The long secondary succession series for a Big Ash Forest in the absence of fire. From pioneer species to the climax community, what will inherit the earth if the Big Ash forest does not burn?
This was originally an assignment for university. If you're wondering, I received a D for it because I did not use enough references.
Montane ash forests are found mostly in central southern Victoria. The forest community forms a tall open forest, dominated by the Big Ash, Eucalyptus regnans. They are the tallest flowering trees in the world, with mature specimens frequently attaining heights in excess of 80m. Other plant species usually form up to four strata below the canopy species; however, the composition of the lower strata depends on the time since the last fire. E. regnans is an obligate seeder, requiring a severe and fatal fire to regenerate from seed (Ashton 2000, Cunningham 1960, Jarrett and Petrie 1929, Ashton 1976). All the species in the community can regenerate after fire, either by resprouting from the unburnt remains of the earlier plant, or by the germination of seeds stored in the soil. From this point, the plant community undergoes changes in structure and species composition as species variously dominate, decline and drop out of the community. After about 350 to 450 years without fire, the dominant Big Ash will also finally senesce and drop out of the community. So what would the plant community look like then? What would dominate the landscape when the Big Ash die? Unfortunately, no fire interval has exceeded the lifespan of E. regnans, so the community composition beyond Big Ash senescence is postulated based on vegetation trends in mature Ash stands (Ashton 2000).
Part 1: The Early Years. The initiating fire to the first century after fire
The Ash-bed Effect, en-masse germinations and resprouting in the early years.
The beginning of secondary succession in the Big Ash Forest requires a crown fire, i.e. a fire severe enough to burn the crowns of mature E. regnans individuals. This, along with increasing the available light and destroying the understorey community, stimulates an en-masse release of the seeds stored in the crown. The seedling densities of E. regnans after fire have been reported as high as 2.5 million seedlings per hectare (Ashton 1976). The mature E. regnans remain standing in the forest, providing a measure of protection for the regenerating flora (Ough 2001).
In E. regnans dominated forests, the species composition for the first hundred years is largely determined by the initial floristic composition after disturbance (Ough 2001). Ashton and Martin (1996) noted that all species present prior to a wildfire in an E. regnans dominated forest had regenerated within 2 years. After 10–12 years, the structure and species composition of the understorey approached its pre-fire condition.
The initiating fire also creates a phenomenon frequently referred to as the ash-bed effect. The heating of the soil during a fire results in sustained and increased releases of N and P, and enhances this by a physico-chemical decrease in the clay colloid of the soil. It may also affect the microbial content of the soil, and decrease the likelihood of seed attack by microorganisms (Chambers and Attiwell 1994). The availability of nutrients in the ash-bed also causes an increased growth rate in E. regnans seedlings (Chambers and Attiwell 1994, Cunningham 1960).
The Initial Growth
In the first two years after the initiating fire, there is an observable seedling stage (Jarrett and Petrie 1929, Ashton and Martin 1996). The understorey consists of Pteridium, E. regnans seedlings, shrub seedlings and regrowth, with abundant herb seedlings and regrowth as well. Acacia dealbata and melanoxylon, Pomaderris aspera, Senecio spp and Cassinia are also likely to appear in the ash-bed (Ashton 1976). Ashton and Martin (1996) identified a floristic group that thrived in the first two years after an initiating fire. It consisted of E. regnans seedlings and Acacia verticillata, Goodia, Goodenia, Cassinia, bracken, Clematis, Oxalis, Acaena and Tetrarrhena. The tree ferns Dicksonia antarctica and Cyathea australis will both resprout quickly after the fire, and smaller ferns like Polystichium and Blechnum will begin growing again shortly after the fire. Resprouting species like Olearia argophylla and Bedfordia arborescens will also regenerate sparsely in the seedbed.
The Acacia species play a vital role in the community at this early stage (Adams and Attiwell 1984). A. dealbata, melanoxylon and verticillata all proliferate profusely after fire, germinating from the soil seedbank. The seedlings grow rapidly, dominating the community for the first two or three years. Such rapid growth utilizes a significant proportion of the available nutrients in the seedling ash-bed ecosystem. After losing the ecosystem dominance to the E. regnans seedlings three years after the fire, the Acacia seedlings gradually succumb to the intense competition of the seedbed and their densities decline. The nutrients stored in their biomass are gradually released back into the ecosystem, as they are decomposed. The brief period of Acacia dominance acts a nutrient conservation mechanism, by storing nutrients that would otherwise be lost in the denuded ash-bed.
There is intense interspecific competition during the early years. Two years after the fire, the community enters the shrub stage (Ashton and Martin 1996), with E. regnans, A. verticillata, Cassinia, Goodia, and Tetrarrhena all forming low shrubs. Most of these will stay to form minor components of the lower strata of the mature forest community, but are quite significant in the earlier years. Four years after the fire, the shrub stage has developed to include a significant component of forbs. By the eighth year, the community has reached the thicket stage, with a distinct E. regnans stratum and the other species forming an understorey. For the dominant E. regnans, there are only two competitors during this early period of development. The competition with Acacia seedlings (Adams and Attiwell 1984) ends after three years with E. regnans seedlings overtopping the Acacias. The other competitor for the early E. regnans is Pomaderris aspera. Pomaderris aspera competes with E. regnans during the first decade due to the similarities in their root systems during that period.
After twenty-five years, immature Pomaderris aspera is likely to dominate the understorey. Scattered Olearia argophylla and Bedfordia arborescens can form a subdominant understorey component (Ashton 2000). By this time, the E. regnans would have reached reproductive age, and have begun producing seeds. By this time, they are likely to be growing about 1m per year (Ashton). The fern layer would generally be thin due to the density of the understorey. The shrub layer of Cassinia and A. verticillata, with bracken, mesophytic herbs and Tetrarrhena would be present where gaps in the understorey provided sufficient light.
Forty five to fifty years after the fire, the shrub understorey stratum should have mostly disappeared or regenerated sparsely, mostly being replaced with ferns, bracken and Coprosma quadrifida (Ashton and Martin 1996). Pomaderris matures after fifty years, and would likely dominate the understorey stratum, having been observed occupying up to two thirds of the understorey (Ashton 2000). Pomaderris was originally thought to be the climax community for the Big Ash (Jarrett and Petrie 1929), but subsequent research (Ashton 2000) shows that the Pomaderris decline in dominance with age, due to a combination of biotic factors (Ashton 2000). Throughout this stage, mature A. dealbata and melanoxylon would be present as emergent or subdominant components of the understorey strata (Ashton 2000).
Between sixty and seventy years after the fire, the shrub stratum Prosthantera lasianthos would be expected to drop out of the community. It would however still be present in soil seed bank (Ashton 2000). E. regnans reaches its full height about this point in the succession (Ashton 1976).
Seventy years after the fire, E. regnans will have reached the spar stage. By this time, the understorey is likely still to have most of its original species, but the community will be undergoing changes in dominance that will have long-lasting effects on the understorey’s future structure and composition. Olearia argophylla is likely to be increasing as a subdominant tree in the understorey, as would Bedfordia. By ninety years after the fire, the Pomaderris would rapidly begin to thin out. Thinning of dense understoreys of P. aspera appears to be inversely related to the density of the E. regnans canopy (Ashton 1976). This would make way for Olearia, Coprosma quadrifia, and the tree ferns. Around this point in time, Acacia dealbata would probably drop out of the community. A. melanoxylon would persist longer, but it is unknown precisely how long. A. dealbata would still be present in the soil seed bank (Ashton 2000), and potentially remain viable for the next two centuries (Ough 2001).
After a century of succession, Pomaderris also drops out of the community. The dominant species in the understorey strata would now appear to be Olearia argophylla and Bedfordia arborescens, along with the tree ferns, bracken, mesophytes and bryophytes forming the lower strata.
Due to their resprouting capacity, and little research on their lifespans, it is uncertain how long Olearia argophylla and Bedfordia arborescens remain as the dominant understorey species. What is known however is that species normally found in riparian rainforest communities are likely to invade the understorey community (Ashton 2000). Montane ash forests sometimes have these rainforests along drainage gullies, where the community ecology is supported. The community is dominated by such species as Atherosperma moschatum and Nothofagus cunninghamii, but has a significant fern component. The community is usually restricted to the riparian area by the region’s fire ecology, the gullies being the only place offering protection from the fire. Atherosperma begins to invade the community around a century after the fire, and it is predicted (Ashton 2000, Ashton and Martin 1996) that the Big Ash forests two hundred years after fire is a seral community, with the mature E. regnans overtopping a rainforest community dominated by Olearia, Atherosperma and fern species.
Given time, the community may continue to change. By two hundred and ninety years, A. dealbata would no longer regenerate from the soil seed bank, as it is unlikely any viable seed remains in the community. By about three hundred and fifty to four hundred years, the dominant E. regnans in the canopy will begin to senesce. As this happens, the community structure will change to a very tall open or woodland community with the rainforest understorey remaining. It is undetermined how the understorey would respond to the increasing senescence of the dominant species, but it is likely some species would begin to out-compete other species. The Olearia would possibly give way to the longer living Atherosperma (Ashton 2000), and if so, would probably drop out of the community after about four hundred and fifty years, if it even persisted that long in the community. Atherosperma, with a lifespan of about three hundred years, would become the dominant canopy species, with an understorey cohort of fern species. Atherosperma readily regenerates under its own canopy, creating a dense overstorey about forty metres tall.
The Climax and the Conclusion
The dominance of Atherosperma in the climax community depended on its proximity to the successionary Big Ash forest. Had it not been subject to propinquity, the dominant species would most likely have been the tree ferns, with an understorey of ground ferns and bryophytes. Not mentioned in this successionary sequence is the small scale regeneration associated with tree smash or fall and the resulting gap dynamics. This results in small vegetation mosaic patches of differing composition (Ashton 2000). It may also stimulate the regeneration of an occasional E. regnans seedling. Only sixteen percent of the seeds released by E. regnans will germinate to the extent of producing a radicle (Cunningham 1960), but most of these are killed in the litter layer by high temperatures (Cunningham 1960, Ashton 1975). Rarely, a seedling might establish in a tree smash gap environment, where the soil and litter layer have been disturbed, and where there is sufficient light (Ashton 2000, Cunningham 1960). Having established, the likelihood of the seedling is inestimably small. The likelihood of any seedling reaching the canopy in a mass germination is about 1 in 500 (Cunningham 1960). For a seedling in an established forest, its fate is unlikely. It may grow to be a suppressed sapling a few metres tall, but suppressed E. regnans tend to only last a few decades before dying, with few surviving beyond thirty years (Ashton 1976).
Since there was little research or field sites available on the later stages of secondary succession in montane E. regnans dominated forests, much of the later community was estimated using observed vegetation trends. It should also be noted that many of the predictions made in the literature are untested.
- Adams MA and Attiwell PM (1984). Role of Acacia Spp. in Nutrient Balance and Cycling in Regenerating Eucalyptus regnans F. Muell. Forests. 1 Temporal Changes in Biomass and Nutrient Content. Aust. J. Bot., 32, 205-15
- Ashton DH (1975). Studies of Litter in Eucalyptus regnans Forests. Aust. J. Bot., 23, 413-33
- Ashton DH (1976). The Development of Even-aged Stands of Eucalyptus regnans F. Muell. in Central Victoria. Aust. J. Bot., 24, 397-414
- Ashton DH (2000). The Big Ash forest, Wallaby Creek, Victoria-changes during one lifetime. Aust. J. Bot., 48, 1-26
- Ashton DH and Martin DG (1996). Regeneration in a Pole-stage Forest of Eucalyptus regnans Subjected to Different Fire Intensities in 1982. Aust. J. Bot., 44, 393-410
- Chambers DP and Attiwell PM (1994). The Ash-bed Effect in Eucalyptus regnans Forest: Chemical, Physical and Microbiological Changes in Soil after Heating or Partial Sterilisation. Aust. J. Bot., 42,739-749
- Cunningham, TM (1960). The natural regeneration of Eucalyptus regnans. School of Forestry, University of Melbourne, Bulletin No. 1, Melbourne.
- Jarrett PH and Petrie AHK (1929). The Vegetation of the Black’s Spur Region: A Study in the Ecology of Some Australian Mountain Eucalyptus Forests: II. Pyric Succession. The Journal of Ecology, 17, 249-281
- Ough K (2001). Regeneration of Wet Forest flora a decade after clear-felling or wildfire—is there a difference? Aust. J. Bot., 49, 645–664.