Managing savanna riparian zones

Riparian vegetation in northern Australia is vulnerable to processes such as changes in water flows, fire, weed invasion and cattle grazing. Monitoring the condition of riparian vegetation is critical for effective adaptive management but can be challenging in northern Australia. Methods such as on-ground quadrats or transects have traditionally been used to survey vegetation but there is increasing interest in using remote-sensing techniques as these methods become more affordable, including drone-based photogrammetry and terrestrial laser scanning.

This component of the project compared two terrestrial laser scanning (TLS) sensors, drone photogrammetry and traditional vegetation transect methods to assess the structure of riparian vegetation in the lower Fitzroy River, Western Australia. The overview of the techniques focuses on the usability and application for management agencies, including associated costs and the level of knowledge, skills and experience required to process and analyse the data collected.

The Fitzroy River catchment map

The Fitzroy River is in the Kimberley region of Western Australia. 

Each of the tested methods has strengths and limitations in relation to scale (both detail/resolution and spatial coverage area), expertise, training and overall costs. Each method has potential to address specific questions in relation to the monitoring of riparian vegetation. TLS was found to be the most accurate and high-resolution method to investigate vegetation structure, but covers a smaller spatial area than drone photogrammetry. There are also a number of potential barriers to the application of TLS methods, including cost of equipment and software, advanced training expertise required, and field conditions. Drone photogrammetry provides TLS-like point-clouds, covers larger areas and has a lower cost to acquire data compared with TLS. However, drone photogrammetry provides less sub-canopy vegetation structure information and also requires high levels of expertise for processing and data extraction.

 

TLS point cloud

TLS point-cloud top down. Riparian vegetation density represented by hyperspectral image.

The TLS point cloud can be visualised at different angles. A side view (top) highlights the height of trees. A view from above (bottom) may better show the cover and distribution of vegetation.

Each of the tested methods produced different types of data, from highly detailed 3D point-clouds using TLS, to estimated vegetation health scores using rapid vegetation transect assessments. There were also differences in the spatial scale assessed by each method and the opportunities for sampling frequency.

Data collection using a drone in the Fitzroy River catchment.

Collection of data using a drone. Photo: Fiona Freestone.

While the technology to collect remotely sensed data is becoming increasingly user-friendly and affordable, processing the data remains a specialised skill. To derive metrics from TLS and drone photogrammetry requires access to appropriate computer processors, software and specialised knowledge, skills and experience. Therefore, the application of TLS or drone photogrammetry should align with the specific aims of the monitoring program and the budget and human resources available.

Completing surveys of riverbank vegetation.

Completing surveys using the transects. Photo: Karen Dayman.

Stinking passionflower (Passiflora foetida) is a weed of significant environment concern along the banks of the Fitzroy River in Danggu (Geikie Gorge) National Park in Western Australia’s Kimberley region. Danggu is included in the West Kimberley National Heritage Place, and the listing specifically notes Danggu’s ‘lush riverine vegetation’. The area is of great significance to Traditional Owners and is a popular tourist location. Danggu also has high conservation values for several species, including the Australian freshwater crocodile (Crocodylus johnstoni).

This component of the project aimed to understand if and how the presence of stinking passionflower impacts on nesting success of freshwater crocodiles, and to develop prioritised weed management recommendations.

This research found no evidence that stinking passionflower currently limits the availability of crocodile nesting space at Danggu – seasonal flooding regularly removes weed biomass and opens up the riverbanks. However, this could change if river flows change in the future – for example, following multiple low-flow years or as a result of water extraction. There are other significant aesthetic, amenity and biodiversity reasons to manage stinking passionflower at Danggu, but this study was confined to the impacts of the weed on freshwater crocodile nesting.

A logical framework detailing timing and techniques of control of stinking passionflower was developed to optimise management. In the long term, a biological control solution is a feasible and effective way to reduce occurrence of the weed in the park and beyond, and Danggu represents a high-value landscape for biological control.

 

Map of Fitzroy River identifying Danggu National Park

Location of Danggu (Geikie Gorge) National Park.

Stinking passionflower covering the banks at Danggu.

Stinking passionflower covering the banks at Danggu. Photo: Ruchira Somaweera.Stinking passionflower monitoring and management model. PDF available in caption.

Framework for decision-making and intervention to control stinking passionflower at crocodile nesting sites at Danggu. PDF available here.

This project surveyed stinking passionflower at Danggu between 2016 and 2019 using field surveys, drones, and analysis of aerial and satellite imagery.

The surveys found that stinking passionflower has a variable but persistent presence in the park, with significant coverage in some years. This variation is likely the result of the intensity and frequency of seasonal flooding, rather than a result of management activities.

There were always suitable areas available for freshwater crocodile nesting throughout the survey period, but nesting site availability could change if several consecutive low-flow years reduced flooding and allowed the weed to proliferate.

Stinking passionflower at Danggu National Park between 2016 and 2019 separated into areas where the vine was consistently present (dark green), occasional (light green) or consistently absent (unshaded).

Stinking passionflower at Danggu National Park between 2016 and 2019 separated into areas where the vine was consistently present (dark green), occasional (light green) or consistently absent (unshaded).

This project also produced an eco-hydrological model to assess and quantify habitat for stinking passionflower and freshwater crocodiles at Danggu. This prototype approach is a combination of an environmental model – simulating hydrodynamics, soil moisture and temperature – and a habitat suitability model for the two species based on a literature review, results from experimental and field research, and expert judgement.

Suitable habitat for both species was successfully estimated using the model, indicating its potential usefulness in assessing scenarios linked to the management of stinking passionflower, and also scenarios associated with flow changes brough about by any water allocation in the catchment.

The model also identified the river interface as the habitat where these two species are most likely to interact.

This project was led by Associate Professor Samantha Setterfield at The University of Western Australia (UWA).

Researchers from Western Australia’s Department of Biodiversity, Conservation & Attractions, UWA, CSIRO and Charles Darwin University are also supporting the project.

This project was completed in June 2021.

Contact
Samantha Setterfield, UWA
[email protected]

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  • Riparian zone in Kakadu National Park, photo Michael Douglas
  • Geikie George (Darngku) National Park, photo Michael Douglas
  • Stinking passionflower in Geikie George (Darngku) National Park, photo Michael Douglas
  • Recording understorey composition, photo Fiona Freestone.
  • Measuring canopy cover, photo Fiona Freestone.
  • Prescriped savanna burn, photo Fiona Freestone.
  • Riparian zone in Kakadu National Park, photo Michael Douglas
  • Kakadu National Park, photo Michael Douglas
  • Kakadu National Park, photo Michael Douglas
  • Kakadu National Park, photo Michael Douglas
  • Riparian habitat in Kakadu, photo Fiona Freestone.