Remote Sensing Assessment of the Expansion of Ips typographus Attacks in the Chuprene Reserve, Western Balkan Range

Abstract

The Chuprene Reserve was created in 1973 to preserve the natural coniferous forests in the Western Balkan Range in Bulgaria. The first infestations by European spruce bark beetle (Ips typographus) were registered in Norway spruce (Picea abies) stands in the mid-1980s. The aim of this study is to assess the damages caused by I. typographus in the Chuprene Reserve using remote sensing techniques – unmanned aerial vehicle (UAV) images, airborne images, and satellite images of European Space Imaging (EUSI), combined with terrestrial verification. High-resolution images in four bands of the electromagnetic spectrum and in a standard RGB channel were taken in 2017 via a multispectral camera ‘Parrot Sequoia’, integrated with a specialized professional UAV system eBee ‘Flying Wing’. The health status of Norway spruce stands in the reserve was assessed with the normalized difference vegetation index, based on the digital mixing of imagery captured in the red and near infrared range. The dynamic of bark beetle attacks was studied in GIS on the basis of maps generated from photographic surveys, airborne images taken in 2011 and 2015, and satellite images from 2020. In the UAV-captured area (314.0 ha), the size of Norway spruce stands attacked by I. typographus increased from 7.6 ha (2.4%) in 2011 to 44.9 ha (14.3%) in 2020. The satellite images showed that on the entire territory of the Chuprene Reserve (1451.9 ha), I. typographus killed spruce trees on 137.4 ha, which is 9.6% of the total area.

1. Introduction

Forest decline due to extreme weather conditions (prolonged droughts, windthrow, snowfall, etc.), outbreaks of harmful pests, and fungal pathogens have been recorded in the last few decades. The process bears characteristics such as the deterioration of the health status of trees and the decrease in their productivity and vitality over large forest areas. The gradual change in climate conditions reduces the resistance of forest stands and makes them susceptible to pest attacks. The use of remote sensing techniques provides information to detect the location of stressed forest stands induced by insect pests and diseases outbreaks.

Satellite imaging remote sensing technologies can provide data for the large-scale monitoring of forest ecosystems to study the causal relationships of disturbance factors across spatial scales and understand relevant feedback processes [1]. The satellite data and other remote sensing techniques cannot directly identify the biotic agents. They obtain information for land cover type, as well as for the health status of vegetation, conducive to the breeding of vectors. This information is then used to identify and characterize the habitat in which pests and diseases spread out.

Unmanned aerial vehicle (UAV) remote sensing has great potential for vegetation mapping in complex landscapes due to the ultra-high-resolution imagery acquired at low altitudes [2]. UAVs have several benefits that make them suitable for forest health monitoring: high-intensity data collection and low operational costs [3]. UAVs can be deployed easily and frequently to satisfy the requirements of rapid monitoring, assessment, and mapping in natural resources at a user-defined spatio-temporal scale. The remote sensing technology, in combination with terrestrial survey techniques and methods, have a huge research potential, especially in protected forest areas with inaccessible and complex terrain [4].

Insects are ubiquitous disturbance agents that play important roles in the long-term dynamics of forest ecosystems. Bark beetles are widely distributed in European forests, mostly as secondary biotic agents, affecting stands that are already weakened [5]. The spruce bark beetle (Ips typographus L.) is the most damaging pest in Norway spruce (Picea abies (L.) Karst.) forests and has caused great ecological and economic disturbances in recent years.

An empirical assessment of the automated identification of mature Norway spruce trees in Finland, suffering from infestation by I. typographus is carried out [6]. Using a novel hyperspectral camera from UAV and aircraft, individual spruces were classified as healthy, infested by the pest, or dead. The results showed that novel, low-cost remote sensing technologies, based on individual tree analysis and calibrated remote sensing imagery, offer great potential for affordable and timely assessments of the health condition of vulnerable urban forests.

The UAV and field verification assessment of the health status of vegetation in urban and peri-urban areas in Bulgaria was carried out in Karlovo – a city in the central part of the country [4]. The results showed that the implementation of this integrated approach was successfully used for the remote monitoring of green systems in settlements with subsequent detailed investigation of calamities of insects and rapid detection of invasive pests and pathogens, in order to prevent their spreading in new areas. Recently, remote sensing techniques based on UAV have also been applied to the assessment of the health status of forest ecosystems and used to detect bark beetle spots caused by I. typographus in two mountain reserves in the Western Balkan Range [7].

The aim of this study is to assess the dynamic of attacks and damages caused by the European spruce bark beetle (I. typographus) on the territory of the Chuprene Reserve (Western Balkan Range) by applying remote sensing techniques.

2. Materials and Methods

2.1. Studied Areas

The studies on the health status of Norway spruce forest stands were carried out as a part of the activities for monitoring of the sanitary condition of the forests stands included in the management plan of the Chuprene Reserve [8]. The reserve was created in 1973 on an area of 1451.9 ha to preserve the natural coniferous forests in the Western Balkan Range in Bulgaria (Figure 1). The area is mountainous, highly rugged, with great differences in the altitudes (between 1140 m and 2021 m). Steep (21–30°) and very steep (more than 31°) terrains prevail in the reserve, at 64.7% and 17%, respectively.

In 1977, the Chuprene Reserve was included in the UNESCO list of Biosphere Reserves under the Human and Biosphere Program. Systematic research on the health status of the natural stands of Norway spruce (P. abies), Silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.) provides the information on the ongoing natural processes in them. The tree vegetation is dominated by Norway spruce stands, which are 100–120 years old. In 2021, the Chuprene Reserve was withdrawn from the list of UNESCO Biosphere Reserves in Europe [9].

2.2. UAV and Aerial Photograph Images

A multispectral camera, ‘Parrot Sequoia’, was integrated with a specialized professional UAV system, eBee ‘Classic’, which is a type of ‘Flying Wing’. The platform, equipped with the Sequoia multispectral camera, was used to take high-resolution images in four channels of the electromagnetic spectrum: green (530–570 nm spectral band), red (640–680 nm), red edge (730–740 nm), and near infrared (770–810 nm), as well as in a standard RGB channel. The health assessment of the tree stands in the reserve was accomplished via the NDVI (normalized difference vegetation index), obtained by the digital mixing of imagery, captured in the red and near-infrared range.

The NDVI allowed the examination of the trees’ vitality by capturing the amount of light that is absorbed and reflected by them and the chlorophyll available. The visualization of the captured area in a different color range shows the vitality level of plants, and indicates the area of healthy spruce stands (NDVI most often over 0.7) and points to bark beetle spots (NDVI below 0.5) and trees infested by I. typographus (NDVI between 0.5 and 0.7).

Seven flights were carried-out between 25 September and 16–17 October 2017, in which a representative part of 314.0 ha (21.6% of the total area of the Chuprene Reserve) was photographed and studied [7]. The GSD of the RGB images was 8 cm, while the GSD of the multispectral images was 20 cm, which resulted in a flight altitude of 127.4 m above elevation distance (AED). The resulting images offered better spatial resolution compared to standard satellite images from the Landsat and Copernicus platforms (Figure 2). This is crucial for the identification of small areas and spots of vegetation, affected by pathogens in relatively heterogenous (from a geomorphological point of view) territories with dense vegetation.

The appearance and enlargement of bark beetle spots was studied in GIS on the basis of maps generated from photographic surveys with UAV, and orthophotos with images taken in 2011 and 2015.

2.3. Satellite Images

Due to the specifics of the period planned for the stands’ health status establishment via remote sensing, and due to difficulties in accessing the terrain, it was impossible to collect the necessary multispectral data through UAV. On this basis, a decision was taken to compensate for the information deficit through the use of satellite images. Various options were considered for the free access to satellite images, such as the United States Geological Survey (USGS) and the European Space Agency-HUB (ESA-HUB) portals. The images available in these portals have extremely insufficient spatial resolution and this makes them inapplicable for the purposes of this study. Taking into account these shortcomings, the European Space Imaging (EUSI), a leader in providing images with spatial resolution up to 30 cm, met the criteria for high spatial, radiometric, and temporal resolution, and was identified as a source of the necessary information. Following a thorough examination of the EUSI multispectral image archives, a satellite image, (generated on 4 August 2020) covering almost the entire territory of the Chuprene Reserve, was provided. The image has a resolution of 50 cm/px and is delivered with four channels, red, green, blue, and NIR (near infrared).

2.4. Processing of Remote Sensing Data

Different platforms were used to process the acquired remote sensing data for the purposes of the study. The UAV-derived data were processed by a Pix4D platform that yielded an orthophoto mosaic and a NDVI raster map of the observed and studied area.

The Sentinel Application Platform (SNAP) and the ESRI ArcGIS Desktop 10.6 software package were used for the processing and analysis of the obtained data. The raw images were loaded into the SNAP platform and RGB and NDVI models were generated sequentially for the entire reserve area. After the calculation of the NDVI, the data were loaded into ArcMap, where it was further refined and a classification was made that clearly distinguished between healthy spruce stands and trees that are either infested by I. typographus or dead. Subsequently, the classified data from NDVI in combination with the RGB image were used to accurately locate and determine the boundaries of areas affected by the pest (Figure 3).

2.5. Terrain Verification

Two field expeditions (25–26 September 2017 and 16–18 October 2017) were carried out in the north-eastern part of the Chuprene Reserve. The objective was to identify bark beetle spots in Norway spruce stands that were severely attacked by Ips typographus.

Four sites were studied in detail: one in the land of Replyana (43°26′33.46″ N, 22°38′41.75″ E, 1459 m) and three in the land of Chuprene (43°27′9.13″ N, 22°38′22.97″ E, 1389 m; 43°27′0.28″ N, 22°38′16.40″ E, 1482 m; and 43°26′54.97″ N, 22°37′57.01″ E, 1484 m) (Figure 4).

At least 10 dead spruce trees were surveyed in each site. I. typographus was identified not only by the presence of larvae, pupae, and adults, but also by the bark galleries typical for the species.

The population density of I. typographus was determined by the number of families and exit holes of adults on the stems of fallen trees on 40 × 20 cm sample plots.

3. Results

3.1. Ips typographus Attacks in Captured Areas

The analyses of data generated from orthophotos, UAV, and satellite images showed that the attacks of European spruce bark beetle, Ips typographus (Linnaeus, 1758) (Coleopterans: Curculionidae, Scolytinae), increased in the captured area in the north-eastern part of the Chuprene Reserve, from 7.6 ha (2.4%) in 2011 to 44.9 ha (14.3%) in 2020 (

Table 1. Dynamics of Norway spruce stands destroyed by I. typographus in captured areas (314.0 ha) in the Chuprene Reserve.

Year	Images	Damaged Areas		Number of Spots	Average Spot Area, ha
		ha	%
2011	Airborne	7.6	2.4	82	0.09
2015	Airborne	22.1	7.0	125	0.18
2017	UAV	41.5	13.2	154	0.27
2020	Satellite	44.9	14.3	287	0.16

).

The number of bark beetle spots increased from 82 in 2011 to 287 in 2020 (Table 1). The average area of bark beetle spots increased between 2011 and 2017. In 2020, the average area decreased due to the appearance of many new infestations and new migration spots of I. typographus that formed in the period of 2017–2020 (Figure 5).

The RGB images showed that a huge number of the bark beetle I. typographus spots expanded (Figure 6A), the oldest spots merged (Figure 6B), but new single or groups of dead spruce trees appeared on the territory of the whole reserve (Figure 6C). The predominant part of them were located in close proximity to rock screes (Figure 6D).

3.2. Ips typographus Attacks on the Entire Territory of the Chuprene Reserve

The satellite images showed that on the entire territory of the Chuprene Reserve (1451.9 ha), I. typographus killed spruce trees on 137.4 ha, that is, 9.6% of the total area. The map shows that the oldest I. typographus attacks, respectively the largest outbreaks and drying of spruce stands, occurred in the northern (lower) part of the reserve (Figure 7).

3.3. Terrestrial Assessment

Under the extreme mountain conditions of the Chuprene Reserve, wind and snow often induced damage to the Norway spruce stands. The presence of fresh thrown or broken stems provoked the development and attacks of I. typographus.

The terrestrial verification of the problem areas showed that damages in spruce stands were caused mainly by I. typographus. As a result of the infestations, many dead trees (Figure 8A) with pest galleries under the bark were registered (Figure 8B). In the upper part of the Chuprene Reserve, numerous migratory bark beetle spots (Figure 8C) appeared around the old ones in the lower parts.

The number of I. typographus in the studied areas varied between 0.3 and 2.6 families/dm², with an average of 1.7 families/dm² and 48.2 specimens per family. Species of parasitic and saprophytic fungi were identified on the roots, stems and branches of trees infested by I. typographus.

4. Discussion

Ips typographus is one of the most destructive xylophagous insect pests of spruce species in Europe. Its outbreaks are usually caused by disturbances or extreme weather conditions [10]. In the twentieth century, several outbreaks of the pest were recorded in various parts of Europe in which over 100 million m³ of spruce trees were killed [11].

In Bulgaria, I. typographus does not usually cause strong damage to the managed spruce forests because of the intensive sanitary and control measures. Conversely, in protected areas, the pest develops on fallen and broken fresh trees, increases in number and attacks healthy stands. In 2001, a tornado affected mature spruce stands on 62 ha in Bistrishko Branishte Biosphere Reserve in Vitosha Mountain, after which the I. typographus population first developed in windthrown timber, and from 2003 to 2005, spruce trees on over 200 ha were killed by the pest near the windthrow in the reserve [12].

The first visible infestations by I. typographus in the Chuprene Reserve appeared in the mid-1980s (unpublished). The strongest attacks were registered in the northeastern part of the reserve, in the outskirts of Replyana and Gorni Lom villages. Since the 2000s, large bark beetle spots and intensive drying of spruce stands have been observed in the northwestern part of the Chuprene Reserve, in the lands of the villages of Stakevtsi and Krachimir.

The terrain investigations in 2017 in the Chuprene Reserve showed that the main damages to Norway spruce stands were caused by I. typographus [13]. Some destructive fungal pathogens, Heterobasidion annosum, Armillaria sp., and Fomitopsis pinicola, were also found on the trees infested by the bark beetle.

In the Chuprene Reserve, the spruce stands are predominately mature. It is well known that I. typographus attacks trees older than 70 years and the dynamics of outbreaks largely depends on higher proportions of spruce trees in a stand, insect abundance, tree susceptibility, weather conditions, and human measures [10]. There are about 120 species of bark beetles in Bulgaria, and over 20 of them are trophically associated with spruces. However, detailed studies on damages caused by spruce bark beetle in the Chuprene Reserve have not been conducted.

The remote sensing image data provided useful information for the detection of the damages caused by bark beetles and root rot disease outbreaks. Analyses of remote sensing techniques in 107 journal papers published between 2015 and 2020 showed that, despite the emergence of new technologies and methods, multispectral data are still the prevalent source in forest health assessment [14]. However, in the Polish part of the Białowieża Forest, the spread of the I. typographus outbreak was estimated by mapping dead spruce stands on a tree level using airborne hyperspectral and laser scanning data obtained from HySpex VNIR-1800 camera and Riegl LMS-Q680i scanner [15,16]. The detailed use of remote sensing data showed that, in just five years (2015–2019), the pest was so destructive that it nearly eliminated Norway spruce as a major tree species in the Białowieża forest [17]. In addition, satellite optical and thermal data of Landsat-8 were also used to investigate the early stage of I. typographus infestation in the Bavarian Forest National Park in south-eastern Germany along the border with the Czech Republic (between 13°12′9″ E and 49°3′19″ N) [18].

In spruce forests attacked by I. typographus, there are mosses on the ground, and abundant grass vegetation appears in the dead stands, which affects the NDVI index. For this reason, the mapping of bark beetle spots based on NDVI data can be quite inaccurate and, therefore, should be used in combination with RGB images. A combined processing of multispectral and RGB photo images was applied for species classification and assessment of the vital status of forest stands in the Chuprene Reserve [19].

5. Conclusions

The present study showed that the use of remote sensing techniques is a useful and effective method for the detection and mapping of I. typographus outbreaks. It is especially applicable for large areas at high altitudes on steep and inaccessible terrain, such as the Chuprene Reserve, where it is extremely difficult to perform classical entomological studies.

The presence of the large bark beetle spots of I. typographus in the Chuprene Reserve, combined with the increasing dynamics of the pest attacks, poses a serious threat to the Norway spruce. This is the only extensive relict spruce forest in the Western Balkan Range and it is of great value. For this reason, after the withdrawal of the Chuprene Reserve from UNESCO’s list of Biosphere Reserves, it is desirable to take urgent preventive and sanitary actions to control the pest and to improve the forest’s health status.