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UCL Home  /  Geography  /  Blog  /  Blog Entries  /  Shining a light on forest structure in Brisbane, Australia

Shining a light on forest structure in Brisbane, Australia

Posted by ucfaam0 at Aug 29, 2013 02:49 PM |

by Mat Disney

One of my main research interests is how we can use remote sensing to measure and understand the state and dynamics of vegetation, particularly forests. By measuring different wavelengths of reflected sunlight e.g. from satellites we can tell a lot about vegetation properties: how much there is (sort of), the rate at which it is photosynthesising and taking up carbon, how it responds to stress or disturbance, and how it is responding to climate and anthropogenic changes.

One of the big difficulties we face is that remote sensing is just that, remote; we never get to measure exactly what we want to and so we have to try and infer it from reflected radiation. This is tricky because the signal we measure is a function not only of the vegetation colour (pigments, water content) but also to its structure i.e. how much vegetation there is, and how it is arranged. This is illustrated below in an image I took using my iphone from a light aircraft window over a rainforest in Australia. The variation in greenness is clear, but so is the variation in ‘lumpiness’ (structure) and density.

A temperate rainforest on Fraser Island, Queensland, Australia (image: M. Disney).

Forest structure encapsulates the biomass of the canopy i.e. how much carbon is stored in the tree, as well as how the leaves and branches are arranged. Current estimates of forest biomass are made using field surveys of tree diameter. This is near-direct, but is labour-intensive and can only cover small areas. Remote sensing potentially provides wide coverage, at the cost of directness. A new technique for measuring tree structure has developed over the last few years, using lidar, or light detection and ranging. Lidar measures the time it takes a laser pulse fired from the instrument to hit something in the canopy and return to the detector, allowing us to build up a 3D point cloud of laser ‘hits’ over the canopy. Lidars have been mounted on aircraft to measure canopy height and structure from above. However there is increasing interest in ground-based instruments, largely developed for surveying where there is a requirement for very precise 3D measurements. Collaborations between academia and industry have resulted in new instruments that have multiple wavelengths, allowing simultaneous measurements of canopy biochemical and structural properties. At UCL Geography we have developed highly-detailed 3D modelling methods, adapted from computer graphics, to understand how these measurements might be exploited (details here). We’re now using these models, along with various colleagues in the UK, Australia, Finland and the US (e.g. see here).

A Eucalyptus tree captured in 3D by the lidar scanner

A Eucalyptus tree (left), captured in 3D by the lidar scanner (centre) with the colours showing height above the ground (blue, low to red, high), and then (right) reconstructed into cylinders, allowing us to estimate volume, height and biomass. Manual measurements of the trunk diameter and height were 0.59m and 24m respectively, while the lidar-derived estimates were 0.55m and 23.9m (images: Andy Burt).

I was fortunate enough to spend Oct-Dec 2012 working with colleagues in Brisbane, Queensland to try out some of our new models and methods in collaboration with Dr. John Armston of DSITIA in Brisbane, and UCL Geography MSc Remote Sensing graduate Kim Calders (now a PhD student at Wageningen). As part of this work, I travelled with Andy Burt, a NERC-funded UCL Geography PhD student, to Brisbane at the start of August to participate in a unique experiment to test new lidar instruments for measuring forest structure. The experiment was organised by John (along with colleagues from DSITIA and UQ) and in collaboration with Terrestrial Laser International Interest Group (TLSIIG), a loose coalition of researchers interested in canopy lidar. The experiment brought together researchers from the UK, US and Australia to deploy five different lidar instruments with different capabilities at the same sites, allowing us to compare how each instrument sees the canopy structure. The only two dual-wavelength systems in the world (SALCA, developed by Prof. Mark Danson at the University of Salford, and DWEL, developed by Prof. Alan Strahler at Boston University and colleagues at CSIRO, U Mass Boston and U Mass Lowell) were joined by other, commercially-developed instruments scanning at single wavelengths, but more rapidly and some with finer detail.

The participants of the Brisbane laser scanner experiment July/August 2013

The participants of the Brisbane laser scanner experiment July/August 2013, standing behind more than $1M worth of various laser scanners!

Despite logistical challenges, the odd snake sighting, a few cuts and bruises, and with some lovely midwinter weather (“only” 20 C), the experiment was a resounding success. Discussions between the teams were very productive and we are already following up on how to use the data to provide new forest measurements. We were also visited in the field by Australian ABC TV news team to record a feature on the experiment (watch here). This unique experiment is the starting point to show how lidar can be used to complement measurements from satellite and those made using field surveys. We can also use the data to derive 3D tree models that we are using to explore ecological scaling laws describing how trees grow. These measurements have only ever been possible via destructive harvesting before.

As part of the next phase of this work, Andy, along with Kim and other colleagues from UCL and Wageningen, are in Gabon, West Africa, scanning tropical forest plots with lidar. More on this in a future post, when they return!

 

 

Left: Mat Disney and Dr. Rachel Gaulton (Newcastle University) measuring bark properties for use in 3D models of the trees which are being produced from the lidar measurements (image: M. Disney).

Right: UCL Geography PhD student Andy Burt scanning a forest plot at Karawatha Forest Park, using the Riegl VZ-400 lidar instrument belonging to Dr. John Armston and DSITIA Queensland (image: M. Disney).

 

 

Many thanks to John Armston for his hard work in organising the Brisbane field experiment – without him it would not have happened. Travel funds for Mat and Andy came in part from the NERC National Centre for Earth Observation and Andy’s NERC CASE studentship (with Infoterra/Astrium GEO-Information Services Division). We are also grateful for the support for this experiment of DSITIA, Queensland, the Terrestrial Ecosystem Research Network for use of site infrastructure, and the other members of the TLSIIG team.