User:TabbyMynx/Secondary succession in Tasmanian lowland forests

Tasmanian lowland forests create a varied mosaic of vegetative communities across the landscape. Disturbance plays a large role in the temporal changes in community structure and composition. Many subterranean disturbances occurring elsewhere in the world such as earthquakes, volcanic eruptions or tsunamis either are rare or are very minor in Tasmania. There are however other types of disturbance that have had a major impact in shaping the forest communities of Tasmania, with fire being a major driver. The main types of Tasmanian lowland forest communities are temperate rainforest, dry sclerophyll, wet sclerophyll, mixed forest and non-eucalyptus forests such as Allocasuarina stands, and the distribution of these can be altered by the presence and frequency of local disturbances.

Dry sclerophyll
Dry sclerophyll consists of a very open eucalyptus canopy, but varies in its possession of an open understorey, shrubs, hedges, grasses and sedges. Most species within this forest type are mesomorphic and/or zeromorphic lending it to have a a high drought tolerance. Eucalyptus species are reliant on fire for regeneration, and optimal fire frequency for dry sclerophyll regeneration should occur between every 4 to 20 years. Less frequent and the eucalyptus canopy may die off, allowing non-eucalyptus canopy species from nearby areas to grow dominant. More frequently than this and there will be no seedbank available for regeneration leading the community to convert into a heathland, grassland or shrubbery. Severe heat, water stress, browsing and insect predation on seedlings directly after fire may have a similar effect.

In the case of low intensity fires where the trunk of the Eucalyptus survives, the trees may be able to regrow via lignotubers or epicormic shoots.

Heavy water logging may cause wet sclerophyll species to take over.

After-fire succession
Early colonisation of the fire created ash beds is by grasses, juncus and weed species, but dry sclerophyll species are present as seedlings. A large percentage of seedlings die withing the first year. Those seedlings left grow quickly and over-top the weed species, limiting the weeds light and reducing their numbers. A rapid rate of self-thinning will occur both in the canopy and shrub layer over the next 5-15 years, resulting in the characteristic dry sclerophyll structure. Understorey and shrub species may differ from the pre-fire community, but still exhibit the mesomorphic and zeromorphic characteristics.

Anthropomorphic disturbance
Anthropomorphic disturbance in this forest type is usually forestry practice associated strip-felling and clump retention, which are followed up by regeneration burns-designed to mimic the community's natural regeneration cycle. In areas where herd animals are grazed on cleared areas this could lead to conversion to grasslands.

Wet sclerophyll
Wet sclerophyll forests possess a layered structure, with an open eucalyptus canopy, followed by a sub-dominant canopy of trees with silver wattle and blackwood being the most common, an understorey of broad-leaved shrubs from 2-15m from ground level, and a dense shrub level with cryptogam ground-cover. This prevents germination of shade intolerant eucalyptus species, resulting in them relying on infrequent but intense fires for germination of successive generations of eucalyptus canopy. Fire must occur between every 20-100 years in order to maintain the community.

Overly frequent fires may cause the forest to revert to a non-eucalyptus forest of the area if the soil-borne seedbank has been killed off and there are no wet eucalyptus forests nearby enough to disperse seed to the area. This may also occur if seedlings are subject to severe frost, heat, drought, fungal pests, browsing or severe insect predation from species such as Chrysomelidae species. Another community type frequent fires may result in is buttongrass moorland.

When fires are infrequent these stands will often see a die-off of understorey species, with rainforest species taking over these positions forming a mixed forest

Drought is likely to cause dry sclerophyll species to grow within the stand, with long-term drought leading to a permanent dry sclerophyll habit.

High rainfall can water log the sclerophyll species, causing them to die and allowing a rainforest canopy to form.

After-fire succession
Initial colonisation is similar to that of dry sclerophyll stands, with wet sclerophyll seedlings over topping the weed species in about twenty years, limiting the weeds light and reducing their numbers. As the seedlings grow light becomes highly limited, with a high density of eucalyptus and understorey stems. A rapid rate of self-thinning will occur over the next 5-15 years, resulting in the characteristic open eucalyptus canopy. Nearly half a century since burning, if no other disturbances occur, the community will be largely restored to its previous structure, and will possess a large abundance of cryptogam species.

The following table shows succession of species in logged and burnt wet sclerophyll forests progressing to mixed forest with age. Note: regeneration burns may differ in succession from natural burns.

Anthropomorphic disturbance
Anthropomorphic disturbance in this forest type are similar to that of dry sclerophyll, though slash and burn is more likely to be used in forestry practices than retention.

Temperate Rainforest
Rainforests occur in areas with fire regimes of greater than 350 years. These forests generally consist of dominant Nothofagus and/or Atherosperma moschatum species, with many ferns and a high diversity of cryptogam species underneath.

Recovery from fire is possible with fires of very low frequency and intensity. Dying canopy enables understorey species to grow taller, allowing the community to continue indefinitely in lieu of any major disturbances. Increased fire frequency is likely to cause the rainforest species to die and enabling growth of bracken, cutting grass or sclerophyllous species.



Mixed Forest
Tasmanian mixed forest possess a wet sclerophyll canopy with rainforest understorey. Fire is required every 100 to 350 years to maintain this community type. Overly frequent fire and it will revert to a wet sclerophyll forest type, bracken, or cutting grass ; too infrequent and the eucalyptus canopy will die allowing rainforest species to become dominant.

Water logging of the eucalyptus' may also lead to rainforest dominance, whilst drought could convert the community to a dry sclerophyll forest type, grassland, sedge, or bracken.

Other
Most non-eucalyptus forests are sensitive to disturbance, and thus exist in areas with negligible to no disturbance. Disturbance will cause a less sensitive forest type to grow, or convert the area to grassland, scrub or brush.