Assessment of defoliation and subsequent growth losses caused by Lymantria dispar using terrestrial laser scanning (TLS)

Martin Jacobs,Jörg Müller,Wolfgang W Weisser,Hans Pretzsch,Sebastian Kienlein,Torben Hilmers,Hannes Lemme,Benjamin M L Leroy

doi:10.1007/s00468-021-02255-z

Abstract

Key messageTLS scans of three surveys before, during and after gypsy moth gradation, allowed high-resolution tracking of defoliation and subsequent inter-annual growth losses on an individual tree level.Foliation strongly determines all tree growth processes but can be reduced by various stress factors. Insect defoliation starts at variable times and is one stress factor that may affect photosynthetic processes and cause immediate reactions like refoliation, which are difficult to detect by surveys repeated at 1-year intervals. This study used a large-scale field experiment in German oak/mixed forests affected by gypsy moths (Lymantria dispar) to test the use of terrestrial laser scanning (TLS) for detecting inter-annual foliation and growth losses at the individual tree level caused by the gypsy moth. The experiment comprised two levels of gypsy moth defoliation risk, high (H) and low (L), as well as two pest control treatment levels: spraying with the insecticide Mimic (M) or unsprayed control (C). The factorial design consisted of four treatment combinations (HC, HM, LC, and LM), applied to 11 spatial blocks with a total of 44 plots. The TLS approach detected the defoliation caused by the gypsy moth, estimated as leaf area and crown perforation parameters. For the first time, TLS-derived tree foliation was evaluated based on inter-annual stem growth. Leaf area and crown perforation showed a correlation of + 0.6 and – 0.35, respectively, with basal area increments. Furthermore, this study revealed subsequent growth losses in the same year due to defoliation. Our results show that TLS can offer new opportunities to develop new indicators that monitor foliation at the individual tree level. The crown perforation can describe defoliation or the tree’s vitality based on one scanning campaign, whereas the leaf area needed at least two.

Highlights

The frequency and intensity of insect-induced forest disturbances is set to rise due to climate change and changes in forest structure and composition (Seidl et al 2011)
In contrast to the terrestrial laser scanning (TLS) leaf-wood separation approaches available, we evaluated our calculated foliation parameters based on inter-annual stem growth since stem growth and leaf area are correlated (Rolland et al 2001)
The distribution of the leaf area and the crown perforation differed between the four combination types visually (HC, HM, LC, and LM)

Summary

Introduction

The frequency and intensity of insect-induced forest disturbances is set to rise due to climate change and changes in forest structure and composition (Seidl et al 2011). One important stress factor is insect defoliation, which causes a significant reduction in tree growth (Piper et al 2015). Whereas stem growth is easy to measure, the exact calculation of a tree’s foliation is difficult. There is no suitable reference for evaluation. Traditional defoliation assessments, e.g., by eye or using fisheye photographs, have been criticised as being highly subjective (Dobbertin 2005), while current foliation determination approaches based on spectral information from TLS return intensity or machine learning are complex and expensive (Calders et al 2020). In contrast to the approaches available, we evaluated our calculated foliation parameters based on inter-annual stem growth since stem growth and leaf area are correlated (Rolland et al 2001)

Methods

Results

Discussion

Conclusion