Friday, December 8, 2023

Plant pests mapped from space

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Satellites capable of identifying wilding pines and mapping systems that monitor their spread and demise once sprayed with herbicide are all part of the new toolbox at hand for combating a pest marring 100,000ha a year of countryside. Richard Rennie spoke to Scion researcher Michael Watt who is working on ways to deal to the wilding pine pest.
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The downside of the introduction of exotic conifers has been the spin-off of wild seedlings establishing in many areas, particularly around Southland, the MacKenzie Country and Canterbury. 

Wilding pines now cover almost two million hectares, a significant amount given New Zealand’s total pastoral area is about eight million hectares, Scion researcher Michael Watt says.

Fortunately, the common radiata pine does not disperse as widely as some varieties but the likes of Douglas fir and lodge-pole pine are among the worst offenders.

“The problem with wilding pines is they can out-compete many native plants for nutrients, light and water and can do well in dryland sites like MacKenzie Country to the detriment of existing vegetation.”

Researchers are encouraged by recent work using manned aircraft  and drones equipped with multi-spectral sensors. The research provided encouraging results for using the technology to identify invasive, wilding pines. 

Work done near Tekapo accurately identified wilding pines before they formed reproductive seed cones, meaning pest managers can find juvenile trees before they spread their seed and deal with them.

“This is absolutely critical for limiting the spread of wilding pine invasions to new areas.”

Work by Scion researchers is also helping improve the accuracy of wilding pine detection from space thanks to new generation satellites like Sentinel-2, operated by the European Space Agency. It can deliver very high resolution images down to 10 square metres.

Its frequent passes over NZ mean it provides a good platform for detecting pest plants. 

Recent work by one of Watt’s colleagues, Jonathan Dash, has developed algorithms capable of enabling the satellite’s technology to recognise wilding pines from space. It uses the spectral properties of the wilding pines to separate them from other vegetation.

“This will prove a far more efficient and cost-effective means of finding the trees compared to just flying around in a helicopter hoping to identify them and spray them when you have.” 

If one satellite is providing the eyes for detection, another might yet bear witness to the demise of those trees identified and dealt to by a dose of Scion’s specially prepared herbicide treatment.

Working with Rebecca Scholten from Trier University in Germany the researchers have spent the last two years using hyper-spectral imaging to better determine the effectiveness of spray programmes on wilding pines.

Taking 2m high pines and spraying them with herbicide in the lab the team could monitor how the trees’ photosynthesis broke down post-spraying. 

A hyper-spectral imager captured invisible changes in the plants’ needles’ light reflectance as the spray took effect. 

The team developed an index that matches the detected spectral signature to the level of degradation from spraying. 

The research results have the team looking forward to the launch late next year of Germany’s EnMAP satellite, also equipped with hyper-spectral sensing equipment.

“This will have reasonably regular passes of up to every four days with spatial resolution of 30m. It could provide a powerful tool for monitoring the efficacy of wilding pine herbicide application across large areas of dense infestation.”

Taking what has been proven in the lab into space will make covering large swathes of the country far quicker and cheaper than trying to over-fly with the hyper-spectral kit attached to either manned or unmanned aircraft.

Watt is also excited by the other applications satellite imaging offers plantation forestry management.

“Satellites can be used for detecting clear-cuts and damage from wind while hyper-spectral imagery could prove useful for detection of disease and nutrient deficiencies in plantation trees. 

“Preliminary research has already highlighted the utility of hyper-spectral imagery for identifying nutrient deficiencies in radiata pine seedlings and further research could be undertaken to scale these results to the stand level,” Watt said.  

Machines see what the eyes can’t

Hyper-spectral imaging uses sensors capable of collecting a narrow wavelength or spectral band of light reflected from an object, includeing those invisible to the human eye. 

They are combined to form a three dimensional data cube that includes the scene’s spatial dimensions and its captured wavelengths. 

The resulting images can provide a high degree of detail about land profiles, crops and even mineral content of land areas.

The technology was originally used in mining because of its ability to detect different minerals and capture the distinct spectral signature of each mineral. 

And it has proved to have multiple applications in agriculture, including mapping farms for nutrient run-off, vegetation type and even for disease detection in broad-acre crops.  

Massey University has pioneered the use of the technology in New Zealand by flying a hyper-spectral imager from a fixed wing plane to measure pasture energy levels on hill country properties.

Further research and trial work is revealing the technology’s ability to help detect, in advance, disease, drought and nutrition issues in crops.

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