328
M.J. Brown and J. Read
aged Nothofagus forest to be found in areas where rainforest fires have
occurred in the past 100 years or more . At the age 'of 200-400 to 600
years , these stands begin to break up through deaths of Nothofagus and
other canopy dominants. The mature forest reaches a dynamic equilibrium
in which the rainforest elements regenerate through time by gap-phase
replacement. A further fire before this stage , but after maturity of the
dominants, would follow the same path.
Mixed forests are maintained in a manner similar to rainforests, except
that the average fire frequency here is 80 to 400 years (Gilbert, 1959).
The eucalypt dominants die after about 400 years and they cannot
regenerate under the closed rainforest canopy. The stand is thus converted
to rainforest. If fires occur in the period 80 to 400 years , the eucalypts are
maintained and can expand into any adjacent patches of burnt rainforest.
The encroachment is step wise, with most eucalypt seed falling within a
radius, of the height of the tree (Gilbert, 1959). However, in the strong
winds which prevail in wildfires, seed can be carried much further, and
eucalypts have been observed to establish at distances to 200 m from the
nearest (dead) trees. There is some documented evidence of rainforest
invasion of long unburnt moorland sites in western Tasmania (Podger,
1990). Ellis (1985) documents establishment of rainforest in long unburned
areas of grassland , shrublands, and eucalypt forest on fertile soils in north
eastern Tasmania, and the old-field succession described by Read and
Hill (1983) follows a similar pathway .
.
Relationships among soil nutrient status, climatic factors , and the fire
regime ' have been described by Jackson (1968) for the vegetation on
oligotrophic soils of western Tasmania. The vegetation there ranges from
sclerophyllous sedgeland and moorland through sclerophyll scrub and
woodland to rainforest. The feedbacks among the vegetation, the physical
environment, and fire profoundly influence the successional pathways.
Jackson described a stochastic process whereby the different structural
forms of vegetation may become fixed through ecological drift , a process
analogous to genetic drift. Thus large areas 'of land in the southwest of
Tasmania are climatically and edaphically suited to the growth of rainforest , but support sclerophyll communities, either pure or in mixture
with rainforest. About 47% of the lowland regions receiving annual
precipitation in excess of 2000 mm and with a precipitation to evaporation
ratio of >2.5 are occupied by sclerophylls (Jackson, 1983). These disclimax
communities are widespread in Tasmania compared with the vegetation
of New Zealand and southern South America. The difference has been
attributed to the characteristics of the plants , the low inherent soil fertility ,
the history of human influences and the postre climate (Jackson , 1968).
Jackson's theory has been analyzed using Markov chains; the predicted
distributions of the rainforest and other vegetation was generally in
accord with the actual distributions (Henderson & Wilkins, 1975; Noble
& Slatyer , 1981 ; Brown & Podger, 1982b). In situ evidence for the
process has been reported by Brown and Podger (1982b) and Podger
et al. (1988).
M.J. Brown and J. Read
aged Nothofagus forest to be found in areas where rainforest fires have
occurred in the past 100 years or more . At the age 'of 200-400 to 600
years , these stands begin to break up through deaths of Nothofagus and
other canopy dominants. The mature forest reaches a dynamic equilibrium
in which the rainforest elements regenerate through time by gap-phase
replacement. A further fire before this stage , but after maturity of the
dominants, would follow the same path.
Mixed forests are maintained in a manner similar to rainforests, except
that the average fire frequency here is 80 to 400 years (Gilbert, 1959).
The eucalypt dominants die after about 400 years and they cannot
regenerate under the closed rainforest canopy. The stand is thus converted
to rainforest. If fires occur in the period 80 to 400 years , the eucalypts are
maintained and can expand into any adjacent patches of burnt rainforest.
The encroachment is step wise, with most eucalypt seed falling within a
radius, of the height of the tree (Gilbert, 1959). However, in the strong
winds which prevail in wildfires, seed can be carried much further, and
eucalypts have been observed to establish at distances to 200 m from the
nearest (dead) trees. There is some documented evidence of rainforest
invasion of long unburnt moorland sites in western Tasmania (Podger,
1990). Ellis (1985) documents establishment of rainforest in long unburned
areas of grassland , shrublands, and eucalypt forest on fertile soils in north
eastern Tasmania, and the old-field succession described by Read and
Hill (1983) follows a similar pathway .
.
Relationships among soil nutrient status, climatic factors , and the fire
regime ' have been described by Jackson (1968) for the vegetation on
oligotrophic soils of western Tasmania. The vegetation there ranges from
sclerophyllous sedgeland and moorland through sclerophyll scrub and
woodland to rainforest. The feedbacks among the vegetation, the physical
environment, and fire profoundly influence the successional pathways.
Jackson described a stochastic process whereby the different structural
forms of vegetation may become fixed through ecological drift , a process
analogous to genetic drift. Thus large areas 'of land in the southwest of
Tasmania are climatically and edaphically suited to the growth of rainforest , but support sclerophyll communities, either pure or in mixture
with rainforest. About 47% of the lowland regions receiving annual
precipitation in excess of 2000 mm and with a precipitation to evaporation
ratio of >2.5 are occupied by sclerophylls (Jackson, 1983). These disclimax
communities are widespread in Tasmania compared with the vegetation
of New Zealand and southern South America. The difference has been
attributed to the characteristics of the plants , the low inherent soil fertility ,
the history of human influences and the postre climate (Jackson , 1968).
Jackson's theory has been analyzed using Markov chains; the predicted
distributions of the rainforest and other vegetation was generally in
accord with the actual distributions (Henderson & Wilkins, 1975; Noble
& Slatyer , 1981 ; Brown & Podger, 1982b). In situ evidence for the
process has been reported by Brown and Podger (1982b) and Podger
et al. (1988).