The ecological biogeography of Callitris vent. in Tasmania

Callitris rhomboidea and Callitris oblonga have a parapatric distribution in Eastern Tasmania. C. rhomboidea occurs on the East Coast and C. oblonga is mainly confined to a few coastal and inland river valleys between Launceston and the East Coast. C. rhomboidea is capable of continuous gap-phase regeneration. Natural stands are often multi-aged but even-aged cohorts often reflect burning. C. oblonga regenerates after fire or after mechanical damage to cone bearing branches (usually during floods). It is therefore an episodic regenerator which does not appear to regenerate continuously in the absence of exogenous disturbance. The two species are ecologically differentiated by frost tolerance, waterlogging and shade tolerance. C. oblonga is more tolerant of frost and waterlogging but C. rhomboidea still occupies a very wide ecological amplitude, although it possibly has a competitive advantage on the driest sites. C. oblonga is well adapted to a riparian and floodplain niche, a distribution which is reinforced by the higher fire frequency in non riparian habitats. Hybrids between the two taxa are rare because timing of their peak pollen release is asynchronous. Climatically suitable areas apparently occur for both species beyond their present range but a temperature reduction during the Last Glacial may have limited C. rhomboidea to the east of the Eastern Tiers and C. oblonga to a small glacial refuge within the northern part of the Eastern Tiers. Expansion beyond these palaeogeographically determined limits has been retarded by the slow overland migration rates of both species, and by the onset of anthropogenic burning. Limited post-glacial dispersal of C. oblonga has been aided by downstream spread of propagules. An increase in fire frequency with the arrival of Aboriginal man about 30,000 years ago may have not only constrained the expansion of Callitris but also caused the extinction of C. rhomboidea in the far north east of Tasmania. A further increase in burning from the early nineteenth century, has caused a decline in the extent of Callitris. Frequent burning has forced C. rhomboidea into fire protected habitats such as cliffs, deep gullies and rocky knolls, and C. oblonga into areas protected by anastomosing channels, cut off meander loops or rocky benches and cliffs. Prior to human colonisation of the \Tasmanian peninsula\" C. rhomboidea was probably found on more xeric sites in juxtaposition with rainforest and wet sclerophyll forest on other sites. Frequent burning is hypothesised to have caused a shift in forest patterns. C.oblonga is inadequately reserved. Management of the species should aim to protect the most upstream stands which provide sources of propagules for downstream habitats. C. oblonga conservation would be helped by fire prevention enrichment planting and weed control. C. rhomboidea is adequately reserved but fire frequency should be reduced throughout its range and outlying stands especially on islands should be reserved and protected."