Abstract
Airborne Laser Scanning (ALS) and Terrestrial Laser Scanning (TLS) systems are useful tools for deriving horticultural tree structure estimates. However, there are limited studies to guide growers and agronomists on different applications of the two technologies for horticultural tree crops, despite the importance of measuring tree structure for pruning practices, yield forecasting, tree condition assessment, irrigation and fertilization optimization. Here, we evaluated ALS data against near coincident TLS data in avocado, macadamia and mango orchards to demonstrate and assess their accuracies and potential application for mapping crown area, fractional cover, maximum crown height, and crown volume. ALS and TLS measurements were similar for crown area, fractional cover and maximum crown height (coefficient of determination (R2) ≥ 0.94, relative root mean square error (rRMSE) ≤ 4.47%). Due to the limited ability of ALS data to measure lower branches and within crown structure, crown volume estimates from ALS and TLS data were less correlated (R2 = 0.81, rRMSE = 42.66%) with the ALS data found to consistently underestimate crown volume. To illustrate the effects of different spatial resolution, capacity and coverage of ALS and TLS data, we also calculated leaf area, leaf area density and vertical leaf area profile from the TLS data, while canopy height, tree row dimensions and tree counts) at the orchard level were calculated from ALS data. Our results showed that ALS data have the ability to accurately measure horticultural crown structural parameters, which mainly rely on top of crown information, and measurements of hedgerow width, length and tree counts at the orchard scale is also achievable. While the use of TLS data to map crown structure can only cover a limited number of trees, the assessment of all crown strata is achievable, allowing measurements of crown volume, leaf area density and vertical leaf area profile to be derived for individual trees. This study provides information for growers and horticultural industries on the capacities and achievable mapping accuracies of standard ALS data for calculating crown structural attributes of horticultural tree crops.
Highlights
The canopy structure of a forest and individual trees influence ecosystem function, carbon cycle, biophysical processes, wildlife habitat, tree health and productivity [1,2,3]
This study demonstrated that Terrestrial laser scanning (TLS) technology has the ability to quantify leave area (LA), leaf area density (LAD) and vertical leaf area profiles for horticultural tree crops and the LA change derived from TLS data were consistent with the expected LA changes caused by canopy management, growth and a severe storm
Our results showed that those crown structure measurements relying on information from the top of the crown were largely similar when using both the airborne laser scanning (ALS) and TLS data (Figure 3)
Summary
The canopy structure of a forest and individual trees influence ecosystem function, carbon cycle, biophysical processes, wildlife habitat, tree health and productivity [1,2,3]. Canopy structure is important, as it relates to yield, tree condition, light interception, pruning requirements, irrigation and fertilizer application, and orchard management practices [4,5,6,7,8]. The geometric character of orchard trees is important, as it can guide pruning, irrigation, fertilization and pesticide applications and indicate tree health condition and vegetative growth [4,10,12]. It is complicated and time-consuming to measure crown structure manually [13]. Remote sensing technologies have been applied to horticultural tree crop environments over the last decades, the main focus of remote sensing technologies on mapping canopy structures is in forestry [4,14]
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