Identifying and quantifying structural characteristics of heterogeneous boreal forests using laser scanner data

Abstract

Structural characteristics of forest stands, e.g. in relation to carbon content and biodiversity, are of special interest. It has been stated in numerous publications that discrete return laser scanner data produce accurate tree canopy information since the quantiles of the height distribution of laser scanner data are related to the vertical structure of the tree canopy. Since some of the laser pulses will penetrate under the dominant tree layer, it is also possible to analyse multi-layered stands. In this study, the existence and number of understory trees were examined. This was carried out by analysing the height distributions of reflected laser pulses. In the laser data of this study, (Toposys Falcon, survey May 2003 in Kalkkinen, Finland, flight altitude 400 m AGL) the average number of laser pulse hits/1m 2 was 12. The reference data consisted of 28 accurately measured field sample plots. These plots include highly heterogeneous structures of boreal forests. The existence of lower canopy layers, i.e. understory trees, was analysed visually by viewing the plotwise 3D-images of laser scanner-based canopy height point data and examining distributions of canopy densities which were computed as proportions of laser hits above different height quantiles. Furthermore, a developed histogram thresholding method (HistMod) was applied to the height distribution of laser hits in order to separate different tree storeys. Finally, the number and Lorey's mean height of understory trees were predicted with estimated regression models. The results show that multi-layered stand structures can be recognised and quantified using quantiles of laser scanner height distribution data. However, the accuracy of the results is dependent on the density of the dominant tree layer.