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Journal of VegetationScience 6. 445-452, 1995
? IAVS;OpulusPress Uppsala. Printedin Sweden
                                                          445



      Impact of grazing and atmospheric nitrogen deposition
        on the vegetation of dry coastal dune grasslands

              ten Harkel, Matthijs J.* & van der Meulen, Frank

 Landscape and Environmenltal Research Group, University of Amsterdam, Nieuwe Prinsengr7acht 130, 1018 VZ
     Amsterdam, The Netherlands; *Correspondingauthor. Tel. + 31 20 5257442; Fax + 31 20 5257431;
                     E-mail mth@fgb.frw.uva.nl


Abstract. A five-year experimentalstudy was carriedout to   1988, 1990; Bobbink 1991; Aerts & Heil 1993). So far,
examine the combined effects of grazing and atmospheric    such systematic studies have not been initiated in the
nitrogen deposition on the vegetation of three dry dune    dunes. The present study was initiated in 1988, in order
grasslands: shortspecies-rich,one shortspecies-poor,and
       one
                                to understand the relations between atmospheric deposi-
one predominated tall graminoids.
          by
  Additional fertilizationwith nitrogen had no significant  tion, grazing and vegetation dynamics. The present pa-
effect, neitherin grazednor in non-grazedplots. Exclusion of  per deals with changes in species composition.
                                  Since the 1970s some tall perennial graminoids,
grazingby rabbitsresultedin an increasein the frequencyof
perennialgraminoidsand a decrease in the frequencyof an-    notably Ammophila arenaria, Avenulapubescens, Cala-
nual graminoids and herbs. Nevertheless, species diversity   magrostis epigejos, Elymus athericus and Carex are-
remainedthe same in the species-richgrassland.During the    naria, have become dominant in many originally short
experiment,the above-groundbiomass increasedin all non-    dry dune grasslands of the Netherlands (C.T.M. Vertegaal
grazed plots and the amount of bare soil and mosses de-    et al. int. report 1991). As a consequence, these grass-
creased.
                                lands show now a relatively low species diversity. From
  The vegetation changes occurredmainly within one year
                                the point of view of nature conservation this is consid-
after the exclusion of grazing. An exception is the grass-
dominatedsite where the amountof Calamagrostisepigejos     ered an unfavourable situation (Westhoff 1985). Nowa-
increased graduallyfrom ca. 20 % in the first two years to   days, nature managers try to counteract this develop-
about50 % in the fourthand fifth year.             ment and try to restore the original vegetation composi-
  Grazingby rabbitsseems essential to preventgraminoids   tion. This requires understanding of the processes respon-
to become predominantin the dry dunes. If graminoidsare    sible for the predominance of graminoids.
dominant,grazingby horses can be an appropriate  methodto     In the past, tall graminoids were usually not domi-
restorethe originalgrasslandvegetation.After six months of   nant because of the low nutrient status of sandy dune
grazingby horsesthe grass-dominated showed a decrease
                   site
                                soils (Willis & Yemm 1961; Boorman & Fuller 1982).
of the frequencyof perennialgraminoids,from 95 %to 80 %,
                                The present dominance of graminoids might be a result
and an increase of the frequency of perennial herbs, from
2.5 % to between 13 and 20 %.                 of the recent increase of atmospheric nitrogen deposi-
                                tion. From 1930 to 1980 the atmospheric nitrogen depo-
                                sition (NH4+ + NO3-) on coastal areas in the Netherlands
Keywords: Horse;Management;  Meijendel;the Netherlands;    increased from 4 kg N.ha-l.yr- to 14 kg N.ha-l.yr-1
Rabbit;Restorationecology; Species diversity.         (Stuyfzand 1993). The predominance of graminoids due
                                to high nitrogen loads is also known from heathlands
Nomenclature: van der Meijden (1990).             (Heil & Diemont 1983; Roelofs 1986; Bobbink et al.
                                1990; Aerts & Heil 1993) and chalk grasslands (Bobbink
                                et al. 1988; Bobbink 1991; Willems et al. 1993). Many
Introduction                          graminoids have a relative higher growth rate than other
                                plant species and are therefore able to benefit more from
  Recently, semi-natural vegetation in the Netherlands    increased nitrogen input. This is shown by fertilizing
has been under increasing influence of atmospheric       experiments in grasslands (Willis & Yemm 1961; Willis
deposition of acid and eutrophic components. This has     1963; Van Hecke et al. 1981; Davy & Bishop 1984;
led to a decrease in nature conservation values.        Bobbink 1991; Mountfordet al. 1993). When grasses
  In chalk grasslands and heathlands the influence of    aretall, light will not penetrate the soil andgermina-
                                                into
atmospheric deposition has been studied to support man-    tion of annualspecies is prevented(Bobbink& Willems
agement programs aiming at restoration of nature values    1991; Olff et al. 1993).
(Heil & Diemont 1983; Roelofs 1986; Bobbink et al.         It seems thatgraminoidswill not dominatewhen the
446                   ten Harkel,M.J. & van der Meulen, F.

vegetation is frequentlygrazedby animalssuch as rab-     with a pH(CaCl2)of 4.2 to 5.4 on similar calcareous
bits (Williamset al. 1974;Bhadresa1977;Zeevalking&      dunesandas the firstsite. The 10 cm thicktop soil has an
Fresco 1977). If therabbitpopulationdecreases,as in the   organic matter content of 3 %. The site representsa
1950s (dueto myxomatosis),graminoids ableto take
                     are         species-poor variant of the Violo-Corynephoretum as-
over (Ranwell 1960; Thomas 1963; Westhoff 1985).       sociation(Westhoff& den Held 1969). A certaindomi-
Theeffect of rabbitgrazingis also experimentally shown   nance of graminoidsoccurredhere from the beginning
in grazedgrasslands  whereafterthe exclusionof grazing   (Table 1). This site has been monitoredsince the sum-
tall graminoidsbecame dominant(Farrow1917; Hill et      mer of 1988.
al. 1992). Even afterrestoration the rabbitpopulation,
                of                The third,grass-dominated is close to the second
                                              site
graminoidsare able to remaindominantfor a long time     site, butthe vegetationhas becometotallypredominated
as rabbitsdo not eat or enter such relatively tall grass   by Carex arenaria, Festuca ovina + F. rubra and
swards.Non-grazedvegetationhas a relativelylow pro-     Calamagrostis epigejos (Table 1). This site has been
tein content in comparison to grazed vegetation and     monitoredsince the summerof 1988.
thereforeanimalsavoid the area (Bakkeret al. 1983).
  The dominanceof graminoidscan be counteracted
by introducinglarge herbivores(Williams et al. 1974).
If heavy grazing is needed to permitseedling establish-
ment of annual species, it can also prohibitsuccessful
                               Table1. Totalnumber speciesandfrequency
                                         of           (averagefor
completion of the life cycle of these annual species     allyears) plant
                                   of   speciesof thecontrol
                                                plotsin thethree
                                                       sites
(Farrow1917; Watt 1962; Silvertownet al. 1992). Re-     (grass-dominated before introduction horses);
                                      site    the     of    Rich
cently, horses and cows have been introduced Dutch
                        in      = species-rich site; Poor = species-poor site; Grass = grass-
naturereservesto reducethe amountof biomassandthe      dominated
                                   site.
vegetation height and thus restore the original species                         Rich    Poor  Grass
composition.
  This paper reports on an experiment in a Dutch      Number of species

coastal dune area where fertilizationwith nitrogenand     Annual herbs                4     3      1
exclusion of grazingwere combinedas a treatment.  The    Perennial herbs              9     3      3
                                Annual graminoids             2           1
purpose was to define the main reason for the domi-      Perennial graminoids            6     6     6
nance of tall graminoids in large parts of comparable    Annual herbs
dune areas. This dominance may be caused by high        Cerastiluml semidecandrllm       1-5     <1      0
atmosphericdeposition of nitrogen, or the absence of      El-odiliuti c(icultariiu        5- 10    <1      0
grazingby rabbits,or a combinationof both.Inaddition,     Se'I'cio) svlvaticus            0         1-5
the effects of the introduction horsesandthe implica-
               of                Teeshiali(a illcicaulis          0    <1      0
                                Veronica arvcnsis            <1      0     0
tions for the managementof dune grasslandswill be                           1-5
                                Violla (cr1'tisii                        0
discussed.                          Perennial herbs                     0
                                Galilum verum              1-5          <1
                                                          1-5
                                                           0
                               Hiera t'i71I pilosellc          <1           0
Material and Methods                     Holcusl /l(anatlus              0         <1
                               Leontodlonl saxatilis          1-5      0     0
                               Lotus corni(iculcatus          1-5           0
Study sites                          Myosotis arvenlsis            1-5           0
                               Ohonis repens             10- 20           0
  The study was carriedout in threegrasslandsites in    Rumnex acetosella              0  10- 20     0
                               Sedumi acre                <1      0     0
Meijendel,a coastal dune areanorthof The Hague, the      Senecio jacobaea             5-10          < 1
                                                          1-5
Netherlands.This area is managedby the Dune Water                           1-5
                               Tarax acum sect. Erythrospe rma            0     0
Companyof Zuid-Holland(D.Z.H.).               Annual graminoids
  The first, species-richsite is located at a distanceof  Aila praecox               1-5    5- 10    1-5
0.5 km fromthe coast. The sandysoil is calcareous,with    Phleuni arenar-iuni            <1      0     0
2- 3 % CaCO3, and with a very low organic matter       Perennial graminoids
                               Agrostis stolonifera + A. capillaris     0   5- 10      0
content(2 %in the 10cm top soil). Phytosociologically,
                               Calamagriostis epigejos         1-5   10 - 20  10 - 20
the vegetationcan be assignedto the Festuco-Galietum     Carex arenaaia             5- 10   5-10    >20
ma-ritimi association(Westhoff& den Held 1969). This     Festuca ovina + F. orubra        1-5    >20   10 - 20
site has been monitoredsince the summerof 1989.        Koeleria macrantha             <1      0   1-5
  The second, species-poor site is situated 2.5 km     Luzula campestris           10 - 20  10 - 20   1-5
                               Poa pratensis              5- 10     < 1    <1
from the coast. The soil is decalcified down to 50 cm
               - Impactof grazingand atmosphericnitrogendeposition-                447

Treatments                          Statisticalanalysis

  At all threesites five homogeneousquadrats 2 m x
                        of        The resultswere analyzedstatisticallywith the com-
2 m were selected. Each quadratwas divided into four     puter package SAS (Anon. 1987). The counts of the
experimentalplots of I m x 1m: (1) control; (2) ferti-    plants were standardizedto frequencies and will be
lized; (3) excluded from grazing by rabbits and (4)     called frequenciesfurtheron.
combinationof fertilizationand exclusion of grazing.       The effect of grazingandfertilizationon the follow-
The treatmentswere allocated randomly to the plots      ing three frequencieswere tested with the SAS-proce-
(randomizedquadrat   design).               dure ANOVA with repeated measurements:species
  In the autumnof 1990, horses were introducedin a     groups (annualor perennialherbs and graminoids),in-
partof Meijendel to reduce severe grass encroachment     dividualspecies, surfacewithoutphanerogamic   species
andto possibly restorethe originalspecies composition.    (baresoil and mosses). The effects on species diversity
The horsesgrazedfor six monthsat the grass-dominated     (see below) and biomass were also tested. A signifi-
site;thereafter horseswereexcludedfromthequadrats
        the                      cance leve of 0.05 was chosen. The effects of the treat-
because they demolishedthe exclosures.            ments were analysed separatelyfor each site. The ef-
  The fertilizerwas appliedto simulatethe effects of    fects of treatmentson the grass-dominatedsite were
high loads of atmosphericnitrogen deposition. It was     analysedtwofold: before the introduction the horses
                                                    of
assumedthatadditionof nitrogenwould speed up grass      and after six months of grazingby horses.
encroachmentand cause the associatedloss of charac-       Species diversity was measured as the Shannon
teristicherbs.                        Wiener index:
  From October 1988 until April 1992 the fertilizer
appliedconsistedof pellets of NH4NO3  (addedin spring    SD =   P ln i                  (1)
and autumn)equalto an extradepositionof 25 kg N/ha/
yr. As weekly unsaturatedsoil water analysis by ce-     where SD = species diversity andPi = the proportionof
ramiccups showed thatlarge amountsleached through      species i (on a frequencybasis).
the surfacesoil within a few weeks, a differentmethod    The moss species were regardedas one group,and were
of fertilizationwas applied. Since April 1992 quadrats    not includedin this calculation.
were fertilized with a 0.0069 M (NH4)2SO4 solution
once every fortnight,which was equal to an extranitro-
gen input of 50 kg N/ha/yr. For osmotic reasons NaCl     Results
(sea salt)was addedin this solutionup to the same NaCl
concentration the rainwater.
        in        The non-fertilizedplots     The species presentin the threesites were dividedin
obtained an equal amount of NaCl solution without      four groups: annual herbs, perennial herbs, annual
(NH4)2SO4. The yearly amount of solution given was      graminoids and perennial graminoids (Table 1). The
equivalentto an extraprecipitation 26 mm/yr,which
                 of             grazedquadrats the species-richsite containthe larg-
                                       of
is only 3 % of the averageyearly precipitation.       est number of herb species, 13. The numbers of the
                               species-poorand the grass-dominated were six and
                                                   site
Measurements                         four respectively. The numberof graminoidspecies is
                               equal in the sites, i.e. seven or eight. However, in the
  Once a year in July, the species composition of the    grazedplots the frequencyof perennialgraminoids(Ta-
vegetation was recordedin each plot by counting the     ble 2) is much higher in the grass-dominated (92 %
                                                      site
presence of all separatephanerogamsand of the rest      priorto grazingby horses) thanin the species-poorsite
(bare soil and mosses) with a point frequencymethod     (63 %) and the species-rich site (44 %). However, the
(Mueller-Dombois& Ellenberg1974), using 100 cross-      amountsof perennialgraminoidspecies in the species-
points of a grid.                      rich and grass-dominatedsystems are the same (Table
  The standingcropbiomass (dead+living)was meas-      1).Thespeciesresponsible thedominance gramino-
                                             for        of
ured each year in July by cutting,drying and weighing    ids in grass encroachedstages are alreadypresentin the
threerandomlyselected subsamplesof 20 cm x 20 cm,      species-richstage.
each at the edge of all plots. The centralpartof each plot
(60 cm x 60 cm) has been left untouchedfor long-term     Effects offertilization and grazing on species groups
monitoring.
                                The impactof nitrogenfertilizationon the change of
                               species groups and the three other variables was not
                               statisticallysignificant at all sites! Thereforefertilized
448                      ten Harkel, M.J. & van der Meulen, F.

Table 2. Average frequency over all years of the species groups in the grazed plots and exclosure plots of the three sites.

Species group          Annual        Perennial     Annual       Perennial
                 herbs         herbs      graminoids      graminoids
                 [%]          [%]        [%]         [%]
Treatment          G    E       G    E     G     E     G     E

Site
Species-rich       7.5 **  1.7 **    41.5*  33.6 *   4.9   2.6    43.8 **  56.0 **
Species-poor       1.2    0.1      24.4*  12.2*   10.5   4.3    62.7**  82.2**
Grass-dominated1     1.1    0.3      2.5   2.5    3.9   3.5    92.4   93.7
Grass-dominated2     0.1    0.2      12.8 *  4.1 *   6.9 **  0.4 **   80.0 **  95.3 **
G = grazedplots; I = before the introduction horses;
                        of
E = exclosure plots; 2 = afterthe introduction horses.
                       of

Significance of differencesbetween grazedand exclosureplots: * = p < 0.05; **= p < 0.01.




and non-fertilized plots were taken together during the           between grazed and non-grazed plots (Table 2). In con-
statistical analysis of the effects of grazing.               trast, the frequency of annual graminoids and perennial
  The effects of the exclusion of grazing by rabbits on          herbs increased (respectively 0.4% to 7%, and 4% to
the species groups are shown in Table 2. The exclusion           13%). The horse-grazed plots showed an increase of the
resulted in an increase of perennial graminoids in both           amount of bare soil and mosses, and of the species
the species-rich and species-poor sites. No increase was          diversity. In the same time the biomass showed a de-
observed in the grass-dominated site, because the fre-           crease.
quency of perennial graminoids was high already from
the beginning (92 %). The frequency of perennial herbs           Eff#cts of fertilization and grazing on indilidual
decreased in both the species-rich and species-poor               is
sites. In the species-rich site the frequency of annual
herbs also showed a decrease.                          Fertilization with nitrogen did not affect individual
  Exclusion of grazing at the species-rich and the            species. Only Luzula cnampestris decreased in the
species-poor site reduced the amount of bare soil and            exclosures of the grass-dominated site. To test the effect
mosses, while the biomass of the stand increased (Table           of grazing on individual species, fertilized and non-
3). Moreover, a decrease in species diversity was seen in          fertilized plots were also taken together.
the species-poor site.                             The significant changes of individual species after
  Before the introduction of horses on the grass-domi-          the exclusion from grazing are shown in Table 4.
nated site, the amount of bare soil and mosses became              Some species showed a decrease at the species-poor
lower in the exclosure plots. Six months of grazing by           site. No increases were detected. However, in the last
horses resulted in a difference of perennial graminoids           paragraph an increase in perennial graminoids was




Table 3. Average percentage of all years of bare soil and mosses, species diversity and biomass of the three sites.

Variable           Bare + mosses              Species diversity             Biomass
                 [%]                 (see formula1)              [g/m-2]
Treatment          G      E               G     E            G       E
Site
Species-rich       43.9 **     8.5 **           1.945   1.983          42 **    264 **
Species-poor       57.1 **     8.1**           1.635 *  1.356*         75 **    467 **
Grass-dominated'     17.8**     5.6**           1.134   1.213         335      536
Grass-dominated2     45.2 **     3.7 **           1.664   1.465 *        256 **    613 **

G = grazedplot;    1 = before the introduction horses;
                         of
E = exclosure plot;  2 = afterthe introduction horses.
                        of

Significance of differencesbetween grazedand exclosureplots: * =p < 0.05; **=p < 0.01.
                - Impact of grazing and atmospheric nitrogen deposition -              449

shown.This contradiction mightbe due to an increaseof     Vegetation change after the exclusion of grazing
differentperennialgraminoidsin each plot.
   In the grass-dominatedsite no changes due to the        Vegetation change was defined as the differencein
exclusion from grazingwere foundbefore the introduc-      frequencybetweentwo yearsof the species thatchanged
tion of grazingby horses. The whole site is dominated     in the non-grazed plots relative to the grazed plots
by perennialgraminoids.                    (Table 4). The difference remainedconstant after the
   Species thatincreasedafterthe exclusion fromgraz-     first year, the changes measured occurred during the
ing or decreased after the introductionof grazing by      first year. The vegetation change was calculated sepa-
horses are all perennialgraminoids.One exception is      ratelyfor frequencieswhich increasedor decreaseddue
the increase of Phleum arenariumat the species-rich      to grazing.
site undernon-grazingconditions.This is unexpectedas        At the species-rich site the vegetation change of
                      is
it is a small annualgrassandits germination normally      species which benefited from the exclusion of grazing
preventedin a vegetationpredominated tall grasses
                     by          was small, viz. from 11 % to 16 %. The vegetation
such as Festuca rubra + F. ovina and Poa pratensis       changeof species which decreasedafterthe exclusion of
(Table 4). Species that showed a decrease after exclu-     grazingall occurredduringthe first year.
sion or an increaseafterthe introductionof grazing by       At the species-poorsite all changes occurredduring
horses,areherbsor smallperennialgraminoids,such as       the first year.
Luzula campestris.                         At the grass-dominatedsite the vegetation change
                                for species which increasedaftergrazingby horses was
                                the same for species which increased after the exclu-
                                sion, i.e. from 3 % to 25 %. Calamagrostis epigejos is
                                the only species which increasedin the exclosures com-
                                paredto the controlplots. In spite of the introductionof
                                horses the frequency of Calamagrostis epigejos remained
                                the same (ca. 20 %) in the grazed plots, therefore it
                                increasedfrom 20 % to ca. 50 % in the exclosures. The
                                exclosures developed from a dominance of several
                                graminoids (Table 1) into a dominance by only one
                                species. The frequencyof the species favouredby graz-
Table4. Plant
      species     after
          development excluding    or
                     grazing        ing showed an increasein the grazedplots from 7 % to
after      of   at        site.
   introduction horses thegrass-dominated          30%, while it remainedlow in the exclosures(3 %).The
                                increaseof these species occurredafterthe introduction
Species-rich site
Festuca ovina +F. rubra          +          of the horses. The vegetation level of the grazed plots
Poa pratensis               +          was lowered and this gave the plants an opportunityto
Phleumarenarium              +
Cerastiumsemidecandrum                    grow.
Erodiumcicutarium
Luzulacampestris
Myosotisarvensis                        Discussion
Seneciojacobaea
Taraxacumsect. Erythrosperma
Viola curtisii                           This study showed thatmaintenanceof species-rich
Species-poor site                        dry dune grasslandsstronglydependson the activity of
Rumexacetosella                        grazinganimals,especiallyunderrelativelyhigh atmos-
Seneciojacobaea
                                pheric nitrogendeposition levels. When grazing is ex-
Grass-dominated site                      cluded, perennialgraminoidsbecome dominantwithin
Before introduction horses
          of          no change        one ortwo years.These graminoids  were alreadypresent
After introduction horses
         of
                                in the three sites before the treatment(Table 1). As in
Aira praecox                +          other grazing experiments (Belsky 1986; Hill et al.
Luzulacampestris              +
                                1992) no new plant species invadedthe plots.
Rumexacetosella              ++
Seneciojacobaea              +
                                  Underrelatively high atmosphericnitrogendeposi-
Calamagrostisepigejos                     tion levels, fertilizationby nitrogenhad no effect on the
                                three sites during the 5 yr of this study. This is not
+  = increase (p < 0.05);
++ = increase (p < 0.01);
                                consistent with fertilizer experiments in other grass-
  =decrease (p < 0.05);                    lands (Willis 1963; Van Hecke et al. 1981; Davy &
 - =decrease (p < 0.01).
                                Bishop 1984; Bobbink et al. 1988; Bobbink 1991;
450                  ten Harkel,M.J. & van der Meulen, F.

Mountfordet al. 1993). Effects of additionalnutrient      These studies show that when no other changes
supply were always registeredwithin 5 yr. In our case    occur, the vegetationis probablynot seriously affected
the amountof nitrogen fertilizerof 50 kg N/ha/yr and    by an increasingavailabilityof nutrients.Whengrazing
background depositionof nitrogenof 10 kg N/ha/yr(ten    is continued,changes will probablybe very slow and
Harkel et al. 1991) should be sufficient to accelerate   cannotbe measuredwithin 5 yr.
changes in species composition if other resources are      The main conclusion of this study is that with high
not limited.Bobbinket al. (1992) gave a criticalload of   nitrogenlevels a dunegrasslanddoes not become grass-
nitrogenfor dune grasslandsof 20-30 kg N/ha/yr.       dominatedas long as the vegetationis grazed,not even
  Some explanations for these deviating results are    when fertilized.When grazing is absent, the relatively
given below.                        high availabilityof nitrogen gives graminoidsthe op-
                              portunityto become dominant.Effects of a decreaseof
Phosphorusas a limitingfictor                grazing intensity were reportedby Ranwell (1960): an
                              increasein the growthand floweringof graminoidsand
  Fertilization experiments with different combina-    a decreaseof herbswas foundafterthe disappearance  of
tions of nutrients(nitrogen,phosphorus, potassium,cal-   rabbits myxomatosisin a sand-dunesystem.Willis &
                                  by
cium) showed that the nitrogencontent of the soil was    Yemm (1961) andWillis (1963) showed, however,that
the most limiting factor (Milton 1940, 1947; Willis     even without grazing a very nutrient-poor sandy dune
1963; Boorman& Fuller 1982; Kachi & Hirose 1983;      system can remain species-rich. However, the system
Doughertyet al. 1990; Olff et al. 1993). However, also   became dominatedby grasses after addition of nutri-
phosphoruscan be an importantlimitingfactor(Milton     ents. Boorman& Fuller (1982) found that the addition
1940,1947; Willis 1963;Boorman& Fuller1982;Kachi      of nutrientsto grazed dune swardscaused a decline of
& Hirose 1983; Dougherty et al. 1990). Phosphorus      annualspecies, but,due to the grazing,perennialgrasses
limitationcan be an explanationwhy nitrogenfertiliz-    did not dominatethe sward.When grazing animalsare
ation did not have any effect in this study. At the study  presentin nutrient-poor systems like chalk grasslands
sites atmosphericphosphatedeposition of 0.3 kg P/ha/    (Bobbink 1991) and heathland(Farrow 1917; Heil &
yr (ten Harkel et al. 1991) and the water soluble soil   Diemont 1983), no dominanceby graminoidsoccurs.In
phosphatecontentof ca. 0.3 kg P/ha in the 8 cm surface   general,it is seen thatmoderategrazing increasesplant
soil (ten Harkel 1992) are very low. However, in this    species diversity (Puertoet al. 1990; Gibson & Brown
study we focused on the role of nitrogenin the vegeta-    1991; Belsky 1992).
tion development.The availabilityof phosphorus the to     The introductionof horses at the grass-dominated
vegetationin the Dutchdunes is currently being studied.  site showed that the dominanceof graminoidscan be
  If phosphatewould have been the limitingfactorin    counteractedby introducinggrazing. Light penetrates
the past,the presentdominanceof graminoidsin partsof                            for
                              into the soil surfaceand providesopportunities ger-
the Dutch dunes cannotbe explained. Therefore,phos-     mination of seedlings (Willems 1983; Bobbink &
phate can be a limiting factor only at high nitrogen    Willems 1991). Herbs increase and species richness
levels. This was also shown by Doughertyet al. (1990).   may be restored.When atmospheric  depositionof nitro-
                              gen remainshigh, grazing will always be necessaryto
Changes in ecosystemsother than nutrlient
                    availability    preservespecies-richdune grasslandsfor the future.

  Research on heathland (Heil & Diemont 1983)
showed that a relatively old stand of Calluna vulgaris   Acknowledgements. thank DuneWater
                                      We   the       of
                                              Company
(ca. 15 yr) was not significantly affected by nitrogen   Zuid-Holland,notably its Departmentof Nature and Land-
application. The closed canopy of Calluna vulgaris     scape Managementfor theirsubstantial    financialsupportand
                              the Department Implementation Managementfor exten-
                                      of          of
preventedFestuca ovina to take advantageof the nitro-    sive technicalfacilities to carryout the field experiments.
                                                           J.H.
gen application.However, when Calluna vulgaris died     van Boxel, G.W. Heil and A. Kooijman assisted with the
aftera heatherbeetle infestationand the vegetationwas    researchandgave valuablehelp duringthe compilationof this
opened up, Festuca ovina became dominantafter re-      article. We thank P.D. Jungerius and J.M. Verstratenfor
peated applicationof nitrogen.               criticalreadingof the text. D. Warmerdam acknowledged
                                                      is
  Chalkgrasslandscan sustainhigh loads of nutrients    for help with the correctionof the English text.
when the managementis appropriate.  Mowing andhar-
vesting of vegetation at a site dominatedby Brachy-
podiumpinnatumwas adequateto restorea characteris-
tic species-richchalk grasslandwithin 5 yr (Bobbink&
Willems 1991).
                - Impact of grazing and atmospheric nitrogen deposition -               451


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                                               Received 23 January1995;
                                            Revision received 23 May 1995;
                                              Accepted 5 September1995.