Academic journal article Geography

Measuring Trees in 3D: What Lasers Can Reveal about Our Forests

Academic journal article Geography

Measuring Trees in 3D: What Lasers Can Reveal about Our Forests

Article excerpt

How do you measure a forest, and why does it matter?

It is widely recognised that forests are vitally important ecosystems on a local to global scale - whether it be for harbouring biodiversity, for carbon storage and cycling, or as an economically valuable timber source. In order to understand, monitor and manage these systems it is essential to be able to quantify the forest in some meaningful and consistent way. This requires measurements. But just how do you measure a forest? Forests are dynamic heterogeneous systems with diverse spatial, structural and biological composition. It is for these reasons that forests are regarded as one of the most important global repositories of terrestrial biodiversity (MEA, 2005). However, their complexity presents a whole host of challenges, and means that many of the measurements that foresters and scientists need to record are often difficult to obtain.

Consider a single standing tree - measuring its key attributes, dimensions and architectural structure is no trivial task. Add a network of adjacent trees and other vegetation layers (including ground cover, shrubs and understorey) and the challenge multiplies. The resultant forest structure can be characterised across horizontal and vertical dimensions. Horizontally, the vegetation creates a mosaic of different sized patches of trees and gaps in the canopy, while vertically the plant material is distributed through multiple height layers. Nevertheless, the capability to quantitatively measure the structural properties of forests at this level of detail is necessary due to the close link with ecosystem functioning, including playing a key role in the carbon cycle (Spies, 1998).

Due to the direct links between vegetation and carbon, forests have a significant role to play in climate change mitigation. In order to understand and respond to the two-way relationship between forests and climate and how changes in one affect the other, we must be able to quantitatively measure both these systems in an accurate and robust manner. Existing approaches can measure atmospheric carbon flux measurements directly from the forest ecosystem. An example of this would be eddy covariance instruments mounted on flux towers above the canopy, which directly measure atmospheric CO2 (e.g. Mizunuma et al., 2013). However, such data only tells part of the story. The central component and key driver of these fluxes is the vegetation. The ability to collect detailed and accurate structural measurements of the forest through time and space would allow a better understanding of the dynamics at play.

This article explores how terrestrial laser scanning (TLS) can address some of the challenges in forestry measurement. TLS is a relatively new and rapidly developing technology that utilises the properties of scattered light to measure distance - a technique known as light detection and ranging (lidar) - to build a three-dimensional (3D) model of its surroundings. Drawing on recent advancements and work in this field, the article will demonstrate the application of this emerging technology. Therefore, the key objectives of this article are three-fold. Firstly, to provide an overview of lidar remote sensing - what it is and how it works - and the development of TLS; secondly, to outline measurements of ecological value that can be extracted from a TLS 3D model; and, thirdly, to briefly discuss the current challenges and future opportunities in this field. The measurement of forest vegetation can be divided into two main areas of study: knowing what is where (forest structure) and how it changes over time (temporal dynamics) - both are explored in the following sections.

Forest structure

Above-ground carbon in forests is divided into two key parts of the plant. The first is the woody material: slow decomposable carbon pools with low metabolic activity where trees accumulate organic material (biomass). Approximately half the dry mass of woody material is carbon (Broadmeadows and Matthews, 2003; Drake et al. …

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