You got me thinking so I went looking...
From
http://www.wadidge.com.au/mallee.htmlMallee (an Aboriginal word) is a growth-form rather than a particular specie, that is, a Eucalypt having many stems arising from a large, underground, woody swelling composed of stem tissue called a lignotuber. The lignotubers are large, woody, convoluted swellings often 0.3-0.6 m in diameter and sometimes up to 1.5 m. The largest recorded is 10 m across, which carried 301 living stems, in Mulette, 1978. Mallee is also widely used to describe the plant communities and regions where these plants predominate.
The Mallee growth-habit can occur in response to a variety of stress conditions, the major stresses involved are shortages of water, fires, termites or felling.