The mature Mountain Ash stands in the O'Shannassy catchment are of State significance as rare surviving examples of the region's historic forested landscape, once a common feature of the forest but now scarce. It was within this forested landscape that many of the valued historic sites were created. The survival of some of these forests enables an appreciation of the historic use of the forest . From a natural heritage perspective, a high value is placed on such remnants stands from a biological and ecological point of view. (Supple 1991)

The O'Shannassy Catchment comprises approximately 5570 hectares of old growth Mountain Ash Forest, probably in excess of 300 years old. A survey in 1977 showed the stands to contain an average of 35 trees per hectare and have a stand height of 67 metres with some trees measuring 85 metres. Gullen et al (1979) describes the catchment as follows:

'Undoubtedly the highest quality vegetation and the largest area of high quality vegetation is encompassed by the O'Shannassy Catchment... The vegetation is old, rich, undisturbed and in many places breathtakingly beautiful... The O'Shannassy is a significant area for the region, the State and the nation as a whole. It is one of the few areas left in Victoria where mature wet sclerophyll forests still exists over a large area. (Supple 1991)