Beneath the Mountain Ash   The winding track is lightly muddy this soggy Belgrave spring. Scrub to the side is dense and unyielding. Eucalypts rear their shaggy, honey trunks to stretch above and straggle. A crimson rosella darts nervously from a hollow; there is almost no birdsong now the nests are charged with eggs.   On the dam below, pacific black and wood ducks cruise with moorhens beneath a shore lined by giant columns of mountain ash that rear like images from past millennia. Along the valley floor a muddy creek snakes past snags and undercut banks. On its sides…