Using fractal analysis of crown images to measure the structural condition of trees

Abstract

Observations of tree canopy structure are routinely used as an indicator of tree condition for the purposes of monitoring tree health, assessing habitat characteristics or evaluating the potential risk of tree failure. Trees are assigned to broad categories of structural condition using largely subjective methods based upon ground-based, visual observations by a surveyor. Such approaches can suffer from a lack of consistency between surveyors; are qualitative in nature and have low precision. In this study, a technique is developed for acquiring, processing and analysing hemispherical images of sessile oak (Quercus petraea (Matt.) Liebl.) tree crowns. We demonstrate that by calculating the fractal dimensions of tree crown images it is possible to define a continuous measurement scale of structural condition and to be able to quantify intra-category variance of tree crown structure. This Page of 28 approach corresponds with traditional categorical methods; however, we recognise that further work is required to precisely define interspecies thresholds. Our study demonstrates that this approach has the potential to form the basis of a new, transferable and objective methodology that can support a wide range of uses in arboriculture, ecology and forest science.

The novel use of proximal photogrammetry and terrestrial LiDAR to quantify the structural complexity of orchard trees

“…The total tree height from the ground to the crowns highest point, the cardinal crown spread (measured from the centre stem to the maximum extent of the four cardinal points), and stem girth at 150 mm above the graft line, were measured. The trees were classified into four condition categories; poor, moderate, good and vigorous, using the visual tree assessment (VTA) and traditional arboricultural assessment (TAA) methods (Mattheck and Breloer 1994;Murray et al 2018). VTA and TAA consider whether the potential of the tree biology, biomechanics or the physiological function have been compromised by any biological (e.g.…”

“…Using the proximal photogrammetric techniques as described in Murray et al (2018), hemispherical imagery was taken from beneath each tree crown, looking upwards towards the zenith viewpoint (Fig. 1a).…”

“…For effective management of orchard crops, precise knowledge of the size, shape and spatial distribution of the canopy allow decisions to be made about operational interventions (Gongal et al 2016). In forestry, the structural assessment of tree crowns has been shown to be a reliable proxy of tree condition (Zhang et al 2007;Murray et al 2018). In agriculture, quantifying individual tree structure is important both for informing pruning regimes and for the precision application of irrigation, fertilisation and other inputs considered necessary to increase fruit yield (Lauri et al 2004).…”

“…Furthermore, Liang et al (2014) describe that for individual tree crown investigations, off-the-shelf digital single-lens reflex cameras can provide equivalent data to laser scanning, including when fitted with a hemispherical lens and used to image the canopy from beneath. This technique allows the capture of subject detail not available from other perspectives (Talbot et al 2017;Murray et al 2018). Processing these images allows the quantification of tree structure through measuring the fractal dimension (D f ) of the image (Fig.…”

“…1 a, b), which as Mandelbrot (1982) describes, provides opportunities to quantify the complexities of patterns within natural structures, particularly in tree crowns which are formed by both the genetic predisposition and environmental influences of "chance, irregularities and non-uniformity". The D f approach has been used as a reliable measure of tree structure complexity in mature trees (Murray et al 2018). Other measures of dimensionality, such as correlation dimension (D c ) are also considered to be effective at quantifying dimensionality with low numbers of data points (Zhang et al 2007;Theiler 1990).…”