaulis, graminoid species, such as Avenella flexuosa (L.) DREJ. and Carex hirta or C. arenaria, and other species, such as Jasione montana, become established forming open plant com- munities. As soon as the surface is consolidated and the substrate exhibits conditions of low water and nutrient availability, mosses, such as Polytrichum piliferum and lichens, mainly of the genus Cladonia and Cetraria, may form a closed vegetation canopy (BIERMANN & DANIELS 1997; VON BRACKEL 2000). In this stage, small soil disturbances by ants and rabbits frequently provide open patches with bare substrate and seem crucial for further seedling establishment of phanerogams (MIL- TON et al. 1997; FriepRICH 2001; JENTSCH et al. 2002c). Cryptogam dominated stages can exhibit long- term dynamic stability, if no nutrients are added to the system (TUXEN 1975; KINDER et al. 1992; TISCHEW & MAHN 1997; JENTSCH 2001; FROMM et al. 2002). In case of more favorable resource conditions, Hieracium pilosella will establish and be a strong competitor to all pioneer species. Along with other species Agrostis The successional trajectories for plant communities of acidic grasslands mainly depend on climatic conditions and soil pH, on water and nutrient availability, on dis- turbance regimes, and on seed pools close by (LACHE 1976; BERGER-LANDEFELDT & SuUKOpP 1965; Ep- MONDSON et al. 1993; RODE et al. 1993; QUINGER & MEYER 1995; WEIGELT 2001; FROMM et al. 2002). Various theoretical concepts regarding general mecha- nisms of succession are discussed in the literature (GLEASON 1926; PICKETT 1976; CONNELL & SLATYER 1977; Mac MaHon 1980; NoBLE & SLATYER 1980; PEET & CHRISTENSEN 1980; TILMAN 1985, 1990a, b, 1993; BAzzaz 1996). Nevertheless, stability and suc- cessional behavior of the plant communities on inland sand dunes are insufficiently known (TISCHEW & MAHN 1997). Merely a description of typical successional stages (Fig. 4) and their characteristic species is given below (compare HOHENESTER 1960, 1967a, 1967b; FELINKS 2000; WEIGELT 2001; JENTSCH et al. 2002c; FROMM et al. 2002).
Fig. 4. Succession in dry acidic grasslands on inland dunes (JENTSCH et al. 2002c, changed after WEIGELT 2001). Initially, bare sandy substrate is loosely covered by pioneer species such as Corynephorus canescens and Rumex acetosella. These first plants reduce surface wind dynamics and contribute to fixing of the sand with a slowly building organic crust (HOHENESTER 1960; PLuis 1994; BUDEL 2001). Then, annuals, such as Spergula morisonii and Teesdalia nudicaulis, graminoid species, such as Avenella flexuosa (L.) DREJ. and Carex hirta or C. arenaria, and other species, such as Jasione montana, become established forming open plant com- munities. As soon as the surface is consolidated and the substrate exhibits conditions of low water and nutrient availability, mosses, such as Polytrichum piliferum and lichens, mainly of the genus Cladonia and Cetraria, may form a closed vegetation canopy (BIERMANN & DANIELS 1997; VON BRACKEL 2000). In this stage, small soil disturbances by ants and rabbits frequently provide open patches with bare substrate and seem crucial for further seedling establishment of phanerogams (MIL- TON et al. 1997; FriepRICH 2001; JENTSCH et al. 2002c). Cryptogam dominated stages can exhibit long- term dynamic stability, if no nutrients are added to the system (TUXEN 1975; KINDER et al. 1992; TISCHEW & MAHN 1997; JENTSCH 2001; FROMM et al. 2002). In case of more favorable resource conditions, Hieracium pilosella will establish and be a strong competitor to all pioneer species. Along with other species Agrostis The successional trajectories for plant communities of acidic grasslands mainly depend on climatic conditions and soil pH, on water and nutrient availability, on dis- turbance regimes, and on seed pools close by (LACHE 1976; BERGER-LANDEFELDT & SuUKOpP 1965; Ep- MONDSON et al. 1993; RODE et al. 1993; QUINGER & MEYER 1995; WEIGELT 2001; FROMM et al. 2002). Various theoretical concepts regarding general mecha- nisms of succession are discussed in the literature (GLEASON 1926; PICKETT 1976; CONNELL & SLATYER 1977; Mac MaHon 1980; NoBLE & SLATYER 1980; PEET & CHRISTENSEN 1980; TILMAN 1985, 1990a, b, 1993; BAzzaz 1996). Nevertheless, stability and suc- cessional behavior of the plant communities on inland sand dunes are insufficiently known (TISCHEW & MAHN 1997). Merely a description of typical successional stages (Fig. 4) and their characteristic species is given below (compare HOHENESTER 1960, 1967a, 1967b; FELINKS 2000; WEIGELT 2001; JENTSCH et al. 2002c; FROMM et al. 2002).
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