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Dead wood in tall, wet E. obliqua forest

An inventory of dead wood in four plots of differing wildfire history in a tall,

wet Eucalyptus obliqua forest in southern Tasmania, Australia.

G.M. Gates1,2, T.J. Wardlaw3, D.A. Ratkowsky1,2*, N.J. Davidson2 and C.L.

Mohammed1

1School of Agricultural Science, University of Tasmania, Private Bag 54, Hobart,

Tasmania 7001, Australia2School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania

7001, Australia3Forestry Tasmania, GPO Box 207, Hobart, Tasmania 7001, Australia

*e-mail: [email protected] (corresponding author)

Abstract

Wildfires in southern Tasmania’s tall wet eucalypt forests generate dead wood of

varying sizes and in many different stages of decay depending on their intensity and

frequency. Large dead wood (coarse woody debris, CWD) and stags were measured

and their attributes recorded in four 50x50 m plots in close proximity but with

differing wildfire histories in a native wet Eucalyptus obliqua forest. Maps were

drawn and are presented here of the CWD in the four plots. In addition, the

diameters at breast height (DBH) of all living higher vascular plants were measured,

and the positions for all stems having DBH≥10 cm were recorded. The maps of the

living vegetation portray the substantial differences between the four plots.

Information from other recent studies of CWD in the same or similar forests

provided a degree of replication to this study. Whereas the CWD volumes in plots of

the same age since wildfire obtained from different studies proved to be very

variable (reflecting the chance location of large fallen eucalypts in the plots) the

proportion of plot basal area occupied by each genus of living tree was remarkably

consistent, and depended upon the time since the last wildfire event. This agreement

lends support to a recent model for canopy structure based upon measurement of the

DBH and relative location of each living tree in the plot, and its identification to

genus level.

Keywords: dead wood, eucalypt, coarse woody debris, stags, wildfire, stand

regeneration, stand development, canopy structure model.

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mailto:[email protected]


Introduction

Dead wood in forest ecosystems has been recognised for some time as being

important in providing a range of ecological niches that foster biodiversity by

maintaining many specialist wood-dwelling and hollow-dependent species, and as a

temporary sink for forest carbon and other elements (e.g. Harmon et al. 1986; Grove

et al. 2002; Stokland and Sippola 2004; Wu et al. 2005). In a forest ecosystem, dead

wood consists of all dead natural structures of woody plant origin, which includes

dead roots, stumps, fallen tree trunks, branches and twigs, and standing dead trees

(stags). Natural mortality of a tree occurs due to ageing and suppression caused by

competition as the stand develops after disturbance. The disturbance may be natural,

e.g. windthrow, wildfire, earthquakes or anthropogenic, e.g. harvesting for timber

and firewood. The quantity of dead wood input into the ecosystem following any of

these disturbances may be large and immediate as with a catastrophic event or a

more gradual temporal process, which may be seasonal, annual or long-term

(Harmon et al. 1986). Spatial input may be within stands or across landscapes and

catchments. Most inputs of dead wood occur at the local scale (e.g. within one tree

length of source) but ecological processes based on dead wood habitats can operate

at a range of scales. Knowledge of the natural dynamics of dead wood provides

important baseline data that can be used for developing and evaluating strategies to

lessen the pressure of anthropogenic disturbance on wood-inhabiting organisms

(Jonsson 2000).

In Australian eucalypt-dominated forests, fire is the major cause of large-scale

natural disturbance. Wildfires vary in type (Luke and McArthur 1978), intensity (Gill

1997), size, frequency and homogeneity (Ashton 1981), resulting in differing starting

points for new stand development. There is generally a lack of knowledge regarding

the log accumulation rate, i.e. the time frame over which trees fall and form logs on

the forest floor, the rates of log decay and how these rates differ between managed

and unmanaged forests in Australia (Lindenmayer et al. 2002). This lack of

knowledge makes it difficult to determine how long it may take logged areas to

accumulate volumes of CWD equivalent to pre-harvesting levels and to establish

silvicultural regimes that ensure that forests are sustainably managed (Lindenmayer

et al. 2002).

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In the commercially important wet lowland eucalypt forests of southern

Tasmania, mature trees of Eucalyptus obliqua L’Hér. frequently attain a height of

over 70 m (Kirkpatrick and Backhouse 1981), enabling these forests to produce a

wood volume per hectare which is amongst the highest produced by any forest in the

world (Woldendorp and Keenan 2005). Mortality in these forests results not only

from catastrophic fire, but also in developing stands through natural selection and

suppression of smaller, weaker individuals. These smaller stems are commonly killed

by insect and fungal attack or a combination of these factors. Smaller diameter CWD

may also accumulate via branch wood that has fallen from the canopy. In these

forests, non stand-replacing wildfires occur more frequently than stand-replacing

wildfires (Alcorn et al. 2001; Turner et al. 2009). This has resulted in a mosaic of

multi-aged forest stands of largely unknown dead wood complexity. A recent study

employing an indirect approach (Grove et al. 2009) suggested that E. obliqua CWD

in Tasmania’s southern forests decomposes very slowly in comparison with CWD

from most other parts of the world.

Volumes of CWD and numbers of stags are two of the attributes used to

measure stand structure and which provide quantitative evidence of habitat that can

be used in biodiversity studies (McElhinny et al. 2005). The volume input,

connectivity (spatial) and continuity (temporal) of CWD are important

considerations in sustainable forest management (Grove et al. 2002). The aim of the

present study was to inventory the dead wood present in a tall wet E. obliqua forest

in southern Tasmania containing stands resulting from different wildfire events and

to compare the CWD volumes therein with available information from other recent

studies carried out in the same forest type (Woldendorp et al. 2002; Sohn 2007). Of

particular interest is a ground-level method for modelling canopy structure in these

same forests (Scanlan et al. 2010), which is based upon identification of each living

tree, measurement of its diameter at breast height (DBH) and its relative location in

the plot. This enables one to examine whether there is a correlation between canopy

structure, as revealed using that methodology, and the quantity of CWD that arises

from natural disturbance, particularly fire, within wet E. obliqua forest.

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METHODS

Study area

The study area was at the Warra Long Term Ecological Research (LTER) site in the

Huon River valley, southern Tasmania, Australia, where four 50x50 m plots were

established in March-April 2006 along the ‘Bird Track’ (see Fig. 1). The plots were

all within ca. 1 km of each other (lat./long. S 43º 06′, E 146º 39′) and had similar

south-facing aspect, altitude, rainfall and temperature, but differed in their histories

of natural disturbance by wildfire. Documented accounts, maps of fire history and

fire scars on E. obliqua trees were used to determine age since fire (Turner et al.

2007). Time since wildfire for each of the four plots was estimated, respectively, to

be at least 200 years (named ‘Old growth’), 108 years (named ‘1898’ as it was burnt

by a fire in 1898, but later it was discovered that parts of it had also been burnt by a

fire in 1934), 72 years (named ‘1934’, burnt by a fire in 1934) and 108 years/72

years (named ‘1898/1934’, burnt by fires in 1898 and in 1934). Plot names are used

for convenience but also reflect, at least to some extent, the disturbance history of the

plot.

Each 50x50 m plot was established in the following way. Star pickets were

placed at 10 m intervals along the outer boundaries of the two opposite sides of the

plot. Twine was strung from the star pickets across the plot and fibreglass rods were

placed at 10 m intervals along the twine to divide the plot into 25 subplots each

measuring 10x10 m. This facilitated mapping of the CWD and stags, and assessment

of the vascular plants.

Vegetation classification and measurement

In each plot, all woody perennial species were identified (following nomenclature of

Buchanan 2009). The plant communities in each plot were described following the

Forest Practices Code (Forest Practices Authority 2005). Within each of the 25

subplots of each plot, all living stems were measured and, if they were at least 10 cm

in diameter, mapped.

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CWD Mapping

CWD as used in this study was defined to be all pieces of dead wood ≥10 cm in

diameter and ≥1 m in length, a modification of the recommendation of Harmon &

Sexton (1996). This definition of CWD included stumps, suspended pieces of wood,

and shards (shattered pieces of larger logs) as well as fallen trunks and branches on

the forest floor. CWD originating from all woody perennial species were included in

the study. CWD was consecutively numbered within each subplot. If a piece of

CWD traversed two or more subplots, its length was measured to the boundary of the

subplot and it was renumbered as a separate piece of CWD in the adjacent plot. The

position, orientation and attributes (see below) of every piece of CWD in each

subplot were recorded for each of the four sites. This information was transcribed

onto large sheets of graph paper (laminated to make it usable in wet weather) marked

with plots and subplots at a scale of 1 mm equal to 10 cm. The following attributes of

each piece of CWD were recorded: 1) CWD length (cm); 2) CWD diameter (cm)

measured at the mid-point of the piece of CWD; 3) CWD decay class (using a scale

from 1 to 5 with intervals of 0.5; see Table 1a); 4) Percent bryophyte cover on each

piece of CWD (a visual score of 0-100%). Stumps were measured for decay class,

height and mid diameter (i.e. the diameter mid-way between the ground and top of

stump).

The system of decay classes used here for CWD (Table 1a) was devised to

accommodate different wood species and to try to overcome the problems associated

with the unevenness of the interval between decay classes 3 and 4. If a piece of

CWD had more than one decay class, an average was taken (after Pyle & Brown

1999). For analysis, CWD was placed into the following diameter classes: ≤15 cm,

15-30 cm, 30-60 cm, 60-90 cm, 90-120 cm, 120-150 cm, >150 cm. These classes

were deemed to be most useful in forest management by Forestry Tasmania (Simon

Grove, pers. comm.; Yee 2005). During analysis, other variables were derived from

length and diameter by calculation, viz. volume and surface area, assuming that the

shape of a piece of CWD approximated a cylinder. For stumps, height replaced

length.

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Stag mapping

Stags were recorded in a similar way to CWD, except 1) height was used in place of

length; 2) diameter was measured at breast height; 3) decay was assessed using a

modified system to that used for CWD, see Table 1b, following Cline et al. (1980),

Spies et al. (1988) and Motta et al. (2006).

Statistical analyses

The majority of the statistical analyses used were of a descriptive nature, producing

summary statistics. Although the CWD was measured within each subplot, enabling

the data to be analysed at a subplot level, which proved useful for a subsequent

survey of the macrofungi growing on wood (see Gates et al. 2011), the analyses

presented here used composite pieces of CWD, obtained by concatenating the

information on pieces of CWD that crossed subplot boundaries. Graphical methods

presented here involved one variable at a time for decay class and % bryophyte cover

on CWD, but we also used profile plots, as devised by Stokland (2001), in which two

explanatory variables (diameter and decay class) were considered simultaneously.

Stags were examined by calculating the number of stags and their diameters in

each plot and by recording the number of stags in each bryophyte cover class.

Results

Vegetation classification

Maps showing the positions of the most frequently occurring vascular species, being

in the genera Acacia, Atherosperma, Eucalyptus, Monotoca, Nothofagus and

Pomaderris, are presented in Fig. 2. In ‘Old growth’, Nothofagus cunninghamii and

Atherosperma moschatum were predominant and Olearia argophylla absent. There

were only two living eucalypts which accounted for more than half the basal area

(Table 2, Fig. 2a). This identifies ‘Old growth’ as close to, but not identical with,

plant community RAIN-CT1 (Forest Practices Authority 2005), the presence of the

eucalypts making the community a mixed forest rather than a true rainforest.

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In ‘1898’, the presence of Olearia argophylla and Phyllocladus aspleniifolius,

the absence of Anodopetalum biglandulosum, and the sparseness of Gahnia grandis

and Anopterus glandulosus (Table 2) places it near plant community WET-OB1000

(Forest Practices Authority 2005). The presence of Pomaderris apetala (193 stems)

was indicative of a second fire in parts of the plot. This is evident from the map (Fig.

2b), where a broad band of living Atherosperma and Nothofagus runs from the upper

right-hand corner towards the lower central and lower left-hand corner of the plot,

with the remainder of the plot containing more than 150 live Pomaderris trees. The

area containing the rainforest vegetation, but lacking Pomaderris, form a partition of

‘1898’ that we designate here as ‘1898R’, and is very close to being considered

‘mature’ forest (Hickey et al. 1999). The second distinct vegetation type into which

‘1898’ was partitioned, designated here as ‘1898P’, is found in the Pomaderris-

abundant regions in the upper left-hand corner and the lower right-hand corner of

Fig. 2b. These areas also contain Acacia and Eucalyptus, but the typifying rainforest

species Atherosperma and Nothofagus can be seen to be absent or extremely sparse.

In the ‘1934’ plot Phyllocladus aspleniifolius, Anopterus glandulosus and

Monotoca glauca were frequent or common and Pomaderris apetala, Olearia

argophylla and Acacia verticillata were sparse or absent (Table 2). This suggests that

the plant community of the ‘1934’ plot is close to, but not identical with, WET-

OB1100 (Forest Practices Authority 2005). The Eucalyptus and Monotoca trees

occurred throughout the entire plot, but not uniformly so (Fig. 2c).

The ‘1898/1934’ plot contained abundant Pomaderris apetala (832 stems) but

also had numerous stems of Eucryphia lucida, Phyllocladus aspleniifolius and

Anopterus glandulosus (Table 2). It is closest to WET-OB1001, which occurs on

humid slopes and gullies on less fertile sites than WET-OB1000 (Forest Practices

Authority 2005). However, the large numbers of Gahnia grandis plants (132) and the

presence of Olearia argophylla (17 stems), both of which reflect disturbance, are

anomalous for this forest type. The distribution of Eucalyptus and Pomaderris in the

left-hand portion of this plot is relatively uniform, but the clusters of rainforest

species in the right-hand portion, reminiscent of the partition ‘1898R’ of ‘1898’ but

on a smaller scale, suggests that the 1934 fire did not burn the plot evenly.

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CWD and stag volumes

For each plot separately, minimum, median and maximum CWD subplot volumes

are given in Table 3, which also gives the total plot volumes. Based on total volume

(m3/ha) of CWD present, ‘1898’ had more than twice the volume of ‘1898/1934’,

whereas for stags (Table 4), ‘1898’ had the lowest volume. If all the stags had fallen

to become CWD, the rank orders of the total amount of dead wood in the plots would

remain unchanged, as the stag volumes in a plot are only a fraction (21%) of the

CWD volumes. That is, ‘1898’ would still be the plot with the most total dead wood,

‘1934’ would remain second highest, ‘Old growth’ would remain next and

‘1898/1934’ would still have the least total dead wood.

CWD maps

Maps showing the positions of CWD in each plot are given in Fig. 3. There were 227

pieces of composite CWD in ‘Old growth’, 138 in ‘1898’, 212 in ‘1934’ and 237 in

‘1898/1934’. The relative sparseness of pieces of CWD in ‘1898’ compared with the

other plots is readily observable.

CWD attributes vs. decay class

Most of the 814 pieces of composite CWD within the four plots fell into the middle

decay classes (DC3 and DC3.5), with 522 pieces of CWD (64.1%) in these combined

classes (Fig. 4). ‘Old growth’ had a very small percentage of CWD in the lower

decay classes (DC≤2.5) compared with younger plots, but compensated for this in

the higher decay classes (DC≥4), reflecting the shift in decay class distribution that

occurs with increasing age of plot. Bryophyte cover tends to increase steadily as

decay class increases in units of 0.5 from DC1–DC5 in all plots combined (Fig. 5).

Slightly deviating from the overall trend is ‘1934’, which reaches a plateau at a

percentage bryophyte cover of ca. 50% in the higher decay classes.

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Profile plots

The percentages of pieces of CWD graphed simultaneously against diameter class

and decay class in profile plots (Fig. 6) shows that there were no pieces of CWD

greater than 60 cm diameter in DC<3 in any of the four plots. In addition, very large

diameter CWD (DBH>120) was generally absent from the highest decay classes. In

‘Old growth’, almost all of the CWD was found in low diameter classes. ‘1898’ had

more CWD in the higher diameter classes than any other plot.

Stag numbers and attributes

The ‘1934’ plot had the greatest number of stags, closely followed by ‘Old growth’,

while ‘1898’ had the least number (Table 4). Although ‘1934’ had 34 stags, all but

one of them was of small diameter (Fig. 7), so that the stags of that plot had the

smallest average diameter of the four plots (Table 4). In contrast, although

‘1898/1934’ had only 20 stags, that plot had the highest average diameter due to four

stags of large diameter, each >100 cm (Fig. 7). ‘Old growth’, with the second highest

number of stags, had two large diameter ones (Fig. 7), giving it the second largest

average diameter (Table 4).With respect to species composition, there is a sharp

contrast between the younger stands, ‘1934’ and ‘1898/1934’, which had 16 and 12

E. obliqua stags, respectively, and the mature forests, ‘1898’ and ‘Old growth’,

which had only one E. obliqua stag each (Table 4). The identifiable stags in ‘Old

growth’ are mainly of Nothofagus cunninghamii, Atherosperma moschatum and

Acacia melanoxylon, typical rainforest species.

The number of stags as a function of bryophyte cover class and plot is shown

in Fig. 8. Increasing bryophyte cover is associated with increasing age of plot, with

few stags in ‘1934’ and ‘1898/1934’ having bryophyte cover of 25% or more.

Basal areas of living vegetation from another study

The four plots of the present study each had a unique wildfire history and therefore

there is no replication with respect to age since wildfire. However, other plots in the

tall wet E. obliqua forests of southern Tasmania are available for comparison. These

plots are derived from the wildfire chronosequence study of Turner et al. (2007),

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with the basal areas measured by Scanlan et al. (2010). Three of these plots, viz.

‘OGS’, ‘1898S’ and ‘1934S’, were located adjacent to plots ‘OG’, ‘1898’ and

‘1934’, respectively, of the present study. It is believed (P. Turner, pers. comm.) that

the whole of ‘1898S’ was affected by a second fire in 1934, but like that of ‘1898’ of

the present study, that was not known at the time of plot establishment. Results for

the basal areas of the living vegetation are given in Table 5, where, following

Scanlan et al. (2010), the woody perennial species are tabulated at the level of genus,

a restriction that affects only Acacia, which combines occurrences of A.

melanoxylon, A. verticillata and A. dealbata. All other genera had only a single

species that was found in the plots.

Because ‘1898’ of the present study is really made up of two fire histories, it

was split into two parts for inclusion in Table 5, with ‘1898P’ based on subplots

where Pomaderris was the predominant understorey species, and ‘1898R’ based on

subplots where rainforest species were predominant. A younger plot (‘1966S’) used

in the chronosequence study of Turner et al. (2007) is also included in Table 5, as it

had a southerly aspect, in common with all other plots in that table. There is

excellent agreement between ‘OGS’ and ‘OG’ in the proportions of all the genera

that appeared in those plots. There is also good agreement between ‘1934S’ and

‘1934’ for Eucalyptus, Nothofagus and Acacia, but there was slightly more

Atherosperma and less Monotoca in ‘1934S’ than in ‘1934’. The three plots,

‘1898P’, ‘1898S’ and ‘1898/1934’, had similar fire histories, as all were burnt twice

(by wildfires in 1898 and in 1934) and therefore might be expected to have had

similar stand compositions. The results in Table 5 indicate that this is largely the

case, with ‘1898P’ agreeing reasonably well with ‘1898/1934’, except for the

presence of slightly more eucalypts and virtually no rainforest species. ‘1898S’ also

broadly agrees with ‘1898/1934’, although the former had more Acacia and

Pomaderris, but less Eucalyptus, than the latter plot.

The youngest plot, ‘1966S’, had the highest proportion of Eucalyptus of any

of the plots in Table 5, and the two oldest plots, ‘OG’ and ‘OGS’, had the lowest

proportion. These results are consistent with a progressive decline of the percentage

basal area of Eucalyptus with increasing plot age of a chronosequence.

Volumes of CWD from other studies

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There are other plots in the tall wet E. obliqua forests of southern Tasmania for

which CWD volumes have been determined. Woldendorp et al. (2002) inventoried

the CWD in two wildfire-affected plots in the Warra LTER, both having a southerly

aspect and which were not amongst the four plots of the present study. The size of

these plots was 1 ha each, i.e. four times the size of each of the plots of the present

study, and the definition of CWD was slightly different, having a lower diameter and

length limit of 15 cm and 50 cm, respectively (instead of the 10 cm and 1 m limits

used here). Additional plots were inventoried by Sohn (2007), viz. ‘1966S’, ‘1934S’,

‘1898S’ and ‘OGS’, all of which were used in the chronosequence study of Turner et

al. (2007), and which were also subsequently used by Scanlan et al. (2010) for

modelling canopy structure. The plots of Sohn (2007) were each 50x50 m, as in the

present study, but the lower diameter limit of CWD was 40 cm and 1 m length

(compared to a 10 cm lower limit in the present study or a 15 cm limit in that of

Woldendorp et al. 2002). The volumes of CWD from the three studies, with ‘1898’

replaced by its partitions ‘1898P’ and ‘1898R’, are graphed in Fig. 9 on a per hectare

basis against age since wildfire (year of wildfire minus year of measurement).

Despite the clearly large variance of CWD volume for plots of the same or closely

similar ages, a quadratic polynomial fitted to the 11 data points (Fig. 9) produces a

pattern that is broadly consistent with the model of Stamm & Grove (unpublished,

but see Grove 2009), where a stand reaches a maximum CWD volume at some

intermediate stage between the initiation of the fire and being designated as an old

growth stand.

Discussion

The sources of CWD are (1) the stand prior to the disturbance, (2) the direct result of

the disturbance itself, and (3) an ensuing gradual input from the current stand,

including mortality caused by disease, suppression and competition, insect attack,

and windthrow. The four plots chosen for this study, although located in relatively

close proximity to each other, have different fire histories and therefore probably

have different mechanisms by which the major part of their CWD was likely to have

originated. In Tasmania, in the long absence of fire and in areas where the rainfall

exceeds 1270 mm, ecological drift occurs (Jackson 1968). This means that the

understory of eucalypt dominated forests progressively becomes dominated by cool

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temperate rainforest (mixed forest) and, as the eucalypts die without regeneration,

the eventual outcome is climax rainforest that may take ca. 400 years to occur. The

‘Old growth’ plot fits the definition of mixed forest (old, even-aged eucalypts, with

an understory of mature rainforest; see Gilbert 1959 and Wells & Hickey 1999). The

live vegetation showed floristic simplification with a preponderance of mature

rainforest species and two very large surviving eucalypts (Table 2, Fig. 2a). Only one

stag in ‘Old growth’ was of E. obliqua origin, compared to 32 stags of rainforest

and/or other species (Table 4). Pieces of CWD in ‘Old growth’ had the highest

percent bryophyte cover of all the plots, a consequence of the direct relationship

between decay class of the wood and percent bryophyte cover (Fig. 5). The most

likely origin of the high percentage of pieces of CWD in high decay classes and of

small diameter in ‘Old growth’ (Fig. 6a) was from branches breaking out of

declining eucalypt crowns and from the tops of stags from climax rainforest species

falling to the forest floor. The sparseness of large diameter CWD in high decay

classes in ‘Old growth’ suggests that sufficient time (>300 yr., Grove et al. 2009)

had elapsed for the CWD resulting from the death and falling of the original mature

eucalypt stand to rot away.

The ‘1898’ plot was made up of two distinct vegetation types. The partition

‘1898R’, an area characterised by living rainforest species, had a CWD volume of

1481 m3/ha, due to some very large pieces of CWD of E. obliqua origin that may

have resulted from trees killed by an intense and possibly stand-replacing fire in the

year 1898. These trees likely fell immediately after the fire or subsequently as a

result of wind or disease. Any small diameter branch wood or suppressed trees of

small diameter from the regenerating stand could have had sufficient time (108

years) to rot away, which may explain the relatively low percentage of pieces of

CWD of small diameter (Fig. 6b). However, the stand may not have been old enough

for the accumulation of small diameter CWD of rainforest species as in ‘Old

growth’. In this rainforest partition of ‘1898’, there were three very old N.

cunninghamii stags consistent with an old growth plot. The partition ‘1898P’ had a

CWD volume almost as high as for ‘1898R’, also due to a few very large diameter

trees, but in a significantly lower decay class, consistent with fallen wood being on

the forest floor for a shorter period of time.

In ‘1934’, the lower average decay class of the CWD reflects the shorter time

(72 years) that the wood has been lying on the forest floor. The high number of small

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diameter stags (Fig. 7) may reflect suppression mortality in the regenerating stand. A

striking difference between the composition of the living stems of ‘1934’ and that of

‘1898/1934’ and of ‘1898P’, plots or parts of plots that experienced a second fire in

1934, is the presence of Monotoca glauca and the absence of Pomaderris (Table 2,

Fig. 2c). This can be attributed to a different underlying geology. Whereas the other

plots are on soils derived from Jurassic dolerite, ‘1934’ is situated on Permo-Triassic

sedimentary rock, which produces a more acidic soil type that favours Monotoca in

place of Pomaderris (M.G. Neyland, pers. comm.).

The ‘1898/1934’ plot had the smallest volume of CWD (Table 3), which is

consistent with the second fire consuming the CWD generated by the first fire.

Alternatively, perhaps the large diameter trees that were killed by the fire of 1898 did

not fall immediately but remained as stags, which survived the fire of 1934 (see

Fig. 7). Any small diameter stags that resulted from regeneration after the first fire,

later suppressed by competition to become small diameter CWD on the forest floor,

were likely to have been consumed by the second fire. Suppression mortality in the

regenerating stand following the second fire was likely to have been responsible for

the many small diameter stags of E. obliqua origin (Fig. 7), similar to ‘1934’.

The close similarity of the basal area percentages for each genus of trees and

shrubs in nearby plots (or partitions of plots) having the same wildfire history (‘OG’

vs. ‘OGS’; ‘1934’ vs. ‘1934S’; ‘1898S’ vs. ‘1898P’) is evidence of a consistency in

the compositions of the higher vascular species resulting from wildfire. This is of

particular relevance because of the existence of a recent ground-level method for

modelling canopy structure that was developed using data from these same forests

(Scanlan et al. 2010). The method requires identification of each living tree and

measurement of its DBH and its relative location in the plot. Using these ground-

based measurements and a series of regression equations developed from empirical

data to predict the crown dimensions of trees of each of the six important canopy

genera, Scanlan et al. (2010) successfully modelled canopy structure. This suggests

that the canopy can be modelled at the stand or landscape level from comparatively

quick and inexpensive measurements made at ground level. The consistency between

the basal area compositions of the present study and those determined in similar plots

by Scanlan et al. (2010) is encouraging, as it suggests that it may be possible to map

and monitor structural variation across forested landscapes from readily and

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inexpensively obtained ground-based measurements without the use of specialised

equipment.

CWD volumes, unlike basal area, vary greatly among forest plots that are

otherwise very similar in wildfire history and stand composition. Discrepancies in

CWD volumes of the order of magnitude observed in Fig. 9 (e.g., a range of 366–

1481 m3/ha for logs + stumps at an age of 72–73 years) cannot be attributed to

differences in the lower diameter limit, i.e. had a lower diameter limit of 40 cm been

used in the present study instead of 10 cm, 92.3% of the total volume would still

have been observed, the total volume being determined mainly by large diameter

logs. The amount of CWD in a 50x50 m plot is largely a matter of chance, the main

contributors to CWD volume being fallen eucalypts, some of which had heights

exceeding the 50 m plot length. A single fallen tree can have a big effect on CWD

volume, e.g. the largest piece of CWD in ‘Old growth’ (clearly visible on the right-

hand side of Fig. 3a) accounted for 42.1% of the CWD volume in that plot. There

were other large senescent trees and stags outside the boundary of the plot that could

have fallen and landed in the plot due to chance. Therefore, the great variance of

CWD volume is not surprising, but despite that variation, the general trend of the

volume data against age since wildfire broadly follows the prediction made by a

stand dynamics model (Stamm & Grove, unpublished).

Conclusions

Identifying the living vegetation to genus level and measuring the DBH and position

of each living tree enables maps to be drawn from which the forest stand structure

can be deduced, as shown in a recent study which modelled canopy structure from

these readily obtainable ground-level measurements. Unlike CWD volume, which

varies considerably, the percentage basal areas occupied by the genera of the living

trees are consistent among replicates of plots subjected to the same or closely similar

fire history. In a small spatial study such as the current study and those of Sohn

(2007) and Turner et al. (2007), the volume of dead wood in the tall, wet E. obliqua-

dominated forests of southern Tasmania does not serve as a useful indicator of stand

structure due to the chance nature of where senescent trees and stags fall.

Nevertheless, the data on CWD volume (for logs plus stumps) obtained from the

present study and two other recent studies using wildfire history broadly support the

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general trend predicted by a model for stand dynamics that is currently under

development.

Acknowledgements

Financial and logistic support for the field work was provided by Forestry Tasmania.

One of the authors (GMG) received financial support from an Australian

Postgraduate Award, the Holsworth Wildlife Endowment Fund, the Cooperative

Research Centre (CRC) for Forestry, the Bushfire CRC, and CSIRO Ecosystem

Sciences. Additional logistic support was provided by the Schools of Agricultural

Science and Plant Science of the University of Tasmania.

References

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Buchanan, A.M. (2009). A Census of the Vascular Plants of Tasmania & Index to The Student’s Flora of Tasmania. (http://www.tmag.tas.gov.au/file.aspx?id=4439).

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Grove, S., Meggs, J. and Goodwin, A. (2002). A Review of Biodiversity Conservation Issues Relating to Coarse Woody Debris Management in the Wet Eucalypt Production Forests of Tasmania. Forestry Tasmania, Hobart.

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Jonsson, B.G. (2000). Availability of coarse woody debris in a boreal old-growth Picea abies forest. Journal of Vegetation Science 11: 51-6.

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Lindenmayer, D.B., Claridge, A.W., Gilmore, A.M., Michael, D. and Lindenmayer, B.D. (2002). The ecological roles of logs in Australian forests and the potential impacts of harvesting intensification on log-using biota. Pacific Conservaion Biology 8: 121-40.

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Motta, R., Berretti, R., Lingua, E. and Piussi, P. (2006). Coarse woody debris, forest structure and regeneration in the Valbona Forest Reserve, Paneveggio, Italian Alps. Forest Ecology and Management 235: 155-63.

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Scanlan, I, McElhinny, C. and Turner, P. (2010). A methodology for modelling canopy structure: an exploratory analysis in the tall wet eucalypt forests of southern Tasmania. Forests 1: 4-24.

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Turner, P.A.M., Balmer, J. and Kirkpatrick, J.B. (2009). Stand replacing wildfire? The incidence of multi-aged and even-aged Eucalyptus regnans and E. obliqua forests in southern Tasmania. Forest Ecology and Management 258: 366-75.

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Table 1. CWD and stag decay classification to accommodate E. obliqua and other

tree species. (a) CWD, (b) Stags.

(a) CWDDecay class Characteristics for classifying CWD1 CWD freshly downed, entire, cylindrical, wood hard, sound, bark intact,

no sign of internal decay or external macrofungal fruit bodies 1.5 Wood has been lying on the ground for some time, cracks appearing in

bark.2 CWD remaining solid, losing some bark, some Basidiomycota fruit

bodies appearing, such as Mycena spp., and some Ascomycota such as Hypoxylon spp. and Hypocrea aff. megalosulphurea, bryophyte cover sparse.

2.5 CWD with many macrofungal fruit bodies, but exhibiting no sign of softening. Category included to accommodate Pomaderris apetala.

3 CWD retaining round shape, bark may be present, bryophyte cover present but variable, some degree of heart rot, still quite firm on the outer surface, many external macrofungal fruit bodies present in season.

3.5 CWD beginning to flatten, becoming softer, often with seedling trees, wood-inhabiting macrofungal genera such as Gymnopilus, Galerina, ectomycorrhizal genera such as Laccaria, Cortinarius and Russula associated with the seedling trees (Tedersoo et al. 2003) and corticioids (resupinate fungi) being commonplace, bryophyte cover substantial. Roots from nursery trees making their first appearance.

4 CWD half its original diameter, often with only the sides remaining but still recognizable as a log or a log that may be prolifically interspersed with roots from nursery trees of considerable size.

4.5 CWD disintegrating into splinters and losing outline. 5 CWD reduced to a pile of humus, still with very small wood fragments

present, outline just visible, mound-like appearance or a ‘cage’ of roots from a nursery log with some woody humus remaining.

(b) StagsDecay class Characteristics for classifying stags1 Stag limbs and branches all present; 100% bark present.2 Stag has some loss of limbs and bark but is sound at base.3 Stag distinctly rotten at base; in E. obliqua the bark can still be intact at

this stage.4 Stag still standing with outer bark intact but obviously very decayed

inside. This category is for Nothofagus stags.5 Stag reduced to a thin central core, no outer wood but still standing.

This category is for Nothofagus stags.

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Table 2. Live tree and woody shrub species in each plot. Numbers in brackets after

each species are respectively: 1Contribution of species to total basal area of plot,

expressed as a percentage of the total; 2Number of living stems; 3Maximum diameter,

cm. Species are listed in decreasing order of their contribution to the total basal

area of the plot.‘Old growth’:Eucalyptus obliqua (54.0%1;22;3503)Nothofagus cunninghamii (24.9%;189;110)Atherosperma moschatum (17.7%;216;80)Acacia melanoxylon (3.0%;5;80)Eucryphia lucida (0.4%;8;35)Anopterus glandulosus (<0.1%;4;5)Coprosma quadrifida (<0.1%;5;6)Phyllocladus aspleniifolius(<0.1%;1;1)Pimelea drupacea(<0.1%;11;1.5)Tasmannia lanceolada(<0.1%;1;1.5)

Total 442 living stems

‘1898’:Eucalyptus obliqua (66.3%;19;300)Pomaderris apetala (11.5%;193;27.5)Nothofagus cunninghamii (9.3%;86;65)Atherosperma moschatum (5.7%;50;50)Acacia melanoxylon (3.6%;10;60)Olearia argophylla (3.3%;37;34)Phyllocladus aspleniifolius (0.2%;4;18.5)Anopterus glandulosus (<0.1%;2;1.5)Coprosma quadrifida (<0.1%;5;4.3)Cyathodes glauca (<0.1%;1;1)Monotoca glauca (<0.1%;2;7.5)Pimelea drupacea (<0.1%;8;1)Pittosporum bicolor (<0.1%;1;7)


‘1934’:Eucalyptus obliqua (85.7%;40;230)Nothofagus cunninghamii (6.0%;167;35)Monotoca glauca (3.5%;174;21)Acacia melanoxylon (2.2%;18;32.5)Acacia dealbata (0.8%;5;31.5)Nematolepis squamea (0.4%;2;25.5)Phyllocladus aspleniifolius (0.4%;22;15.5)Tasmannia lanceolata (0.3%;29;12.5)Pittosporum bicolor (0.2%;2;23)Cyathodes glauca (0.1%;32;8.5)Eucryphia lucida (0.1%;3;18)Anopterus glandulosus (0.1%;33;11.5)Atherosperma moschatum (<0.1%;3;3)Coprosma quadrifida (<0.1%;7;2.5)Pimelea drupacea (<0.1%;7;1.5)


‘1898/1934’:Eucalyptus obliqua (73.9%;39;250)Pomaderris apetala (17.2%;832;25.5)Acacia melanoxylon (3.5%;13;70)Eucryphia lucida (2.2%;78;36)Nothofagus cunninghamii (1.6%;155;33)Atherosperma moschatum (0.5%;18;17,5)Phyllocladus aspleniifolius (0.4%;54;27)Olearia argophylla (0.3%;17;9.5)Acacia verticillata (0.1%;3;12)Anopterus glandulosus (0.1%;29;10)Coprosma quadrifida (0.1%;28;5)Cyathodes glauca (0.1%;29;7)Aristotelia peduncularis (<0.1%;4;3)Monotoca glauca (<0.1%;2;1)Pimelea drupacea (<0.1%;20;1)Pittosporum bicolor (<0.1%;1;3)Tasmannia lanceolata (<0.1%;11;6.5)


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Table 3. Subplot minimum, median and maximum volume of CWD (in the 25

subplots) for each of the plots ‘Old growth’, ‘1898’, ‘1934’ and ‘1898/1934’, and the

total volume and total volume/ha for the same plots.

Plot‘OG’ ‘1898’ ‘1934’ ‘1898/1934’

Min volume m3 0.627 0.429 0.261 0.263Median volume m3 3.715 13.534 9.680 6.707Max volume m3 34.359 45.939 37.065 21.098Total volume m3 209.462 361.717 272.729 175.302Total volume m3/ha

(((m3ha?, m3/ha

837.8 1446.9 1090.9 701.2

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Table 4. The total number of stags, number of E. obliqua stags, average stag diameter and total stag volume in the four plots (‘1898’ is divided into its two component parts).

Plot‘OG’ ‘1898R’ ‘1898P’ ‘1934’ ‘1898/1934’

Total no. of stags 33 7 3 34 20Volume of stags (m3/ha) 226 4.4 42.3 192 176No. of E. obliqua stags 1 1 0 16 12No. of non-eucalypt stags 32 2 7 18 8Ave. stag diam., eucalypts 150 2.5 – 34.7 23.2Ave. stag diam., non-eucs 45.2 20.5 47.6 23.0 90.8

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Table 5. Proportion of total basal area contributed by the woody perennial genera in the plots of the present study and in other similar plots (having the same stand type with the same southerly aspect) located in the tall, wet E. obliqua forests of southern Tasmania.

Plot name; approximate plot age, years

Genus‘1966S’

41‘1934S’

73‘1934’

72‘1898/1934’

72‘1898S’

73‘1898P’

72‘1898R’

108‘OG’>200

‘OGS’>200

Acacia 0.7 3.7 3.0 3.6 13.5 1.5 10.0 3.0 4.9Atherosperma 2.2 <0.1 0.5 1.7 11.9 17.7 15.8Eucalyptus 93.1 87.1 85.7 73.9 62.8 80.5 55.8 54.0 54.9Eucryphia 0.1 0.4 0.1 2.2 0.4Leptospermum <0.1Monotoca 0.6 3.5 <0.1 0.2Nematolepis 4.6 0.2 0.4Nothofagus 5.5 6.0 1.6 1.0 <0.1 20.7 24.9 24.2Olearia 0.3 0.1 1.0 1.3Phyllocladus 0.2 0.4 0.4 0.1 0.1 <0.1 <0.1Pomaderris 1.5 17.2 20.8 16.7Tasmannia 0.1 0.3 <0.1 <0.1

Notes: plots ‘1934’, ‘1898/1934’ and ‘OG’ are those of the present study; ‘1898P’ and ‘1898R’ are partitions of ‘1898’ of the present study, the former being based upon subplots where Pomaderris predominates the understorey, and the latter being based upon subplots where rainforest species are predominant; ‘1966S’, ‘1934S’, ‘1898S’ and ‘OGS’ are plots used in the chronosequence study of Turner et al. (2007), with the basal areas determined by Scanlan et al. (2010). Blank entries indicate the genus was absent from that plot or contributed less than 0.05% to the column total, which would be 100% were it not for this fact. The age of ‘1898S’ and of ‘1898P’ reflects the fact that their most recent fire occurred in 1934.

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Fig. 1. Warra LTER site location (from http://www.warra.com) and the position of

the plots used in this study along the ‘Bird Track’.

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http://www.warra.com/


(a) (b)

(c) (d) Fig. 2. Maps of tree species with stems 10 cm diameter, Acacia (gold diamonds), Atherosperma (pink squares), Eucalyptus (green triangles), Monotoca (violet squares), Nothofagus (orange triangles), Pomaderris (blue squares). Other genera formed only a minor component of the live vegetation and are omitted from the figure. (a) ‘Old growth’, (b) ‘1898’, (c) ‘1934’, (d) ‘1898/1934’.

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(a) (b)

(c) (d)

Fig. 3. CWD maps for the four plots at the Warra Bird Track. (a) ‘Old growth’, (b)

‘1898’, (c) ‘1934’, (d) ‘1898/1934’.

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45

6

2


0

5

10

15

20

25

30

35

40

1 1.5 2 2.5 3 3.5 4 4.5 5

Decay class

% o

f CW

D

All plots'Old growth''1898''1934''1898/1934'

Fig. 4. Percentage of pieces of CWD in each decay class for each plot separately,

and for all plots combined. The percentages add up to 100% within a plot.

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4

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0

10

20

30

40

50

60

70

80

90

100

1 1.5 2 2.5 3 3.5 4 4.5 5

Decay class

% B

ryop

hyte

cov

erAll plots'Old growth''1898''1934''1898/1934'

Fig. 5. Average percent bryophyte cover in each decay class for each plot separately

and for all plots combined.

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Fig. 6. Profile plot of percentages of pieces of CWD versus decay class (DecayC)

and diameter class (DiamC, cm) for each of the four plots: (a) ‘Old growth’, (b)

‘1898’, (c) ‘1934’, (d) ‘1898/1934’. See Table 1a for an explanation of the decay

classes.

11.5

22.5

33.5

44.5

5

D≤15

15<D

≤30

30<D

≤60

60<D

≤90

90<D

≤120

120<

D≤15

0D>

150

0

5

10

15

20

25

CWD

DecayC

DiamC

(a)

11.5

22.5

33.5

44.5

5

D≤15

15<D

≤30

30<D

≤60

60<D

≤90

90<D

≤120

120<

D≤15

0D>

150

0

5

10

15

20

25

CWD

DecayC

DiamC

(b)

11.5

22.5

33.5

44.5

5

D≤15

15<D

≤30

30<D

≤60

60<D

≤90

90<D

≤120

120<

D≤15

0D>

150

0

5

10

15

20

25

CWD

DecayC

DiamC

(c)

11.5

22.5

33.5

44.5

5

D≤15

15<D

≤30

30<D

≤60

60<D

≤90

90<D

≤120

120<

D≤15

0D>

150

0

5

10

15

20

25

CWD

DecayC

DiamC

(d)

28

1

12

3

4

5

6

7

8

9

2


2040

6080

100120

140160

180200

220240

'1898P''1898R'

'1898/1934''Old growth'

'1934'

02468

101214161820

Frequency

Diameter, cm

Fig. 7. Diameter distribution of stags in the four plots at the Warra Bird Track

(‘1898’ is divivded into its two component parts).

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'189

8P'

'189

8R'

'193

4'

'189

8/19

34'

'OG

'

0≤B

≤25

25<B

≤50

50<B

≤75

75<B

≤100

0

5

10

15

20

25

30

35

Frequency

Bryophyte %cover

Fig. 8. Stag numbers in bryophyte percentage cover classes (‘1898’ is divivded into

its two component parts).

30

1

12

3

2


0

200

400

600

800

1000

1200

1400

1600

0 25 50 75 100 125 150 175 200 225 250 275Years since wildfire

CW

D v

olum

e

Woldendorp et al. (2002)Sohn (2007)Gates (2009)

Fig. 9. Volume of CWD (m3/ha) as a function of time since wildfire, comparing data

from the present study with two other studies that used plots having the same stand

type and the same southerly aspect in the tall, wet E. obliqua forests of southern

Tasmania.

31

1

1

23

4

5

6

78

2

an inventory of dead wood in four plots of differing ...€¦ · web viewan inventory of dead...

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