Living Land

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Satellite laser altimetry: a powerful tool to

enhance the capability of global forest inventory

Yoshiki Yamagata, Masato Hayashi and Habura Borjigin, National Institute

for Environmental Studies, Center for Global Environmental Research, Japan

A schematic illustration of observations by

satellite laser altimetry

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LiDAR Return

Signal end

Signal start

Source: CGER

lobal forest inventory is required for Reducing

Emissions from Deforestation and Forest

Degradation (REDD+) implementations. However,

conventional methods depending on field measurements are

extremely labour-intensive. Satellite laser altimetry can make

it possible to efficiently collect enormous measurement data

about forest carbon stocks without in-situ measurements. As

a result of verification in Borneo, laser altimetry was shown

to successfully and accurately measure canopy height and

forest biomass. Furthermore, laser altimetry could be used

to estimate the rate of forest loss, the distribution of forest

biomass, and the total amount of biomass in Borneo.

Among various ecosystem services of forests, the regulating

services such as carbon storage have attracted public atten-

tion, because they could contribute to mitigating climate

change. The REDD+ scheme has been developed in recent

years to prevent global warming resulting from deforestation.

Large-scale deforestation and forest degradation decreases the

water-holding capacity of soil, and it is one of the principal

causes of desertification. Therefore, REDD+ is also effec-

tive for combating desertification. The national report for

REDD+ implementations requires the measurement of activ-

ity data (extent of deforestation activities) and the emission

factor (greenhouse gas emissions per unit area). The emis-

sion factor is classified into three levels of complexity, from

Intergovernmental Panel on Climate Change default values

(Tier 1) to precise values based on a national forest inventory

(Tier 3). A higher tier can result in a more accurate carbon

budget estimation, but it may generally require labour-inten-

sive field measurements. Therefore, if satellite remote-sensing

can serve as a substitute or assistance for forest inventory,

it will be extremely valuable to REDD+ implementations by

reducing costs and labour.

For the estimation of forest carbon stocks using remote-

sensing technology, estimating forest biomass is usually

adopted as a substitute. Biomass represents the dry weight of

trees and about half of it is carbon weight. Therefore, forest

biomass can be easily converted into forest carbon stocks.

However, measuring belowground (root) biomass is difficult

for remote-sensing technology, so the aboveground biomass

(AGB) is usually adopted as a measuring object.

In remote-sensing technology, various sensors are used

on-board satellites. An optical imaging sensor is the most

orthodox sensor and has a long history; however, an accurate

estimation of forest biomass cannot be expected. Synthetic

aperture radar is a sensor using radio waves (microwaves) and

has an ability to measure forest biomass, but it has a weakness

in that the sensitivity is saturated in high-biomass forests over

150 t/ha. Laser altimetry, which is also called light detection

and ranging or LiDAR, is a sensor using laser beams, and it

can accurately measure forest biomass without signal satura-

tion. However, its weakness is that it cannot fully observe a

large area because its observation points are discrete.

The only satellite laser altimetry so far has been the Ice

Cloud and land Elevation Satellite (ICESat)/Geoscience Laser

Altimeter System (GLAS), operated by NASA from 2003 to

2009. GLAS transmitted laser pulses with a frequency of 40

Hz, and its footprints were spaced at intervals of 172 m on

the ground along ICESat’s orbital track. The footprint was

elliptical with a 60 m nominal diameter, but the size varied

according to the observation period. The GLAS observations

iving

and