Estimation of the vertical distribution of tree biomass using last significant return laser altimetry returns from Eucalypt trees in New South Wales, Australia

Snow mass and soil moisture are important features of the environment governing the availability of drinking and irrigation water, food and hydro-power. They are estimated globally by measuring the microwave emission of the Earth’s surface. Soil’s microwave emissivity is strongly affected by its liquid moisture content, and the attenuation of soil-emitted radiation by snow is a function of the snow mass. The high water content of vegetation affects these results through attenuation, scattering and emission, so improving knowledge of vegetation mass and distribution will enable more accurate global characterisation of snow and soil moisture. Airborne laser altimetry systems acquire information about the environment by pulsing laser light at the ground, and interpreting the returning light curve using onboard differential GPS and inertial navigation systems. Such systems are primarily used to acquire topography by assuming that the last light returning to the sensor has been returned from the ground. By continuously recording the intensity of the light returned before the ground return, information about the vegetation between the aircraft and the ground can be derived. Recording the full intensity curves consumes a large volume of recording space, however, and is a relatively new instrument innovation, having in the past usually been combined with a reduced pulse rate to conserve storage. Deriving some information from systems which only record the last light returned would be of use in characterising large areas without using complete light curve return recording systems, allowing greater spatial cover and resolution with the same instrument and resources. Our previous work has characterised the error budget of single-return laser altimetry systems, and used this to distinguish different soil roughness at the centimetric scale, and show vegetation density variations within crops around 2m high. This work shows the vertical information on vegetation density that can be derived from trees, by comparing the elevation return frequency from a number of individual Eucalypt trees in New South Wales, Australia, and comparing this to photography of the same trees. Airborne laser altimetry last significant return data was acquired in November 2006, which included ten Eucalypt trees. This was analysed using the assumption that probability of beam interception within a fixed height interval was proportional to vegetation area, and, assuming constant leaf thickness and angular distribution, vegetation mass. This provided a layered estimate for vegetation mass, relative within the tree. This was compared with analysis of photographs acquired in 2007 and 2010, indicating a change in structure and growth within the canopy over the intervening period for the younger, shorter trees. A potential problem with this form of analysis is the loss of information below the point at which all laser pulses have been intercepted, however in this case, no trees showed this, with at least 26% of pulses penetrating to ground level.