Determination of differences in temperature regimes on healthy and bark-beetle colonised spruce trees using a handheld thermal camera ...
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iForest Research Article
doi: 10.3832/ifor3531-014
vol. 14, pp. 203-211
Biogeosciences and Forestry
Determination of differences in temperature regimes on healthy and
bark-beetle colonised spruce trees using a handheld thermal camera
Andrej Majdák (1), In this study, we compared the daily temperature regimes of healthy unin-
fected trees in the interior of a forest stand and at the fresh forest edge with
Rastislav Jakuš (1-2), infested trees at the forest edge in an area affected by a bark beetle out-
Miroslav Blaženec (1) break. We estimated the potential of a handheld thermal camera for early
identification of bark-beetle infested trees. We show that infested trees have
significantly higher trunk temperatures than uninfested trees, which is more
visible on the shine side of the trunk, and we report the differences in tem-
perature between the shine and shadow sides. The differences are more no-
ticeable on a warm, bright, and sunny day than on cold and cloudy day. The
different intensity of solar radiation does not affect the distinction between
infested and uninfested trees. The handheld thermal camera shows potential
for identifying bark-beetle infested trees by scanning tree trunks on bright
sunny days.
Keywords: Bark-beetle Infested Trees, Handheld Thermal Camera, Incoming
Solar Radiation, Norway Spruce, Solar Radiation Modelling, Temperature Dif-
ferences
Introduction 2013). Higher air temperatures at cleared tion rates, and may eventually lead to trees
Stands of mature Norway spruce (Picea edges, associated with increased solar radi- being subjected to drought stress (Kautz et
abies [L.] Karst.) are often attacked by Eu- ation, predispose such edge trees to infes- al. 2013, Marešová et al. 2020).
ropean spruce bark beetle (Ips typogra- tation by providing favourable flight condi- To some extent, trees can influence their
phus L.). In response to climatic changes tions for bark beetles and by reducing host thermal environment and thereby avoid
and especially to increases in maximal tem- resistance mechanisms due to drought or excessive heat or enhanced temperatures
peratures, the extent of damage caused by thermal stress on dispersing beetles (Kautz during otherwise hot conditions (Leuzinger
these beetles is significantly increasing et al. 2013). Higher temperatures on sun- & Körner 2007). According to Helliker &
(Mezei et al. 2017). In mountainous condi- exposed bark can increase the emission of Richter (2008), the morphological and
tions, I. typographus prefers to attack trees primary bark beetle attractants (Baier & physiological characteristics of a tree can-
on fresh forest edges or in small openings Bader 1997, Hietz et al. 2005, Marešová et opy work to maintain leaf temperatures
caused by wind damage, salvage cutting, al. 2020). During host selection, bark bee- near the optimal value for photosynthesis
or previous bark beetle infestation (Jakuš tles prefer warmer trees (Kautz et al. 2013). (about 21 °C), irrespective of tree species,
et al. 2011, Kautz et al. 2013). Undisturbed Another factor is connected with sun-re- latitude, or average growing season tem-
spruce stands are usually well protected lated effects. A clear cut of a recent infesta- peratures. In warmer climates, leaf temper-
against direct insolation by individual and/ tion patch will lead to sudden sun expo- atures are lowered by evaporative cooling
or collective shading (Jakuš et al. 2011). sure to large parts of the stems of the edge and mechanisms that reduce the absorb-
Natural disturbances, such as windthrows trees, which were previously located with- ance of solar radiation, such as decreased
or bark beetle infestation gaps, create new in a closed stand and are thus charac- leaf angles. However, the architecture of
forest edges within a stand, which show a terised by relatively short crowns. Sun ex- an individual tree changes only marginally
sharp microclimatic gradient. The altered posure without any prior adaptation to the during a few weeks of drought. On bright,
microclimate is characterised by increased altered microclimate, particularly on south- warm days, the difference between canopy
solar radiation, wind exposure, higher tem- facing cleared edges, may result in heat- foliage and air temperature is mainly the
peratures, and lower humidity (Kautz et al. damaged phloem and increased transpira- result of transpiration. Longer drought pe-
riods increase canopy foliage temperatures
(Scherrer et al. 2011). Stress induced by an
(1) Institute of Forest Ecology, Slovak Academy of Sciences, Ludovíta Štúra 2, 960 53 Zv- invasion of insects or the onset of disease
olen (Slovak Republic); (2) Faculty of Forestry and Wood Sciences, Czech University of Life leads to, among other symptoms, stomatal
Sciences, Kamýcká 1176, 165 21 Praha 6 - Suchdol (Czech Republic) closure and, consequently, to increased
leaf and canopy temperatures (Smigaj et
@ Andrej Majdák (andrej.majdak@gmail.com) al. 2015). This temperature rise can often
be detected at the leaf level at an early
Received: May 21, 2020 - Accepted: Feb 23, 2021 stage of infection by thermal imaging.
However, research on the use of thermal
Citation: Majdák A, Jakuš R, Blaženec M (2021). Determination of differences in temperature remote sensing approaches for tree health
regimes on healthy and bark-beetle colonised spruce trees using a handheld thermal camera. monitoring has been limited (Smigaj et al.
iForest 14: 203-211. – doi: 10.3832/ifor3531-014 [online 2021-05-02] 2015). The development of handheld ther-
mal cameras has allowed thermal imaging
Communicated by: Carlotta Ferrara to be extended to the diagnosis of the
physiological status of both woodland
© SISEF https://iforest.sisef.org/ 203 iForest 14: 203-211Majdák A et al. - iForest 14: 203-211
trees and individual urban trees (Jones have been so thoroughly girdled that mor- Materials and methods
iForest – Biogeosciences and Forestry
2004). tality was assured long before any effects
The cooling effect of sap flow can help on sap velocity were observed. Research site
keep tree bole temperatures lower. Ac- Early detection of infested trees is crucial The study plots were set up in a 90-year-
cording to Hietz et al. (2005), during day- for the effective management of bark bee- old Norway spruce stand (Picea abies [L.]
light hours, the bark is warmed by the sur- tle outbreaks (Latifi et al. 2014). Infested Karst.) on a moderately southwest-expos-
rounding air and direct radiation. The high trees begin to undergo changes in their ed slope at 650-730 m a.s.l. in the Western
water content of sapwood dampens the spectral characteristics (Abdullah et al. Carpathians, Central Slovakia, Michalová
daily temperature fluctuations in the bole; 2018), which can be effectively detected us- forest district (48° 45′ 55.85″ N, 19° 47′
transpiration water passing through the ing remote sensing (RS) techniques, e.g., 31.18″ S; WGS84). The mean annual climate
sapwood, which enters the stem base at unmanned aerial vehicles (UAV), satellites, indicators of the research site were as fol-
the temperature of the soil, will remove and thermal imaging (Lausch et al. 2016, lows: air temperature, 4-6 °C; precipitation,
heat and cool the cambium and the inner Lausch et al. 2017). Thermal imaging, based 900-1000 mm; sum of global radiation,
parts of the secondary phloem. on terrestrial sensors or handheld thermal 1100-1150 kWh m-2 (Lapin et al. 2002). The
During attack and colonization, bark bee- cameras, has been used for a wide range of soil is classified as Dystric Cambisols and
tles disrupt two basic life-sustaining trans- conditions and with diverse plant species. Cambic Umbrisols, associated with Lep-
port processes of the trees they infest. In The technique can be applied at different tosols, from weathering products of acid
the early phase of overcoming tree host scales: from single seedlings/leaves (Buitra- to neutral rocks (Lapin et al. 2002) accord-
defences, adult beetles enter a suitable go et al. 2016) to whole trees or field crops ing to the World Reference Base (WRB)
host by boring through the outer bark and and regions (Costa et al. 2013). It shows sig- classification system (IUSS Working Group
into the phloem, and consume phloem tis- nificant potential for studying plant-envi- 2014).
sue to build egg galleries; developing lar- ronment interactions and specific phenom-
vae consume phloem for food until matu- ena, such as stomatal closure, stress toler- Research design and measurements
rity. Together, phloem feeding by adults ance, and the effects of different manage- For our observations, three groups of
and larvae contributes to some amount of ment strategies on crop water status and trees were selected. Two groups contained
phloem girdling, disrupting the transport tree health (Jones 2004, Reinert et al. 2012, healthy trees, not infested by bark beetles,
of photosynthate from the canopy to other Costa et al. 2013). However, using a hand- and one contained infested trees. There
tissues within the tree (Paine et al. 1997, held thermal camera for identifying bark- were five trees in each group. The first
Wullschleger et al. 2004, Hubbard et al. beetle infested trees based on trunk tem- group of healthy trees was selected in the
2013). Later, phytopathogenic fungal perature sensing has not yet been de- forest interior (plot A, Interior_Uninfested)
spores introduced by bark beetles germi- scribed. and the second at the newly created forest
nate, and the spreading fungal hyphae In our study, we aimed to compare daily edge (plot B, Edge_Uninfested). The group
penetrate the water-conducting xylem tis- temperature regimes of healthy uninfested of five infested trees (plot C, Edge_In-
sue in the sapwood and block water trans- trees in the stand interior and at a fresh fested) was selected at another newly cre-
port from the soil to the canopy (Wull- forest edge and infested trees at the forest ated forest edge close to the two groups
schleger et al. 2004, Hubbard et al. 2013). edge in areas affected by a bark beetle out- of healthy trees, about 150 m apart. The
Transpiration can begin to decline within break. We also estimated the potential of a trees were selected from the dominant
the first weeks of beetle infestation. After handheld thermal camera for early identifi- and codominant trees in the forest stand,
several weeks, pre-dawn water potential cation of bark-beetle infested trees. We and the mean values of selected tree char-
drops significantly as water transport to were specifically interested in the follow- acteristics of the observed tree groups are
the canopy significantly declines. In the ing: (1) What is the proper height for sens- listed in Tab. 1. The measurements were
early stages, there may be no visible signs ing trees for mutual comparison (base, 1.3 recorded in two days. The first day (20 Au-
of needle discoloration (Kirisits & Offen- m, 4 m, or crown) to find the differences gust 2012) was sunny with high tempera-
thaler 2002, Hubbard et al. 2013). The sub- between uninfested and bark-beetle in- tures, with a maximum air temperature in
sequent death of the tree (staining or oc- fested trees? (2) Is it possible to distinguish the open area close to the forest edge
clusion of sapwood, fading of foliage) may between uninfested and bark-beetle in- reaching 31.7 °C and the sum of global radi-
occur long after the critical interactions be- fested trees based on temperature? (3) ation totalling 5588 Wm-2 (Fig. 1a). The sec-
tween the tree and beetle/fungus complex How much is the temperature difference ond day (10 October 2012) was cloudy and
(Paine et al. 1997). Wullschleger et al. affected by direct solar radiation and does cold, with a maximum air temperature of
(2004) showed that when xylem function that prevent the identification of infested 9.2 °C and sum of global radiation of 1452
begins to fail, the progeny of attacking trees? (4) Can phloem temperature be de- Wm-2 (Fig. 1b). Twenty days before the first
beetles would have already departed the rived from thermal imaging by handheld day of measurement, there was almost no
tree, and the phloem and cambium would thermal cameras? precipitation; the total for this period was
2.6 mm. This means that the conditions
were not only hot but also relatively dry
before the first day of measurement. The
Tab. 1 - Mean values (± standard error, SE) of selected tree characteristics between
weather characteristics in the 20 days be-
the plots and results of ANOVA (F[1, 2], α = 0.05). (*): diameter at breast height; (**):
fore each measurement day are presented
trees creating the surrounding forest stand and assessed for the purpose of model-
in more detail in Tab. 2.
ling the light distribution in the MIXLIGHT model.
Images of stem and crown temperatures
were captured using a WH-8 thermal cam-
DBH* Height Crown length era (Wuhan Guide Infrared Co., Ltd.,
Stats Plot
(cm) (m) (m)
China), with a focal plane array (FPA) and
Mean tree A 38.6 ± 1.80 37.2 ± 1.11 18.2 ± 0.92 uncooled microbolometer with a resolu-
characteristics B 34.0 ± 1.70 37.0 ± 1.08 16.7 ± 0.44 tion of 384 × 288 pixels. The spectral range
(± SE) was 8-14 μm and the sensitivity was ≤ 0.08
C 36.0 ± 2.26 38.6 ± 0.93 17.6 ± 0.85
°C at 30 °C. The emissivity of imaging was
other** 36.8 ± 0.49 38.7 ± 0.26 17.5 ± 0.27 set to 0.96. For trees in plot A (Interior_Un-
ANOVA results F[3, 103] 0.90 1.16 0.30 infested, two trees) and plot B (Edge_Un-
p-value 0.45 0.33 0.82
infested, three trees), the temperature of
phloem close to the place sensed by the in-
204 iForest 14: 203-211Temperature differences on healthy and bark-beetle-colonised trees
iForest – Biogeosciences and Forestry
Fig. 1 - Daily course of
global radiation and air
temperature in an open
area close to plots and in
the forest interior during
measurement days: (a)
warm and sunny day and
(b) cold and cloudy day.
Air temperature in the
open area is represented
by a solid black line and
by a solid grey line in the
forest interior. Global
radiation is represented
by a dotted black line.
frared camera was recorded continuously Tab. 2 - Course of weather characteristics in the 20 days before each day of measure-
using thermal sensors inserted into the ment (20 August and 10 October 2012).
phloem (EMS, Brno, Czech Republic), con-
nected to a datalogger and stored in 15-
Parameter Stats 31 July - 19 Aug. 2012 20 Sept. - 9 Oct. 2012
minute intervals. The obtained infrared pic-
tures were processed with IR Analyser Air temperature Avg 17.6 11.8
software (Wuhan Guide Infrared Co., Ltd., (°C) Min 4.5 -0.5
China). To eliminate local inaccuracies of Max 34.2 24.3
stem temperatures caused by sunlight pen-
etrating through the canopy, we selected Relative air humidity Avg 72 80
an area of about 20 × 20 cm of the ob- (%) Min 32 37
served stem, from which the average tem- Max 100 100
perature was obtained (Fig. 2).
Precipitation (mm) Sum 2.6 30.6
Thermal pictures of each tree trunk were
captured at three heights: at the base and Global radiation Avg 4722 2847
at 1.3 and 4 m on both the shine and (W m-2 day-1) Min 1047 786
shadow sides; the temperature of the tree
Max 6532 5299
crown was taken from the ground. The
shine side was the part of the tree trunk
facing direct sun every time a thermal im- shining on the plots. The last interval lasted surements on all plots on the second day
age was captured, and the shadow side two hours. We captured nine measure- (10 October 2012). In the open area, air
was on the opposite side. The images were ments on plots A (Interior_Uninfested) and temperature and global radiation were
captured in approximately one hour inter- B (Edge_Uninfested) and eight measure- measured at half-hour intervals using a
vals from morning hours after sunrise to ments on plot C (Edge_Infested) on the Minikin RT® (EMS, Brno, Czech Republic).
evening hours when the sun was no longer first day (20 August 2012), and nine mea- Air temperature was simultaneously mea-
Fig. 2 - Thermal image
and RGB photo from
1.3 m height captured
using a thermal cam-
era: (a) shine side of a
tree; and (b) shadow
side of a tree. On the
thermal image, an area
of approximately 20 ×
20 cm in the middle of
the observed stem was
selected for obtaining
the average tempera-
ture. On the right or
left side of the stem is
an auxiliary scale visi-
ble for easier identifica-
tion of the required
height on the thermal
image.
iForest 14: 203-211 205Majdák A et al. - iForest 14: 203-211
iForest – Biogeosciences and Forestry
Fig. 3 - Vertical mean temperature profile from measurements on both shine and shaded sides of trees on (a) warm and sunny days
and (b) cold and cloudy days. Mean temperatures from uninfested trees in the forest interior (plot A) are represented by dotted
grey lines, from uninfested trees on the forest edge (plot B) by dashed grey lines, and from bark-beetle-infested trees on the forest
edge (plot C) by solid grey lines. Solid black lines represent total mean temperatures from all plots. Statistically different groups of
total mean temperatures between the heights on both shine and shaded sides of trees within each measuring day separately were
determined according to Tukey’s test (α = 0.05) and are highlighted by different lowercase letters.
sured in the forest interior using a Minikin exported individual solar radiation values and plot B (Edge_Uninfested), where the
TH® (EMS, Brno, Czech Republic). at the selected height and for a particular phloem temperature was measured, were
measurement time for each tree measured used in the analysis.
Solar radiation modelling by the thermographic camera.
The amount of solar radiation at each Results
measurement point (height on the stem) Statistics
was recalculated using the MIXLIGHT mod- The data were tested by Levene’s homo- Vertical temperature profile and
el (Stadt & Lieffers 2000), which is used to geneity test and the Shapiro-Wilk normality optimal height for thermal
calculate solar radiation passing through test. Given the test results, the data met measurement
tree crowns. The input to the model was the assumptions for parametric statistical The total mean temperature of the stem
data obtained with an Ilris HD ® terrestrial methods and were statistically evaluated in base on a warm and sunny day on the
laser scanner (Teledyne Optech, Canada). R. shine side in all plots was the highest (Fig.
The scanning method was chosen to pro- (1) To determine the appropriate height 3a) and significantly different (F = 141.9, df
duce the best possible representation of for thermal camera measurements, we = 7, p < 0.001) from temperatures at other
the forest stand structure. The focus was used repeated-measure ANOVA (a = 0.05) heights. With increasing measurement
not only on selected trees in individual for testing the temperatures at each tree height, temperatures decreased, whereas
plots, but on all trees at a distance of at height: at the base, at 1.3 and 4 m, and at at heights of 1.3 and 4 m did not show sta-
least 3 times the average height of trees the crown, separately for the two mea- tistically significant differences among
from experimental areas. The point cloud surement days. The relevance of the tree groups but differed from crown tempera-
was processed in the Polyworks™ v. 12 soft- to the individual plot (plot A, Interior_Unin- ture. Temperatures on the shady side were
ware environment (InnovMetric Software, fested; plot B, Edge_Uninfested; and plot balanced along the full height profile; to-
Canada), where the scan was then georef- C, Edge_Infested) and the sensing time gether with the crown temperature on
erenced into the WGS84 geodetic system. was not considered. Tukey’s test was used shine side, they did not show statistically
Geometric characteristics of the trees were to determine statistically different groups. significant differences among groups. On
obtained manually from the created point (2) To identify the temperature differ- the cold and cloudy day (Fig. 3b), the mean
cloud. The parameters that were detected ences in 1.3 m height between trees on temperatures also decreased with mea-
were tree height, diameter at breast height each of the three plots based on individual surement height on both sides of the trees.
(DBH), trunk coordinates, crown deploy- time of measurement and sunny and shady The highest mean temperature was ob-
ment height, crown parameters, and tree sides, we used ANOVA (α = 0.05) followed served at the stem base from the shine
social status. After entering the tree char- by Tukey’s test. side and did not differ significantly from
acteristics and other characteristics, such (3) To determine the role of incoming so- temperatures at heights of 1.3 and 4 m, but
as slope, orientation, geographic position lar radiation on the temperature of the differed from crown temperature (F =
of the area, and time (or time interval), the trees in each individual plot, we used out- 28.51, df = 7, p < 0.001). The mean stem
model quantifies the percentage of solar puts from the MIXLIGHT model containing temperatures on the shady side together
radiation passing through the forest stand. information on the recalculated global radi- with crown temperatures on both sides did
We used mean values from 10 minute inter- ation for individual trees and the individual not show statistically significant differ-
vals of solar radiation at the time of ther- measuring times as covariates in the AN- ences among groups.
mographic camera measurement. Using COVA test (α = 0.05).
real solar radiation data from the weather (4) To determine the dependence of tree Differences between healthy and bark-
station converted with MIXLIGHT data, we surface temperature and phloem tempera- beetle infested trees
were able to calculate the real amount of ture, we used linear correlation analysis. The temperature course on the warm and
solar radiation. Using a Python script, we Trees from plot A (Interior_Uninfested) sunny day (Fig. 4a) shows that bark-beetle
206 iForest 14: 203-211Temperature differences on healthy and bark-beetle-colonised trees
iForest – Biogeosciences and Forestry
Fig. 4 - Mean temperature ± standard error
(SE) at 1.3 m above the ground from the
shine and shaded sides of trees and temper-
ature difference ± SE between both sides
separately for both days of measurements:
(a) warm and sunny; (b) cold and cloudy.
Statistical differences in temperatures
between plots according to Tukey’s test (α =
0.05) at particular measurement times are
separately highlighted by different lower-
case letters at the top of subgraphs. The X-
axis is divided for each plot (A, B, C) within
each measurement, indicated by corre-
sponding measurement times. Uninfested
trees in the forest interior (plot A) are repre-
sented by solid black lines, uninfested trees
on the forest edge (plot B) by solid grey
lines, and bark-beetle infested trees on the
forest edge (plot C) by dotted black lines.
infested trees (plot C) had statistically sig- ments than the uninfested trees on the for- edge (plot B) and interior (plot A) trees, re-
nificantly higher temperatures on the shine est edge (plot B). The temperatures of un- mained similar for the first four measure-
side since the first measurement than the infested trees from the shine side, both ments. They began to differ significantly
uninfested trees on the forest edge (plot
B) and in the forest interior (plot A). The
temperature remained above 50 °C for
most of the day, with first and last mea-
surements being above 40 °C, which dif-
fered from the uninfested trees in the for-
est interior, whose temperatures did not
exceed 33 °C. The temperatures from the
first six measurements were significantly
higher compared to those of uninfested
trees on both the shine and shaded sides.
The temperatures of uninfested trees,
both edge and interior, were similar for the
first two measurements, with minimal dif-
ferences between shine and shaded sides.
The temperatures of uninfested trees on
the forest edge and the interior began to Fig. 5 - Mean crown temperature ± SE from the shine side of trees for both measure -
differ from the third measurement on the ment days: (a) warm and sunny; (b) cold and cloudy. Statistical differences in temper -
sunny side and the fourth on the shadow ature between plots according to Tukey’s test (α = 0.05) at particular measurement
side. times are separately highlighted by different lowercase letters at the top of sub-
The temperature course on the cold and graphs. The X-axis is divided for each plot (A, B, C) within each measurement, indi-
cloudy day in Fig. 4b shows that bark-bee- cated by corresponding measurement times. Uninfested trees in the forest interior
tle infested trees (plot C) had statistically (plot A) are represented by solid black lines, uninfested trees on the forest edge (plot
significant higher temperatures on the B) by solid grey lines, and bark-beetle-infested trees on the forest edge (plot C) by
shine side during the first three measure- dotted black lines.
iForest 14: 203-211 207Majdák A et al. - iForest 14: 203-211
from the fifth measurement. The tempera-
iForest – Biogeosciences and Forestry
Tab. 3 - Results of ANCOVA comparison (F, df, p) at α = 0.05 between plots for shine ture courses of both sides of uninfested
and shadow sides of trees and different measurement days. trees in the interior were similar, with the
difference not exceeding 1 °C. The results
ANCOVA results
Day Side Plot comparisons from the cold and cloudy day showed that
df F p-value the infested trees also had significantly
Interior_Uninfested (A) - Edge_Uninfested (B) 1 1.02 0.316 higher temperatures on the shine side
Shine
warm and sunny
Interior_Uninfested (A) - Edge_Infested (C) 1 49.77Temperature differences on healthy and bark-beetle-colonised trees
Relationship between tree stem surface distance). Based on our results, we deter- trees on the forest edge (plot B) and in the
iForest – Biogeosciences and Forestry
temperature and phloem temperature mined the optimal height of this measure- forest interior (plot A), we report a strong
A significant coefficient of determination ment to be 1.3 m. An important factor is correlation between phloem and bark sur-
(R2 = 0.92) for the shine side with a mean the angle of measurement, which should face temperatures. This agrees with the re-
temperature of 22.9 °C from the thermal be perpendicular to the measured surface, sults reported by Powell (1967) and Hietz
camera and 18.5 °C from thermal needles as recommended by the manufacturer, to et al. (2005), and also demonstrates the
was obtained (Fig. 6a). The most sun-ex- achieve the highest accuracy. However, we precision of thermal camera measurement,
posed were the uninfested trees on the recorded almost equal temperatures on which has potential for tree physiology and
forest edge (plot B) with the mean surface the tree trunk 1.3 m above the ground on bark beetle development modelling. The
temperature of 40.5 ± 2.54 °C (Tab. 4) and the shine side. This indicated that for other most extreme values of surface bark tem-
mean phloem temperature of 34.4 ± 1.61 °C. thermal camera applications, the whole peratures on uninfested trees were re-
However, starting with the fourth mea- profile of the tree trunk can be used. For corded on the forest edge (plot B) from
surement, the surface temperatures were example, scanning forest edges from the shine side during the warm and sunny
above 40 °C with a maximum of 54.4 °C and greater distances would be possible, focus- day, especially during noon and afternoon.
a mean of 48.6 °C. In contrast, phloem tem- ing on the part of the tree between the During this period a maximum of 54.4 °C
peratures were above 33 °C with a maxi- trunk base and the crown deployment was recorded on the bark surface, but the
mum of 43.5 °C and a mean of 39 °C. The height. temperatures recorded in the phloem were
mean difference between the stem surface On the shaded side, the soil surface has a on average 9.6 °C lower. This agrees with
and the phloem temperatures during this stronger influence, heating the tree stems. the study of Nicolai (1986), who described
period was 9.6 °C. For the shadow side, a The temperature is highest at the base and the insulating properties of bark in differ-
significant coefficient of determination (R 2 gradually decreases upward along the ent tree species and types of bark. Accord-
= 0.92) with a mean temperature of 17.72 °C stem. On the shine side, the temperature is ing to Annila (1969), not only temperature
from the thermal camera and 15.34 °C from more directly determined by direct solar ra- but also relative humidity affects the sur-
thermal needles was obtained (Fig. 6b). diation. vival of adults and offspring of Ips typog-
raphus. The relative humidity is naturally
Discussion Differences between trees in forest higher in the galleries under the bark and
stand interior and at newly created thus reduces the risk of mortality from dry-
Vertical temperature distribution forest edge ing out. Larvae, as well as pupae and
Our results showed that the temperature Healthy or infested trees on forest edges adults, survived at 45° and 46 °C for 6
on the trunk base is influenced not only by had higher temperatures than trees in the hours, at 47 °C for 4 hours and at 48 °C for 2
solar radiation, but also by the local micro- stand interior. This corresponds with the hours. Only temperatures exceeding 47 °C
climate. A strong effect is related to dis- preference of I. typographus to attack trees appear to be fatal in the wild, but such
tance from the soil surface. The tempera- on forest edges (Jakuš et al. 2011, Kautz et temperatures only stay for a few hours and
ture at the trunk base is much higher com- al. 2013). A higher level of solar radiation the bark beetles survive this without loss.
pared to other measurement heights. Tem- generally increases surface temperature. Karasev et al. (2017) used semiconductor
peratures are higher on the sunny part of Due to the effect of transpiration, the sur- thermoresistors to detect trends in stem
the trunk. This agrees with our observa- face temperature of relatively healthy temperatures of Scots pine in different
tions of early spring bark beetle attacks on trees on the forest edge did not increase groups based on the physiological state of
forest edges in NP Šumava (Jakuš R, un- directly with exposed irradiation, as ob- trees. The temperature was found to have
published observations). In spring swarm- served for the infested ones. Our results a strong inverse correlation with the state
ing, bark beetles first attack trees on forest correspond with those of Hietz et al. of the trees, confirming that the method of
edges at the base of the trunk (Zumr 1985). (2005), especially with the described cool- assessing physiological state using temper-
Contrary to our findings, Powell (1967) re- ing effect of transpiration on sapwood and ature parameters allows early diagnosis of
ported lower temperatures at the base of cambium temperature. decreased viability of conifers.
lodgepole pine trunks than the rest of the With lower irradiation, healthy trees on
bole. Although it is a different species from the forest edge maintained their surface Canopy layer
Norway spruce, both species have similarly temperatures near those of trees in the in- Only a few studies have focused on bark
finely scaly bark with similar thermal prop- terior. In the case of interior trees, we beetle detection using UAVs (Senf et al.
erties (Nicolai 1986). So, according to Pow- found a similar temperature pattern for 2017, Hall et al. 2016). For example, Näsi et
ell (1967), possible contributing factors to shine and shaded parts of the trunk. This is al. (2015) and Näsi et al. (2018) distin-
the lower temperatures at the base in- caused by low irradiation in the stand inte- guished between healthy and infested and
cluded the lower temperature of xylem wa- rior and the forest stand microclimate. dead trees with overall accuracies of 76%
ter coming from the roots, conduction of and 81%, respectively, using hyperspectral
heat between the trees and ground, in- Differences between healthy and bark- UAV-borne images in Finland; however,
creased bark thickness nearer to the beetle infested trees they did not distinguish between healthy
ground, and lower air temperature near Bark-beetle infested trees on the forest trees and those in an early stage of infesta-
the ground that persisted for most of the edge had higher trunk temperatures than tion.
24 h period within the forest stand. It healthy trees on the forest edge and in the Using a handheld thermal camera, we
seems that the base of the trunk is strongly interior, whereas both groups of trees on measured the bottom, and shady part of
influenced by many factors that affect the the forest edge were exposed to similar the crown, and our study is thus limited.
microclimate of the tree. Powell (1967) in- amounts of incoming solar radiation. This Measuring crown temperature from the
dicated higher trunk sections as being agrees with the results of Powell (1967) on bottom is not the most appropriate
more similar in temperature. We concluded lodgepole pines infested by Dendroctonus method. We cannot be sure of the temper-
that the temperature at the trunk base was ponderosae. According to that study, fresh- atures on the sunny/upper part of the can-
not appropriate for further analysis. ly infested trees had subcortical tempera- opy because significant temperature differ-
Crowns tend to maintain stable tempera- tures similar to those of uninfested ones. ences could exist between the upper and
tures, even on infested trees. The measure- However, as the beetle broods developed, lower parts of the crown (Reinert et al.
ments of crown temperatures were not the water content of the sapwood low- 2012).
suitable for further analysis of these differ- ered, and the brood experienced higher We can assume that the healthy canopy
ences because this would involve different subcortical temperatures and greater ex- layer of uninfested trees tends to cool by
types of measurement (i.e., surface and tremes of temperature. On uninfested transpiration and maintains stable and ho-
iForest 14: 203-211 209Majdák A et al. - iForest 14: 203-211
mogeneous temperatures (Helliker & Rich- tic-information systems for improving the IUSS Working Group WRB (2014). World refer-
iForest – Biogeosciences and Forestry
ter 2008). Air flow could also be an impor- efficiency of management”, supported by ence base for soil resources 2014. World Soil
tant factor regulating the temperature of the Research & Development Operational Resources Reports no. 106, FAO, Rome, Italy,
the crown layer. Even the canopies of bark- Programme funded by the ERDF; (3) Grant pp. 192.
beetle infested trees had temperatures Agency of the Ministry of Education and Jakuš R, Edwards-Jonášová M, Cudlín P, Blaenec
similar to healthy trees. This may be due to the Slovak Academy of Sciences (VEGA M, Jeík M, Havlíček F, Moravec I (2011). Charac-
bark beetles attacking phloem (Wermelin- 2/0176/17); (4) EXTEMIT-K grant no. Z.02.1. teristics of Norway spruce trees (Picea abies)
ger 2004). Sap flow and cooling by transpi- 01/0.0/0.0/15_003/0000433, financed by surviving a spruce bark beetle (Ips typographus
ration can be affected only later by fungal Operational Programme Research, Devel- L.) outbreak. Trees - Structure and Function 25:
infection initiated by bark beetles (Paine et opment and Education (OPRDE). 965-973. - doi: 10.1007/s00468-011-0571-9
al. 1997, Wullschleger et al. 2004, Hubbard Jones HG (2004). Application of thermal imaging
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