Date:  07 August 2024

The use of the small high-resolution cameras – fitted to a fixed-wing aircraft – is a new method for collecting forest canopy data, an important measure for the DOC’s work to assess forest health.

DOC Monitoring Science Advisor Terry Greene says the cameras allowed for a comparison of forest canopy health between two adjacent tracts of land in the Papakai area of the park, totalling 2016 ha.

“The aircraft’s flight path took it over two distinct areas of Papakai – a 1300 ha strip we’ve treated with 1080 to control possums, and a 716 ha area where we haven’t,” Terry says.

“We analysed the images to determine the extent of dead and living trees in the canopy.

“The differences between treated and untreated areas are stark – with more than eight times the amount of dead wood seen in the untreated area compared to the Papakai predator control area,” Terry says.

The high-resolution images allow individual trees to be identified within the forest canopy. Images were collated into a seamless picture or ‘orthomosaic’ and analysed using computer algorithms to group pixels with similar characteristics, enabling trees to be classed as either dead or alive.

The difference in the size of the treated and untreated areas (1,300 ha compared to 716 ha) was factored into the analysis so they could be more equally compared.

The trial results correlate with long-term predator control in the Papakai area. Where possums are controlled, the forest canopy is much healthier in the images captured and where there is no possum control there is significantly more dead wood.

Possums are known to go for the most appetizing species first – and in Coromandel, that’s tōtara and kāmahi species. Possums will continually target these trees, eating all their foliage and eventually killing the tree. Over time, possums will impact the structure and composition of a forest as the canopy trees are replaced by less possum-palatable species.

The reduction of healthy forest canopy trees means the wider forest is more susceptible to disease and storm events which cause further dieback and can ultimately lead to complete collapse of the forest ecosystem.

Possum control keeps the possums from building up to levels where they are causing canopy dieback which means the forest is more resilient.

Terry says more work is needed to assess the relative contribution of other factors such as drought and disease to assessments of forest canopy mortality.

Background information

The protected Coromandel forest is home to important native species such as Archey's frogs, Coromandel striped gecko and Coromandel brown kiwi.

Forest canopy monitoring is part of DOC’s broader programme to monitor the outcomes of its predator control work for the native species and forests that it’s designed to protect.

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