Journal Number 106
December 2007

Pterostylis aenigma
by Mike Duncan
From ANOS (Vic) Bulletin 39 (11), June 2007

Many orchid species flower profusely after a bushfire, but wildfires are not always good for orchids. The Great Alpine Fire of 2003 burned for 59 days and devastated 1.2 million hectares
of bushland, including the only known site of Pterostylis aenigma in the Omeo hills. Before the fire, the orchid occurred in a broad, flat, seasonal creekline. At the last survey before the fire,
80 individual plants were recorded when they flowered in December.

The wildfire that swept through the area in January 2003 was a very hot burn that decimated all the undergrowth and turned the soil into a very fine, powdery dust. In March 2003, a very severe thunderstorm occurred, dumping 80mm of rain in less than an hour. The powdery soil was washed down the hillside and deposited a mass of sediment up to one metre deep in the creekline. While in the vicinity of the P. aenigma colony, the sediment was about 20cm deep.
(The creek has since cut itself a new path.)

So what happened to the P. aenigma colony? Surprisingly, many rosettes appeared in November 2003, and 50 flowering plants were recorded in December. However, in 2004, only ten plants appeared and no plants were seen in 2005 or 2006.

Why did this rapid decline occur? Well, in the tons of sediment that rolled onto the site were thousands of seeds of both indigenous and weed species. This led to vigorous, thick regrowth along the creekline, which is probably smothering the P. aenigma plants. Weed species such as blackberries and English broom have been particularly problematic. Intensive, fine-scale weed management is ongoing at this site to try and give P. aenigma a chance to flower again.