- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 143Applied Vegetation Science 9: 143-154, 2006© IAVS; Opulus Press Uppsala.

Abstract.Question: Does understorey richness, cover and biomasschange during succession in abandoned Castor canadensisimpoundments of riparian Nothofagus forests?Location: Magellanic Nothofagus forests at Tierra del FuegoNational Park (54°50'32.4'' S, 68°32'11.5'' W), Argentina.Methods: Five meadows of different time since abandonment(1, 5, 6, 9 and 20 years ago) and two controls (pure N. pumilioand mixed N. pumilio - N. betuloides forests) were sampled.Understorey variables (species richness, cover and biomass)in beaver meadows were measured at eight plots, as well assapling and seedlings age and height. In control treatments, tenplots on each forest type characterized forest structure.Results: Beavers alter vegetation dynamics, modifying bio-mass and composition of the original forest communities.Richness, cover and biomass were significantly modified whencompared to the original understorey. Ferns are the mostaffected group, while grasses became more abundant. Manyspecies established in the impacted sectors, which did notgrow in primary forests. Trees did not regenerate in impactedareas for long periods, and many understorey original speciesare missing. Nothofagus forests are not adapted to support along-term beaver impact.Conclusions: Beavers modify the original ecosystem fromclosed forest to a grass- and sedge-dominated meadow, due tothe lack of adaptive regeneration strategies in the Nothofagusforests. The maintenance of the present level of the beaverpopulation is not sustainable over time, due to utilized andimpacted tree biomass which could not be replaced by thenatural dynamics of the forest ecosystem.

Keywords: Beaver; Diversity; Exotic species; Meadow;Nothofagus betuloides; Nothofagus pumilio; Dynamic cycle;Regeneration.

Nomenclature: Correa (1969-1998); Moore (1983).

Abbreviations: FA = Forest stand; GIS = Geographical infor-mation system; LF = Nothofagus pumilio forest; LGF = N.pumilio/N. betuloides mixed forest.

Understorey succession in Nothofagus forestsin Tierra del Fuego (Argentina) affected by Castor canadensis

Martínez Pastur, Guillermo1*; Lencinas, M. Vanessa1; Escobar, Julio1; Quiroga, Paula2;Malmierca, Laura3 & Lizarralde, Marta1

1Centro Austral de Investigaciones Científicas (CONICET), cc 92 (9410) Ushuaia - Tierra del Fuego, Argentina;2Laboratorio de Ecotono (CRUB) Unidad Postal Universidad (8400) Bariloche - Río Negro, Argentina; 3Administración

de Parques Nacionales, San Martín 1395 (9410) Ushuaia – Tierra del Fuego, Argentina;*Corresponding author; Fax +54 2901430644; E-mail cadicforestal@cadic.gov.ar

Introduction

The invasion and spread of intentionally introducedexotic species has a major impact on natural ecosystemsand is one of the most difficult problems in naturemanagement (Vázquez 2002). Invasion of vertebratesmay lead to a reduction in biodiversity and ecosystemfunctioning that both impoverishes and homogenizesthe world’s biota (Sala et al. 2000). Tierra del Fuego(Argentina) has not escaped the problems of numerousintroductions since the colonization of the region at theend of the 19th century. The most harmful species forthe biodiversity of the native forests in the region areMustela vison, Pseudalopex griseus and Castor cana-densis (the American beaver) which was released in1946 (Lizarralde 1993).

Beavers are ecosystem engineers that directly orindirectly control the availability of resources for otherorganisms by causing changes in the physical state ofthe ecosystem, primarily by mechanical means (Jones etal. 1994). They are, in a way, allogenic engineers be-cause they are not necessarily a permanent part of thesystem (Jones et al. 1997) and, as a result, patternscreated by beavers can persist after the animals have leftthe area (Wright et al. 2004). The modification of bothupland and riparian environments leads to floodingassociated with dam building activity, canal diggingand selective foraging of woods within the riparianzone (Terwilliger & Pastor 1999). Through the crea-tion of ponds, the structure and functioning of bothmeadows and stream communities structure can bechanged (Johnston & Naiman 1987). Such impactshave been well documented, notably on riparian trees(Barnes & Dibble 1986; Johnston & Naiman 1990a;Breck et al. 2003), biogeochemical characteristics ofstream water and soil (Naiman & Melillo 1984; Naimanet al. 1994; Johnston et al. 1995; Lizarralde et al. 1996),riparian plant communities (McMaster & McMaster2000; Wright et al. 2002) and vegetation succession

144 MARTÍNEZ PASTUR, G. ET AL.

(McMaster & McMaster 2001). Following abandon-ment, beaver ponds drain and often form persistentmoist patches with graminoids in the forested landscapewith variable hydrologic regimes and vegetation com-position (McMaster & McMaster 2000; Wright et al.2002). Novel habitat types can be created in the riparianzone when species capable of exploiting the resourcesprovided by these engineered patches are present (Wrightet al. 2002). This depends on the importance of localand landscape-level controlling mechanisms on plantspecies richness (Wright et al. 2003).

Sub-Antarctic ecosystems appear to be very vulner-able to invasion by introduced species, and beavers area great threat to the largely primary forests in one of theworld’s largest and most pristine remaining wildernessareas (Mittermeier et al. 2001). Beaver impacts on theforest ecosystem include, according to Morris et al.(1993) (1) alteration in microclimatic conditions; (2)introduction of further exotic species; (3) suppressionof native species; (4) changes in natural dynamics; (5)removal of nutrients and (6) changes in soil physics(Naiman & Melillo 1984; Francis et al. 1985; Pinay &Naiman 1991). There are few studies on the impact ofCastor canadensis (beavers) on primary Nothofagusforests in Tierra del Fuego (Sielfeld & Venegas 1980;Lizarralde 1993; Lizarralde et al. 1996, 2004; Coronatoet al. 2003). Consequently, little information is avail-able on the impact on forest components such as under-storey diversity, structure and natural dynamics in theflooded zones of abandoned ponds. Therefore, the ob-jective of the present study was to evaluate changes inunderstorey richness, cover and biomass during succes-sion in formerly beaver-influenced, but now abandoned,impoundments of riparian Nothofagus forests in Tierradel Fuego (Argentina).

Study system: Magellanic forests

Nothofagus is the main component of the SouthPatagonian forests, with a wide range of natural distri-bution from 36°50' S to 55°02' S. The southern forestsare predominantly deciduous, with N. pumilio and N.antarctica as dominants, but there are also someevergreen forests, consisting mainly of N. betuloides(Barrera et al. 2000). There are also forests with amixture of these main species, called mixed magellanicforest. In Tierra del Fuego, forests are used mainly fortimber, cattle grazing and tourism. The management ofthis natural ecosystem is primarily economic (MartínezPastur et al. 2000), while conservation is of secondaryimportance (Martínez Pastur et al. 2002a).

The genus Nothofagus contains heliophytic specieswhich are medium shade-tolerant (Donoso 1993) andwhich pass through gap regeneration cycles (Veblen1989; Holz & Veblen in press; Urlich et al. 2005).Where a canopy tree was felled, groups of Nothofagusindividuals regenerate, due to the increase in light.However, these forests have problems regeneratingunder systematic or permanent disturbances becauseof ungulate grazing and the impact of beavers (Raedake1980; Lizarralde 1993; Pulido et al. 2000; Kitzbergeret al. 2005). The understorey of the deciduous forest ispoor in species, and consists mainly of the generaOsmorhiza, Dysopsis and Cardamine (Moore 1983;Roig 1998). The understorey of the evergreen forestcontains more shrubs, e.g. Berberis ilicifolia, Maytenusdisticha and Ribes magellanicus, as well as herbaceousplants, e.g. Blechnum, Gunnera and Acaena.

Fig. 1. Location of thestudy area.

- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 145

Study area: Beaver meadows and primary forests

This study was conducted in the southern part ofGrande Island of Tierra del Fuego. The study site was a20 ha Nothofagus forest in Tierra del Fuego NationalPark (54°50'32'' S, 68°32'11'' W), situated 15 km west ofUshuaia city (Fig. 1). The area belongs to the southernanti-boreal zone as defined by Tuhkanen (1992), theclimate of which can be assigned to the domain of sub-polar climates in the southern hemisphere with short,cold summers and long, snowy and freezing winters.This area is subjected to maritime influence, with per-manently cloudy days with a high humidity, and theabsence of a freezing season. Summer temperaturesvary between 4 °C and 15 °C, while winter temperaturesrange from –3 °C to 6 °C. Predominant winds are fromthe SW with a mean speed of 6 - 7 km/h and maximumspeeds of over 100 km/h on unusually stormy days.Mean annual rainfall is 530 mm, homogeneously dis-tributed throughout the year, with abundant snowfall(160 mm) that covers the forest floor from May toSeptember. The forest floor freezes during winter (Mayto August) from 10-20 cm down to ca. 1 m deep.

The history of beaver activities in the study area arewell known, as the Administración de ParquesNacionales and Centro Austral de InvestigacionesCientíficas (Tierra del Fuego – Argentina) monitorthem. This area has numerous abandoned beaver ponds,that were naturally drained to become wet meadowsduring the last 20 years. Analysis of aerial photographs

(Instituto Geográfico Militar de Argentina, BuenosAires, AR, February 1970, 1:20 000) and satellite im-ages (SPOT, February 1995) were useful to identifyand compare beaver impoundments, and to evaluatetheir colony dynamics. In the riparian area, beaverswere responsible for the change of 2.53 ha of originalforest to meadows, which were used by several beavergenerations along a small, east to west 800-m longstream. Data from January-February 2001 were in-cluded and analysed into a GIS (Geographic Informa-tion System) program. Five meadows were selectedaccording to the time since abandonment: 1 year (2000- A1; 0.251 ha); 5 years (1995 - A5; 0.430 ha); 6 years(1994 -A6; 0.256 ha); 9 years (1991 - A9; 0.123 ha)and 20 years (1980 - A20; 0.260 ha). The width ofbeaver impact along the stream is between 15 m and 95m (Fig. 2) from riverside to the closest primary forestwithout flooding impact surrounding the affected area.Topography and soil properties were originally notdifferent along the study area.

Two representative Nothofagus primary forests sur-rounding the beaver impoundment (Fig. 2) were se-lected to evaluate the original forest structure beforethe introduction of beavers. These were considered ascontrol situations and were compared with influencedareas. One of these (LF), a 2-ha old growth N. pumilioforest, was located upstream in the eastern area. Theother stand (LGF), a 3 ha, mixed old growth N. pumilio- N. betuloides forest, was located downstream in thewestern area.

Fig. 2. Forest stands (LF and LGF) and beaver meadows (A1 to A20) selected along the Cruz stream in Tierra del Fuego NationalPark. Dotted line is the National Route 3 and grey lines indicate old dams and boundaries of the flooding areas.

146 MARTÍNEZ PASTUR, G. ET AL.

Methods

Forest structure characterization and understoreysampling

Forest structure in the control treatment was char-acterized using angle count sampling method (BAF 8)(Bitterlich 1984), by ten plots randomly located ineach forest type. DBH, basal area, number of trees andtotal bark volume estimated by Martínez Pastur et al.(2002b) were obtained. In addition, dominant height(mean of three close dominant trees) was measuredusing an Impulse 200 Laser Tech. Inc. clinometer andsite index was defined according to Martínez Pastur etal. (1997). Understorey variables in beaver meadowswere measured in eight plots randomly located in eachtreatment. Every plot was a combination of four 0.25-m2 subplots, 5 m from the central point. Subplots con-stituted the unit of statistical analysis. Central plotpoints were located in the field using a GPS (± 5 m),once defined by a polar co-ordinates system. Thisconsisted of randomly selected azimuth (0° to 359°)and distance (0 to the maximum ratio in m of eachtreatment) starting from the central point of each treat-ment. The central co-ordinates for the forest stands(LF and LGF) and beaver meadows (A1 to A20) weredetermined into the GIS (Fig. 2).

All vascular plants (Dicotyledonae, Monocotyl-edonae and Pteridophytae) in the plots were identifiedat species level, the lower plants were identified ac-cording to the main taxonomic groups (mosses andliverworts). Individual age of tree saplings and seed-lings was estimated by counting bud scales and density(plants/m2) by counting individuals within the plot;plant height was measured with a cm tape. Forest floorcover (understorey, woody debris and bare floor) wasregistered with the help of a grid of 100 points/m2 inevery subplot. All live above-ground plant materialwas collected and separated into groups (tree regenera-

tion, other dicots, monocots and lower plants) for bio-mass determination, using a drying oven at 70 °C untilconstant weight was achieved using a precision scale(± 0.01 g). Biomass was expressed in dry weight (g.m–2).During sampling, plant specimens were collected to maxi-mize the range collected. Each species was classifiedas sampled when it was found in the plots, or detectedwhen it was found in the sampled stand or meadow, butnot in the plots. A herbarium was made, and wasannexed to the ‘Herbario de Tierra del Fuego’ at theCentro Austral de Investigaciones Científicas (CADIC-CONICET) in Ushuaia, Argentina.

Data analysis

Treatment comparisons were carried out byANOVA (F-test) and means were separated using Tukeysignificant difference test (p < 0.05). When varianceheterogeneity (Cochran test) suggested lack of nor-mality, the non-parametric Kruskal-Wallis test wasused, through means separated by the confidenceinterval for the median test (p < 0.05). Complete link-age amalgamation rule and Euclidean distance meas-urement, based on presence or absence of the species,were used to group the treatments. Then, an overlap-ping analysis was carried out using species richnessinformation, between forests, recent installed mead-ows and abandoned ponds of more than five yearsold, to evaluate the influence of time. In addition,DCA (Detrended Correspondence Analysis) was car-ried out, using PC-ORD (McCune & Metford 1999),with richness and frequency data, with weighting ofrare species.

Finally, indexes for !-diversity (communityspecies richness), "-diversity (qualitative Jaccard in-dex) and #-diversity (based on total species richness)permitted characterization and comparison of treat-ments. The Jaccard index was calculated as

IJ = c / (a + b – c) (1)

where a is the species number of treatment A, b is thespecies number of treatment B, and c is the number ofcommon species between A and B.

The "-index follows from

" = $ qj (Sr – Sj) (2)

Here qj is the proportional area of the community j;Sr is the total species number over all communitiesand Sj is the species quantity in community j.

The #-index was obtained by addition of mean !and ".

Table 1. Characterization of the structure of the forest standsby one-way ANOVA. LF = Nothofagus pumilio forest; LGF =mixed forest of N. pumilio and N. betuloides; N = tree density;BA = basal area; TOBV = total over bark volume. Site indexaccording to Martínez Pastur et al. (1997), with I-II corre-sponding to the best site quality index and IV-V to the worstsite quality index. F = Fisher test, P = probability.

Variable LF LGF F-value P

Site index I-II IV-V 111.15 0.000DBH (cm) 41.3 21.4 15.25 0.003N (trees/ha) 457 1281 8.38 0.016BA (m2.ha–1) 61.3 48.0 2.50 0.144TOBV (m2.ha–1) 903.1 373.9 24.71 0.001

- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 147

Fig. 3. A. Cluster analysis of forest stands and beaver mead-ows, based on a species presence-absence matrix. B. Speciesnumbers overlapping among forest stands and beaver mead-ows. LGF = mixed forest of Nothofagus pumilio and N.betuloides; LF = N. pumilio forest; A1 to A20 = beavermeadows abandoned one to 20 years ago; FA = forest stands.

Species richness was highest in A5 (36 species) andlowest in A1 (eight species); and between beaver andnon beaver situations the influenced sites were richer(55 species in A5 to A20). "-diversity in the foreststands was 0.42, against 0.47 - 0.62 in the older mead-ows (A5 to A20). A1 was significantly different fromthe others, with values from 0.09 to 0.23. Recent andolder meadows were more similar amongst them-selves (0.40) than forests compared to impoundmentsabandoned 5 to 20 years ago (0.14), or to A1 (0.12).Species richness of the landscape (#-diversity) was68.3, based on ! = 28.7 and " = 39.6. The #-indexcalculated on the basis of environmental groups was61.5 (34.3 + 27.2). These indices highlight the resultsshown in Fig. 3 and 4, where numerous species werenon specialists, being found in all study environ-ments, but many others needed particular conditionsto live.

Results

Primary forest structure in control treatments

Forest structure presented significant differences(Table 1), except for the occupation degree (basal area)where both possessed a complete density. Nothofaguspumilio forest, LF, showed a regular age structure witha bell shaped distribution in only one tree layer. On theother hand, the mixed forest (LGF) was uneven-aged,reverse j-size distribution, with more than one tree layerand numerous individuals with small diameters. Moreo-ver, LF had better site quality, fewer individuals, highermean DBH and higher volume per ha.

Beaver effects on vegetation diversity

In the whole study area, 68 plant species were iden-tified: 43 dicots, 22 monocots and three ferns, of whichseven dicots and one monocot are introduced species(App. 1, Tables 1 and 2). Primary forests had manydicots (28 species), few monocots (nine species) andferns (three species). Of these, 7.5% are introduced, e.g.Poa pratensis in LF, and Taraxacum officinale andCerastium fontanum, found in LGF.

After one year following beaver abandonment (inA1), meadow formation was initiated: eight native spe-cies were observed (three monocots and five dicots),and three of these were not identified in the primaryforest stands: Carex curta, Juncus scheuchzerioides andRanunculus biternatus. The 20-year abandoned pondA20 had 33 dicots, 19 monocots and three ferns, manyof them not found in the primary forests, including 11native monocots and 14 dicots. In A20, 13 native speciesof the original forests did not return to these sectors.These were the grass Bromus araucana, forest orchidsCodonorchis lessonii and Gavilea lutea, the tree Drimyswinteri, shrubs Maytenus disticha and Berberis ilicifoliaand herbaceous species Adenocaulon chilense, Dysopsisglechomoides, Galium aparine, Macrachaenium gracile,Senecio acanthifolius, Viola magellanica and V. reichie.On the other hand, mosses and liverworts with manyspecies in the primary forests, were observed one yearafter abandonment, and they developed faster under theshelter of the other species.

Forests without impact were very similar (Fig. 3A),but were different from the understorey of the beavermeadows, which was related to the time of abandon-ment; 13 species disappeared after the introduction ofbeavers, and only grew in closed canopy forests (Fig.3B). Further, only five species were found in all studysites. This was also observed in the ordination (Fig. 4),where many species were only found in primary forests(LF and LGF) or related environments (A5 to A20).

148 MARTÍNEZ PASTUR, G. ET AL.

Fig. 4. Ordination of environmental parameters and species by DCA. FA = forest stands; A1 to A20 = beaver meadows abandonedone to 20 years ago. Species codes appear in Tables 1 and 2 of App. 1.

Beaver effects on vegetation cover and biomass

Beaver activity and pond building produces a di-rect impact on under and overstorey, with completeremoval of the forest. Ground cover in beaver mead-ows showed significant differences, related to thetime since abandonment (Table 2). The forest floorcover was 28% - 46% of litter in primary forests, andincreased to 92% one year after drainage (A1). As timewent by, many plants occupied beaver meadows, reach-ing the bare floor minimum value (13%) in 20 yearabandoned ponds. Total vegetation cover showed aninverse trend as the litter floor (46-62% in forests and83% in 9- to 20-year old beaver meadows). However,vegetation composition varied depending on the speciesrichness and their re-colonization dynamics. Ferns inthe primary forests were important in the understoreycover with 16% - 34%, followed by the dicots (10% -18%). Monocots (1% - 3%) and mosses + liverworts(1% - 8%) were less important. In the abandonedponds, monocots acquired a major importance startingwith a substantially higher cover (7%) than in primaryforests, which increased over the years to 53%-64% inthe oldest meadows. On the other hand, dicots in-creased slowly reaching only 27%. Ferns had very lowcover in the sectors under impact (< 0.5%), whilemosses and liverworts reached variable values, butsimilar to those found in the primary forest (2% -15%).

Vegetation biomass changed along the studied en-vironments; in the beaver meadows this was relatedto the time since abandonment (Table 3). Primary

forests produced more than 190 g.m–2 of above-groundbiomass. Beaver meadows had a sparse biomass (22g.m–2) one year after pond abandonment, but the old-est meadows duplicated the values of primary forest(386 - 406 g.m–2). Lower plants and ferns representedthe major percentage of understorey biomass in forestswithout flooding impact (46% in LF and 20% in LFG),followed by dicots (13%) and monocots (3%). In theone-year-old abandoned pond, lower plants (69%) andmonocots (27%) were the most important components.As time passed, grass¡ increased in percentage of bio-mass (from 25% to 69%). This left the lower plants insecond place in A1, but these were replaced by dicots(33% in A20) in the following years. Finally, fernsdiminished their relative importance in beaver mead-ows compared to primary forests, to less than 1% oftotal biomass.

Forest regeneration dynamics

Primary forests presented a seedling bank withinthe understorey (10.5 trees/m2, SE = 7.8), with a spe-cific composition related to the forest type (100% ofNothofagus pumilio in LF; 72% of N. pumilio and 28%of N. betuloides in LFG). The mean age of the seedlingbank was 4.8 years with a maximum of 40 years (treesunder 1.3 m total height), and the mean height was 15.5cm with a maximum of 92.0 cm. Regeneration indi-viduals made up 1% - 12% of the forest understoreycover, and contributed 18% to the total biomass (4.8 -67.6 g.m–2) (Tables 2 and 3).

Through the impoundments following the building

- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 149

of dams by beavers, all the Nothofagus individuals inthe upper and lower stratum (trees, saplings and seed-ling bank) were killed. Re-colonization started afterone to three years, and occurred mainly near the dam ofthe abandoned ponds. In the one-year-old abandonedpond no seedlings were observed, but in five-year-oldbeaver meadows 1.33 seedlings/m2 (SE = 1.86) oc-curred with an age of 1.5 years and height of 2.6 cm;13% of the seedlings were N. pumilio and 87% N.betuloides. In A6, 0.18 plants/m2 (SE = 0.40) werefound (means of 3.0 years and 9.0 cm height); thesewere all N. antarctica (three years old and 10 cmmaximum height). In A9, there were 0.33 seedlings/m2

(SE = 0.81), 6.5 years old and 55.0 cm mean height, allof them N. antarctica (8 years old and 80 cm heightmaximum). Finally, no regeneration was detected inthe pond that was abandoned 20 years ago. The rege-neration represented 0.1% and 2.0% of the meadowvegetation cover, contributing less than 4% to theirtotal biomass (less than 11.5 g.m–2) (Tables 2 & 3).

Discussion

Old-growth Nothofagus pumilio and N. betuloidesforests comprise of mature trees, with a high crownclosure, high basal area, low density of large trees andhigh stand volume (Martínez Pastur et al. 2002a), andpossess either an irregular or a regular structure. Theirregular structure is found in many overlapping even-aged patches, which originate from gaps (Veblen 1989).Large-scale disturbances such as fires, wind-throw,avalanches or landslides, lead to even-aged forests(Donoso 1993; Rebertus et al. 1997), which are morehomogeneous but conserve many characteristics of theuneven-aged forests (high proportion of mature indi-viduals, stocking levels and high crown closure). Ac-cording to their structure, old-growth forests selected

as reference sites correspond to the two typical foresttypes of the southern zone of Tierra del Fuego,although the N. pumilio forests we studied havebetter site quality than most on the Island (MartínezPastur et al. 2000; Collado 2001). These structurescould be analogous to the previous forest beforebeavers arrived, with a comparable composition ofthe understorey.

Beavers manipulate their native ecosystems by re-moving trees to create dams (Crooks 2002). They alsoinvade forest streams, flooding lands by pond forma-tion and felling trees for food and building material.Above the dams, soils are saturated, stream velocity isreduced and sediments accumulate. Below the dams,discharge rates decline and sediment loads are reduced(Naiman et al. 1988). Additionally, beavers create apatch structure around the pond where they harvesttrees and shrubs, the magnitude of which is determinedby its geomorphology (Johnston & Naiman 1987;1990b). They often cut all stems growing near theirwinter food caches, dams and lodges, but become moreselective as foraging distances increase (Baker et al.2005). The amount of biomass harvested by beaversexceeds by far the total actually ingested, and is muchgreater than any other herbivore harvests (Johnston &Naiman 1990a). Simultaneously, beaver activities al-ter ecosystem processes, due to changes in the nutrientcycles and dynamics of the decomposition (Naiman &Melillo 1984; Francis et al. 1985; Naiman et al. 1988;Johnston et al. 1995; Johnston 2001). All riparianbeaver effects are the result of their ‘engineering’activities, with the possible exception of changes in thecomposition of riparian trees, partly due to their herbi-vory (Wright et al. 2004).

In Tierra del Fuego, beaver biological invasionalters the physical structure of the ecosystems, thecommunity level responses to engineer induced altera-tions of habitat complexity and the ecosystem level

Table 2. Characterization of the ground cover of the forest stands and beaver meadows. LF = Nothofagus pumilio forest; LGF =mixed forest of N. pumilio and N. betuloides; A1 to A20 = beaver meadows abandoned one to 20 years. Means followed by differentletters are significantly different at P < 0.05.

Treatment Litter Debris Total Seedlings Other dicots Monocots Ferns Other lowerfloor(%) (%) understorey(%) and saplings (%) (%) (%) (%) plants (%)

Kruskal 26.23 18.75 28.51 26.86 23.08 33.82 33.26 20.80Wallis test (0.000) (0.005) (0.000) (0.000) (0.001) (0.000) (0.000) (0.002)

LF 27.9ab 9.70ab 62.4b 12.5b 10.3b 1.4a 34.1b 4.2aLGF 45.9b 8.1ab 46.0b 1.1a 18.5bc 2.7a 15.7b 8.0abA1 91.9c 0.3a 7.80a 0.0a 0.5a 7.1a 0.0a 0.3aA5 19.4ab 22.4b 58.2b 0.1a 7.9ab 35.0b 0.0a 15.3bA6 25.3ab 2.2a 72.6bc 2.0a 8.2ab 60.1b 0.0a 2.2aA9 15.3a 1.7a 83.0c 0.2a 16.0b 63.7b 0.0a 3.1aA20 13.0a 3.7a 83.3c 0.0a 27.0c 53.5b 0.5a 2.3a

150 MARTÍNEZ PASTUR, G. ET AL.

Table 3. Biomass understorey of the forest stands and beaver meadows. LF = Nothofagus pumilio forest; LGF = mixed forest of N.pumilio and N. betuloides; A1 to A20 = beaver meadows abandoned one to 20 years. Means followed by different letters aresignificantly different at P < 0.05.

Treatment Total Seedlings and Other dicots Monocots Ferns Other lower(g.m–2) saplings (g.m–2) (g.m–2) (g.m–2) (g.m–2) plants (g m–2)

Kruskal 28.51 24.83 27.53 33.73 33.46 20.10Wallis test (0.000) (0.000) (0.000) (0.000) (0.000) (0.003)

LF 196.5b 67.6b 10.1b 1.3a 58.8c 58.8abLGF 193.4b 4.8a 40.0bc 8.4a 20.3b 119.9cA1 22.4a 0.0a 0.7a 6.2a 0.0a 15.5aA5 354.7bc 0.1a 25.1bc 100.0b 0.0a 229.6cA6 318.2bc 11.5a 44.7bc 218.9c 0.0a 43.0abA9 405.6bc 8.9a 82.7c 232.7c 0.0a 81.3abA20 385.6c 0.0a 126.0d 169.9bc 2.2a 87.5ab

(Mc Master & Mc Master 2001; Wright et al. 2004). Intorrential streams of Tierra del Fuego, the occupationdepends on water availability, which becomes sea-sonal in small watercourses. Once abandoned, damsdisintegrate causing ponds to drain and expose accu-mulated sediments (Wright et al. 2003). Eventually(e.g. when impoundments flood roads), man elimi-nates beaver colonies, even though these places shouldbe settled again in the future (as A1). The re-coloniza-tion depends on the availability of suitable trees as pro-viders of food and building material (Basey et al. 1988),preferably less than 15 cm DBH, and of the geo-morphologic conditions of the environment (Johnston &Naiman 1987; Mella 1995; Coronato et al. 2003).

Vegetative succession following abandonment andpond drainage may follow multiple pathways influ-enced by factors such as surrounding vegetation, hy-drology, geomorphology, herbivory and re-coloniza-tion by beavers (Naiman et al. 1988; Terwilliger &Pastor 1999). Often, grass- and sedge-dominated bea-ver meadows persist in the landscape long after aban-donment, because succession in flooded drained pondsappears to follow an altogether different pathway thanthose of other disturbance caused openings (Terwilliger& Pastor 1999). Nutrient and carbon accumulationprocesses during stream impoundment produce long-term effects on soil structure and functioning, even along time after the pond has been drained, resulting inproductive wetland and meadow patches in an other-wise nutrient impoverished forest (Pinay & Naiman1991; Johnston et al. 1995).

Changes in nutrient cycles and decomposition dy-namics (Naiman & Melillo 1984; Francis et al. 1985;Naiman et al. 1988; Johnston et al. 1995; Johnston2001) modify the biomass and composition of the newcommunities (Barnes & Dibble 1986; Johnston &Naiman 1990a). Plant species richness typically in-creases after a patch is created, but later declines due to

formed by a mosaic of different environments, whereclosed stands of different species are mixed with others(wetlands, riparian forests, rangelands or peat-lands).These forests support a relatively low number of plantunderstorey species (Moore 1983; Roig 1998), andalmost all of them are generalist, with many introducedplants in anthropized environments. However, eachforest type possesses particular biotic and abiotic com-ponents that generate different ecological conditions,which allow the installation and survival of distinctivespecies. For example, stream shores and wetlands in-side the forests have a higher species richness andbiomass than the other forest environments (Lencinas2005). Riparian ecosystems with an original floristiccomposition are scarce in Nothofagus woodlands, dueto the current levels of beaver populations (Lizarralde1993). Beaver activities change the narrow streamcorridor among the forest matrix into a pond by damcreation, which has different properties than the re-placed forest ecosystem, as was cited by Johnston &Naiman (1987) for Populus tremuloides woods. Bea-ver engineering is responsible for the loss of manyspecies (secondary trees, shrubs, orchids and manyherbaceous plants) along the shores of rivers andstreams, which do not establish in the meadows. Streambank impact could be different depending of geomor-phology, because wetlands and floodplains are alreadyunder saturated conditions before impoundments, andecosystem level processes are principally anaerobic(Johnston & Naiman 1987).

Beavers mainly invade initially all downstream ar-eas and eventually upstream ones; each colony canbuild more than one pond (Johnston 1994). Usually,colonies occupy areas for five to 15 years (Lizarralde1993; Wright et al. 2003), and then migrate to anothersite when food supply dwindles, ponds have becomefilled with sediments, or when the animals have beenremoved by trapping, natural predation or disease

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the dominance of competitively superior species(Wright et al. 2003). However, beaver meadows in thestudy area presented higher species richness in olderponds than in the youngest. This increase is thought tobe the result of the accumulation over time of poorlydispersing, late successional species that temporarilyco-exist with better dispersing early successionalspecies (Wright et al. 2003). Nevertheless, there willbe a positive relationship between habitat complexityand biotic diversity and abundance (Crooks 2002),which could be related to spatial heterogeneity, whichincreases over time, since unconsolidated sedimentserode to a micro-landscape of pits and hummocks,particularly in older meadows (McMaster & McMaster2001; Wright et al. 2002, 2003). On the other hand, soilmoisture also has a large effect on herbaceous plantspecies richness and composition (Wright et al. 2003).Old, drier meadows should have greater species rich-ness than young, wetter meadows, as was observed inthe study area.

Large numbers of plant species settled in aban-doned ponds, many of them that do not grow in theprimary forests, because species able to live in theinvader modified habitat take advantage of the ecosys-tem engineering, while those living in the unmodifiedhabitat are inhibited (Crooks 2002). Lizarralde et al.(2004) indicate that beaver altered sites in Tierra delFuego have higher levels of organic and inorganicnitrogen. Some plants are characteristic of associatedenvironments, such as rangelands, peat bogs or N.antarctica forests, and develop vigorously, while block-ing the establishment of regenerating species, whichshould restore the ecological characteristics of theflooded areas. In addition, changes in the environmentopens possibilities for introduced plant species, whilethese early invaders can modify the habitat and facili-tate further invasions (Crooks 2002). There are fewintroduced species in the primary forests, but thesespecies establish abundantly after any impact, e.g.during harvesting (Martínez Pastur et al. 2002a). Someof these invaders possibly arrive through roads orcattle, e.g. Poa pratensis and Taraxacum officinale, orby dispersion along riversides, e.g. Cerastium fontanum.Nevertheless, not only is the understorey richness modi-fied, significant changes also appear in cover, bio-mass, frequency and abundance in the newly settledcommunities, compared to the original old-growthforests.

At a scale that encompasses both patches affectedby ecosystem modification and patches of unmodifiedhabitat, there is an increase in regional species richness(Jones et al. 1997). In the Huntington Wildlife Forest,Wright et al. (2002) estimated that beaver-modifiedpatches may contribute as much as 25% of the total

herbaceous plant species richness of the riparian zone.It is expected that Nothofagus will colonize aban-

doned ponds after a short time, because these speciesare adapted to catastrophic disturbances (Donoso 1993;Rebertus et al. 1997). In this way the forest can berestored in a wide range of other natural conditions(Martínez Pastur et al. 2002a; Cuevas 2000). However,the flooding associated with the dams drowns all theseedlings and trees in a few years (McMaster &McMaster 2000). In addition, deposited sediments com-pletely cover the original forest floor and impede seedgermination, making new input of seeds to the aban-doned ponds necessary. For this reason, forest speciesdo not completely regenerate in meadows for a longtime (more than 20 years) related to the beaver occupa-tion period. In the absence of reproduction from relictseeds or vegetative propagules such as stumps androots, invasion of woody plants requires the presenceof viable seeds in the disturbed area and the successfulgermination, growth and competition of the resultingseedlings (Terwilliger & Pastor 1999). New seeds mustreach the abandoned ponds, and the close proximity tomature, seed productive stands is essential. In thestudy meadows, only A5 has N. pumilio and N.betuloides seedlings and saplings, which are locatedclose to the LF stand (Fig. 2). However, N. antarcticaoccurs in older ponds (> 6 years), these are not presentin the studied area as tree dominant species, but growaround peat bogs 1 km upstream. This species is con-sidered as a pioneer of non-forested environments(rangelands and borders of peat bogs), creating thenecessary conditions for the establishment of otherNothofagus species. On the other hand, other factorscould influence the establishment of seedlings, such asthe annual variation in mass seed production of Notho-fagus (Schmidt & Urzúa 1982), the lack of ecto-mycorrhizal symbionts (Terwilliger & Pastor 1999)and the fast development of vegetation, which in-creased in cover from 46% to 83% in our study. In theHutcheson Memorial Forest, Myster (1993) found thatconifer invasion into grass-dominated meadows isslowed down by the physical effects of grasses, wherethe few seeds which make contact with the soil areshaded out by rapidly growing grasses. In addition,seedlings must survive resource limitations, anoxicsoils, frost heaving, physical damage and herbivores inthe formed meadows (Terwilliger & Pastor 1999;McMaster & McMaster 2001).

The population size of American beavers in Tierradel Fuego is estimated at 35 000 - 50 000, which haveapparently colonized all streams in the Andean andextra-Andean areas of the island (Lizarralde et al.2004). This population could potentially exert a strongimpact on the species richness at the landscape level.

152 MARTÍNEZ PASTUR, G. ET AL.

Beavers can create landscapes dominated by aban-doned patches, and can produce large numbers ofcolonizers by rapidly depleting the resource levels andleaving abandoned patches that recover very slowly(Wright et al. 2004). The flooded areas represent ca.3.0% of the total woodlands (Collado 2001); they aremore extensive in some forests of the Chilean sector ofthe Island (3.8% Vicuña and 28.8% in Río Bueno)(Mella 1995). In the forests where the beaver is anative species, the foraging alters larger areas (e.g. 12-15% in Voyageours National Park, United States)(Naiman et al. 1988).

Regeneration strategies of Nothofagus are the mainreasons for a lack of adaptation of these forests tolong-term beaver impact. Beaver activity modifiesthe original Nothofagus ecosystem from a closedforest to a grass and sedge dominated meadow. Thisagrees with Naiman et al. (1988), who suggest that thelong time impacted sectors for beavers reaches adifferent stable condition from the original one inCanada. These areas could have returned to theiroriginal structure, but the beavers modified the hy-drology and vegetation patterns. Longer periods oftime than for other impacts, e.g. a harvested or wind-blown forest in south Patagonia (Schmidt & Urzúa1982; Rebertus et al. 1997), are necessary for theforest to recover. In North America, meadows canpersist for over 70 years once formed and rarely, ifever, revert back to the original forested riparian zone(Wright et al. 2003). For these reasons, the mainte-nance of the present levels of beaver population inTierra del Fuego is not sustainable in time, due to theutilization and impact on tree biomass which couldnot be replaced by the natural dynamic forest ecosys-tem (Johnston & Naiman 1990a).

Acknowledgements. The authors gratefully acknowledge fieldassistance by Patricia Ríos, and helpful comments provided byEsteban Carabelli, Robert Naiman, James Hellfield andChristopher Anderson.

References

Baker, B.W., Ducharme, H.C., Mitchell, D.C.S., Stanley, T.R.& Peinetti, H.R. 2005. Interaction of beaver and elk herbi-vory reduces standing crop of willow. Ecol. Appl. 15: 110-118.

Barnes, W.J. & Dibble, E. 1986. The effects of beaver inriverbank forest succession. Can. J. Bot. 66: 40-44.

Barrera, M., Frangi, J., Richter, L., Perdomo, M. & Pinedo, L.2000. Structural and functional changes in Nothofaguspumilio forests along an altitudinal gradient in Tierra delFuego, Argentina. J. Veg. Sci. 11: 179-188.

Basey, J., Jenkins, S. & Busher, P. 1988. Optimal central-place foraging by beavers: tree-size selection in relation todefensive chemicals of quaking aspen. Oecologia 76: 278-282.

Bitterlich, W. 1984. The relascope idea. Relative measure-ments in forestry. Commonwealth Agricultural Bureaux.London, UK.

Breck, S.W., Wilson, K.R. & Andersen, D.C. 2003. Beaverherbivory and its effect on cottonwood trees: influence offlooding along matched regulated and unregulated rivers.River Res. Appl. 19: 43-58.

Collado, L. 2001. Los bosques de Tierra del Fuego: Análisisde su estratificación mediante imágenes satelitales para elinventario forestal de la provincia. Multequina 10: 1-15.

Coronato, A., Escobar, J., Mallea, C., Roig, C. & Lizarralde,M. 2003. Características geomorfológicas de ríos demontaña colonizados por Castor canadensis en Tierra delFuego, Argentina. Ecol. Austral 13: 15-26.

Correa, M.N. 1969-1998. Flora Patagónica. Instituto Nacionalde Tecnología Agropecuaria. Vol. 8. Part II, III, IVb, V,VI & VII. Buenos Aires, AR.

Crooks, J.A. 2002. Characterizing ecosystem-level conse-quences of biological invasions: the role of ecosystemengineers. Oikos 97: 153-166.

Cuevas, J.G. 2000. Tree recruitment at the Nothofagus pumilioalpine timberline in Tierra del Fuego, Chile. J. Ecol. 88:840-855.

Cuevas, J.G. & Arroyo, M.T.K. 1999. Ausencia de banco desemillas persistente en Nothofagus pumilio (Fagaceae) enTierra del Fuego, Chile. Rev. Chil. Hist. Nat. 72: 73-82.

Donoso, C. 1993. Los bosques templados de Chile y Argen-tina. Variación, estructura y dinámica. Editorial Univer-sitaria, Santiago de Chile, CL.

Francis, M., Naiman, R. & Melillo, J. 1985. Nitrogen fixationin subarctic streams influenced by beaver (Castorcanadensis). Hydrobiologia 121: 193-202.

Hall, J.G. 1960. Willow and aspen in the ecology of beaver onSagehen Creek, California. Ecology 41: 484-494.

Holz, C.A. & Veblen, T.T. In press. Tree regeneration re-sponses to Chusquea montana bamboo die-off inNothofagus sub-alpine forests in the southern Andes. J.Veg. Sci.

Johnston, C. 2001. Wetland soil and landscape alteration bybeavers. In: Richardson, J.L. & Vepraskas, M.J. (eds.)Wetland soils: genesis, hydrology, landscapes and classi-fication, pp. 391-408. Lewis, Washington, DC, US.

Johnston, C. & Naiman, R. 1987. Boundary dynamics at the

- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 153

aquatic-terrestrial interface: The Influence of beaver andgeomorphology. Landscape Ecol. 1: 47-57.

Johnston, C. & Naiman, R. 1990a. Browse selection by bea-ver: effects on riparian forest composition. Can. J. For.Res. 20: 1036-1043.

Johnston, C. & Naiman, R. 1990b. Aquatic patch creation inrelation to beaver population trends. Ecology 71: 1617-1621.

Johnston, C. 1994. Ecological engineering of wetlands bybeavers. In: Mitsch, W.J. (ed.) Global wetlands: Old Worldand New, pp. 379-384. Elsevier, Amsterdam, NL.

Johnston, C., Pinay, G., Arens, C. & Naiman, R. 1995. Influ-ence of soil properties on the biogeochemistry of a beavermeadow hydro sequence. Soil Sci. Soc. Am. J. 59: 1789-1799.

Jones, C.G., Lawton, J.H. & Shachak, M. 1994. Organisms asecosystem engineers. Oikos 69: 373-386.

Jones, C.G., Lawton, J.H. & Shachak, M. 1997. Positive andnegative effects of organisms as physical ecosystem engi-neers. Ecology 78: 1946-1957.

Kitzberger, T., Raffaele, E., Heinemann, K. & Mazzarino,M.J. 2005. Effects of fire severity in a north Patagoniansubalpine forest. J. Veg. Sci. 16: 5-12.

Lencinas, M.V. 2005. Biodiversidad en el bosque productivode Nothofagus pumilio y sus ambientes asociados enTierra del Fuego. Doctoral Thesis. Universidad Nacionalde Sur, Bahía Blanca, AR.

Lizarralde, M. 1993. Current status of the introduced beaver(Castor canadensis) population of Tierra del Fuego,Argentina. AMBIO 22: 351-358.

Lizarralde, M., Deferrari, G., Alvarez, S. & Escobar, J. 1996.Effects of beaver (Castor canadensis) on the nutrientdynamics of the Southern Beech forest of Tierra del Fuego.Ecología Austral. 6: 101-105.

Lizarralde, M., Escobar, J. & Deferrari, G. 2004. Invaderspecies in Argentina: A review about the beavers (Castorcanadensis) population situation on Tierra del Fuego eco-system. Interciencia 29: 352-356.

Martínez Pastur, G., Peri, P., Vukasovic, R., Vaccaro, S. &Piriz Carrillo, V. 1997. Site index equations for Nothofaguspumilio Patagonian forest. Phyton 61: 55-60.

Martínez Pastur, G., Cellini, J., Peri, P.L., Vukasovic, R. &Fernández, C. 2000. Timber production of Nothofaguspumilio forests by a shelterwood system in Tierra delFuego (Argentina). For. Ecol. Manage. 134: 153-162.

Martínez Pastur, G., Peri, P., Fernández, M.C., Staffieri, G. &Lencinas, M.V. 2002a. Changes in understory speciesdiversity during the Nothofagus pumilio forest manage-ment cycle. J. For. Res. 7: 165-174.

Martínez Pastur, G., Lencinas, M.V., Cellini, J.M., Díaz, B.,Peri, P. & Vukasovic, R. 2002b. Herramientas disponiblespara la construcción de un modelo de producción para lalenga (Nothofagus pumilio) bajo manejo en un gradientede calidades de sitio. Bosque 23: 69-80.

McMaster, R.T. & McMaster, N.D. 2000. Vascular flora ofbeaver wetlands in western Massachusetts. Rhodora 102:175-197.

McMaster, R.T. & McMaster, N.D. 2001. Composition, struc-ture, and dynamics of vegetation in fifteen beaver-im-

pacted wetlands in western Massachusetts. Rhodora 103:293-320.

Mella, J. 1995. Castores. Informe del Subproyecto 94-14.Estudio de Línea Base – Proyecto Río Cóndor, PuntaArenas, AR.

Mittermeier, R., Mittermeier, C., Robles-Gil, P., Pilgrim, J.,Fonseca, G., Brooks, J. & Konstant, J. 2001. Wilderness:Earth’s last wild places. Conservation International, Wash-ington DC, US.

Moore, D. 1983. Flora of Tierra del Fuego. Anthony Nelson-Missouri Botanical Garden. London, UK.

Morris, L., Bush, P. & Clark, J. 1993. Ecological impacts andrisks associated with forest management. In Cairns, J.,Niederlehner, B. & Orvos, D. (eds.) Predicting ecosystemrisk. Advances in modern environmental toxicology, Vol.XX, Chapter 10, pp. 153-213. Princeton Scientific, Pub-lishing, NJ, US.

Myster, R.W. 1993. Tree invasion and establishment in oldfields at Hutcheson Memorial Forest. Bot. Rev. 59: 251-272.

Naiman, R. & Melillo, J. 1984. Nitrogen budget of a subarcticstream altered by beaver (Castor canadensis). Oecologia62: 150-155.

Naiman, R., Johnston, C. & Kelley, C. 1988. Alteration ofNorth American streams by beaver. BioScience 38: 753-762.

Naiman, R.J., Pinay, G., Johnson, C.A. & Pastor, J. 1994.Beaver-induced influences on the long-term biogeo-chemical characteristics of boreal forest drainage net-works. Ecology 75: 905-921.

Pinay, G. & Naiman, R. 1991. Short-term hydrologic varia-tions and nitrogen dynamics in beaver created meadows.Arch. Hydrobiol. 123: 187-205.

Pulido, F.J., Díaz, B. & Martínez Pastur, G. 2000. Incidenciadel ramoneo del guanaco (Lama guanicoe) sobre laregeneración temprana en bosques de lenga (Nothofaguspumilio) de Tierra del Fuego, Argentina. InvestigaciónAgraria, Sistemas y Recursos Forestales 9: 381-394.

Raedake, K. 1980. Food habitats of the guanaco (Lamaguanicoe) of Tierra del Fuego, Chile. Turrialba 30: 177-181.

Rebertus, A.J., Kitzberger, T., Veblen, T.T. & Roovers, L.M.1997. Blowdown history and landscape patterns in theAndes of Tierra del Fuego, Argentina. Ecology 78: 678-692.

Roig, F. 1998. La Vegetación de la Patagonia. In: Correa, M.(ed.) Flora Patagónica, Tomo VIII, Vol. I, pp. 48-174.Instituto Nacional de Tecnología Agropecuaria. ColecciónCientífica, Buenos Aires, AR.

Sala, O.E., Chapin, F.S., Armesto, J.J., Berlow, E., Bloomfield,J., Dirzo, R., Huber-Sanwald, E., Huenneke, L.F., Jackson,R.B., Kinzig, A., Leemans, R., Lodge, D.M., Mooney,H.A., Oesterheld, M., Poff, N.L., Sykes, M.T., Walker,B.H., Walker, M. & Wall, D.H. 2000. Global biodiversityscenarios for the year 2100. Science 287: 1770-1774.

Schmidt, H. & Urzúa, A. 1982. Transformación y manejo delos bosques de lenga en Magallanes. Ciencias Agrícolas11: 1-62.

Sielfeld, W. & Venegas, C. 1980. Poblamiento e impacto

154 MARTÍNEZ PASTUR, G. ET AL.

ambiental de Castor canadensis Kuhl, en Isla Navarino,Chile. Ans. Ins. Pat. 11: 247-257.

Terwilliger, J. & Pastor, J. 1999. Small mammals, ecto-mycorrhizae, and conifer succession in beaver meadows.Oikos 85: 83-94.

Tuhkanen, S. 1992. The climate of Tierra del Fuego from avegetation geographical point of view and its ecoclimaticcounterparts elsewhere. Acta Bot. Fen. 145: 1-65.

Urlich, S.C., Stewart, G.H., Duncan, R.P. & Almond, P.C.2005. Tree regeneration in a New Zealand rain forestinfluenced by disturbance and drainage interactions. J.Veg. Sci. 16: 423-432.

Vázquez, D.P. 2002. Multiple effects of introduced mamma-

lian herbivores in a temperate forest. Biol. Invas. 4: 175-191.

Veblen, T. 1989. Tree regeneration responses to gaps along atransandean gradient. Ecology 70: 541-543.

Wright, J.P., Jones, C.G. & Flecker, A.S. 2002. An ecosystemengineer, the beaver, increases species richness at thelandscape scale. Oecologia 132: 96-101.

Wright, J.P., Flecker, A.S. & Jones, C.G. 2003. Local vs.landscape controls on plant species richness in beavermeadows. Ecology 84: 3162-3173.

Wright, J.P., Gurney, W.S. & Jones, C.G. 2004. Patch dynam-ics in a landscape modified by ecosystem engineers. Oikos105: 336-348.

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