Monday, 14 July 2014

Summer of Scan 2014: Alice Holt revisited

We've been gradually getting to grips with the Riegl VZ-400 and ironing out some of the quirks, while we go about scanning. We are very lucky to be able to work with colleagues at Forest Research, the research branch (ahem) of the UK Forestry Commission, based at Alice Holt on the border of Surrey and Hampshire. A longstanding collaboration with Dr. Eric Casella, who has mutual interests in TLS and 3D tree structure, has led to us being able to piggyback on his destructive harvest activities in the Alice Holt forest. We've also been working on this with Prof. Mark Danson, from Salford and his SALCA instrument, and in early July we all turned up at the forest to measure and harvest some trees.

The experiments we've been doing are aimed at characterising the above-ground biomass (carbon) of trees using our laser instrument non-destructively and rapidly. This is important as we know forests sequester atmospheric CO2 as carbon in wood, but estimating this for larger areas is hard and uncertain, and relies on simple empirical models drawn from very small samples of trees.

So we think we can measure all this pretty well  with our laser scanner techniques - our results from Brisbane, Gabon and elsewhere have demonstrated this. But ironically, the best way to really convince people that it works is to cut down some trees and weigh and measure them in full detail. Which is a lot of work!

Here we can see the moment one of the oaks was felled by the FR foresters.

So our experiments in Alice Holt have involved:

  • Scanning three, selected oak (and other) trees, with leaves off in spring
  • Scanning the same trees with leaves on in late spring/early summer
  • Felling the selected trees, weighing all of the woody components both wet and (oven) dry
  • Collecting every (every!) leaf from each tree, in each canopy layer
  • Sampling the spectral response of leaves in each canopy layer
  • Measuring the leaf area of a sample of harvested leaves
  • Weighing the dried leaves
To do this for 3, moderately sized oak trees has taken (so far), I would estimate around 70 person days; it took 10 of us 3 full days just to strip and bag the leaves from the three felled trees.

L to R: Dr. Rachel Gaulton (Univ. of Newcastle), Jingjing Yan, Meng Yu (both UCL MSc students), Lucy Walker (Univ. of Salford), Andy Burt (UCL), with the last leaf from the first, large tree. The second tree beckons enticingly from the tarp in the background.
Having done all this we will know the dry mass (and density) of all the carbon in the wood of each tree and the total mass of all the leaves. For the woody carbon, we can then compare this with what we get using the laser scanner to (hopefully!) show that the scanner can be used to estimate it nearly as accurately, but faster, and without having to cut anything down.

For the leaves, given that we know the area of a sample of leaves, we can calculate the specific leaf area, or SLA (the leaf mass per unit area). If we multiply the total leaf dry mass by 1/SLA we will know the total leaf area of each trees, as well as the amount of material in each layer. This is another extremely important property as the leaf area controls transpiration, CO2 exchange and perhaps most importantly, photosynthesis. The leaf area is therefore an important factor in the signal we can measure from remote sensing. Our colleagues at Salford and Newcastle are using their SALCA instrument, which has two laser wavelengths, to help separate leaf and wood material from forest measurements. Our instrument is only single wavelength, and so can't do it this way, but we are trying to use other information, related to the laser waveform, return intensity and photographic RGB values, to try to do this.
This movie shows a fly-through of the leaf-off scans from Alice Holt, done in early April. The point cloud is overlaid with the photographic RGB values. The white points are where the camera cannot see the point, due to occlusion (eg the upper side of leaves).

So far, the results of all these measurements look very promising and many thanks to Eric in particular for organising the field campaign, and to all those who helped out.