Terrestrial Scanning / Forestry Assessment & Agriculture

Kim Calders et al.: Laser scanning reveals potential underestimation of biomass carbon in temperate forest 19.12.2022

Quantifying climate mitigation benefits of biosphere protection or restoration requires accurate assessment of forest above-ground biomass (AGB). This is usually estimated using tree size-to-mass allometric models calibrated with harvested biomass data.

Using three-dimensional laser measurements across the full range of tree size and shape in a typical UK temperate forest, the scientific team show that its AGB is 409.9 t ha−1, 1.77 times more than current allometric model estimates. This discrepancy arises partly from the bias towards small trees in allometric model calibration: 50% of AGB in this forest was in less than 7% of the largest trees (stem diameter > 53.1 cm), all larger than the trees used to calibrate the widely used allometric model.

For data acquisition a RIEGL VZ-400 3D Terrestrial Laser Scanner was used. Terrestrial laser scan data were collected in leaf- off conditions throughout late November 2015, December 2015 and January 2016. Windy days were avoided to ensure data quality.

The instrument has a beam divergence of 0.35 mrad and operates in the infrared (wavelength 1550 nm) with a range up to 350 m. The pulse repetition rate for each scan was 300 kHz, the minimum range was 0.5 m and the angular sampling resolution was 0.04°. This resulted in 22,500,000 outgoing pulses for a single scan, resulting in a beam diameter of 2.45 cm and beam spacing of 3.5 cm at 50 m (for instance). The azimuth angle range was 0°–360°, and the zenith angle range was 30°–130°. Therefore, an additional scan was acquired at each scan location with the scanner tilted at 90° from the vertical to complete sampling of the full hemisphere at each location. Scans were done in a larger 6 ha area using an approximate 20 m × 20 m grid to ensure the best possible data quality within our 1.4 ha study area. Trees which had at least more than half of their stem at tree diameter 1.3 m inside the boundaries of the study area were included.

This paper presents new empirical evidence that the fundamental assumption of tree size-to-mass scale-invariance is not well-justified for this kind of forest. This leads to substantial biases in current biomass estimates of broadleaf forests, not just in the UK, but elsewhere where the same or similar allometric models are applied, due to overdependence on non-representative calibration data, and the departure of observed tree size-to-mass from simple size-invariant relationships.

The full article was published in the Ecological Solutions and Evidence Volume 3, Issue 4 and can be found here.