The European spruce bark beetle
The use of this tool has already quite a long history – pheromone baited traps were proposed for monitoring bark beetle populations shortly after those synthetic attractants were introduced into the forestry practice (Bakke 1985). This concerns both short-term survey within a single growing season (Faccoli and Buffo 2004), as well as the long-term monitoring aimed to the definition of changes in the population density between subsequent seasons (Holly 2005; Pontuali et al. 2008). This approach seems to have higher importance in the protected areas, especially when the active protection treatments are limited or excluded. In such a case, the pheromone traps could serve mainly (or exclusively) as the source of information about
In the Tatra National Park (TPN) in Poland, the pheromone traps for monitoring
The study design is based on the set of pheromone traps located in the whole area of the TPN and operated in the years 2010–2019. Only the traps of precisely known location within the study period were taken into consideration. Finally, the data from 23 locations (only the traps operated in all years) were used (Fig. 1); 10 traps were located in the zone between 860 and 1000 m a.s.l. (zone 1), 11 traps in the zone between 1001 and 1200 m a.s.l. (zone 2) and 2 traps – above 1200 m a.s.l. (zone 3), approximately at the upper limit of the lower montane belt (Piękoś-Mirkowa and Mirek 1996). Most of the traps were located on eastern and northern slopes (15 and 8, respectively), while only 5 on western, and 3 on southern ones. The slit traps were usually (except one) installed by 2 pieces in one locality, pheromone dispenser was inserted at the beginning of May, a new one was added at the beginning of July, and the data about the number of captured beetles were collected monthly by the professional staff of the TPN. The data on the captures of beetles were then compared with two kinds of data concerning the mortality: the area covered by standing dead trees in the no-intervention zone, and the volume of trees infested by bark beetles processed in the intervention (active protection) zone. Due to instability of the range of individual protection zones, and the lack of precise spatial data concerning the sanitary/ salvage felling in adequate resolution, it was not possible to include these two kinds of data in one analysis. Therefore, some approaches were applied.
The data concerning standing dead trees were derived from the numerical layer containing the spots with such trees delimited and digitized by Marcin Bukowski (TPN) based on the airborne photographs of the Park, taken in the autumns of 2011, 2012, 2013, 2014, 2015 and 2017 (the appropriate images from 2016 were not available). Mapping was conducted manually by comparing aerial orthophotos taken in individual years in ESRI ArcGIS 10.2. (Sproull et al. 2017). For every year, only the newly detected clusters of dead trees were delimited and digitized. For each locality of concern, three circular concentric zones of 100, 200 and 500 m radius were delimited and the total area covered by dead trees was calculated for each zone around each locality in all the years. It was assumed that the tree mortality derived in such a way was caused exclusively or at least mainly by bark beetle infestation, as no terrestrial verification was done.
The data concerning the infested trees processed in the stands in active protection zone come from the official reports that are supplied yearly by the TPN to the Forest Research Institute for evidence/forecast of threats to forests. The data from the entire active protection zone in the TPN area were used for general characteristics of the dynamics of spruce mortality. As the data are organized by the protection ranges (no retrospective data at higher resolution are accessible), such figures were used for the interpretation of captures. For this purpose, two characteristic areas (called “western” and “eastern”) comparable in extent and represented by equal number of traps, but affected by extended wind damage in different years, were chosen (Fig. 1). The “western” one (A) with the area of 3725 ha, located in the Kościeliska Protection Range, includes 2326 ha of forests growing between 920 and 1550 m a.s.l. (up to the upper timberline) and the non-forested zone of 1399 ha (mainly rocks above the upper timberline and meadows), and was affected by wind damage in 2013 (Grodzki and Gąsienica Fronek 2017). The “eastern” one (B) with the area of 2695 ha of forests, located in the Brzeziny and Kośne Hamry Protection Ranges and growing between 820 and 1550 m a.s.l., does not include the zone above the upper timberline, and was affected by wind damage in 2017 (Grodzki and Gąsienica Fronek 2019). Both study areas remain partially under active (lower parts), and partially – active protection regime. The data from the pheromone traps (5 in each, as the most northern trap in area B, see Fig. 1, was excluded) exposed in those two areas in comparable altitudinal range (940/925–1150/1240 m a.s.l., respectively) were related to the respective data concerning sanitary felling in those areas. The information about the location of wind damage was kindly provided by the TPN administration.
In case of bark beetle captures, the data from the entire trapping seasons (May–August) in 2010–2019 were used. As traps were usually located in pairs, the mean values per 1 trap were used for each location – thus, the data concerning one location will be called “trap”. The raw data was kindly provided by the TPN administration.
The data processing was done on 3 levels: at the whole TPN area level and all the traps, at the level of the selected “eastern” and ”western” area, and at the level of individual locations of traps and surrounding circular zones. The data from 3 above described sources (standing dead trees, processed infested trees, captures of beetles) were compared and discussed. For the statistical analyses (descriptive statistics, linear and Spearman rank correlations, Kruskal-Wallis tests) Statistica 13 (TIBCO Software Inc. 2017) was used.
The spatial analyses of the occurrence of standing dead trees in circular zones around all 23 traps located in the entire TPN area revealed that the results obtained for 100 and 200 m radius are very scarce and not representative for further comparisons (Tab. 1). In case of 100 m radius, the number of localities with standing dead trees varied in individual years within the range 0–7 (out of 23), for 200 m radius it was 1–16 locations, while in case of 500 m radius, it was 6–21 locations. Therefore, the last ones were used for analysis.
Area of detected occurrence of new standing dead trees detected in subsequent years in the concentric zones around the 23 locations of pheromone traps in the entire TPN area, using 100, 200 and 500 m radius
Radius of concentric zone (m) | Area (ha) with standing dead trees within the zones in the year | |||||
---|---|---|---|---|---|---|
2011 | 2012 | 2013 | 2014 | 2015 | 2017 | |
100 | 0.00 | 0.06 | 0.00 | 0.00 | 0.08 | 1.15 |
200 | 1.30 | 0.28 | 0.49 | 0.35 | 0.10 | 12.48 |
500 | 28.10 | 8.37 | 13.57 | 3.62 | 1.17 | 126.07 |
The detected tree mortality defined in the circular zones around all 23 traps varied between years, reaching relatively high level at the beginning of the period covered by the analysis, then decreasing gradually till 2015 and dramatically increasing in 2017 (Tab. 1). However, it should be pointed out that the area detected in 2017 results from 2-years mortality and the proportion between 2016 and 2017 remains unknown. The between-years variability in the main area with dead trees around the trap reveals the same pattern (Fig. 2).
The spots with dead trees detected in the entire range of the two selected study areas occurred most distinctly at the beginning and the end of the period covered by the analysis. In 2011, the total area of spots with dead trees in the entire “western” area was 3 times as large as in the “eastern” one, while in 2017 – more than twice, but in the meantime, those areas were comparable, with slightly higher values in the “eastern” area (Fig. 3).
During the decade 2010–2019, the yearly volume of trees infested by bark beetles, processed in the active protection zone of the TPN varied between 7.1 and 33.6 thousand m3 with two peaks – in 2012 and 2016, and 2 periods on relatively low level – in 2014–2015 and 2017–2019 (Fig. 4). The active protection zone was not stable during this decade, initially covering about 27% of the TPN area and reduced to about 16% starting from 2017. It should be pointed out, that the active measures aimed at the reduction of bark beetle populations (removal of trees with living insects under the bark) were not applied in all those areas, formally belonging to active protection zone.
The dynamics of sanitary felling was not homogenous in the entire active protection zone of the TPN. In the “western” area, the volume of removed trees identified as “infested by bark beetles” was relatively high in 2010–2012, then decreased in 2013–2015, increased in 2016 and decreased again in 2017–2019. At the same time, the share of removed trees already left by the bark beetles in 2010–2012 remained between 55% and 70%, while in 2015, it was only 1%. During the second half of the analysed decade, the share of such trees was between 1% and 36% (Fig. 5A). In the “eastern” area, the volume of removed trees infested by bark beetles was also the highest in the first years (2010–2012), then decreased in 2013–2015 and increased again starting from 2016. The share of removed trees already left by bark beetles during the first 3 years did not reach more than 27%, while in 2017–2019, those trees prevailed (57%–77%) (Fig. 5B).
The mean numbers of
The captures in individual months of the studied years remained in quite stable pattern in all the years, when the swarming (and number of caught beetles) had the highest dynamics in the first half of the growing season. The number of beetles captured till the end of June in individual years was between 23.6% in 2015 and 70.2% in 2018, but this share was much higher till the end of July, reaching between 71.6% in 2017 and 87.0% in 2010. Both the captures recorded till the end of June and till the end of July were highly correlated with the captures from the entire trapping period (Tab. 2).
Share of beetles captured in all the 23 pheromone traps in individual years till the end of June and July and correlation coefficients related to the captures from entire trapping period (May–August). All the correlations are significant at p < 0.01
Year | Beetles captured till the end of June | Beetles captured till the end of July | ||
---|---|---|---|---|
share [%] | correlation r | share [%] | correlation r | |
2010 | 52.6 | 0.845 | 87.0 | 0.987 |
2011 | 57.3 | 0.980 | 86.4 | 0.998 |
2012 | 60.4 | 0.975 | 85.4 | 0.991 |
2013 | 49.8 | 0.916 | 76.2 | 0.980 |
2014 | 45.8 | 0.942 | 77.8 | 0.983 |
2015 | 23.6 | 0.815 | 79.9 | 0.964 |
2016 | 32.2 | 0.846 | 76.3 | 0.981 |
2017 | 31.8 | 0.519 | 71.6 | 0.969 |
2018 | 70.2 | 0.933 | 85.9 | 0.990 |
2019 | 54.7 | 0.935 | 81.6 | 0.969 |
Regarding the altitudinal distribution, the highest mean (± std. deviation) number of beetles calculated from all the traps and years were recorded in the zone 2 (9342 ± 8799), lower in the zone 1 (7576 ± 6972) and the lowest in zone 3 (6106 ± 4500), however, the effect of altitude was not significant (
The dynamics of
The captures in both areas are correlated with the area of spots with dead trees in the 500 m circle around the traps, however, this correlation is not statistically significant (Tab. 3). There is no relation of captures to the volume of processed infested trees in both areas (Tab. 3). Nevertheless, the mean captures from individual years, calculated for “eastern” and “western” area, are correlated (r = 0.69;
Spearman rank correlation coefficients between mean yearly captures per trap and tree mortality
Tree mortality expressed by | Western area | Eastern area | ||
---|---|---|---|---|
rs | p | rs | p | |
Area of spots with dead trees | 0.714 | n.s. | 0.657 | n.s. |
Volume with and of left processed by insects trees | 0.018 | n.s. | 0.055 | n.s. |
Volume with insects of processed trees | 0.430 | n.s. | 0.067 | n.s. |
Pheromone traps are often recommended as the source of data for monitoring
We used two kinds of data to describe the effects of bark beetle activity, that is, the mortality of Norway spruce trees. However, the data of these two kinds were not available in comparable resolution: the volume of trees removed in sanitary felling concerned only the stands in the active protection zone, contrarily to the data on standing dead trees on relatively large areas, containing the stands under (formally) both active and passive protection. Therefore, the data on sanitary felling concerned only a part of the study areas, mainly the stands growing on lower altitudes (according to the TPN zonation). At the same time, in the other parts of the study areas, the bark beetle populations remained out of control, and the emerged beetles of new generations were able to freely disperse and infest/kill the trees surrounding those in which they completed their development. It is obvious that
At this point, the question arises concerning the radius of the zones used for the analysis. Initially, we started from the 100 m radius, according to the knowledge concerning the attraction range
Another important phenomenon affecting the definition of possible patterns is the damage caused by the wind. The distinct collapse in the captures, recorded in individual study areas (in 2014 in “western” and in 2017 in “eastern” area) correspond with the wind damage that occurred at the end of 2013, and in March 2017, respectively (Grodzki and Gąsienica Fronek 2017, 2019). In both cases, the fresh broken and fallen trees represented an attractive and “easy” breeding material for bark beetles in the first growing season after the damage (thus: 2014 and 2018, respectively). It is known that fresh broken and fallen trees attract
It is commonly known that both wind damage and high temperatures during the growing season are the main factors contributing to bark beetle outbreaks in spruce forests (Mezei et al. 2017; Biedermann et al. 2020), however, in case of the Polish part of the Carpathian mountains, the spruce mortality due to bark beetle infestation, expressed by the volume of processed infested trees, started to decrease in 2017 (Sierota et al. 2019). Therefore, the observed increase of captures starting from 2017 may be the effect of the change in the extent of active protection measures (Grodzki and Gąsienica Fronek 2018, 2019) or insufficient effectiveness of applied treatments (Vanická et al. 2020). The infested trees were not removed even in the stands formally located in the active protection zone. All traps were placed in the areas (locations) that remained formally in active protection zone, but at the same time, the spots with standing (i.e., not removed) dead trees have been detected in 500 m circular zones around almost all (21/23) traps, thus, in fact, the active protection measures were not applied there. On the other hand, also the cutting of infested trees in some stands, but with high share of trees that were already left by insects, indicates that those – not timely removed – trees represented additional source of beetles to be captured in the traps.
The analysis if the data collected during 10 subsequent growing seasons revealed high percentage of beetles captured in the first half of the growing season (till the end of July), regardless the installation of fresh dispensers at the beginning of July, and very high correlation of those with the results from the entire growing season (till the end of August). This finding remains in accordance with the results of Faccoli and Stergulc (2004, 2006), who suggested that the spring catches (till the end of July) can be used for forecasting damage in the current growing season. Also, Grodzki (2007) demonstrated that in three national parks in the Carpathians (Bieszczady NP, Gorce NP, Tatra NP), the share of
Pheromone monitoring of
Pheromone traps represent valuable source of information about the bark beetle
The results of pheromone trapping are affected by several factors that influence the number of captured beetles. The most important are: wind damage, defence potential of trees resulting from their physiological status and nature protection regime. Another important circumstance disturbing the data analysis results from the lack of coherent data reflecting the effect of bark beetle activity, that is, tree mortality.
As most of the beetles are captured in the first half of the growing season, the data collected till the end of July is sufficient for monitoring purposes; in order to minimize human intervention in the protected areas, the monitoring, at reasonable number of localities, should be reduced to the period May–July.