Changing forests
Natural regeneration pathways
This process accounts for how and why forests change. Since Australias beginnings as part of Gondwana, major environmental changes in Tasmania have been:
A shift from wet to much drier climatic conditions:
- Seasonal rather than all-year-round distribution of rainfall, with extremes of temperature including frost, and an alternating drought and flood pattern that eroded the landscape and leached nutrients from the soil; and
- In recent times greater frequency of fire as a result of drier conditions and the presence of humans
- Tasmanias forests have developed regeneration cycles which respond to differing scales of disturbance brought about by these factors and especially to the presence or absence of fire in forests
Natural regeneration pathways - cool temperate rainforest
Fire in cool temperate rainforest occurs perhaps only once every few hundred years and so regeneration is not geared to major disturbance.
Tree species in this type of forest are geared to gap-phase regeneration cycles. Parent trees produce large quantities of seed, some-times every year like leatherwood, sometimes every two or three like myrtle.
Seeds sprout and stay alive for a number of years in the small amount of light that reaches the floor of an undisturbed forest. They can then take full advantage of any extra light that becomes available when a gap appears in the canopy, such as that caused by an old tree falling or by storm damage.
Low light levels prevent seedlings from other forest types from growing, even though their seeds may be present, carried in by the wind or animals or by bird droppings.
When wildfire does occur in cool temperate rainforest the area takes 100 - 200 years without further major disturbance to regenerate fully to rainforest. If a eucalypt seed source is present.
Mixed forest will establish and pure rainforest will only form again after about 350 years. A typical pattern of succession if a Eucalypt seed source is present might be:
Mixed forest. Old eucalypt canopy over a pure rainforest understorey over 200 years since the last fire.
- First few months - mosses, liverworts and eucalypt seedlings First one or two years - fireweed and bracken with young eucalypts.
- From 60 to 80 years - dogwood, musk, stinkwood and wattles form an understorey. Over 80 years - rainforest species develop as an understorey.
- Over 350 years - eucalypts die and pure rainforest forms.
Developing rainforest understorey in wet eucalypt forest.
Pure rainforest almost 400 years since the last fire.
Natural regeneration pathways - wet eucalypt forest
Wet forest eucalypt regeneration depends on fires for rapid seed release, for removing understorey vegetation and forest litter, (thus reducing competition for light), and allowing seedling root access to adequate moisture and mineral soil.Populations of seed harvesting insects and browsing mammals are also reduced Fire in wet forests is infrequent, occurring in severe drought every 80 to 200 years.
Huge amounts of fuel build up in the litter layer both because wetter areas produce more growth and it has longer to accumulate. Low nutrients in the litter means it is broken down very slowly. Intense fire kills everything leaving only an ash bed for new eucalypts to develop in. (Seed is protected from the heat in capsules high in the canopy.) This fire regime results in trees of the same age making up the forest.
Understorey species such as musk, dogwood and blanketbush recolonise the burnt forest by spreading seed on the wind from patches of unburnt forest which may be kilometres away.
The dense understorey of the wet eucalypt forest prevents it from drying out and also allows immense accumulation of litter. (Low nutrients in this litter means it breaks down slowly.) Fires, occurring once every 80 to 200 years, destroy everything but leave an ash bed ideal for the growth of new eucalypts. These arise from seed falling from dried eucalypt capsules in the burnt canopy.
Young and old forests of the wet eucalypt type can look quite different because trees are all the same age dating from the last fire. A whole forest may be composed of either very young, mature or old trees depending on how long it has been since the last major fire.
Natural regeneration pathways - dry sclerophyll forest
Fire in dry sclerophyll forest occurs perhaps every 25 years or less. Fire conditions are the opposite to those in the wet eucalypt forest: because there are more frequent fires in dry areas and less growth, forest litter doesn’t build up to such levels.
Fires may burn only at ground level leaving taller under-storey trees untouched. This means that young and old dry sclerophyll forests can look quite similar, (Unlike young and old wet eucalypt forests), because they have many age classes of trees which have survived successive fires.
If fires occur more than once every five to seven years, some shrubby species may disappear as they take up to seven years to reach seed-bearing age. Spring fires result in grassy groundcover while autumn fires generally produce heathy groundcover in dry sclerophyll forests.
The results of a spring fire in a dry sclerophyll forest. The moist ground has meant that the fire was not very hot and scorch height is low. Young eucalypts scorched to their crowns have not been killed and will re-sprout from epicormic buds protected under the bark on the trunk of the trees. The bare ground will allow a new generation of eucalypts to germinate and grow in the high light levels.
The last fire in this patch of dry sclerophyll forest was around 10 years ago. This patch of forest is very close to the patch of burnt forest in the other picture. Apart from the fire scars on the older trees it is not evident that fires are a regular visitor to this forest. All plants here are well adapted to the regular fires passing through the area.