Epsilon Archive for Student Projects

Abstract

Today Holmen Skog finds and selects suitable thinning stands using data on upper height and basal area. These figures are often based on basic forest data that has been updated with the help of growth prediction models. The problem is that these figures are not always correct. The aim of this study was to investigate if a combination of data from a profiling laser and a digital camera for low elevation photography, can be used for collecting data on number of stems per ha, tree species mixture as well as basal area weighted mean height and upper height for stands. The possibility to use these data to assess the need for thinning cutting was also investigated.

Laser data and aerial photographs were collected from a helicopter operating at an approximate height of 100 meters. In total, data were collected for 48 of Holmen Skog’s young stands in Västerbotten and 21 of these were selected for the study (including two stands that were removed because they had been thinned after the air photo acquisition). A majority of the stands were dominated by Scots pine. All stands were young forests that according to Holmen Skog were ready for their first thinning cutting in the following five year period.

The estimation of number of stems per hectare was based on about 10 aerial photographs per stand with the help of the FotIT-GIS software. The photographs were first automatically divided into 9 equally sized squares. Then, the trees and their species were marked in three randomly selected squares in each selected photograph. In the last step, FotIT-GIS calculated the total number of stems per tree species and hectare for each stand.

Upper height and basal area weighted mean height were calculated for each stand using two functions, which were created with the help of field data and laser data. The basal area weighted mean height derived from laser data was 3,7% lower than corresponding field measured height. The difference was significant. The upper height estimates based on laser data was 1,2% lower than the upper height according to the field inventory, this difference was not significant.

The comparison showed that the number of stems per hectare estimated from the aerial photographs were 7,1% lower than the objectively collected stem numbers, and it was significantly lower than the estimate obtained from the objectively collected field data.