How does surrounding vegetation affect the course of succession: A five-year container experiment

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  • How does surrounding vegetation affect the course of succession:A ve-year container experiment

    Lanta, Vojtech1,2 & Leps, Jan3,4

    1Institute of Botany, Czech Academy of Sciences, 37005 Trebon, Czech Republic;2Section of Ecology, University of Turku, 20014 Turku, Finland;

    3Department of Botany, Faculty of Science, University of South Bohemia, Branisovska 31, 37005 Ceske Budejovice,

    Czech Republic; E-mail suspa@prt.jcu.cz;4Biology Centre, Institute of Entomology, Czech Academy of Sciences, Branisovska 31, 37005 Ceske Budejovice,

    Czech Republic;Corresponding author; Fax1420 384 721 136; E-mail lanta@butbn.cas.cz

    Abstract

    Question: How does location and time of insertion affectthe course of succession in experimental containers?

    Location: Benesov nad Lipou, Ceskomoravska vrchovina(Czech-Moravian uplands), Czech Republic

    Methods: We designed a 5-year container experiment inwhich plant succession started from scratch. Soil conditionswere constant and all containers were lled with homoge-neous substrate containing no propagules. We placed thecontainers in two contrasting habitats (meadow and ood-plain) under identical climatic conditions but differing insurrounding vegetations and hence seed input. New con-tainers were installed (and hence succession started) in twosubsequent years, twice in each year (spring and autumn).We assume that the individual dates would lead to differ-ences in propagule input and weather conditions.

    Results: Although both year and season of successioninitiation considerably affected the initial species compo-sition, we observed a pronounced convergence within theset of containers located in each habitat. However, thesimilarity of containers initiated at the same time butlocated in different habitats decreased over the course ofsuccession. Final composition of the meadow and ood-plain containers was therefore mostly determined bypermanent seed input from their nearby neighborhood.

    Conclusions: This study demonstrated that propaguleavailability is an important determinant of the course ofsuccession, and that differential seed input leads to differ-ent pathways of succession, even when all otherenvironmental conditions are equal.

    Keywords: Convergence; Divergence; Habitat effect; Pro-pagule availability; Seed rain.

    Nomenclature: Kubat et al. (2002).

    Introduction

    The course of succession within any regiondiffers among individual habitat types. The threebasic determinants of the successional pathway aresite conditions, composition of propagules arrivingto the newly opened site or already present in thesoil, and climatic/weather conditions. Under naturalconditions, these three sets of factors are closely re-lated and, consequently, their effects are difcult todistinguish. Moreover, the disturbance that opens anew area and starts succession in that area onlyrarely completely destroys that habitat and, even inthe case of a major disturbance, succession does notstart from complete scratch (so that biotic interac-tions play an important role from the verybeginning). In the case of secondary succession, soilbiota are always present and can affect the course ofsuccession.

    It is well known that seedling establishment is oneof the most sensitive phases of the plant life cycle(Fischer & Matthies 1998; Isselstein et al. 2002) andthat there is large variability in seedling establishmentamong individual years (e.g. Spackova & Leps 2004;Pakeman & Small 2005). The regeneration niche the-ory (Grubb 1977), that seedling establishmentrequirements differ among individual species, meansthat different availability of safe sites (Harper et al.1965) could determine establishment of individualspecies. Similarly, the time when succession startsmight, to a large extent, determine the whole course ofsuccession. Both weather conditions and seed rainchange between seasons over the course of a year andbetween individual years. Although differences be-tween seasons (in both weather and seed rain) aremore or less predictable, the differences between in-dividual years are not.

    Journal of Vegetation Science 20: 686694, 2009& 2009 International Association for Vegetation Science

  • Classical successional theory (Clements 1916)suggests that community composition shouldconverge toward climax, as determined by environ-mental conditions (the monoclimax theory). Incontrast, Eglers (1952) Initial Floristic Composi-tion concept predicts that the course of succession is,to a large extent, determined by the species that wereinitially dominant; thus we would expect divergence.Leps & Rejmanek (1991) predict that convergent ordivergent behaviour can be determined by thevariability in habitats observed with respect to fac-tors limiting the distribution of early and latesuccessional species, which are differentially sensi-tive to environmental factors. If the habitats arehighly variable in factors to which the early succes-sional species are sensitive (typically the type ofsuccession initiating disturbance) and rather homo-geneous with respect to factors to which the latesuccessional species are sensitive (typically soil orclimatic conditions), we expect convergence. We ex-pect divergence under the opposite scenario.Examples of both types of development have beenrepeatedly reported from natural communities(Matthews 1979; Barbour & Minnich 1990; Sjors1990; Nilsson & Wilson 1991; Inouye & Tilman1995; del Moral 2007); however, we are not aware ofan experimental study able to disentangle the effectof site conditions, weather conditions and differ-ences in seed rain (propagule input).

    We cannot manipulate climate conditions orkeep them constant from year to year, but in ex-periments on a small spatial scale (e.g. in plasticcontainers), we are able to keep soil conditions con-stant. We can change seed input simply by sowingseeds, but we do not have a clear idea how thequantity or composition of seed rain might differbetween two relatively close sites or among in-dividual seasons and years, i.e. what is the real seedinput. Consequently, we designed a container ex-periment in which we started the succession fromscratch. We placed the containers in two contrastinghabitats under identical climatic conditions. Thecontainers in the two habitats were thus subjected totwo differing but realistic types of seed rain. We in-serted sets of containers in two seasons in each oftwo subsequent years. From this we hoped to de-monstrate that the effects of seed rain are notaffected by variations in soil conditions or seasonaland annual variations during the course of succes-sion. The effect of insertion time includes both theeffect of weather in a season/year and varying seedrain between seasons/years.

    Our experiment enabled the study of experi-mental succession over 5 years where initial soil

    conditions were kept constant, and any variabilitywas caused only by differences in propagule inputand weather conditions. Our aim was to separatethese two effects. By following the course of succes-sion for several years, we can examine which factorsdetermine the initial variability. Specically, we ex-amined:

    (1) How the course of succession was affected by thedifferential availability of propagules in differenthabitats.

    (2) How the course of succession was affected by thetime of succession initiation (in various seasonsand various years).

    (3) Whether the initial differences increase or de-crease during succession, i.e. whether and underwhich conditions the course of succession isconvergent or divergent.

    Methods

    Locality

    The experiment was conducted within an or-ganic farm at Benesov nad Lipou, a site in thesoutheastern part of the Czech Republic, in theCeskomoravska vrchovina (Czech-Moravian up-lands, 491920N, 151000E, 665m a.s.l.). This area hasa temperate climate, with a mean annual tempera-ture of 6.71C and annual precipitation of 759mm(Cernovice meteorological station).

    The experiment was established in two habitatswithin the farm. The rst (here called meadow)was in a strip that formed the border between twoarable elds and an extensively grazed pasture. Thisstrip was 3-m wide and faced northwest-southeast.Young trees of Sorbus aucuparia, Salix caprea andother woody species were planted there at regulardistances of 4-5m. The herbaceous vegetation be-tween the trees formed permanent grasslandcontaining several ruderals or arable weed species.Common species included the grasses Agropyron re-pens, Poa trivialis, Phleum pratense, Dactylisglomerata and the forbs Taraxacum sect. Ruderalia,Trifolium repens, Artemisia vulgaris and Veronicaarvensis. The second habitat (here called ood-plain) was located in the oodplain of theVcelnicka rivulet. (However, oods are rare and theplot was not ooded during our experiment.) Plantstypical of moderately wet meadows and pasturesprevailed (grasses Phalaris arundinacea, Festuca ru-bra, Agrostis tenuis, Holcus lanatus and the forbs

    - How does surrounding vegetation affect the course of succession - 687

  • Ranunculus acris, Myosotis nemorosa, Galium uligi-nosum, Anthriscus sylvestris). Both habitats wereunmanaged during the course of the experiment.The whole area of meadow is rather at with a slopeof o51, the oodplain is completely at. The twosites were about 500m from each other and sepa-rated by a small wood; the difference in elevationwaso10m. Consequently, the habitats share nearlythe same weather conditions. (Originally, the ex-periment included a third habitat type: a newlyabandoned eld, but all the containers were stolenand we therefore had to restrict ourselves to just twohabitat types.)

    In each habitat type, we selected one location(locations within habitats are not replicated). Theexperiment does not intend to represent the succes-sional course in the respective habitats ourintention is only to demonstrate the differences insuccessional pathways due to differential seed input.Consequently, we selected two locations that areclose enough not to differ in climatic conditions, butfar enough apart to receive different seed rain.

    Experimental design

    Circular plastic containers (0.55m dia-meter0.25m deep) lled with garden substrate(without any seeds) containing active humus (pH inwater suspension between 5.5 and 7.0; 5% of parti-cles bigger than 20mm) were inserted in the eld. Ateach starting time, 10 containers (ve with and vewithout perforated bases) were inserted into the soilwithin each of the two habitats, dated and coded:April 2002 (T1), September 2002 (T2), April 2003(T3) and September 2003 (T4). As the water regimediffered between the two sites, containers with per-forated bases enabled vertical water movement toprevent desiccation and introduce differences in soilmoisture. This was insurance in the case of an ex-tremely dry period that could have led to completedrying out of containers with non-perforated bases.Fortunately, no such conditions occurred. Further,in the analyses, the effect of perforation was alwaysnegligible and non-signicant.

    Containers were inserted into the soil at regularintervals (i.e. 2m) with the upper margins of a con-tainer being level with the soil surface. In total, 80containers were inserted in the two habitats (20 per-forated and 20 non-perforated in each habitat) overthe rst 2 years of the experiment. The use of con-tainers helped prevent penetration of rhizomes andstolons of surrounding vegetation into the gardensoil. In a limited number of cases (ca. 10) in theoodplain, we found aboveground stolons of plants

    tended to spread into the containers. These were re-moved from the closest neighbour to the containers.Consequently, the succession in each container re-ected establishment only of plants that hadgerminated from the seed rain.

    The cover of all present plant species was re-corded twice per year, in the spring and late summer,in ve subsequent years, 2002-2006.

    Data analysis

    The changes in total species composition wereanalysed using multivariate ordination methods(Canoco for Windows, ter Braak & Smilauer 2002).Data on species richness were analysed using ANO-VA models. Cover data from each time census wereused as response data for ordination, yielding a ma-trix of 600 samples and 88 plant species. Values ofspecies cover (in %) were log(X11)-transformedprior to analyses. Each vegetation record was char-acterized by the location of the container (meadowor oodplain), year of container insertion, season ofcontainer insertion (spring or summer), year of datarecording, and season of data recording. All of thesefactors were considered as environmental variables(in Canoco terminology).

    Data were rst subjected to detrended corre-spondence analysis (DCA) in order to assess theoverall variation pattern in species composition,with the environmental variables projected passivelyto the ordination plane. The interaction among fac-tors, time of container insertion (T; four dummyvariables)position of container (P; two dummyvariables)census for data recording (R; sevendummy variables) was passively projected to the or-dination plain (yielding 4275 56 centroids).Note that the ordination is solely based on speciesdata. The passive projection of interaction displays,where the centroids of containers inserted in in-dividual habitats and at individual times are atindividual census dates in ordination space, was de-ned by species composition. Successional trendsare displayed as lines connecting the time series ofthe centroids of the same position and insertiontime. Canonical correspondence analysis (CCA) wasthen used to evaluate the species-environment re-lationship. Environmental variables were subjectedto forward selection (FS), mainly to see the sequenceof contributions of individual variables to explana-tion of species composition. The amount ofvariability explained by individual variables wascalculated by dividing corresponding eigenvalues bytotal inertia. The signicance of each variable wasevaluated using the Monte Carlo permutation test

    688 Lanta, V. & Leps, J.

  • (499 permutations). Four related CCA analyseswere used: CCA 1 for the whole dataset, CCA 2 for1-year-old containers (recorded in spring of the yearfollowing insertion, i.e. the set contains records fromvarious years), CCA 3 for the 3-year-old containers(similarly to the previous, but recorded 2 years la-ter), and CCA 4 for data from the nal census(summer 2006). In all analyses, the Position, Year ofcontainer insertion and Season of container inser-tion were used (all binary variables); in addition, inCCA1, the Year and Season of data recording wereused. Year was in this case used as quantitativevariable and explains the successional trend. InCCA 2 and CCA 3, containers were of the same age,but recorded in various years, so the differencesmight be caused either by habitat, or by year of in-sertion, which was not confounded by thesuccessional age, but might be affected by weather inthe year of recording. In CCA 4, time of insertionwas confounded with successional age of the con-tainer, but all the samples were recorded at the sametime.

    Changes in species richness were anal...

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