University of Twente Student Theses


Effect of crown size and shape of different temperate tree species on modelling AGB and AGC using UAV images

Torres Rodriguez, Alejandra (2020) Effect of crown size and shape of different temperate tree species on modelling AGB and AGC using UAV images.

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Abstract:The improvement in the quantification of aboveground biomass (AGB) and aboveground carbon stock (AGC) is highly relevant for the optimisation in forest management and conservation initiatives worldwide, like REDD+. UAV RGB images can estimate AGB/AGC in diverse forest ecosystems. The stem diameter or Diameter at Breast Height (DBH) is the most influential tree variable to determine AGB and AGC. The measurement of some tree variables is more straightforward than others, but the relationship between them can be used to estimate one of them indirectly from the other. Crown Projection Area (CPA) and Crown Diameter (CD) have been used to estimate DBH. In the field of remote sensing, RGB images have used these relationships to estimate DBH. The advance in UAV high resolution images has rapidly improved, allowing more details in the interpretation of tree parameters like CPA or CD from which DBH can be estimated. This study focuses on the effect of DBH acquired from the relationships of DBH-CPA and DBH-CD on the estimation of AGB/AGC. A species-specific DBH model (i.e., 6 species), as well as a General Broadleaves and Conifers DBH model, were built from both DBH-CPA and DBH-CD relationships in a temperate mixed forest in the Netherlands. The results of this study showed that both DBH-CPA and DBH-CD relationships could estimate DBH from UAV with high accuracy and with no significant difference compared to field measurements. Also, the difference between the accuracy results from both relationships was minimal. The general Conifers and Broadleaves DBH model validation brought similar accuracy results, but broadleaves have a much higher residual, related with a higher crown size variation. In the case of the species-specific models, Spruce resulted in the highest accuracy and the lowest residuals. Moreover, in all cases, DBH-CD relationships estimated DBH with a lowest variance. Once the DBH estimations are used to calculate AGB and AGC plot-wise, then the model has to deal with the variation from the accumulative effects of the influence of endogenous and exogenous factors on the crown size and hence, on the DBH estimation. Overall, the general models and species-specific models from both relationships were proved to estimate AGB and AGC with no significant difference compared to the biometric AGB/AGC. A few plots presented important differences (under and overestimations), and this was proven to be highly influenced by Beech species, due to its high crown flexibility to deform itself according to the external conditions (plasticity). Consequently, the sensitivity limitations of Beech species-specific models (DBH-CPA and DBH-CD) should be acknowledged. Both relationships lead to results with no significant difference when compare against DBH field measurements. Nevertheless, this study has found the species-specific DBH models from DBH-CD resulted in higher accuracy and less variation (except for Beech) on estimating AGB/AGC than DBH-CPA relationship.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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