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II. Scientific Evidence Regarding Rainforest Ecology and Protection
5. TIME SCALES OF RAINFOREST ECOLOGICAL PROCESSES
An important aspect in the evaluation of protection measures for
Victorian rainforests is the recovery time of mature rainforest
and ecotonal vegetation from disturbances of different kinds.
If the consequences of edge effects are transitory on the time
scale of harvesting rotations then they may safely be ignored.
Similarly, if the kind, frequency and intensity of disturbances
that result from human activities are similar to those that could
be expected in a pre-industrialised environment, then we may expect
that the rainforest communities are sufficiently resilient to
cope with modern disturbances. Certainly, natural disturbance
regimes are a part of the ecology of Nothofagus forests
elsewhere in the world (Veblen 1989, Stewart et al. 1991,
Armesto et al. 1992).
There has been widespread disturbance of much of Victoria's rainforest
in the past, especially in the Central Highlands, the Strzelecki
Ranges and the Otway Ranges associated with logging activities
and access to European settlements. These disturbances varied
greatly in their nature, extent and persistence. However, there
has been no systematic, retrospective study of the short or long
term effects of these disturbances. As a result, it is not possible
to use this information to make any inferences on the resilience
or stability of rainforest stands. The few studies of rainforest
disturbance in Victoria have focused on regeneration dynamics
following fire. Howard (1973a) found in cool temperate rainforest
at Mt Donna Buang that rapid recovery of the N. cunninghamii
forest understorey from fire precluded eucalypt regeneration from
seed. Howard (1981) suggested that the widespread fires in Victoria
in 1939 reduced the distribution of rainforest to its most sheltered
enclaves, and that local canopy closure on burnt rainforest sites
required 40 years for canopy closure and will require a further
30 years before there is a continuous rainforest canopy beneath
emergent eucalypts. If there is another fire within 40 years,
before crown closure, then tree and understorey plants are susceptible
and eucalypt recruitment would be abundant, resulting in a mixed
forest, if in fact Nothofagus survives a second fire.
There is evidence that Acmena, Pittosporum and Tristaniopsis
were killed outright by a single fire in some stands (McMahon
1987). There are no data on the resilience of other rainforest
species to repeated fires at short intervals.
There are many studies of the recovery from disturbance of Australian
temperate rainforests, some based on inferences made from regeneration
dynamics and growth rates, and others based on direct evidence.
Horne and Gwalter (1987) observed that the time to achieve overstorey
recovery in warm temperate rainforest following 25%, 65% and 75%
removal of basal area by selective logging is <60 years, 60-100
years and >100 years respectively. Eucalyptus regnans
stands in the Florentine Valley of Tasmania that were not burnt
for 100-300 years normally have an understorey dominated by rainforest
trees. Stands that have survived a fire less than 100 years ago
have a sclerophyll understorey originating from the fire (Cremer
and Mount 1965). Mount (1979) suggested that wet sclerophyll
forests adjacent to rainforest generally have a fire frequency
of 20-120 years. Bowman and Jackson (1981) suggested fire-free
intervals of 200-400 years are necessary for the maintenance of
cool temperate rainforest, and that intervals greater than 400
years result in dominance of Tasmanian rainforest by fire sensitive
gymnosperms. In cool temperate rainforest in Tasmania, the time
taken for myrtle beech and celery-top pine (Phyllocladus aspleniifolius)
to attain 60cm dbhob was up to 200 and about 400 years respectively,
and the time for Huon pine (Lagarostrobus franklinii) to
attain 60 cm was at least 500 years, based on growth functions
determined from stem analysis (Hickey and Felton 1987). McMahon
(1987) speculated that floristic and structural changes in warm
temperate rainforest ecotones that result from hot fires will
take 100 years for the demise of the sclerophyll understorey and
a further 200 years for the senescence of the eucalypt overstorey
and replacement by Acmena smithii (see also, Ashton and
Frankenberg 1976, Howard 1973a, McMahon 1987). Such fire-free
periods are unlikely in sclerophyll forest. Woodgate et al.
(1994) suggested that in damp forest, early senescence of eucalypts
only begins after about 250 years. In such circumstances, frequent
recurrent low intensity fires will not affect the eucalypt overstorey
but would eliminate or reduce the A. smithii understorey.
Chambers et al. (1977) suggested the regeneration time
of temperate rainforest ecosystems is likely to be of the order
of 200 years or more. Attiwill (1994b), citing the work of Cremer
(1960), Mount (1979) and Ashton (1981a) among others, suggested
that fire intervals ('return times') of 350-400 years are close
to the life span of most eucalypts, allowing the development of
rainforest. With fire intervals of 100-350 years, eucalypts are
retained with an understorey of rainforest species. Fire intervals
less than 100 years result in sclerophyll forest or other vegetation
types.
Long-term changes in cool temperate rainforest composition are
possible, depending on the initial floristic composition and on
the spatial scale of disturbance. This is because some canopy
species are able to regenerate vegetatively (eg., N. cunninghamii,
A. moschatum, E lucida), some establish by vigorous
seedling growth (E. lucida) and some species are able
to dominate through long life spans following a single recruitment
opportunity, even if recruitment by conspecifics is unlikely (eg.,
P. aspleniifolius; Read and Hill 1988). Chesterfield
et al. (1990) found that 5 years after a severe fire in
warm temperate rainforest in East Gippsland, vegetation composition
was dominated by species that characterise mature rainforest.
There were also large increases in populations of Acacia melanoxylon,
eucalypt species and sclerophyllous shrubs. They suggested that
earlier fires had influenced the species composition, facilitating
invasion by sclerophyllous species and that frequent fires (<40
to 50 years) would result in the replacement of the rainforest
by a fire-dependent disclimax sclerophyllous community. Such
changes, if they occur, are unlikely to be easily detected in
the short term, even over the period during which clear felling
has been the most important harvesting technique in Victoria.
Careful sample design will be necessary to discriminate these
changes from the many other confounding processes that determine
species composition at a site.
The regeneration time of rainforest ecosystems is likely to be
of the order of several centuries. The incursion of fire into
ecotones on a shorter time scale and harvesting in ecotones where
they extend beyond 20m will effectively change the interactions
between rainforest and adjacent sclerophyll forest. Harvesting
in mixed forest will have important consequences for dynamics
within these communities. The interpretation of the importance
of these impacts depends on the amount of harvesting that occurs
in an ecotone or a mixed forest and on our ability to exclude
fire from the ecotone. There may be cumulative effects in which
repeated disturbance will favour some species, leading to some
permanent changes in floristic composition.
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