Forest Ecology and Management 258 (2009) 366–375

Stand-replacing wildfires?The incidence of multi-cohort and single-cohort Eucalyptus regnans and E. obliquaforests in southern Tasmania

Perpetua A.M. Turner a,b,*, Jayne Balmer c, J.B. Kirkpatrick d

a School of Plant Science, Private Bag 55, University of Tasmania, Hobart 7001, Tasmania, Australiab Bushfire CRC, Level 5, 340 Albert Street, East Melbourne, Victoria 3002, Australiac Biodiversity Conservation Branch, Department of Primary Industries and Water, GPO Box 44, Hobart 7001, Tasmania, Australiad School of Geography and Environmental Studies, Private Bag 78, University of Tasmania, Hobart 7001, Tasmania, Australia

A R T I C L E I N F O

Article history:

Received 8 May 2008

Received in revised form 20 April 2009

Accepted 27 April 2009

Keywords:

Wildfire

Multi-cohort forest

Stand replacing

Tasmania

A B S T R A C T

The natural age structure of wet eucalypt forest has important implications for biodiversity conservation

and the mode of wood production. Southern Tasmanian wet eucalypt forests were sampled to describe

age class variation and to test the following hypotheses that relate to it: (1) Eucalyptus regnans stands are

more likely to be single-cohort than Eucalyptus obliqua and mixed stands; (2) old-growth trees are

associated with multi-cohort wet eucalypt forests; (3) the E. regnans stands are more multi-cohort than

Victorian E. regnans stands. Data from 762 stands, all with either E. obliqua or E. regnans were analysed to

determine how stand characteristics related to forest type and to the presence of old-growth trees. Over

half the stands studied were multi-cohort. Stands with E. regnans had a lesser tendency towards multi-

cohortness than stands lacking this species, although most old-growth stands, including those

dominated by E. regnans, were multi-cohort. In contrast, most regrowth stands of all species

combinations were single-cohort. The proportions of E. regnans stands that were multi-cohort were

similar to some estimates from the same type of forest in Victoria. Modifications of forestry regimes in

wet eucalypt forests could help to maintain the existence of these biodiverse multi-cohort forests in the

landscape.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

For both silvicultural and nature conservation management it isimportant to understand natural forest regeneration processes.These are usually deduced from age or size structures. Populationsof species of trees in natural forests can vary from single-cohort tomultiple-cohorts. We define single-cohorts (equivalent to even-age; Husch et al., 1982), as stands in which dominant canopy treeshave regenerated in response to a single event, such as fire. Single-cohort stands result from disturbance events that create theconditions for mass regeneration, while killing surviving indivi-duals from the previous generation (Veblen et al., 1981; Franklinet al., 2002). The production of large single-cohort stands underregimes of high severity fire is termed a ‘stand-replacing fire’,defined as infrequent high severity fires that kill canopy trees(Turner et al., 2007).

* Corresponding author at: School of Plant Science, Private Bag 55, University of

Tasmania, Hobart 7001, Tasmania, Australia. Tel.: +61 3 6248 5681;

fax: +61 3 6233 8292.

E-mail address: Perpetua.Turner@utas.edu.au (Perpetua A.M. Turner).

0378-1127/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2009.04.021

Fire is influenced by many variables such as topography,weather and fuel loads, leading to considerable spatial variation infire intensity, influencing tree survival and subsequent treeregeneration and stand density (Turner et al., 1994; Turneret al., 1997; Schoennagel et al., 2006; Bradstock, 2008). This maylead to the origin of multiple-cohort stands (Turner et al., 1994;Turner et al., 1997; McCarthy and Lindenmayer, 1998; Bradstocket al., 2002; Schoennagel et al., 2006). Multiple-cohort stands(equivalent to multi-aged; Husch et al., 1982) can consist of either(a) mixed ages resulting from continuous recruitment which lackdefinite boundaries in the size-class distribution or (b) standswhere definite ages classes can be identified although theboundaries in the size-class distributions may be unclear. Multi-cohort stands resulting from continuously regeneration species inresponse to endogenous ecological processes are typical of manyprimary (climax) rainforests including both tropical and temperate(Adam, 1992; Oliver, 1981; Oliver and Larson, 1996).

Eucalyptus species of both wet and dry sclerophyll forests inAustralia do not typically form primary forests, relying principallyon exogenous disturbance for their regeneration (Ashton, 1981a;Gill, 1997). This can lead to multi-aged and single-aged cohortstand structure, in both wet and dry sclerophyll forest. Multi-

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375 367

cohort forests are typical of the dry sclerophyll forests throughmuch of Australia (Duncan and Brown, 1985; Groves, 1994). Incontrast, the dominant fire paradigm for wet eucalypt forests inSouthern Australia is that the infrequent intense conflagrationsassociated with fire in these forests results in the death of mosttrees and hence the production of single-cohort eucalypt stands(Gilbert, 1959; Jackson, 1968; Florence, 1996). However, multi-cohort stands with discreet age cohorts result from repeatedexogenous disturbance events that do not kill all of the previousgenerations, as in the case of the response of stands of Eucalyptus

delegatensis ssp. tasmaniensis to repeated fires (Bowman andKirkpatrick, 1984).

Where forests are thought to be typically constituted of asingle-cohort, silvicultural management often takes the form ofperiodic catastrophic intervention, that has been argued to bepositive for both forest regeneration and native biodiversity in thatit ‘mimics nature’ (Attiwill, 1994) in kind, if not necessarily type,frequency and scale. McCarthy and Lindenmayer (1998) statedthat ‘‘the current silvicultural system in mountain ash forests isbased on the paradigm that mountain ash trees rarely survivefires’’. The silvicultural imposition of an even-aged stand structureon a naturally multi-aged forest has been argued to be likely toresult in negative effects on native biodiversity (Kirkpatrick andBowman, 1982; McCarthy and Lindenmayer, 1998).

After the pioneering work of Gilbert (1959) and Cunningham(1960) demonstrated that successful natural regeneration ofEucalyptus regnans required close to full sunlight and a mineralseed bed free of competing established plants, all potentiallyprovided by fire, the wet eucalypt forests of Southern Australiahave been largely managed by clearfelling followed by highintensity burning and aerial seeding (Hickey et al., 2001).

Prior to 1960 Tasmania’s wet eucalypt forests had a historydating back to the early 1800s of being selectively logged withoutactive management of regeneration (Kostoglou, 1996; Hickey andWilkinson, 1999). Despite the long history of logging, unloggedforests still remain. Most of these unlogged eucalypt forests havebeen regarded in Australia as ‘old-growth’ where they aredominated by eucalypts older than 110 years and lack noticeablepast human disturbance (Woodgate et al., 1994). This definition isnot that used elsewhere in the world, where subclimax forests,such as those dominated by eucalypts, are not regarded as old-growth (Oliver and Larson, 1996). The old-growth eucalypt forestsof Australia have high conservation importance because bydefinition they contain mature old trees which typically containhollows required by many vertebrate species for breeding(Lindenmayer et al., 1991a, 1991b). These hollows are usuallysmaller and rarer in younger trees.

E. regnans is atypically fire-sensitive for a eucalypt, lacking alignotuber and having thin protective basal bark for two to fourdecades after germination (Ashton, 1981a). Intense crown fires cancause the death of all individuals of this species, while less intensefires may eliminate young and senescent trees. Where all trees arekilled outright the post-fire seedlings regenerate and a single agedcohort develops, in the absence of further fire this cohort growsinto a single aged old-growth stand (Lindenmayer et al., 2000).However, not all fires that penetrate E. regnans forest kill all trees ofthe species, a fact noted by the earliest researchers (Gilbert, 1959;Cunningham, 1960; Ashton, 1976) and mentioned in later reviews(Ashton, 1981a; Gill, 1997; Mackey et al., 2002). More recently,Ough and Ross (1992) and McCarthy and Lindenmayer (1998) havedocumented the proportions of multi-cohort and single-cohort E.

regnans stands in some Victorian forests. The 30% of multi-cohort E.

regnans stands in the central Victorian study area of Lindenmayeret al. (1999) was associated with steep slopes and the presence ofold-growth trees. Such stands occur in places where fires have notbeen uniform in their intensity (Ashton and Martin, 1996;

Williams et al., 1994). Variation in intensity has been inferredfrom variation in tree mortality after fire with estimates of at least34% of Victorian E. regnans trees surviving after a fire (Mackey et al.,2002, p. 42). The proportion of Tasmanian E. regnans stands that aremulti-cohort, and the nature of the correlates of variation in agestructure are questions that have not been addressed.

Eucalyptus obliqua, also a major dominant of lowland weteucalypt forests in south-eastern Australia (Kirkpatrick et al.,1988), contrasts with E. regnans in its greater ability to resist firedamage and to recover by epicormic and lignotuberous shootsafter fire (Ashton, 1981a). It could therefore be expected that,within similar wet forest environments, E. obliqua would have agreater number of cohorts than E. regnans. E. delegatensis ssp.tasmaniensis, a taxon often concurrent with either or both of E.

obliqua and E. regnans is intermediate between them in its ability tosurvive fire and known to have both multi-aged and single-agedstands (Bowman and Kirkpatrick, 1986).

Ashton (1981b) postulated differences in relative growth ratesand relative distribution patterns between E. obliqua and E. regnans

between Victoria and Tasmania. Modelling of the persistence of E.

regnans forest predicts a relatively sharp change in occupancy asthe mean fire interval decreases (Mackey et al., 2002, p. 42). This isconsistent with observed sharp boundaries between E. regnans andE. obliqua in Victoria. Sharp boundaries between these two speciesare not commonly observed in Tasmania suggesting a difference instand structure and forest composition as a consequence of fireregime compared to Victorian forests. It may be that thepredominance of single-cohort stands documented for E. regnans

in Victoria does not pertain to Tasmania, invalidating the wide-spread generalization that E. regnans forests are predominantlyproduced by stand-replacing fires (Squire et al., 1991). The forestmanagement paradigm should reflect these fundamental ecologi-cal processes.

In the present paper we document variation in cohort structureof southern Tasmanian wet eucalypt forests, and test the followinghypotheses: (1) Eucalyptus regnans stands are more likely to besingle-cohort than E. obliqua and mixed stands; (2) old-growth(Australian definition) trees are associated with multi-cohortforests; (3) the E. regnans stands are more multi-cohort thanVictorian E. regnans stands. Finally, we discuss the managementimplications of the results.

2. Methods

2.1. Study area

The study area constituted the wet forests of south-eastTasmania including the Huon and Derwent forestry districts(Forestry Tasmania) and the eastern boundary of the TasmanianWilderness World Heritage Area (WHA) (Fig. 1). Extensivewildfires occurred in the area in 1898, 1906, 1914, 1934 and1967 (Luke and McArthur, 1978; Alcorn et al., 2001). There are noreliable spatial records for previous fires. Both selective andsalvage logging activities have been undertaken in the southernforests in the past. Selective logging was prevalent up until the1960s (Kostoglou, 1996), where after clearfelling techniquesbecame established (Hickey and Wilkinson, 1999). Salvage loggingwas undertaken after the 1967 conflagration primarily usingclearfelling techniques (Hickey and Wilkinson, 1999).

2.2. Data collection

A total of 762 sites (Fig. 1) were located using a stratifiedrandom approach to ensure that they sampled the full range oftopographic (e.g. ridges vs gullies), and edaphic (elevation, slope,aspect) variation in apparently unmodified wet eucalypt forests

Fig. 1. Map of the study area showing plots by forest type and single-cohort or multiple-cohort.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375368

Fig. 2. Wet eucalypt forest in southern Tasmania last burnt by wildfire in 1934, with trees from more than one cohort. A large old E. obliqua tree with fire scars, second from the

left, is in the background. Regrowth E. obliqua are scattered in the foreground.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375 369

whose dominant age of cohort ranged from 5 to >450 years old),with the presence of either or both of E. regnans and/or E. obliqua

being required. The 762 plots included 67 plots sampled in 1990within or adjacent to the south-eastern boundary of the WHA byone of the other authors (JB) and 695 permanent forest inventoryplots sampled between 1967 and 2002 within the Southern forestsby Forestry Tasmania.

Plots were excluded from the dataset if they lacked both E.

obliqua and E. regnans or were located in an area mapped onForestry Tasmania Aerial Photograph Interpretation (API) maps ashaving been clearfelled or thinned (Stone, 1998). There is apossibility that some of the permanent forest inventory plots hadsome history of selective and salvage logging not recorded in theForestry Tasmania dataset or API mapping.

The 67 plots located within or adjacent to the south-easternboundary of the WHA were sampled using 20 m � 20 m plots. Iftrees of particular size ranges (e.g. scattered trees of more than onemetre diameter at breast height over bark [DBH]) were present inuniform surrounding forest within 60 m of the centre of the plot,but absent or poorly sampled by the plot, then the distance to theclosest tree of that size-class in each quarter was measured fromthe plot centre together with the DBH of the tree. The plot size ofthe Forestry Tasmania permanent inventory plots, varied depend-ing on establishment date and use: 312 plots = 0.1 ha(20 m � 50 m), 351 plots = 0.2 ha (20 m � 100 m) and 32 plot-s = 1 ha. The latter plots are angle count plots. Angle count plotsincorporate six transects, 50 m long and 50 m apart.

Within all plots stem diameters of all live eucalypts >10 cm atbreast height (DBH) over bark at 1.3 m above ground level (orabove the buttress, if present) on the upside of a slope wererecorded, together with notes on the presence of fire damage (e.g.charcoal and or scars). Our aims required that we identify whetherthere was one, or more than one cohort of eucalypts in each plot.We did not attempt to identify all cohorts. Plots were attributed toa single-cohort if there were no fire damaged eucalypt treespresent. Plots were identified as having more than one cohort(multi-cohort) if fire damaged eucalypt trees were mixed togetherwith smaller eucalypt trees that were undamaged by fire. Signsdefined as fire damage included basal scarring and/or hollows

(termites are not active in Tasmania and charcoal is found in thehollows) and charcoal on the persistent bark. In addition to thepresence of fire damage, the older cohort was usually alsoidentifiable by a massive break in the size-class distribution anddifferences in canopy characteristics (Fig. 2).

Cohorts were allocated to a regeneration year on the followingbasis; (a) the regeneration year of the youngest cohort wasdetermined by cutting down the fire-sensitive understorey treesthat are known to establish after fire (Nematolepis squamea,Pomaderris apetala, Leptospermum, Acacia species) and countingannual rings. Where the fire history of the site was known fromhistorical records this was used to confirm estimated ring counts.Where the youngest cohort consisted of mature/old-growth trees(greater than 110 years) and the understorey was one ofcontinuously regenerating rainforest species, the regenerationyear was usually imprecise and followed the method used forolder tree cohorts; (b) Trees belonging to older cohorts wereidentified by the presence of fire damage including fire scars,charcoal and epicormics (Fig. 3). The method of identifying theyear of regeneration of older cohorts was usually estimated lessprecisely and involved using a variety of methods such asdiameter–age relationships, counting of rings on stumps inadjoining areas, well-known fire history back to the extensive1898/1899 fires (Marsden-Smedley, 1998) and tree form/canopystructure. Harper (1977) rightly observes that diameter is a poorindicator of age and Dean et al. (2003) found that the DBH of E.

regnans in one estimated 321 year old single-cohort Tasmanianstand varied between 1.4 m and 4.0 m, thus demonstrating thattree size distribution within a stand may be highly variable.Nevertheless a significant relationship has been found to existbetween diameter and age for these eucalypt species (e.g. E.

obliqua: Koch et al., 2008, E. regnans; Dean et al., 2003, eucalyptspecies; Forestry Tasmania unpublished growth models). Ouraims required only that we distinguish accurately between old-growth trees (older than 110 years) and regrowth tree ages. Whilesuppressed trees may exist and therefore not all trees in the samecohort will look like they are from the same cohort, the presence offire damage on older trees allowed discrimination betweenyounger and older cohorts i.e. older trees that have been

Fig. 3. Eucalyptus obliqua old-growth tree, showing upper canopy structure with

tree hollow. This tree was present in a stand occupied by old fire scarred trees and

younger trees without fire scars.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375370

suppressed will not have been put into the young cohort due to thepresence of fire damage.

2.3. Data analysis

The densities and basal areas of cohorts sampled using the pointcentred quarter method was calculated following the methoddescribed by Mitchell (2007). Tree densities and basal areas for allplots were adjusted to the 1 ha scale. The error associated with theuse of data obtained from different plot sizes were consideredminor in comparison to natural stand variation in tree density andbasal area. However, the 1 ha plots were adjusted by applying a0.67 multiplier to stem basal area (David Mannes, pers. comm.).Each stand was allocated into one of four forest floristic classes:pure E. regnans; pure E. obliqua; mixtures of E. regnans and E.

obliqua; E. obliqua and/or E. regnans with one or more of E.

delegatensis, E. globulus, E. johnstonii and E. viminalis. Each standwas also allocated into one of four forest structural classes single-cohort stands with regrowth age trees (Trees 110 years or youngerin age), single-cohort stands of old-growth trees (trees over 110years); Multi-cohort stand with only regrowth trees and multi-cohort stands including old-growth trees (Fig. 4).

Analysis of variance (ANOVA) was used to test for significantdifferences between forest floristic types and structural types for anumber of variables, listed in Tables 1 and 3. The same analyseswere undertaken using the non-parametric Kruskal–Wallis test ofdifference between medians, as some of the plots of the residualsvaried from normality.

Aerial photographic interpretation map (API) data (ForestryTasmania 2002 unpublished data, Stone, 1998) was used toindependently validate the survey data results of tree allocation tocohorts. The API classes were allocated into the same fourstructural classes as the forest stands i.e. single-cohort regrowth,single-cohort old-growth, multi-cohort regrowth and multi-cohortincluding old-growth. The association between the structuralclasses attributed to plots on the basis of the ground survey and APImapping were tested using the Chi-square test for independence.The Chi-square test was also used to test the association betweenthe forest structural types and possible selective logging history.The logging history was determined by overlaying the plots on theRegional Forest Agreement (RFA) forest disturbance map whichrecorded logging history at a low resolution scale (1:100,000,Tasmanian Public Land Use Commission, 1996). The mappedclasses included unlogged, selectively logged and clearfelled. Plotsoccurring within polygons for which no information was providedwere excluded from the analysis.

An approximate fire interval was estimated on the basis of thedifference between the age of the youngest cohort and the nextoldest cohort. This parameter is of limited accuracy where the agesof old-growth trees were determined with low precision. Howeverit provides an indication of the relative differences in fire intervalbetween multi-cohort regrowth forest and forest with old-growthtrees.

3. Results

Of the 762 plots 39% had only E. obliqua, 19% had only E. regnans,19% had both E. obliqua and E. regnans with no other eucalypt, 9%consisted of E. obliqua in various combinations with E. delegatensis,E. globulus, E. viminalis and E. johnstonii, 9% consisted of E. regnans

in various combinations with E. delegatensis, E. globulus, E. viminalis

and E. johnstonii, and, 5% consisted of both E. obliqua and E. regnans

in various combinations with E. delegatensis, E. globulus, E. viminalis

and E. johnstonii.Forty-four percent of the plots had only a single-cohort. The

majority of plots with pure E. regnans and a mixture of E. regnans

with E. obliqua (mixed) had only one age cohort, whereas themajority of other plots had multiple-cohorts (Table 1). Thedifferentiation of single-cohort or multi-cohort by forest typedeviated strongly from randomness (p < 0.0001). There is nostrong spatial pattern in the variation (Fig. 1).

The mean age for stands with a single-cohort was 61 years(S.D. = 59 years); for multiple-cohorts the mean age of the youngestcohort was 51 (S.D. = 25) but the mean age of the oldest cohort was162 (S.D. = 72). Only 22 out of the 332 single age cohort stands wereolder than 110 years. Eleven of these were pure E. obliqua, nine werepure E. regnans and two were the complex mixtures. None of thesingle-cohort mixed stands of E. obliqua and E. regnans had old-growth trees. In contrast, 124 out of the 430 multi-cohort stands hadan oldest cohort younger than 110 years (Table 2).

Pure E. regnans stands had significantly fewer stems per hectarethan the other forest types, but were similar to the mixture of E.

regnans with E. obliqua and the other mixed category in basal areaper hectare (Table 1). The E. obliqua stands had a significantlyhigher basal area per hectare than the other forest types (Table 1).

Eucalyptus obliqua stands tended to have older cohorts thanmixed stands and other species stands (Table 1). The percentage ofstems in the youngest cohort for E. regnans and mixed stands wassignificantly different to E. obliqua and other mixed stands(Table 1). The percentage of total basal area in the youngestcohort was significantly higher in E. regnans stands and standsconsisting of a mixture of E. regnans with E. obliqua than in E.

obliqua stands; the opposite being true for the oldest cohort(Table 1).

Fig. 4. Percentage frequency histograms of tree diameter at breast height (DBH) in 10 cm classes for eight plots: (a) E. obliqua 40-year-old single-cohort stand

(density = 755 stems/ha) and 38-year-old single-cohort E. regnans stand (density = 580 stems/ha); (b)�200-year-old single-cohort old-growth E. obliqua stand (density = 45-

trees/ha) and �300-year-old single-cohort old-growth E. regnans stand density = 30 trees/ha); (c) 36–69-year-old multi-cohort regrowth E. obliqua stand

(density = 520 stems/ha) and 36–69-year-old multi-cohort regrowth E. regnans stand (density = 500-stems/ha) and (d) 36 to �200-year-old multi-cohort old-growth E.

obliqua stand (density = 270 stems/ha) and 38 to �200-year-old multi-cohort old-growth E.regnans stand (density = 460 stems/ha). X-axis: diameter class in 10 cm

increments; Y-axis: percentage frequency of stems per diameter class. Pure E. obliqua stands, black; pure E. regnans stands, grey.

Table 1Characteristics of stands by forest type.

Variable E. regnans

(n = 146)

Mixed

(n = 147)

E. obliqua

(n = 295)

Other

(n = 174)

p

(ANOVA)

p

(Kruskal–Wallis)

Single cohort (%) 57a 54a 36b 37b <0.0001 <0.00001

Mean (median) number of

stems per hectare

240a (143) 439b (307) 400b (270) 400b (283) <0.0001 <0.0000001

Mean (median) total

basal area (m2/ha)

51a (34) 44a (37) 62b (55) 47a (42) <0.001 <0.00001

Data for the youngest cohort:

Mean diameter of youngest

cohort (cm) (median)

50a (40) 35b (28) 41b (30) 35b (29) <0.001 <0.01

Mean age of youngest cohort

(years) (median)

58ab (42) 47c (39) 60a (53) 52bc (44) <0.05 ns

Stems in youngest cohort

(%) (median)

92a (100) 93a (100) 87b (96) 85b (96) <0.001 <0.00001

Basal area in youngest cohort

(%) (median)

73ac (100) 75a (100) 61b (66) 65bc (77) <0.001 <0.0001

Data for the oldest cohort(s):

Mean diameter of oldest

cohort(s) (cm) (median)

(see note 1)

115a (61) 85b (54) 102a (95) 85b (67) <0.001 <0.01

Mean maximum age of oldest

cohort(s) (years) (median)

123ac (75) 98b (71) 129a (120) 113bc (91) <0.01 <0.001

Mixed: a mixture of only E. obliqua and E. regnans. Other: a mixture of other eucalypt species as well as E. obliqua and/or E. regnans. Underlining indicates a value more than

expected. The presence of the same letter adjacent to a figure in a row indicates that the two values are identical at p > 0.05 using Tukey’s family error rate. ns = not significant.

(1) Note that more than one cohort of trees may be included within the older cohort tree group and for single-cohort plots the data from the single cohort has been analysed as

the oldest cohort for this variable.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375 371

Table 2The number of plots in structural classes defined by the number of cohorts and the presence or absence of old-growth trees versus the forest floristic type.

Forest type Single-cohort Multi-cohort p (Chi-square; df)

Regrowth

cohort

Old-growth

cohort

Regrowth cohorts only Including

old-growth cohort

E. regnans 74 (10%) 9 (1%) 13 (2%) 50 (7%)

Mixed 80 (10%) 0 19 (2%) 48 (6%)

E. obliqua 94 (12%) 11 (1%) 50 (7%) 140 (18%)

Other 62 (8%) 2 (0%) 42 (6%) 68 (9%)

n (% of total) 310 (41%) 22 (3%) 124 (16%) 306 (40%)

Chi-square = 49; df = 9; p < 0.0001

Regrowth-cohort: youngest cohort with an estimated age of less than 110 years. Old-growth: oldest cohort with an estimated age of greater than 110 years old. Mixed: a

mixture of E. obliqua and E. regnans, Other: a mixture of other eucalypt species as well as E. obliqua and/or E. regnans.

Table 3Mean time since last fire, fire interval, diameter and other forest structural attributes for each forest structural class (as defined by the number of cohorts and the presence or

absence of old-growth trees).

Variable Single-cohort Multi-cohort p (ANOVA) p (Kruskal–Wallis)

Regrowth cohort

n = 310

Old-growth

cohort n = 22

Regrowth

cohorts only

n = 124

Including

old-growth

cohort n = 306

Mean time since last fire

(years) [i.e. age of youngest

cohort] (median)

48a (39) 227b (193) 40c (38) 55d (53) <0.0001 <0.00001

Mean interval between most

recent fire and previous fire

Not known Not known 37a (33) 122b (114) <0.0001 <0.0000001

Mean diameter (cm)

(median diameter)

37a (32) 180b (164) 44c (42) 55d (50) <0.0001 <0.0000001

Variance of mean diameter

(standard deviation)

499 (22) 7844 (89) 326 (18) 865 (29)

Mean stem density of the

youngest cohorts (median)

(trees/ha)

423a (280) 91b (40) 273c (160) 322a (200) <0.0001 <0.0000001

Mean stem density of the

oldest cohorts (median)

(trees/ha)

423 (280) 91b (40) 63b (26) 53b (20) <0.0001 <0.0000001

Mean basal area of the

youngest cohort (median)

(m2/ha)

33a (28) 111b (87) 19c (14) 25d (19) <0.0001 <0.00001

Mean basal area of the

oldest cohorts (median)

(m2/ha)

33a (28) 111b (87) 20c (16) 51d (38) <0.0001 <0.00001

Regrowth-cohort: youngest cohort with an estimated age of less than 110 years. Old-growth: oldest cohort with an estimated age of greater than 110 years old. Mixed: a

mixture of E. obliqua and E. regnans, Other: a mixture of other eucalypt species as well as E. obliqua and/or E. regnans. The presence of the same letter adjacent to a figure in a

row indicates that the two values are identical at p > 0.05 using Tukey’s family error rate.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375372

Mean time since last fire was significantly different (p < 0.01)for each structural class with the greatest time since last fire beingobserved in single-cohort old-growth stands (Table 3). Theprevious fire interval could not be determined for single-cohortstands but for multi-cohort stands there was a strong associationbetween the persistence of old-growth trees and longer fireintervals (Table 3).

The Tasmanian Regional Forest Agreement disturbance map(Tasmanian Public Land Use Commission, 1996) provides anindication that up to 42% of the permanent forest inventory plotshave had some selective logging within the local area. The scaleof mapping and the dispersed nature of selective loggingoperations make it impossible to determine precisely if plotswere logged without visiting each site. However there was astrong association between stand structure and a possiblehistory of logging (p < 0.0001) with old-growth trees morelikely to be present in plots for which no logging history wereidentified (Table 4).

The association of logging history and stand floristic classeswere also found to differ significantly from random (p < 0.0001).The least likely stands to have been logged being E. regnans (only

27% of E. regnans with logging history mapped were mapped aslogged) compared with 37% for E. obliqua, 51% for stands in whichE. obliqua was mixed with E. regnans and 60% of stands with speciesother than E. obliqua or E. regnans.

Cohort allocation of the survey data was validated by the strongcorrelation (p < 0.0001) with structural classes independentlydetermined by aerial photographic interpretation (p < 0.0001,Table 4). The many differences can be attributed to lowerresolution scale of the aerial photographic interpretation mappingand the heterogeneity in the forest landscape.

4. Discussion

4.1. Variation between forest types in age structure

The hypothesis that E. regnans suffers greater mortality after firethan the other eucalypts in our plots is supported by the highproportion of single-cohortness in both pure stands of the speciesand in its mixture with E. obliqua. Just over half (57%) of E. regnans

plots were single-cohort, providing some support to the stand-replacing fire paradigm for these forests.

Table 4Number of plots (PIP: permanent forest inventory plots) in structural classes defined by the number of cohorts and the presence or absence of old-growth trees versus;

mapped history of selective logging or not (Regional Forest Agreement [RFA], Tasmanian Public Land Use Commission, 1996); and aerial photographic interpretation (API,

1:25,000) mapping.

Variable Single-cohort Multi-cohort p (Chi-square, df)

Regrowth cohort

n = 309

Old-growth

cohort n = 23

Regrowth cohorts

only n = 124

Incl. old-growth

cohort n = 306

PIP plots in polygons

mapped as selectively

logged on RFA

disturbance map

87 (18%) 0 (0%) 43 (9%) 70 (15%)

PIP plots in polygons

mapped as never

logged on RFA

disturbance map

47 (10%) 19 (4%) 30 (6%) 175 (37%)

Total n = 471 n = 134 (31%) n = 19 (2%) n = 73 (21%) n = 245 (46%) <0.0001 (69, df = 3)

API single-cohort

regrowth only

154 (27%) 1 (0%) 53 (9%) 72 (13%)

API single-cohort

including old-growth

9 (2%) 7 (1%) 3 (1%) 16 (3%)

API multi-cohort

regrowth only

2 (0%) 0 (0%) 2 (0%) 1 (0%)

API multi-cohort

including old-growth

10 (2%) 2 (0%) 58 (10%) 170 (30%)

Total n = 560 n = 175 (31%) n = 10 (2%) n = 116 (21%) n = 259 (46%) <0.0001 (240, df = 9)

Regrowth-cohort: youngest cohort with an estimated age of less than 110 years. Old-growth: cohort with an estimated age of greater than 110 years old. Underlining

indicates a value more than expected.

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375 373

Our data indicate that more than half of the wet forests notsubject to clear-fell burn and sow logging in south-easternTasmania are multi-cohort stands. The proportion of multi-cohortness is highest for E. obliqua forests (64% multi-cohortcompared with 53% for all forests). Forests not subjected toselective and/or salvage logging practices are likely to have an evenhigher proportion of multi-cohort stands (our data suggestsaround 76% of stands).

These multi-cohorts forests may have arisen/persisted byvarious mechanisms. E. obliqua is known as a facultative resprouter(Gill, 1981). Unlike E. regnans, E. obliqua has the ability to resist firedamage and to recover by seed and epicormic and lignotuberousshoots after fire (Ashton, 1981a). Previous studies have found thatresprouting trees retained in the overstorey have a direct effect inrestricting seedling establishment in the understorey particularlyfor obligate seeding species e.g. E. regnans resulting in a mix of treeages (Ashton, 1981a, 1981b; Bowman and Kirkpatrick, 1986;Vivian et al., 2008). Fire severity is also considered important in thesurvival and development of a stand (Jordan et al., 1992; Ashtonand Martin, 1996; Turner et al., 1997; Turner et al., 2003; Vivianet al., 2008). Due to variation in wind, vegetation type, weatherconditions, fuel loads and topography, there is substantial spatialvariation in fire intensity and residence time, resulting in aheterogeneous vegetation response (Smith and Woodgate, 1985;Turner et al., 1994). Low fire intensity or low residence time islikely to be responsible for the creation of multi-cohort forest, suchas those observed in the present study, where old-growth treesremain in the post-fire population.

4.2. Old-growth trees and multi-cohortness

Very few (7 %) of the stands with a cohort of trees more than 110years old (old-growth) were single-cohort, compared to over 70%of the multi-cohort forests. Selective logging appears to havereduced the extent of old-growth trees in the forest and may haveimpacted further by indirectly leading to an increased firefrequency and intensity. Little is known about pre-European firefrequencies in these wet forests but it is likely to have altered withthe inception of logging and other European development in

adjacent areas. Several major conflagrations have affected much ofthe study area between the 1890s and the 1970s. These burnt mostplots (Hickey et al., 1999).

Ashton (1981a) and Lindenmayer et al. (1999) observed thatfires occurring in old forests of E. regnans are less likely to be acomplete stand-replacing event than fires occurring in youngforests. McCarthy and Lindenmayer (1998) developed a model thatpredicted that a multi-cohort forest is more probable as the meanfire interval increases. This model was validated by Lindenmayeret al. (1999). Our data also support the accuracy of this prediction.The longer the fire interval the more likely the understorey is tohave developed a substantial rainforest element (Gilbert, 1959;Jackson, 1968). The rainforest species are known to inhibit fireintensity due to the low volatility of the plants and the highdecomposition rate of the litter from these non-sclerophyllousspecies (Hill and Read, 1984; Dickinson and Kirkpatrick, 1985;Jordan et al., 1992). Survivorship of mature eucalypts is thereforelikely to be higher in wet eucalypt forests with a rainforestcomponent.

The scope of this paper precluded an analysis of the distributionof multi-cohort stands in the landscape. However previous studiessuggest that multi-cohort forests in the landscape may beassociated with: (a) stands with low fuel loads and low fireintensity (associated with short or very long fire intervals) and (b)fire protected niches where longer fire intervals are more probable(Jackson, 1968; Jordan et al., 1992; Mackey et al., 2002). Furtherstudy of these associations and the floristics associated with thesestands may well support the inclusion of multi-cohort forests as adistinct phase in the succession of vegetation from wet eucalyptforests with sclerophyllous understoreys to rainforest.

4.3. Differences in the prevalence of multi-cohort stands of E. regnans

in Victoria and southern Tasmania

It is difficult to compare the results of Ough and Ross (1992),McCarthy and Lindenmayer (1998) and Lindenmayer et al. (1999)with ours; given that the Victorian analyses were based on rulestructures rather than our direct determination. However thefigure we gain for the proportion of multi-cohort pure E. regnans

P.A.M. Turner et al. / Forest Ecology and Management 258 (2009) 366–375374

stands of 34% is close to the estimates of Ough and Ross (1992,� 40%) and Lindenmayer et al. (1999, 7.2–22.7%) and well above theestimate of McCarthy and Lindenmayer (1998, 9–11 %). The figurefor all stands with E. regnans of 50% is much higher than anyVictorian estimate. However, given the strong influence of age asrelated to fire history on the development of multi-cohortness,there is little reason to believe that any fundamental difference inecological processes will be found between the two strongholds ofE. regnans.

4.4. Management implications of results

There are data that suggest that multi-cohortness in forests isvaluable for the conservation of many elements of biodiversity(Lindenmayer et al., 1991a, 1991b; Lindenmayer et al., 1994; Bar-Ness et al., 2006). Certainly, the presence of an old-growth cohort,whether by itself or with younger cohorts, is important for theconservation of many elements of biodiversity. Research inTasmania on wet eucalypt forests suggests that this is true forvascular plants (Hickey, 1994), bryophytes (Turner and Pharo,2005; Turner et al., 2006), invertebrates (Michaels and McQuillan,1996; Bar-Ness et al., 2006), and vertebrates (Koch et al., 2008). Inthe case of the southern forests, it seems that multi-cohortnessincluding old-growth trees, is more widespread than previouslyacknowledged in the literature. The sparsity of the old-growthtrees within multi-cohort forests has probably led to theirdismissal and treatment of the forest as single-cohort. Howeverwhilst the density of old-growth trees in these multi-cohort forestsis lower than single-cohort forests of comparable age, thecontribution of these old-growth trees to total basal area issubstantial. Given the conservation importance of these trees forthe maintenance of biodiversity the assumption of single-cohortedness is dangerous. The presence of surviving canopytrees in multi-cohort forest reduces the density of tree regenera-tion following fire and results in the production of forest with morestructural diversity compared to single-cohort forests (Linden-mayer et al., 1999).

Although it is recognised that multi-cohort forests occur, theextent of these has been poorly documented and the implicationsfor wet forest silvicultural practice have been largely ignored. Ourdata suggest that stand-replacing fires may be more the exceptionthan the rule in unmanaged wet eucalypt forests. Stand-replacingfire is a principle upon which much of current fire and silviculturalmanagement in wet eucalypt forest is based and appears to havebeen inappropriately applied to both the E. obliqua and E. regnans

forests of southern Tasmania. The last 50 years of clearfellsilviculture and the selective logging prior to that, has, and stillis simplifying forest structure, through the assumption of standreplacement and short rotation times (e.g. 90 years, Whiteley,1999). Clearly these management approaches do not replicate thenatural ecological processes that determine the structure andfunction of wet eucalypt forests, and carry the possibility of a lossin biodiversity.

Current management prescriptions in Tasmania are seeking tomaintain structural diversity within a harvested area, particularlythrough the retention of patches of forest called ‘aggregates’(Hickey et al., 2001). These aggregates are often surrounded byclearfelled forest. In principle these aggregates should enable theretention of existing and/or future old-growth trees in thelandscape, but they will only do so if they are not killed duringthe regeneration burn or by a subsequent wildfire. The placementof aggregates in naturally fire protected niches and the protectionof these from management burning is desirable.

Where multi-cohort forest, occurs, partial harvesting andlonger rotation times may facilitate the retention of biodiversityvalues.

Acknowledgements

The majority of data were collected and summarised byForestry Tasmania for use in this project. Inspiration for thisresearch is due to Drs Mick Brown and Simon Grove. We aregrateful to Murray Haesler and Forestry Tasmania staff for datacollection; Casey Balmer and Forestry Tasmania for data entry andto Darren Turner for data manipulation. Forestry Tasmania staff(especially David Mannes and Ruiping Gao) provided a substantialamount of assistance with data; Tony Blanks commented on themanuscript. The research was initiated by the Bushfire Co-operative Research Centre (CRC) and Forestry Tasmania throughthe Wildfire Chronosequence Project. Research funding wasprovided by the Commonwealth Government of Australia-WorldHeritage Area project funds (JB) and the Bushfire CRC/ForestryTasmania/University of Tasmania (PT). Three anonymousreviewers greatly improved the manuscript.

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