The existence of patches of rainforest embedded in tracts of Eucalyptus forests have long perplexed and sometimes astonished field biologists. The abrupt rainforest boundaries often rise up like ‘a dark wall’ (Figure 1.1) in the relatively open Eucalyptus forest (Herbert 1932) and literally confront ecologists with the question ‘what determines the position of the boundaries?’ The floristic differences between rainforest and Eucalyptus forests can be ‘so great as to suggest separate geographic and historical origins in spite of their growing side by side’ (Herbert 1932). The purpose of this book is to investigate the deceptively simple question of why rainforests have such limited and fragmentary coverage in Australia (Figure 1.2). This basic geographic question raises other questions such as:

1. (i) Why should Eucalyptus and Acacia dominate the great bulk of Australian woody vegetation (Figure 1.3)?

2. (ii) How should rainforest be defined in Australia?

3. (iii) What environmental factors control the local extent of rainforest?

As will become apparent in this book, these questions are central issues in Australian vegetation science. The rainforest-boundary question occurs in nearly all the major arguments about Australian woody vegetation. Indeed, a number of important theories about the ecology of Australian woody vegetation have explicitly sought to resolve this problem of sharply contrasting forest types growing side by side. I recognise that the Australian rainforest boundary problem is a subset of the global biogeographic question concerning the cause of the differentiation between forest and savanna.