Indicator 1.1d: Fragmentation of forest cover (2025) - Supporting information

This indicator reports a simple set of spatial variables that can form the basis of tracking forest fragmentation nationally and regionally over time. Analysis of fragmentation involves measuring one or more of several parameters derived from spatial analysis of the configuration of forest cover (Tickle et al. 1998; Lindenmayer et al. 1999). Fragmentation parameters can include the relative amounts of edge (forest-exterior) and interior forest, and the size and shape of forest patches.

Australia’s forests are mapped at a one-hectare scale in the National Forest Inventory (See About Australia’s State of the Forests Report for the definitions of forest used in Australia), with each one hectare unit or ‘pixel’ across Australia being scored as Native forest, Commercial plantation, Other forest or non-forest (Indicator 1.1a-i). This dataset is suitable for analysis of forest fragmentation at the national scale with limitations. For example, at the scale of the forest mapping undertaken for the National Forest Inventory, narrow breaks in forest cover (for example, a minor road), may not sufficiently interrupt the forest cover when converted to a 100-metre unit to influence fragmentation analysis; fragmentation metrics may also not capture forest that is subject to disturbance but continues to meet the definition of forest.

Two sets of metrics were calculated using only the native forest component of the National Forest Inventory forest dataset (ABARES 2023), fragmentation classes (forest-interior and forest-exterior) derived from the number of forest units that each native forest unit has as (edge-to-edge) neighbours, and patch size which is derived from the size of patches of native forest in which every unit is a neighbour (edge-to-edge) to another native forest unit.

The fragmentation class metric distinguishes two classes (Figure 1.1d-3):

• ‘Forest-interior’ units are native forest units that have all their four neighbouring (edge-adjacent) units as native forest. A higher proportion of forest-interior units implies a forest that is relatively unfragmented, and not affected by any nearby non-forest and non-native forest area.
• ‘Forest-exterior’ units are native forest units that have one or more non-forest or non-native forest neighbouring units, and are therefore at a boundary between forest and non-forest or non-native forest. They could also be named ‘forest-edge’ units. A higher proportion of forest-exterior units and a lower proportion of forest-interior units implies a forest area that is relatively more fragmented.

The area represented in Figure 1.1d-3 comprises two patches of forest. The top-right unit is a patch containing one hectare of forest, not being edge-connected to any other forest unit, while the remaining forest units are all edge-connected and make up a 12-hectare patch of forest.

The 13 units in this area of forest comprise 11 forest-exterior units (coloured orange) and 2 forest-interior units (coloured green and containing the number ‘4’): the latter are the units that have all four of their neighbouring (edge) units as forest. In this area of forest, the proportion of forest-interior units is 2 of 13 units = 15%.

The non-forest adjacent to forest-exterior units may be land cleared for agricultural land use, urban development or infrastructure, with potential to affect the forest ecosystem; may be commercial plantation or other forest; may be woody non-forest vegetation such as sparse woodland with under 20% crown cover, and representing a natural vegetation transition with a lesser impact on the native forest area; or may be other non-forest vegetation.