Darwin’s large woodland trees – a preliminary study

1.  Introduction

The tropical savanna woodland (hereafter referred to as woodland), and the diverse wildlife it supports, helps make Darwin a special place for residents and visitors. Many of the animal and bird species found around Darwin rely on tree hollows for shelter or nesting. The oldest and largest trees of the woodland provide the highest number and quality of hollows (see tree hollows) as they have experienced a long life where branches are broken by storms or burned by fire, exposing trucks hollowed by termites. Unfortunately, most of these large trees have been lost through clearing during the development of Darwin, resulting in large declines in the abundance of the wildlife requiring this important habitat.

This report presents the preliminary findings of a study undertaken by Friends of Lee Point investigating the current and future state of Darwin’s savanna woodland containing important mature trees with hollows.

Approach – Current woodland assessment

Darwin’s woodland (Figure 1) consists mainly of Darwin Stringybarks (Eucalyptus tetradonda) and Darwin Woollybutts (Eucalyptus miniata), with Bloodwoods (Corymbia spp.) which tend to be in the lower, wetter areas. These woodland stands with mature, large trees generally occur on elevations greater than 14 m above mean sea level (MSL), which reflects the rise in landform from the low dunes and mangrove habitats.

Figure 1. A native mix-species stand of large Darwin Stringybarks, Darwin Woollybutts and Bloodwoods at Lee Point.

Previous scientific research (see tree hollow abundance, and Woinarski and Westaway 2008) has found a significant relationship between hollow abundance and tree characteristics such as size and species, as well as other factors. The density per hectare of large trees meeting the criteria from this research was used in this study and we identify “large trees” any of the main species (E. minita, E. tetradonta and Corymbia spp.) with a diameter of the trunk at breast height (DBH, 1.3 metres above ground level, in cm) of 35 cm or more. All other tree types were excluded. The location and abundance of large trees was determined by a targeted field survey.

All areas of natural woodland more than 14 metres above mean sea level were identified from satellite imagery (Google maps) and surveyed by foot or bike in late 2022. A reduced effort was applied at Charles Darwin National Park as it is expected to become more isolated for small mammals with any wildlife corridor ceasing to exist by 2050. A mobile survey tool (Mergin Maps App) and QGIS was used to collect spatial data and map habitat quality for display in Google Maps.

The study area (Figure 2. inside white line) was chosen to include housing areas no further than 30 minutes’ drive to Lee Point and at least 14 metres above mean sea level. It also includes Darwin and Palmerston which is over half the Territory’s population.

Figure 2. Darwin Study Area (187 sq km) – 14m above mean sea level with Defence Land shown in purple and woodland habitat in green.

In addition to locating individual trees, the various natural woodland habitats surveyed were rated for the quality of hollow providing habitat (i.e., the presence of large trees). Only parks or buffer areas that had or could support the selected large woodland trees were rated using the categories below:

  • POOR: 0-2 large trees per hectare.
  • FAIR: 2-5 large trees per hectare and an area larger than 1ha (small isolated areas were ignored)
  • GOOD: 5-10 large trees per hectare with evidence of hollows >5cm.
  • VERY GOOD: >10 large trees per hectare with evidence of hollows >5cm.

2. Future land use

The NT Government’s Darwin Regional Land Use Plan guides land use in Darwin and shows areas intended for development. These areas are typically 14 m above mean sea level and therefore cover the same elevation as the woodlands considered here.

Future woodland assessment

We used historic satellite imagery (Google Maps) to provide an indication of woodland habitat area in 2006 and the scientific literature and the following assumptions to predict the state of woodland with old trees in 2050:

  • All woodland areas 14 metres above mean sea level except where currently in reserves or buffers will be developed.
  • No further clearing of woodland areas will occur at Lee Point.
  • The number of large trees will not change significantly by 2050 in current woodlands.
  • The surveyed large woodland trees reflect the relative abundance of tree hollows.

Findings

Future land use

An excerpt from Darwin Regional Land Use Plan (for Greenfield development, Figure 3), with current Defence Force bases added, shows the future housing areas of Berrimah (2,000 houses) and Holtz (1,500 houses). Weddell (10,000 houses) is planned to be the major housing area in future. Planning for Lee Point describes the benefits of housing defence personnel at Berrimah instead of Lee Point. All future land use is above 14m above mean sea level and so includes the woodlands of this study.

Figure 3. Darwin Regional Land Use Plan – Greenfield development with Defence Force bases added and showing the future residential developments.

 

3.  Woodland assessment

Most of the large trees (>=35cm DBH) observed were Darwin Stringybarks. In areas missing large trees, but with conditions expected to support mature woodland, these trees were generally less than 25 cm DBH and not expected to reach 35 cm DBH by 2050 based on growth rates reported in the scientific literature for these species in Top End savannas (approximately 1.6mm DBH per annum – references to be added).

A number of large trees of other native species were observed, but these have been excluded for the following reasons:

  • Acacias, Melaleucas and other Eucalypts – very few hollows were observed in trees over 50 cm DBH, making tree hollows in these tree types uncommon.
  • Northern Milkwood (Alstonia actinophylla) – no hollows were observed, and they are known to be termite resistant
  • Dead trees – these trees are not expected to exist in 2050 given the current fire regime and occurrence of storms.

We estimate eight square kilometres of woodland in parks and buffers from 2006 and these are assumed to be protected through to 2050 (Table 1). Other areas of native woodland have declined from an estimated 48 km2 in 2006 to 21 km2 today, and unless specifically protected there is no reason to suggest these will be present in 2050 based on the current land use plan.

Table 1. Historic (2006), current (2022) and predicted future (2050) Darwin woodland habitat area (km2)

  Year 2006 Year 2022 Year 2050
  sq km sq km sq km
Parks and buffers (see Table 2) 8 8 8
Native woodland 48 21 0
Total 56 33 8

The location of woodland areas identified are provided in Figure 4 with the area of woodland classified in each of the rating levels provided in Table 2. The POOR rating relied on aerial photography plus general field observations while FAIR, GOOD and VERY GOOD ratings were based on the field surveys and GPS-based spatial observations. Field-work load was reduced by only using selected main tree types with a minimum DBH of 35cm, ignoring isolated small areas <1ha with only a couple of large trees, and estimating the number of tree hollows (in place of a detailed assessment).

The majority of spatial survey records were identifying individual trees (with tree type, DBH and approx. number of hollows), while the remainder of GPS marks provided information on the ratings.

The majority of the very large trees (>= 50 cm DBH) were located in GOOD or better areas. The majority of the GOOD and VERY GOOD areas are in the DHA project area on Lee Point. While Charles Darwin National Park may well have the largest VERY GOOD area, it was only a quick assessment and warrants further assessment.

Figure 4. Location of parks and buffer areas that were rated (see Table 2)

Table 2. The amount of woodland habitat (ha) in each area surveyed based on rating levels assigned in 2022 with locations provided in Figure 4.

Location
POOR or better
FAIR or better
GOOD or better
VERY GOOD
Records using GPS
 
ha
ha
ha
ha
no.
1. CCR (not in Lee Point)
33
4
1
0
10
2. Lee Point DHA project area
110
30
20
2.5
100
3. Lee Point (outside DHA project)
161
25
4
0.8
110
4. Sewerage buffer
71
10
1
0
10
5. Karama dump buffer
105
11
4
0
25
6. Holmes Jungle Reserve
113
41
3
0
50
7. Knuckey Lagoon Reserve
39
4
2
0
15
Sub-total
614
125
35
3.3
320
8. Charles Darwin National Park
180
114
32?
na
4
TOTAL
794
239
   
 

 

Lee Point woodland, DHA project and offset area

This section provides results specific to Lee Point, the DHA project area. A 21.5 ha offset area has been allocated to compensate for the loss of the endangered Black-footed Tree-rat (Mesembriomys gouldii) habitat from planned clearing of stages 1-8. While Stage 1 included suitable habitat, it was cleared before this assessment and hence only Stages 2-8 were included here. Figure 5 indicates the amount of fair, good and very good habitat with respect to large trees and hollows that is present at Lee Point and the flow of this habitat providing a corridor from Casuarina Coastal Reserve in the east to Buffalo Creek in the west.

Figure 5. Woodland habitat rating for Lee Point with DHA project area (red) and offset areas (purple)

Table 3. The rating of woodland quality across Lee Point and that encompassed by the DHA project area and the 21.5 ha Black-footed Tree-rat offset area. See locations in Figure 5.

  POOR FAIR GOOD Very GOOD
  ha ha ha ha
(A) DHA project area 87 24.0 9.3 2.1
(B) 21.5 ha Offset area 6.6 3.9 0.9 0
Net loss in ha (A – B) 80 20.1 8.4 2.1
% of B/A 8% 16% 9% 0%

 

4.  Discussion

This study represents a preliminary assessment of the state of woodland containing large trees in the Darwin region. It shows that natural woodland stands providing hollows for wildlife are limited around Darwin with Lee Point providing probably the best tree stands outside the few relatively small National Parks and Reserves. We acknowledge several other woodlands, but these are on Defence land including the airport where access is restricted, and we were unable to undertake any assessment of their quality and suitability.   

This study relied on published scientific research for the relationship between tree DBH and likelihood of tree hollows (refer to Woinarski and Westaway 2008), and for growth rates for savanna tree species (reference to be added). Accepting these relationships meant that work could be focussed on locating/recording large tree areas.

Large trees (>=50 cm DBH) are important for providing large tree hollows for wildlife. The majority of the trees >=50cm DBH occurred in the GOOD areas. Conserving GOOD or VERY GOOD areas is an absolute priority because of the significant large trees they contain.

Without considering the quality woodland lost through the clearing of Stage 1 (personal observations), the DHA project has the potential to destroy some of the best and last savanna woodland habitat in the Darwin City and beyond.

The proposed “21.5 ha offset” for the endangered Black-footed Tree-rat contains less than four hectares of woodland rated FAIR or better. In essence, the DHA project blocks the corridor making access to the offset area more difficult for the Black-footed Tree-rat (an area it had access to anyway) and removes 30 ha of woodland (rated FAIR or better) for this small mammal, suggesting the offset is not fit for purpose in conserving this endangered species.

The identification of large trees on Lee Point and subsequent rating of woodland habitat clearly identifies the corridor of highly valuable habitat linking Casuarina Coastal Reserve to Buffalo Creek and supporting wildlife in the northern suburbs. This corridor includes the Lee Point Dam and may explain why this dam is so utilised by a wide variety of species including the endangered Gouldian Finch.

Regardless of the Darwin Land Use planning map, whether all land more than 14 m above MSL will be developed (causing habitat loss) by 2050 is open to speculation. More likely is that the current woodland areas for small land animals will become more and more fragmented resulting in a similar fate, a decline in numbers. Habitat loss and fragmentation are leading causes of biodiversity loss in urban areas. Without protection, it is highly unlikely that any valuable savanna woodland habitat will remain outside the parks and reserves in 2050 given the current rate of land clearing for development. This will have dramatic consequences for the wildlife that relies on this habitat and may have follow on impacts on Darwin’s biodiversity in general affecting the ecological value of the city for residents and the potential ecotourism market.

This study highlights the need for further research into the remaining woodland tree stands around Darwin and Friends of Lee Point encourage funding into further valuable ecological research, tree mapping and inventories, and improved fire and weed management for this internationally significant tropical savanna.  

 

5.  Key findings

  1. The number of large woodland trees in the Darwin study area (187 sq km) will be substantially reduced by 2050 and Charles Darwin National Park will become more isolated from the other woodlands for small mammals.
  2. The Lee Point corridor is expected to have a significant proportion of the remaining large woodland trees (and tree hollows) in Darwin by 2050 – assuming no further land clearing of the Lee Point corridor occurs.
  3. The DHA project (if it proceeds) will remove 20ha of the GOOD or better woodland area – this represents over 60% of GOOD or better areas outside Charles Darwin National Park and over 90% of the GOOD or better woodland areas from Lee Point.
  4. The proposed 21.5ha offset area (for the Black-footed Tree-rat) contains less than one hectare of woodland rated GOOD or better with hollows critical for the survival of this species.
  5. The only corridor likely to connect large woodland tree areas for small mammals in 2050 is located north of Darwin’s northern suburbs.

Recommendations

  1. The Lee Point corridor be retained to conserve high quality woodland habitat.
  2. A wildlife corridor suitable for Black-footed Tree-rats and other small mammals be developed north of Darwin’s northern suburbs to conserve Darwin’s biodiversity.