Figure 1.
The study area in the Central Highlands of Victoria, south-eastern Australia.
Figure 2.
Sequential development of tree forms in Mountain Ash trees.
Form 1: Mature, living tree; Form 2: Mature living trees with a dead or broken top; Form 3: Dead tree with most branches still intact; Form 4: Dead tree with 0–25% of the top broken off; branches remaining as stubs only; Form 5: Dead tree with top 25–50% broken away; Form 6: Dead tree with top 50–75% broken away; Form 7: Solid dead tree with ≥75% of the top broken away; Form 8: Hollow stump. In subsequent surveys we added a ninth category – Form 9: Collapsed tree.
Table 1.
The overarching modelling framework used to quantify relationships between period and fire and the mortality and collapse of large cavity trees.
Figure 3.
Temporal changes in the numbers of trees of different forms.
The relative composition of populations of large trees with cavities in different forms (sensu Figure 2) is shown for 1997–2009.
Table 2.
Percentage rates of mortality of large cavity trees by fire category adjusted (standardized) for the duration of each measurement period.
Table 3.
Percentage rates of collapse of large cavity trees by fire category adjusted (standardized) for the duration of each period.
Figure 4.
Historical death and collapse rates of trees on sites which did not experience wildfire in 2009.
Note (a) corresponds to a dataset comprised of 286 large trees with cavities on 29 sites measured in 1988 and1993, (b) refers to 744 large trees with cavities measured on 109 sites in 1988 and 1993, and (c) corresponds to a dataset comprised of 399 large trees with cavities on 49 linear strips measured in 1993 and 1998.
Figure 5.
Projected relative composition of forms of large trees with cavities in 2011, 2039 and 2067.
The latter date is when existing 72 year old trees will reach 120 years of age and regularly begin developing cavities – see text. This assumes no logging and no further fire on the 156 sites used to make the projection. In addition, we assume there are no changes in moisture index.
Figure 6.
Map of disturbance in Mountain Ash forest in the Central Highlands of Victoria.
The map includes the small remaining areas of unlogged and unburned old growth forest – a forest type that encompasses ∼1.16% of the total ash forest resource in the Central Highlands of Victoria. The map shows areas that have been clearcut since the 1960s as well as the 17 655 ha of ash of forest that planned for clearcutting in the coming 5 years under the Victorian Government's Timber Release Plan (TRP) [85]. The two squares are magnified in Figure 7 to show projected abundance of large cavity trees in different disturbance classes.
Figure 7.
Projected abundance in 2039 and 2067 of large trees with cavities.
The projection is for unlogged forest that was either unburned in 2009, subject to moderately severe fire in 2009 or subject to high severity fire in 2009. These projections are based on information contained in Figure 5. We assumed that no large trees with cavities would occur in areas that have been clearcut and slash-burned in the past 40 years or in areas that are planned for clearcutting in the coming five years. We made this assumption because past work [60] has shown that the small numbers of trees retained on harvested sites have a high probability of collapsing after logging. For these projections, we also assumed no further wildfires between 2011 and 2067, and no logging on any of our 156 long-term sites where we are quantifying tree mortality and tree collapse. The star * against the clearcut and regenerated image indicates that we did not study the death and collapse of large trees with cavities on logged sites. This study was not done because large trees with cavities are largely absent or rare in logged sites and/or rapidly collapse in these areas (see text). White areas on the map correspond to non-ash forest.
Figure 8.
Conceptual model of the key processes influencing large cavity tree death and collapse.
The processes include (1) natural disturbance (fire), (2) human disturbance (logging), (3) fire-logging interactions, (4) climate (although the effects of the moisture index on tree death were marginal in this study as indicated by the star * in the model), (5) productivity, and (6) time (as indicated by time varying death and collapse rates). These factors are underpinning accelerated large cavity tree loss and impaired large tree recruitment in Mountain Ash forests.