Influence of Site-Specific Conditions on Estimation of Forest above Ground Biomass from Airborne Laser Scanning

Abstract

Forest aboveground biomass (AGB) is an important variable in assessing carbon stock or ecosystem functioning, as well as for forest management. Among methods of forest AGB estimation laser scanning attracts attention because it allows for detailed measurements of forest structure. Here we evaluated variables that influence forest AGB estimation from airborne laser scanning (ALS), specifically characteristics of ALS inputs and of a derived canopy height model (CHM), and role of allometric equations (local vs. global models) relating tree height, stem diameter (DBH), and crown radius. We used individual tree detection approach and analyzed forest inventory together with ALS data acquired for 11 stream catchments with dominant Norway spruce forest cover in the Czech Republic. Results showed that the ALS input point densities (4–18 pt/m2) did not influence individual tree detection rates. Spatial resolution of the input CHM rasters had a greater impact, resulting in higher detection rates for CHMs with pixel size 0.5 m than 1.0 m for all tree height categories. In total 12 scenarios with different allometric equations for estimating stem DBH from ALS-derived tree height were used in empirical models for AGB estimation. Global DBH models tend to underestimate AGB in young stands and overestimate AGB in mature stands. Using different allometric equations can yield uncertainty in AGB estimates of between 16 and 84 tons per hectare, which in relative values corresponds to between 6% and 37% of the mean AGB per catchment. Therefore, allometric equations (mainly for DBH estimation) should be applied with care and we recommend, if possible, to establish one’s own site-specific models. If that is not feasible, the global allometric models developed here, from a broad variety of spruce forest sites, can be potentially applicable for the Central European region.

Canopy Top, Height and Photosynthetic Pigment Estimation Using Parrot Sequoia Multispectral Imagery and the Unmanned Aerial Vehicle (UAV)

“…In this study, three tree stands were selected in the LYS catchment (LYS 1K, LYS 2K, and LYS 4K) and three stands in the PLB catchment (PLB 2K, PLB 3K, and PLB 5K), these stands were the object of previous long-term research, thus the soil conditions were known as well as tree height information, which was measured in-situ and modelled using the LiDAR data [62].…”

“…The tree characteristics-height, crown, and top-derived from the CHM raster (Figure 7) were compared with the in-situ measurements that were collected in 2015 [62]. Table 6 summarizes the evaluation statistics.…”

Canopy Top, Height and Photosynthetic Pigment Estimation Using Parrot Sequoia Multispectral Imagery and the Unmanned Aerial Vehicle (UAV): Norway Spruce Forest Case Study

“…In the field it was observed that tree cuts were common in this particular forest stand. It will be interesting to test in the future, if this approach can be further employed or adjusted for estimating the forest aboveground biomass in a similar way as airborne laser scanning has been employed [45].…”

“…In this study three tree stands were selected in the LYS catchment (LYS 1K, LYS 2K, LYS 4K) and three stands in the PLB catchment (PLB 2K, PLB 3K, PLB 5K), these stands were the object of previous long-term research, thus the soil conditions were known as well as tree height information which was measured in-situ and modelled using the LiDAR data [45].…”

“…The tree characteristics -height, crown and top -derived from the CHM raster ( Figure 8) were compared with the in-situ measurements collected in 2015 [45]. Table 7 summarizes the evaluation statistics.…”