Research results: iPad mobile laser scanning can help assess forest carbon stocks, but is more effective in mature, regular stands

By Arne Van Wolputte, KU Leuven

Measuring the amount of plant material (biomass) in forests is important for two main reasons: it helps us understand how climate change is affecting forests, and it allows us to calculate how much carbon these forests are absorbing, which helps counteract and offset emissions. In a previous blog post, we introduced the research of KU Leuven MSc student Arne Van Wolputte, exploring the transition from traditional aboveground biomass assessment methods to innovative, digital techniques such as mobile laser scanning. We also discussed the potential of the novel Arboreal Forest iPad app to make carbon Monitoring, Reporting and Verification effective and affordable while maintaining its simplicity. In this new article, Arne presents the results of his investigation, looking at how his use of the app in biomass inventorying affected the accuracy and efficiency of biomass measurements.

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Accurately assessing above-ground biomass in forests is crucial for understanding the impacts of climate change and quantifying carbon sequestration in offsetting projects. Traditional forest inventory methods, involving a tape measure and height estimation devices, are accurate but time-consuming and expensive to conduct. The integration of laser scanning technology into commercial devices, such as the iPad Pro, presents a promising avenue for the forestry sector to reduce inventory costs. These systems offer the potential to efficiently and accurately assess biomass while ensuring accessibility for stakeholders.

We explored the transition from traditional methods to Arboreal Forest, a forest inventory app with laser scanning functionality, in two forest environments covered by the INFORMA project’s case studies: the Atlantic and Mediterranean biogeographic regions.

Main findings

Our findings revealed that the Arboreal Forest app’s effectiveness varied depending on the forest environment. In the Atlantic case study, measuring the above-ground biomass took about 60 minutes per plot with the traditional method, while only 16 minutes were required when Arboreal Forest was used. The biomass estimations by Arboreal Forest were nearly as accurate as those observed with traditional tools, with a 0.58% difference in measured biomass.

The Mediterranean case study presented a different narrative. A less consistent performance was observed here due to larger densities and higher share of small diameter trees; larger densities increased surveyor errors whereas small diameter trees posed a challenge for the built-in laser scanner. Here, traditional methods required 54 minutes whereas Arboreal only required 30 minutes. However, a 27% underestimation by Arboreal was observed.

Overall, accuracy improved with tree size, and efficiency gains were particularly pronounced in environments with large-diameter trees.

Arboreal Forest as an alternative?

The use of high-tech applications in forestry is increasing. The introduction of Terrestrial Laser Scanning systems in the forestry sector was met with enthusiasm due to the numerous possibilities and advantages they could offer. However, the price tag, weight and requirements of expertise for gathering and processing the collected data hindered widespread application of the technique. So, the search for alternative solutions continued. The introduction of laser scanning systems in mobile devices provided some solutions.

Contrary to many other Mobile Laser Scanning applications, Arboreal Forest does not generate 3D point clouds, limiting co-registration errors and post-processing requirements. The collected data can be easily downloaded as .CSV files through Arboreal’s website, making the implementation of Arboreal in forest inventories even more user-friendly.

Tips and tricks for in-field use

  1. In both case studies, we noted that both the accuracy and the efficiency of Arboreal Forest improved if a lower Diameter at Breast Height (DBH) threshold was applied. We recommend setting the lower limit to 7 cm.
  2. Trees growing perpendicular to steep slopes can pose a challenge for Arboreal Forest. As per good inventory practices, the DBH should be measured from the uphill side of the tree. Slanted trees should be measured from the side from/to which they grow.
  3. Arboreal Forest allows DBH measurements of irregularly shaped trees from two different viewpoints. From our experience, this practice resulted in a more accurate DBH assessment for irregularly shaped trees.
  4. Using technology for fieldwork brings along an important challenge: battery life. In this study, the iPad’s battery would survive about half a day before recharging was required. We recommend having a least 20% battery charge remaining before starting a new plot, of course depending on forest metrics such as tree density. We recommend bringing an appropriate power bank.

Conclusions

The Arboreal Forest app emerged as a viable option for forest inventories, offering significant time savings and cost-effectiveness, particularly in mature, regular stands. Recommendations for optimizing its usage include implementing a lower DBH threshold (7 cm) and avoiding its use in young, dense, and highly diverse forest stands. Our research contributes valuable insights into the application of mobile laser scanning systems in forestry and underscores Arboreal Forest’s potential to improve inventory practices and efficiency, aiding in climate change mitigation efforts.