Fire-induced colonization of a Flooding Pampa grassland by thistles: Remnant litter and interference...

- FIRE-INDUCED COLONIZATION OF A FLOODING PAMPA GRASSLAND BY THISTLES - 35

Applied Vegetation Science 6: 35-44, 2003© IAVS; Opulus Press Uppsala.

Abstract. Winter and spring burnings constitute a traditionalmanagement practice of the Flooding Pampa grasslands whichare dominated by the tussock grass Paspalum quadrifarium.In addition to increasing the primary productivity and thenutrient quality of the regrowth, this burning favours theestablishment of opportunistic species, especially the legumeLotus glaber and the thistles Cirsium vulgare and Carduusacanthoides. The aims of the present study were to assess theeffects of burning and those of the remnant litter on L. glaberand thistle recruitment, as well as the effects of L. glaberdensity on the emergence, survival and flowering of thistles.Two field experiments were carried out: 1. A completelyrandomized factorial design with occurrence of L. glaber andfire was applied in a mature Paspalum stand; 2. A completelyrandomized design with four L. glaber seed densities oversownon a burned Paspalum stand, as the treatments. Thistle and L.glaber recruitment within the Paspalum stand was dependenton fire, but their emergence in burnt plots was reduced by thepresence of remnant litter. Remnant litter and interferencefrom L. glaber provide complementary mechanisms to reducefire-cued colonization by thistles. While seedling emergenceof thistles was mostly affected by the presence of remnantlitter, seedling survival depended on local density of L. glaber.By the end of the first post-fire growing season, the survivorshipof established thistles was linearly reduced with increasingsowing density and above-ground biomass of L. glaber. Themaintenance of a significant litter coverage and a high densityof L. glaber in the pre-fire seed bank, as well as the oversowingof L. glaber after fire, may be useful tools in fire managementof Paspalum stands aimed to improve their forage value withreduced dependence on chemical control of weeds.

Keywords: Carduus; Cirsium; Fire management; Flooding;Lotus; Paspalum; Weed.

Nomenclature: Cabrera (1953).

Fire-induced colonization of a Flooding Pampa grasslandby thistles: Remnant litter and interference effects

Ortega, Elizabeth1,2 & Laterra, Pedro1*

1Unidad Integrada Balcarce: Facultad de Ciencias Agrarias, Universidad Nacional de Mar del Plata -EEA Balcarce, INTA, CC 276, (7620) Balcarce, Buenos Aires, Argentina;

2Current address: Facultad de Agronomía y Zootecnia, Universidad Nacional de Tucumán, Av. Roca 1900,(4000) S.M. de Tucumán, Tucumán, Argentina;

*Corresponding author; Fax + 54 1 2266 439101; E-mail [email protected]

Introduction

Paspalum quadrifarium is a warm-season grass na-tive to the Flooding Pampa. It is found in the highestparts of low and floodable lands and is usually regardedas a weed. It forms tall (0.8 to 1.5 m) and dense tussocksof low palatability which, with the exception of theregrowth period (Vervoorst 1967), provide forage oflow nutritional value (Sacido et al. 1995). Its vigoroussummer growth, its capacity to maintain great quantitiesof standing dead tissue and a significant amount of sur-face litter make this a dominant grass. A similar situationis observed in Australian grasslands dominated byThemeda triandra which, in the absence of disturbancesoutcompetes other species because of its height, litteraccumulation and lateral tillering (Lunt 1990).

Cattle stocking rates of Paspalum stands may beincreased during summer through winter burnings, from0.6 to 2.1 LU.ha–1 (Cahuépé 1990) (LU = livestockunit). Fire usually removes the complete canopy, so thebetween-tussock areas of soil surface remain partiallybare until the regrowth onset of Paspalum tussocks.These patches are colonized by opportunistic speciesamong which the most abundant are exotics: a foragelegume, Lotus glaber (ex L. tenuis, bird-foot threefoilclover), and the thistles Carduus acanthoides and Cirsiumvulgare (Laterra et al.1994; Juan et al. 2000; Laterra etal. 2003).

Seed availability as well as the availability ofmicrosites appropriate for establishment (Harper 1977)can limit the incorporation of new individuals into apopulation (Eriksson & Ehrlén 1992). Both limitationscan be removed with fire, which increases seedproduction (Daubenmire 1968) and/or generates adequatesites for establishment (Cauhépé 1990; Laterra 1997;Juan et al. 2000).

Seedling survival of opportunistic species can beaffected by competitive and/or allelopathic interactions.For example, the abundant cover provided by the leg-

36 ORTEGA, E. & LATERRA, P.

ume canopies is known to reduce the amount of freespace for the establishment of other species (Altieri1987). In particular, the negative impact of L. glaber onthe thistles could be explained by the occurrence ofinterference effects between L. glaber, C. acanthoidesand C. vulgare in burnt stands (Laterra 1977). Studiesperformed under controlled conditions have highlightedthe importance of seedling and seed density of L. glaberon the emergence, survival and development of thethistles (Laterra & Bazzalo 1999; Ortega et al. 2001).Differences in initial densities, establishment time, and/or the competitive ability of the competing species leadsto asymmetry in plant size as well as in resource capture,affecting competition dynamics (Harper 1977; Grace1987; Weiner 1990; Wedin & Tilman 1993).

However, it is difficult to predict the way in which thedensity of L. glaber will affect thistle invasion because ofinteractions with other factors which usually affect post-burn establishments. One such factor is the unburnt litterwhich remains on the ground surface of burnt Paspalumstands. Unlike the situation in arid or semi-arid grasslands,a layer of litter of varying thickness can remain in burntPaspalum stands after the fire, whose depth varies ac-cording to the amount of pre-fire litter and fire conditions.In arid environments, the litter usually increases indi-vidual establishment because it reduces evaporation(Fowler 1986). In humid and sub-humid habitats, littermay reduce the germination and individual establishmentbecause it mechanically prevents the emergence of seed-lings on the ground surface (Grime 1979; Hamrick & Lee1987; Facelli & Pickett 1991). Seedlings germinatedunder a thick litter coverage are weaker and many diebefore reaching the soil surface as a result of the extraenergy required to lengthen their shoots towards thesurface (Harper 1977). The litter can also strongly affectthe light environment of the soil surface, retarding emer-gence. Besides, litter can reduce competition by actingnegatively on the density as well as on the biomass of thedominating species (indirect effect) (Facelli 1994). Thus,the thickness of the remnant litter after burning wouldconstitute a determining factor for L. glaber and thistleestablishment and growth.

The aims of the present study were to assess theeffects of burning and those of the remnant litter on L.glaber and thistle recruitment, as well as the effects of L.glaber density on the emergence, survival and flower-ing of thistles. Relationships between the abundance orpresence of thistles, L. glaber, and remnant litter wereexplored at different spatial scales through two fieldexperiments representing different conditions duringthe first growing season after burn.

Material and Methods

Study site

Two experiments were carried out in a Paspalumstand located at San Ignacio (37∞20' S; 58∞25' W, 68 ma.s.l.), Ayacucho, province of Buenos Aires, Argentina.This site was selected because of its accessibility as wellas its physiognomic homogeneity. It has a flat relief andpoorly drained soils, which were classified as fine,illitic, thermic Natraquolls with a natric horizon(Sakalauskas et al. 2001). The average annual rainfall inthe last 13 yr in the locality of Ayacucho was 1022 mm(data obtained from Ayacucho station) and the isothermrunning through that locality is 14 ∞C.

The study site had not been burnt during the last tenyears, so biomass accumulation was high. No L. glaberwas recorded and cover of other species (visually esti-mated in each of the plots) amounted to less than 10%.Accompanying species were identified as: Cirsiumvulgare, Bromus spp., Geranium sp. ¥ spec., Festucaarundinacea and Eryngium pandanifolium. Aerial biomassimmediately before burning the first experiment (August1994) was 1847 g.m–2 (S.D. = 1005 g.m–2; n = 4), with90.4% (S.D. = 6.07%) of dead standing tissue, and 1874g.m–2 (S.D.= 1243 g.m–2) of litter, while immediatelybefore burning the second experiment (three monthslater), standing dead tissues represented 86% (S.D.=13%) of the aerial biomass, and there was 1108 g.m–2

(S.D. = 419 g.m–2) of litter. No significant variation inaerial biomass was expected between the first and thesecond burnings since both were made before the growthseason onset of P. quadrifarium. Immediately beforethe first burning, the mean density of germinable seedsin the soil seed bank was 0.37 (± 0.98), 0.12 (± 0.41),and 0.12 (± 0.34) seeds/dm2 L. glaber, C. acanthoidesand C. vulgare, respectively (Laterra & Vignolio 1997).

Experiment 1

This experiment was aimed to test fire and L. glabereffects, as well as to analyse the influence of remnantlitter on the establishment of thistles. It consisted of acompletely randomized design with a factorial arrange-ment, with presence and absence of L. glaber and fireoccurrence as factors, with five repetitions. In August1994, 20 4 m ¥ 4 m plots were demarcated, with a 2 mwide fire barrier separating one another. Selected plotswere completely oversown with L. glaber, and twocentral between-tussock areas were selected withincorresponding plots and sown with C. vulgare on06.08.1994. Litter in the between-tussock areas wasremoved immediately before sowing and L. glaber wassuperficially oversown with a density of 3 seed.cm–2.


Next, 64 C. vulgare seeds were buried at 1 - 3 mm deep,at a rate of 0.44 seed.cm–2, following a uniform spatialarrangement (8 rows per 8 columns). Immediately aftersowing, seeds of both species were covered with thelitter previously removed from the sowing area and, topromote contact of the seeds with the soil, they weretrodden upon. Burning was applied five days aftersowing, aiming to approximate the imbibition status ofsown seeds to that of seeds belonging to eventually pre-existent seed banks. Cirsium vulgare seeds wereharvested the year before in a population near the placewhere the experiment was carried out and kept at 4 ∞C.Germinability of the C. vulgare seeds at the sowing timewas 97% (assessed in a growth chamber with day/nighttemperatures of 27/17∞C, 14-hr photoperiod). Seeds ofthe L. glaber belonged to a commercial line (TresurChajá), and presented a 100% viability and a 86%hardness at sowing time.

A removable grid was used as a guide for precisepositioning of the sown seeds within a quadrat definedby fixed stacks. By repositioning of grids before eachseedling counting, any seedling emerging 3 mm awayfrom the sowing points,was considered as emergingfrom the seed bank. After early mortality of thistleseedlings within grids, the sampling area was extendedby placing additional 1-m2 plots next to the grids, whichprovided for enough seedlings needed to allow con-struction of survivorship curves.

At the time of the fire, atmospheric temperature was15 ∞C and relative humidity 77%. The humidity contentof both the standing (live plus dead) biomass of Paspalumquadrifarium and the litter immediately before the firewas 39% and 54%, respectively. The maximum tem-peratures reached at soil surface during the fire wereestimated using ceramic pieces marked with Tempil-sticks™ thermosensitive crayons (Big Three Industries,Inc, Hamilton Blvd, South Plainfield, NJ 07080, USA)whose melting points ranged from 69ºC to 677∞C atnearly regular intervals of temperature. Two ceramicpieces were placed in each plot in the between-tussockareas and below the litter. The plots were burnt on11.08.1994 at 2:30 p.m. against the wind in order toenhance material combustion and for improved firecontrol. This expanded homogeneously over the overallsurface of each plot, leaving a heterogeneous distributionof remnant litter (mean: 483 g.m–2, S.D. = 4 g.m–2; n =10). Flames reached ca. 2 m in height. The mean of thefire-front speed per plot was 0.50 m.min–1 (S.D. = 0.14m.min–1). Maximum temperatures reached during thefire at ground level in the between-tussock areas rangedfrom 111 to 427 ∞C, and the mode was 163 ∞C.

Experiment 2

A completely randomized design with L. glaberseed density as the only factor was applied on acompletely burned Paspalum stand to analyse theemergence and survival responses of thistles to L. glaberdensities. Additionally, the influence of remnant litteron seedling emergence was analysed like in Experiment1. Treatments included the addition of high (4 seed.cm–2),medium (0.4 seed.cm–2) or low (0.1 seed.cm–2) densitiesof L. glaber seeds within 52 4 m ¥ 4 m plots; there wasalso a null density control. Eight plots were randomlyassigned as replicates to each of the treatments, and tenadditional plots were randomly assigned to each of thehigh and null densities, as part of a study of biomassproduction (which will be reported elsewhere). On02.11.1994 (16 days before burning), 64 C. vulgareseeds per plot (from the San Ignacio population) weresown at the centre of the plots, following the proceduresdescribed for Experiment 1. The L. glaber seeds had thesame origin as those used for Experiment 1, but incontrast with that experiment, L. glaber was oversownafter burning (November 22) to simulate the usual man-agement procedure (Juan et al. 2000).

With the aim to improve the combustion of litter,burning of this experiment was performed ca. threemonths after burning of the previous experiment, whenless humidity in litter and in aerial biomass favoured amore intense fire. Immediately before burning, thehumidity content of the aerial biomass was 30.5 % (S.D.= 10.6 %; n = 6), atmospheric humidity was 72% and theatmospheric temperature was 26 ∞C. During burning,average wind speed was 5 m.sec–1 and the mean of thefire-front speed was 0.80 m.sec–1 (S.D. = 0.14 m.sec–1; n= 3). The fire temperature, estimated as in Experiment 1,ranged from 152 to > 677 ∞C with a mode of 566 ∞C, butit generally surpassed in the upper limit of the rangeprovided by the thermosensitive crayons used.

At burning time, mean densities (± S.E.) of germinableseeds within the upper 2 cm of the natural soil seed bankof C. acanthoides, C. vulgare and L. glaber were very low(12 ± 6 seed.m–2, 12 ± 5 seed.m–2 and 37 ± 14 seed.m–2,respectively; Laterra & Vignolio pers. comm.), incomparison with the sowing densities.

In addition to differences in fire conditions andsowing time of L. glaber, this experiment also differedfrom Experiment 1 in its post-fire conditions. WhileExperiment 1 was not disturbed after the fire, a canopyremoval by cattle grazing was simulated in three oppor-tunities in all plots of this experiment. In each of theseopportunities all the aerial biomass surpassing 25 cmand 2-3 cm from the ground, in the tussock and in theinter-tussock areas, respectively, was harvested from a1 m ¥ 1 m subplot which contained the thistle sowing


area, and the harvested biomass was air-dried andweighed. Cutting heights were determined according toad hoc observations in neighbour grazed fields. The firstharvesting was made on 31.01.1995, when the meanmaximum height of the P. quadrifarium regrowth was87 cm (SE = 4.2), and the intermediate and final harvestswere done on 02.02.1995 and on 13.05.1995, respect-ively. Low stature of thistle seedlings preserved themfrom defoliation in the first cutting date; but most ofestablished thistles were partly defoliated by the secondharvest date because of their palatable leaves (as observedin neighbouring fields) reached the cutting height.

Measurements

Emergence and survival of thistle seedlings wasregistered from both sown seeds (C. vulgare) and fromthe seed bank of thistle species (C. vulgare and C.acanthoides). Both experiments were visited after burn-ing with a 7 - 10 day frequency. Between 30 and 40 daysafter burning of both experiments, thistle and L. glaberseedlings started to emerge. All the thistle seedlingsemerged within the 1-m2 plots were tagged with thincoloured straws next to each specimen. When the this-tles had the first pair of true leaves, species identity ofthe previously marked seedlings was determined. In thefollowing measurements new and survivor seedlingswere counted, and their largest blade was measured asan indicator of the size of the seedlings previouslytagged. The first counting of emerged seedlings wasmade after a rapid decline in the rate of emergence, andwhen nearly all the previously marked seedlings werestill alive. In Experiment 1, counting and measurementswere repeated in five observations that took place at 74(spring, seedling stage), 131 (late spring), 167 (earlysummer), 188 (mid-summer) and 278 days (autumn)after burning. In Experiment 2, observations were per-formed at 43 (early summer, seedling stage), 68 (mid-summer), 84 (mid-summer) and 175 (autumn) daysafter burning.

The relationships between the presence of thistlesand the presence of L. glaber or the presence of remnantlitter were studied for both experiments at two differentspatial scales. The experiments were sampled when L.glaber and thistle seedlings had not more than two andthree true leaves, respectively, that is, three months afterburning in Experiment 1 and two months after burningin Experiment 2. Two 56-square grids with cells of 3.5cm ¥ 3.5 cm each, arranged in 7 rows and 8 columns,were randomly placed in each plot between-tussockareas with a distance of at least 20 cm from the plot edge.Presence and absence of L. glaber, C. vulgare, C.acanthoides and litter was registered for each cell of

each grid. Presence of L. glaber and thistles was regis-tered for any cell with one or more seedlings of thesespecies rooted within the cell, and presence of litter wasregistered for any cell partly or completely covered bylitter. Assessment of spatial associations at grid scale(13.7 dm2) was done by using the sum of cells of bothgrids per plot, whereas relationships at cell scale (0.12dm2) were explored by considering the joint presenceand absence of thistle, L. glaber and/or remnant litter foreach cell of the grids. Additionally in Experiment 2, thethickness of postburn remnant litter was measured foreach cell.

Emerged seedlings of L. glaber within Experiment 1were estimated 40 days after burning by randomly placingtwo 8 cm ¥ 8 cm frames within the permanent plots.When L. glaber plants nearly completely covered thesoil surface (29.10.1994), their effect on the soiltemperature was measured at the sub-surface level (0.5cm deep) in presence and absence of litter. Seedlingcounting was not possible in Experiment 2 due to adelayed recording date and the consequent difficultyposed by profusely branched individuals to be counted.As a consequence, L. glaber abundance in each treatmentof Experiment 2 was only characterized by means of thebiomass harvested at the end of the experiment. Finally,the number of thistle individuals that reached thereproductive stage and the number of inflorescencesproduced per plant, were recorded.

Statistical analysis

Examination of thistle, L. glaber and litter relation-ships at both ‘grid’ and ‘cell’ scales required seedlingsof both thistle species to be pooled into a single class, toavoid a too low number of cases with thistle species.Conditional frequencies of cells with or without thistleseedlings given the presence or absence of L. glaberand/or given the presence or absence of remnant litterwere calculated separately for each plot (both gridspooled) in each experiment.

Litter and L. glaber influences on soil temperaturewere analysed by a two-way ANOVA. Relative fre-quency of cells were compared by using non-parametricmethods because of large departures from normality inall datasets could not be overcome by the usual datatransformations. Both the Kruskal-Wallis ANOVA andMann-Whitney U-tests were applied to the same sets ofdata, but only the results of the last test were shownsince the significance of both tests at the a = 0.05 levelagreed for all the cases. The number of grids used forcomparison of conditional frequencies varied betweentests because of lack of heterogeneity in L. glaber and/orlitter occurrence at cell scale. Litter and L. glaberinfluences on emergence were also analysed at the grid


scale applying simple and multiple regression analysis,using the sum of cells of both grids per plot with thistleas dependent variable and the sum of cells of both gridsper plot with L. glaber and/or remnant litter, as inde-pendent variables.

No important cohorts were detected after the firstcounting of thistle seedlings, so, while thistle density atthe first counting closely reflected the total emergenceof thistle seedlings, the following counting approxi-mately reflected the survival of the initial cohort. There-fore, total seedling density instead of the density bycohort was plotted against time, as a simple surrogate ofthe survival curve of thistle seedlings. Since no emer-gence of seedlings was observed in the unburnt treat-ments of Experiment 1, the effects of L. glaber on size,density and reproduction of thistle, were analysed usingStudent’s t-tests. Density data of Experiment 2 wereanalysed by means of ANOVA and Duncan post-hoccomparison of means. The relationship between thefollowing factors was analysed by means of linear andnon-linear regressions: (1) seed density and final aerialL. glaber biomass vs thistle survival; (2) L. glaberdensity vs its final biomass and (3) L. glaber density vsnumber of leaves of both thistle species. Thistle densityvalues were expressed as a proportion of the initialdensity of plants, and their angular transformation wasperformed before their variance analysis, but the rawdata rather than transformed data were presented. Allmean values were expressed ± the standard error of themean (S.E.).

Results

Experiment 1

Burning did not affect the litter cover at the begin-ning of the growing season, which averaged 18 ± 7%and 17 ± 7% of unburnt and burnt grid areas, respectively.Accordingly, ca. 20% of cells into the sown grids werecovered by litter up to 3 cm thick (Fig.1a).

In the unburnt plots, an apparently (not quantified)high proportion of sown L. glaber seeds managed togerminate (30 days after the burn) but, although thesedeveloped very long stems, they did not emerge throughthe litter layer and no L. glaber emergence occurred.This pattern was only observed for thistles in somecontrol plots. In the burnt plots where L. glaber wassown, a density of 2.45 seedling.cm–2 was recorded 40days after the burn. No spontaneous L. glaber establish-ment occurred within the control plots nor outside thesown area within the experimental plots.

Soil temperature at subsurface level was signifi-cantly (p < 0.0001) affected by the interaction of L.

glaber and litter presence. In absence of L. glaber, litterreduced both the mean and the amplitude of temperaturesregistered during the observation period, but when L.glaber was present the inverse was true (Fig. 2).

The influence of L. glaber on the emergence of C.vulgare could not be assessed, since the germinationpercentage of sown seeds of C. vulgare was negligible(0.6%) and nearly all registered seedlings of thistlespecies emerged from the natural seed bank. No thistleemergence form any source was registered in the unburntplots. The relative abundance of seedlings of C. vulgareand C. acanthoides in burnt plots in the Spring countingwas 54% and 46%, respectively. In this experiment, thepresence of other species was not quantified but theiroccurrence in similar conditions has been recorded(Laterra 1997).

At a small scale (3.5 cm ¥ 3.5 cm), the presence ofthistle seedlings in cells with presence of remnant litterwas lower than in those where litter was absent (p = 0.04)(Fig. 3). No significant association was observed betweenthe presence of thistle seedlings and the presence of

Fig. 1. Percentage of cells within grids occupied for differentthickness categories of remnant litter after the burning ofExperiments 1 (a) and 2 (b).


seedlings of L. glaber (p = 0.27). At the grid scale, nosignificant relationships were detected between the fre-quency of cells with thistle and the frequency of cellswith remnant litter (p = 0.92) or the frequency of cellswith L. glaber (p = 0.70) by using simple regressionanalysis, nor by simultaneously considering the fre-quency of cells with remnant litter and with L. glaberthrough multiple regression analysis (p = 0.11). Norelationships between the relative frequency of cellswith L. glaber and the relative frequency of cells withlitter were analysed because only five plots with seedlingsof this species, those burnt and sown with L. glaber,were available.

As from 155 days post-burn onwards, L. glaberoccurrence significantly reduced (p < 0.05) the jointdensity of both thistle species. From the 51 seedling.m–

2 initially recorded in absence of L. glaber, 73% sur-vived by the end of the first post-burn growing season,but only 22% of the initial thistle density (56 seedling.m–

2) survived after the same period in presence of L. glaber(Fig. 4). No significant effects of L. glaber presencewere detected on seedling densities when tests wereseparately performed for each thistle species.

For C. vulgare, the establishment of a second cohortwas observed. However, this represented a low percent-age in relation to the initial cohort (7% and 18% of theinitial number, for treatments with and without L. glaberrespectively), all individuals dying before the secondobservation (third date).

The presence of L. glaber did not have a significanteffect (p > 0.05) on the number of thistle leaves per plant(4.6 and 4.2 with and without L. glaber, respectively),but leaf length significantly increased (p < 0.05) in thetreatment with L. glaber (15.7 cm and 12.9 cm with andwithout L. glaber, respectively). Only 7.5% (with L.glaber) and 6.9% (without L. glaber) of the survivingindividuals completed the reproductive cycle, and nosignificant differences were found between treatments(p > 0.05).

Experiment 2

As in Experiment 1, no emergence of sown thistleseeds was observed but the spontaneous establishmentof both thistle species did occur after fire in the between-tussock areas of the experimental plots. Despite thehigher fire intensity in this burning, compared to that inExperiment 1, cover and thickness of remnant litter afterfire was similar in both experiments (Fig. 1).

The density of seedlings which emerged during thefirst period was higher for C. vulgare than for C.acanthoides in the different treatments (Table 1). Asecond cohort of C. vulgare emerged between the firstand the second observations, representing 3%, 3%, 1%

Fig. 2. Soil temperature at 0.5 cm depth after burning inExperiment 1 in presence or absence of litter and /or seedlingsof L. glaber.

Fig. 3. Distribution of the conditional frequency ofcells with thistle given the presence or the absenceof remnant litter among grids in Experiment 1. Cellclasses 1, 2, 3 and 4, correspond to the followingupper limits of relative frequencies: 0, 0.05, 0.1,and 1, respectively.


and 10% of the initial density for treatments with high,medium, low and null density of L. glaber. All theseedlings which belonged to the second cohort diedduring the following period.

The relative frequency of cells with thistle given thepresence of remnant litter (2.1 ± 1.2%; n = 31) wassignificantly lower (p = 0.002) than the relativefrequency of cells with thistle given the absence oflitter (3.7 ± 0.72%; n = 52) (Fig. 5). Therefore, therelative frequency of cells with L. glaber in the presenceof remnant litter (2.5 ± 0.48%; n = 20) was signifi-cantly lower (p = 0.002) than the relative frequency ofcells with L. glaber given the absence of litter (5 ±0.64%; n = 28). The number of seedlings of L. glaberby cell and by plot, for those cells with one or moreseedlings of L. glaber, was significantly higher (p =0.01) for cells without litter (2.25 ± 0.25%; n = 25)than for cells with litter (1.58 ± 0.27%; n = 19) (Fig. 6).

No significant differences (p = 0.08) were observedbetween the relative frequency of cells with thistle

given the presence of L. glaber (3.2 ± 1.4%; n = 28)and the relative frequency of cells given the absence ofL. glaber (mean = 3.3%, ± 0.7%, n = 51). The presenceof L. glaber seemed to exert a different trend on thepresence of thistle, according to the presence or absenceof litter (Fig. 6), suggesting an interaction between L.glaber and litter on thistle presence. However, the fre-quency of cells with thistle given the presence vs theabsence of L. glaber did not significantly differ withinany level of litter cover (p > 0.12).

No significant relationships were detected at gridscale between the frequency of cells with thistle andthe frequency of cells with remnant litter (p = 0.46) orthe frequency of cells with L. glaber (p = 0.24), byusing simple regression analysis, nor by simultaneouslyconsidering the frequency of cells with remnant litterand with L. glaber through multiple regression analysis(p = 0.30).

The pooled density of both thistle species in testplots showed a significant difference (p < 0.05) inrelation to the plots sown with high L. glaber densityonly (Fig. 7). While only 55% of the thistle seedlingsemerging in these plots survived 5 months postburn,thistle survival was 85% during this same period in thecontrol treatment (without L. glaber).

Different accumulations of aerial biomass of L. glaberwere still observed by the end of the experiment(13.05.1995) as a consequence of different sowingdensities: 105 ± 22 g.m–2, 42 ± 13 g.m–2, 34 ± 21 g.m–2

for the high, medium and low density treatments, re-spectively. A significant linear relationship between L.

Table 1. Cirsium vulgare and Carduus acanthoides densities± S.E. at 60 days after the burn.

Treatments Thistle density(seeds/m2) (seedlings/m2)

L. glaber C. acanthoides C. vulgare

0 11 ± 11.2 2 ± 3.4N = 18 N = 18

0.1 10 ± 6.6 9 ± 12.8N = 8 N = 8

0.4 13 ± 13.4 3 ± 2.6N = 8 N = 8

4 8.2 ± 6.2 2 ± 1.7N = 18 N = 18

Fig. 4. Pooled density of thistle (Cirsium vulgare plus Carduusacanthoides) seedlings (from 73 days after burning), in pres-ence (�) or absence ( ) of Lotus glaber. Vertical bars represent2 SD.

Fig. 5. Distribution of the conditional frequency of cells withthistle given the presence or the absence of remnant litter and/or Lotus glaber among grids in Experiment 2. Cell classes 1, 2,3, and 4 correspond to the following upper limits of relativefrequencies: 0, 0.05, 0.1 and 1, respectively.


glaber final biomass (g.m–2) and the sowing density(seed.cm–2) was obtained. However, most of the variationin the final L. glaber biomass could not be explained bythat single variable:r2 = 0.389; p = 0.01; y = a + bx; a =14.240; b = 48.46.

Survival of the seedling pool of both thistle species,expressed as the ratio between their initial and finaldensities, showed a linear decrease with the sowingdensity (seed.cm–2):r2 = 0.25; p =0.003; y = a +bx ; a =84.84; b = –49.5.The decrease of thistle survival with L. glaber biomass(g.m–2) by the end of the experiment was linear as well:r2 = 0.12; p = 0.04; y = a + bx; a = 1.17; b = – 0.002).The number of leaves per plant of surviving thistles wasnegatively affected by the sowing density of L. glaber:r2 = 0.19; p = 0.026; y = a + bx; a = 5.73; b = –1.13.

Thistle flowering within the first growing season

after fire was very rare in all treatments, involving lessthan 1% of surviving plants of each treatment at the finalobservation date (May 13, 1995).

Discussion and Conclusions

Burning of dense Paspalum stands has proven to be anecessary condition for their colonization by thistles andL. glaber, confirming previous observations (Laterra 1997;Juan et al. 2000). Germination of thistle species in thecontrol treatment was negligible, probably because theirheat and/or light requirements for germination were notreached under the litter layer and the Paspalum canopy(Klinkhamer & de Jong 1988). The emergence failurewhich occurred from the sown seeds of C. vulgare afterburning, might be due to lethal temperatures reached atthe sowing depth during the fire, as demonstrated byOrtega (1998), so thistle recruitment depended on seedspresent in the soil and situated deep enough to be protectedfrom severe heating during fire.

While the emergence of thistles and L. glaber in thePaspalum stand was triggered by fire, it was not asufficient condition for emergence and/or seedlingsurvival, since the frequency of thistle seedlings afterfire depended on the cover of remnant litter. A litterlayer may directly inhibit thistle germination or seedlingsurvival by reducing light intensity, by altering lightquality, and/or by avoiding contact with soil. In particular,the amount of light reaching the surface was shown akey factor promoting C. vulgare germination in disturbedsites (Klinkhamer & de Jong 1988, 1993). These lightrequirements were also observed by Meed & Lovett(1978) in Carduus nutans germination. In general,germination is reduced in darkness or if the red-far redratio does not meet the seed requirements (Grime et al.1981). Litter also can affect germination rates by simplemechanical effects, precluding the contact of the seeds

Fig. 6. Distribution of the conditional frequency of cellswith Lotus glaber given the presence or the absence ofremnant litter among grids in Experiment 2. Cell classes1, 2, 3, 4, and 5, correspond to the following upper limitsof relative frequencies: 0, 0.25, 0.5, 0.75, and 1, respec-tively.

Fig. 7. Percent of the initial density of thistle seedlings (Cirsiumvulgare plus Carduus acanthoides) surviving during the firstgrowth season after fire, under different sowing densities ofLotus glaber: 0,1 seeds/cm2 (▲), 0,4 seeds/cm2 (◆), 4 seeds/cm2 (■), and 0 seeds/cm2 ( ). Different letters indicatesignificant differences (p < 0.05).


ance in thistle density and the L. glaber abundanceestimators could arise because: (1) the competitive envi-ronment of the thistles did not only comprise L. glaberbut also other species; (2) L. glaber sowing density doesnot represent a good estimator of its impact on thistle;(3) there is uncontrolled predation of thistle seedlings byinvertebrates and small vertebrates; (4) non-uniformdistribution patterns of thistle and L. glaber may intro-duce uncontrolled variation of intra- vs. inter-specificcompetition; (5) the two thistle species have variableproportions. In spite of all these sources of error, densityeffects of L. glaber were strong enough to be detectedwith high significance values. Thus, functional relation-ships obtained between thistle survival and density of L.glaber seeds may be utilized as a guide to aid manage-ment decisions aimed at a certain degree of thistlecontrol and reducing the dependency on chemical con-trol.

In summary, the results support the hypothesis thatfire-cued colonization of Paspalum stands by thistlespecies is negatively affected by the cover of remnantlitter as well as by the density of L. glaber. Significantreductions of thistle density by L. glaber may occur byboth pre-existent L. glaber seeds (underneath the litterlayer) and oversown L. glaber seeds after fire, evenwhen defoliation by grazing of the postburn regrowthwas simulated. While seedling emergence is mostlyaffected by the presence of remnant litter, seedlingsurvival depends on local density of L. glaber. Thus,remnant litter and interference from L. glaber providecomplementary mechanisms to reduce fire-cued coloni-zation by thistle, which improves the forage value of thePaspalum stand with reduced dependence on chemicalcontrol of weeds.

Acknowledgements. We thank M.F. Buckley for field andlaboratory assistance, and R. Sampedro for the English revi-sion. The manuscript was improved by the comments of P.Dixon and P. Mordelet. The research was supported by theUniversidad Nacional de Mar del Plata.

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Received 8 March 2001;Revision received 29 April 2002;

Accepted 2 December 2002.Coordinating Editor: T.A. Watt.

Fire-induced colonization of a Flooding Pampa grassland by thistles: Remnant litter and interference...

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