Estimating Changes in Leaf Area, Leaf Area Density, and Vertical Leaf Area Profile for Mango, Avocado, and Macadamia Tree Crowns Using Terrestrial Laser Scanning

Abstract

Vegetation metrics, such as leaf area (LA), leaf area density (LAD), and vertical leaf area profile, are essential measures of tree-scale biophysical processes associated with photosynthetic capacity, and canopy geometry. However, there are limited published investigations of their use for horticultural tree crops. This study evaluated the ability of light detection and ranging (LiDAR) for measuring LA, LAD, and vertical leaf area profile across two mango, macadamia and avocado trees using discrete return data from a RIEGL VZ-400 Terrestrial Laser Scanning (TLS) system. These data were collected multiple times for individual trees to align with key growth stages, essential management practices, and following a severe storm. The first return of each laser pulse was extracted for each individual tree and classified as foliage or wood based on TLS point cloud geometry. LAD at a side length of 25 cm voxels, LA at the canopy level and vertical leaf area profile were calculated to analyse tree crown changes. These changes included: (1) pre-pruning vs. post-pruning for mango trees; (2) pre-pruning vs. post-pruning for macadamia trees; (3) pre-storm vs. post-storm for macadamia trees; and (4) tree leaf growth over a year for two young avocado trees. Decreases of 34.13 m2 and 8.34 m2 in LA of mango tree crowns occurred due to pruning. Pruning for the high vigour mango tree was mostly identified between 1.25 m and 3 m. Decreases of 38.03 m2 and 16.91 m2 in LA of a healthy and unhealthy macadamia tree occurred due to pruning. After flowering and spring flush of the same macadamia trees, storm effects caused a 9.65 m2 decrease in LA for the unhealthy tree, while an increase of 34.19 m2 occurred for the healthy tree. The tree height increased from 11.13 m to 11.66 m, and leaf loss was mainly observed between 1.5 m and 4.5 m for the unhealthy macadamia tree. Annual increases in LA of 82.59 m2 and 59.97 m2 were observed for two three-year-old avocado trees. Our results show that TLS is a useful tool to quantify changes in the LA, LAD, and vertical leaf area profiles of horticultural trees over time, which can be used as a general indicator of tree health, as well as assist growers with improved pruning, irrigation, and fertilisation application decisions.

A Review of Current and Potential Applications of Remote Sensing to Study the Water Status of Horticultural Crops

“…Thermal-based sensors capture the temperature of the crop's surface, which indicates the plant's stress (both biotic and abiotic) [53]. Generally, digital RGB camera and LiDAR can be used to quantify 3D metrics, such as the plant size and shape, via 3D pointclouds with sufficient accuracy for canopy level assessment [111][112][113][114][115][116][117][118].…”

“…Similarly, recent advancement in UAS technology and lightweight sensors have enabled multirotor (payload 5-6 kg readily available) to onboard multi-sensors. Water status of crops is a complex process influenced by a number of factors including the physiology of the crop, available soil moisture, the size and vigour of the crop, and meteorological factors [30,108,116,156,157]. For this reason, a multi-sensor platform is used to acquire measurements of the different aspects of the crop for water status assessment [34,102,108].…”

“…Using the structural measurements, such as the canopy height, canopy size, the envelope of each row, LAI, and porosity, among others, the water demand of the crop may be estimated. Generally, larger canopies tend to require more water than smaller canopies with less leaf area [116,157]. Using the temporal measurement of the plant's 3D attributes, the vigour can be computed.…”

“…Canopy structure metrics are closely related to horticultural tree growth and provide strong indicators of water consumption, whereby canopy size can be used to determine its water requirements [234]. Other 3D attributes, such as the crown perimeter, width, height, area, and leaf density, have been shown to enable improved pruning of horticultural plants [116,119].…”

Measuring Canopy Structure and Condition Using Multi-Spectral UAS Imagery in a Horticultural Environment

“…More recently, UAS have been considered as a useful platform to acquire suitable remotely sensed data for measuring horticultural tree crop structure, as it provides both higher spatial and temporal resolution and the flexibility of operations [6][7][8]18]. Airborne image data, initially stereo-photography, were analyzed using soft-copy photogrammetry to provide information for extracting ground and canopy heights [2][3][4][5][6][7]24], while more recently this has been done by collection and processing of LiDAR point clouds [1,[25][26][27][28]. Similar products are now generated from UAS multi-band image data using multiple forms of softcopy photogrammetry, such as structure from motion (SfM).…”