Effect of diurnal solar radiation regime and tree density on sap flow of Norway spruce (Picea abies [L.] Karst.) in fragmented stands

doi:10.21203/rs.3.rs-3262723/v1

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Effect of diurnal solar radiation regime and tree density on sap flow of Norway spruce (Picea abies [L.] Karst.) in fragmented stands

https://doi.org/10.21203/rs.3.rs-3262723/v1

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The continuous threat of ongoing climate changes and related weather anomalies pose a significant challenge to forest ecosystems. The phytosociological structure of forests plays a crucial role in determining their resilience to various biotic and abiotic stressors. Moreover, stand density, which regulates the allocation of resources within individual trees, is a vital aspect for comprehending forest functioning. This study was conducted in Norway spruce forests located in the Czech Republic, where we investigated the influence of tree density on sap flow rates within three predefined directions corresponding to sun position during the morning (5:00–11:10 hours; East), noon (11:10–15:10 hours; South), and evening (15:10–21:10 hours; West) intervals. Tree density was calculated within a 10m radius buffer around each tree using high spatial resolution aerial imagery acquired by Unmanned Aerial Vehicle (UAV). We measured the sap flow in 10-minute intervals for 25 selected trees during the nine hottest days of the summer in 2019. We normalized sap flow measures using the abundance of tree foliage, which was qualitatively evaluated in the field as a reverse estimate of defoliation. The normalized data were used in further statistical analyses. Our findings reveal a strong negative correlation between sap flow and tree density, underscoring the substantial impact of neighboring tree density on tree transpiration. This relationship was most pronounced during midday, followed by the evening and morning hours, suggesting that sparser stands experience greater water deficit. The interaction between stand density and incoming solar radiation may constitute a crucial factor allowing forests to endure and adapt to climate change and other stressors such as bark beetle infestations.

solar radiation

sun positioning

azimuth angle

bark beetle outbreaks

transpiration

competition effect

Forests around the world are becoming more vulnerable to dieback due to the combined effects of temperature and drought-induced physiological stress, which are often accompanied by increase in the population of forest pests (McDowell et al., 2008; Allen et al., 2010; Kolb et al., 2016). The combination of recurring summer drought events and rising temperatures are believed to be a significant contributor to bark beetle outbreaks having direct effects on insect population dynamics and indirect effects on the growth and defense mechanisms of host plants (Jactel et al., 2012; Weed et al., 2013; Bentz and Jönsson, 2015; Raffa et al., 2015).

Typically, outbreaks are preceded by windthrow events since windfalls provide a development substrate for large numbers of pest beetles (Hanewinkel et al., 2008). The susceptibility of the forest ecosystem to wind damage depends on various factors, such as the attributes of individual trees, the composition of the overall stand, and the site characteristics (Mezei et al., 2014b). Spruce stands become susceptible to wind damage after reaching a certain age, with unstable trees being the first to be eliminated. Initially, individual trees fell down without showing a domino effect, but subsequent wind events or damage caused by bark beetles can lead to further individual damage (Schütz et al., 2006; Mezei et al., 2014b). Only after some dissipation of the main networking framework, from so-called "skeleton-trees" (the most stable ones), are gaps created, followed by a total dissolution of stands when wind events continue. These larger gaps act as forest edges and are shown to be attractive sites for initiating bark beetle infestations (Marešová et al., 2020; Střibrská et al., 2022). The small canopy opening serves as interspace gaps (Zabihi et al., 2021); however, the presence of larger gaps or forest edges likely induces destabilization by wind (Schütz et al., 2006) or by a combination of wind and bark beetle-caused damage (Jakuš, 1998; Mezei et al., 2014a). Fragmented open-canopy forests with lower density tend to have higher solar radiation loads on individual stands than close-canopy forests (Emmel et al., 2013; Vanderhoof et al., 2014), fostering beetle-favorable microclimate (Hroššo et al., 2020). Abrupt changes in microclimatic conditions, such as elevated temperatures, increased solar radiation, reduced humidity, increased vapor-pressure demands, and wind exposure, can have significant impacts on species composition (Kautz et al., 2013) and affect the physiological responses of trees, including sap flow (Herbst et al., 2007; Marešová et al., 2020; Özçelik et al., 2022).

The ecosystem response to changes in resource availability and competition is still a persisting challenge in ecology (Gleason et al., 2017). The phytosociological structure of the forest, determined by the number and size of individual trees, regulates how much light, water, and soil nutrients are available to each tree (Kholdaenko et al., 2022). Several studies have shown that reduced forest density increases the productivity of the remaining trees by increasing average resource availability while reducing the vulnerability of forests to wildfires and insect infestations (Latham and Tappeiner, 2002; Fettig et al., 2014). In contrast, others have shown that thinning can increase drought vulnerability due to evaporative demands (Aussenac, 2000; Lagergren et al., 2008; Brooks and Mitchell, 2011), as well as increased competition for soil moisture (Nilsen et al., 2001).

Tree growth and its ability to respond to environmental abiotic and biotic drivers are significantly affected by the density of trees in a given area and the competition for resources (Bello et al., 2019; Steckel et al., 2020). For instance, when the number of trees per unit area increases, it leads to greater competition for resources, such as water, nutrients, and sunlight, affecting photosynthesis and sap flow. In contrast, when tree density is lower, there is more space between the trees, allowing increased solar radiation exposure and heat loads on tree canopies and forest grounds, leading to increased evapotranspiration (Zabihi et al., 2023). Previous research has explored the effect of tree density on water stress (Eastham et al., 1990; D'Amato et al., 2013), resistance towards pathogens (Zausen et al., 2005; Zhang et al., 2013), and scarcity of nutrients (Latham and Tappeiner, 2002; Perevalova, 2019). However, it remains unclear how the transpiration rate is influenced by the competition effect caused by its surrounding tree populations. Therefore, understanding the interplay between tree density and transpiration patterns is of paramount importance in shaping forest management practices toward the establishment of a sustainable ecosystem, particularly in the context of prevailing occurrences of extreme climatic events.

Given the changing climatic scenario and limited evidence elaborating the interplay between tree stress and competition effects exerted by neighboring tree density, the first goal of the presented research was to analyze the effect of neighboring tree density on the sap flow rates of spruce stands. Additionally, we examined how temporal solar radiation influences sap flow rates during the day. We combined sun positioning data, sap flow measurements from selected hot days of the 2019 summer, and neighboring tree density around each target tree recorded using airplane-acquired imagery to understand the relationship between these variables.

2.1 Study Area

This study was carried out in a Norway spruce-dominated area located near Kostelec nad Černými Lesy (49.9940° N, 14.8592° E) in the eastern district of the Central Bohemian region of the Czech Republic (Fig. 1).

The study site contained six plots of mature (90–100-year-old) Norway spruce stands situated in a commixture of oak–beech vegetation zone. The vegetation of this area had a predominance of conifer forest with an admixture of fir and beech, but at the time of study spruce (50%) and pine (16%) were the dominant species (Trubin et al., 2023). The forest stands have been extensively managed through classical silvicultural practices, mainly thinning. In addition, the stands have experienced intensive sanitary interventions in recent years due to wind and bark beetle-induced tree damage resulting in canopy fragmentation.

2.2 Climatic data and meteorological measurements

Climatic data were collected by an automated meteorological station (Minikin Group, EMS Brno, Czech Republic) located in an open space between experimental plots 3 and 4 (< 200 m, Table 1). Soil water potential (SWP, Ψw) was recorded at topsoil (at 10 cm depth; Teros 21, Meter group, München, Germany) using five sensors in each subplot as single measurements. Relative air humidity (%) and air temperature (°C) were recorded at 10-minute intervals using Minikin QTHi (EMS Brno, CZ). The photosynthetically active radiation (PAR) was measured by an EMS12 quantum sensor (EMS Brno, CZ), which was used to calculate global radiation (W.m^− 2) for the selected days of the study period. The vapor pressure deficit (VPD) was estimated for each sector and all selected days according to the Monteith equation (Goudriaan and Monteith, 1990). To calculate potential evapotranspiration (ET₀), we utilized the "PE_Oudin" function from the 'airGR' package in R, which employs Oudin's formula (adjusted PE model). This model is known for its high accuracy and requires fewer environmental variables than other models (Oudin et al., 2005; Singh et al., 2023).

We selected days with high solar radiation during June and July, i.e., the four hottest days in June (24, 25, 26, and 30) and the five hottest days in July (6, 23, 24, 25, and 26). The average temperature was 26.5 ^oC across the selected days, while the maximum air temperature of 35.5°C was recorded on the 30th of June 2019. The average Ψw was high throughout the study period, and the lowest Ψw was observed at one subplot (− 1377 kPa). However, values did not drop below the permanent wilting point of − 1500 kPa, indicating no significant drought during the study period. The mean global radiation during this period was 6303 W.m^− 2. The mean daily relative humidity and VPD were 48.4% and 2051.6 Pa, respectively. The ET₀ for the selected days of the study period was 42.4 mm (Table 1).

All meteorological, SWP, and sap flow data collected from the field were stored in 10-minute intervals in a single data logger (GreyBox N2N 3P, EMS Brno, CZ) connected to a cloud system that delivers the data online.

Table 1

Average, maximum, and minimum values of selected variables measured and calculated at the study site during the target days of the study period. ‘2019 selected days' represents the nine days used in the study. E, S, and W represent the sectors defined by the time intervals of the sun transition. As SWP and ET₀ cannot be computed for hourly intervals, and thus the rows are left empty.
Variables		2019 selected days	E 5:00–11:10	S 11:10–15:10	W 15:10–21:10
Air temperature (°C )	avg	26.45	22.26	31.01	27.83
	min	11.85	11.85	25.19	20.05
	max	35.56	32.05	35.37	35.56
Relative air humidity (%)	avg	48.44	63.71	34.28	41.75
	min	17.78	30.14	17.98	17.78
	max	94.32	94.32	56.83	79.29
Global radiation (W. m^− 2.day^− 1)	avg	6303.68	2403.05	2943.50	957.13
	min	5996.47	2242.81	2834.01	888.50
	max	6654.42	2539.82	3072.25	1058.79
VPD (Pa)	avg	2051.63	1148.65	3038.95	2346.56
	min	96.09	96.09	1671.41	557.52
	max	4802.86	3192.85	4579.32	4802.86
SWP (Ψw, kPa)	avg	-102.52
	min	-1377.40
	max	-9.60
Potential Evapotranspiration (ET₀, mm)	avg	4.73
	min	4.35
	max	5.34

2.3 Tree buffer and neighboring tree density estimation

The determination of the apparent sun position trajectory during the day is closely related to estimating the solar azimuth angle (α). Generally, azimuth angle changes with latitude and time of year (Soulayman, 2018). The solar azimuth angle, sun trajectory, and sunlight phases (time of sunrise, sunset, dusk) for the selected days were determined using Suncalc.org (SunCalc sun position and sun phases calculator, 2017, Fig. 2). The daylight was observed from 5:00 to 21:10 hours (α = 52^o–308^o) on selected days, with the sun entering the East sector at 5:00 hours and leaving the West sector around 21:10 hours.

A 10-m radius buffer around each tree geo-position was created by overlaying the UAV imagery acquired at 20-cm spatial resolution using ArcGIS Desktop (10.8.1; ESRI 2020). The buffer was further divided into sectors (North, East, South, and West) according to the sun’s positioning. The number of trees located entirely in each sector was counted, while some trees positioned on the border of two neighboring sectors were assigned a decimal number, depending on the crown portion within each sector (Table 2). The sap flow sum was calculated for each sector; for example, when the sun was in the East (5:00–11:10 hours, α = 52^o–129^o), South (11:10–15:10 hours, α = 129^o–234^o), and West (15:10–21:10 hours, α = 234^o–308^o). The sap flow data were recorded at 10-minute intervals. Therefore, we rounded up the time difference between the sun entering and leaving each sector to align the data uniformly.

Table 2

The number of individual trees from each direction and their respective density used for balanced categories (25 trees for each sector) in this study.
Tree density in sectors	Sectoral direction
Tree density in sectors	East	South	West
1	2	0	3
1.5	1	0	4
1.8	0	0	1
2	4	4	4
2.2	0	2	0
2.5	4	4	4
3	4	5	4
3.5	4	5	3
4	4	4	0
4.5	0	1	0
5	1	0	1
5.5	1	0	1
Weighted Average	2.96	3.02	2.47
‍Median	3	3	2.5
Crown length (Mean ± SE, %)	57.8 ± 1.71	59.4 ± 1.74	58.6 ± 2.21
Foliage (Mean ± SE, %)	64.4 ± 0.78	64.0 ± 0.82	65.4 ± 0.76

2.4 Sap flow, tree, and crown characteristics measurements

Each plot consisted of four subplots with trees monitored for sap flow and soil water potential. The distance between the subplots ranged from 60 m to 120 m. Sap flow sensors (EMS 81, EMS Brno, CZ) were installed at the height of 2 m in the year 2018 on mature trees, considering their similarities in height, crown area, and diameter at breast height (DBH). The sap flow data was measured for the entire vegetation season in 2019. The year 2019 marked the first complete data collection for the entire vegetation season. Subsequently, in 2020, experimental clear-cuts were conducted on the designated study plots. The effects of clear-cutting are not included or analyzed as part of this study.

Additionally, macroscopic crown characteristics, such as crown length and total crown defoliation rates, were recorded during a field survey (Polák et al., 2007; Cudlín et al., 2017). The total crown defoliation, an important proxy of the tree's physiological condition, was visually estimated as the percentage of missing foliage in the total crown volume (Cudlin, 2015 in Jakuš et al., 2015). The length of the juvenile and productive parts of the tree crown, which determines photosynthetic rates, and defoliation pattern was used to calculate the unit of sap flow (Fig. 3).

2.5 Data processing and analysis

For a few trees from Plot 2, sap flow data were missing because a technical error prevented us from taking measurements on target days during our study period. Therefore, the data collected from these trees were not further processed. The sap flow data of selected trees were pre-processed by removing night-time fluxes and setting a baseline using the system software according to the manufacturer's instructions (Mini32 ver. 10.2.17.0; EMS Brno, CZ). Sap flow sums calculated over the selected days for individual trees were split into intervals from 5:00–11:10 hours, 11:10–15:10 hours, and 15:10–21:10 hours according to sun positioning in different sectors (East, South, and West).

The data were recalculated to relative crown length and abundance of foliage (1 − total defoliation). It was necessary to calculate the level of sap flow to a unit (relative to individual crown length) where all processes which affect sap flow occur. This recalculated unit was essential to compare the sap flow rates among individual trees. This data were used for the calculation of mean sap flow per tree and day. Subsequently, we normalized the data according to the best-performing tree on a particular day, as a result of this normalization, we obtained relative sap flow values of the trees in the dataset on a selected day. In the next step, we split these relative sap flow values according to time interval with the sun position in the sector and get the relative sap follow values for the East, South, and West sectors on that particular day. The normalization workflow is depicted in Fig. 3. The normalization allowed us to account for the temporal dynamics in radiation intensity over and within the selected days and prevented undesired variability that could potentially bias the results of direct comparisons among individual trees over each day of the study period.

Further, the extreme values (highest and lowest 10) were identified, and these trees were excluded from the analyses. The remaining 41 trees were then categorized based on the number of neighboring trees in particular directions, and 25 trees with balanced neighborhood densities in each sector were selected (Table 2). Maintaining balanced categories in categorical data analysis is crucial since each category's sample size plays a significant role. The analysis results can be significantly distorted if the sample sizes are unbalanced.

We calculated the weighted average and median for each group to test if selected tree density categories in each sector are uniformly distributed. The weighted averages in the East and South sectors were very close to three (E = 2.96 and S = 3.02, Table 2) and median was three (Table 3). However, for the West sector, the weighted average was 2.47 and median 2.5 (Table 2). Therefore, we modeled sap flow response in the sector using sap flow daily means of 25 selected trees, weighted average values, and regression equations presented in Fig. 4 to compare the measured and modeled sap flow rate values and to quantify the fit of the obtained regression equation.

Generalized Linear models (GLM) were used to define the relationship between tree groups and sap flow sums of each sector using the "glm" function from the "stats" package (RStudio Team, 2020). Further, Pearson's product-moment correlation test was performed using the "cor.test" function to examine the association between these two sets (RStudio Team, 2020).

3.1 Temporal dynamics of sap flow in response to solar radiation regime

Our data shows that diurnal sap flow variations in the particular sectors differed significantly (Table 3). In the morning, the sap of spruce trees started to flow at 05:00 hours with the beginning of solar radiation; a gradual increase was seen in sap flow rates corresponding to solar radiation in the East sector. The maximum values of sap flow were reached during midday between 11:10 to 15:10 hours, the period when the sun was at its zenith in the South sector. To better describe the changes, we used this sector as a reference in relative measure and considered the total sap flow in the South as 100%. Compared to this, there was a significantly lower mean sap flow rate in the early morning hours, which reached only 57.3% of the sap flow rates during midday. However, in the evening hours, when the sun was in the West sector, the total sap flow only decreased slightly by 11.8% compared to the South sector.

For validation, we modeled sap flow response by using balanced categories of trees in each sector. The modeled sap flow values for particular sectors show that the regression equation used in the calculation slightly underestimates the results compared to the measured values, while the distribution between sectors remains approximately the same.

Table 3

Hourly intervals for each sector according to sun positioning and the calculated variables used in the study. The % changes in sap flow in the East and West sectors are calculated by taking the South as a reference, assuming that the total flow in the South equals 100%.
Sector	Hourly intervals (Hours)	Azimuthal angle (α)	Sap flow measured (mean ± SE kg.day^− 1)	% Change in SF	Sap flow modeled (mean ± SE kg.day^− 1)	% Change in SF
East	5:00–11:10	52^o–129^o	45.16 ± 2.41	57.3	39.28 ± 0.98	57.8
South	11:10–15:10	129^o–234^o	78.78 ± 3.34	100	67.95 ± 1.68	100
West	15:10–21:10	234^o–308^o	69.48 ± 2.30	88.2	60.76 ± 1.51	89.4

3.2 Tree density and solar azimuthal angle effects on sap flow

We investigated the effect of tree density, i.e., the number of trees in the neighborhood, on sap flow rates by counting the number of trees in a 10 m radius buffer around each target tree. The sap flow rates responded inversely to tree density, suggesting lower sap flow in denser populations and higher sap flow rates in sparser stands over the entire study period (p < 0.01, Fig. 3).. Among all the directions, the strongest relationship between sap flow rates and tree density was observed in the West sector (R² = 0.43, p < 0.01, Fig. 3C). Furthermore, sap flow response to tree density for the South sector was slightly stronger (R² = 0.39, p < 0.01, Fig. 3B) than in the East (R² = 0.32, p < 0.01, Fig. 3A).

Tree density in forests can significantly affect the physiology, growth, and overall health of individual trees, as well as the functioning of the forest ecosystem. Sap flow and transpiration are primarily influenced by air temperature and solar radiation, among other environmental factors. However, studies examining the effects of tree density on physiological processes are less common than dendrological measurements, likely because of the greater complexity involved in field assessments (Gabira et al., 2023). To the best of our knowledge, no studies have attempted to describe the influence of tree density by means of ecophysiological processes, particularly sap flow, because these measurements are costly in terms of equipment as well as data post-processing. Here, we used a novel approach by calculating sap flow to a unit entirely based on individual trees' crown lengths and defoliation parameters and by counting neighboring tree density around each target tree using high-resolution imagery. The proposed relative sap flow measure is quantified per unit of available foliage and crown length as a ratio of sap flow recorded for a tree receiving the highest amount of solar radiation on a particular day, accounting for the diurnal sun position and tree-specific neighborhood competition in a stand.

4.1 Significance of data normalization relative to the crown length and defoliation

The transpiration rate is mainly influenced by the amount of solar radiation intercepted by the canopy. The physical and physiological functions of the canopy are determined mainly by the overall foliage within the tree crown (Wang and Jarvis, 1990), and failure to account for these factors can lead to biased interpretations of tree physiological processes and their relationship to environmental drivers. Therefore, it is essential to consider crown length and defoliation when analyzing and comparing data among different trees or populations to ensure a fair and robust assessment of ecological processes.

4.2 Temporal dynamics of sap flow in response to solar radiation regime

Our results demonstrate that transpiration rates were maximum at midday compared to early- and late hours. The major driving forces of tree transpiration are soil water content and atmospheric evaporative demand (Gartner et al., 2009), and significant factors that determine evaporative demand are radiation, air temperature, wind speed, and air humidity (Allen et al., 1998; Zhang et al., 2021). Our study sites had similar climatic conditions, and no significant drought period was observed; thus, the sap flow was mainly controlled by evaporative demands (ET₀). The average air temperature and Vapor Pressure Deficit (VPD) values followed similar patterns to sap flow, where maximum sap flow was observed when the evaporative demands were the highest in the South sector.

The tree transpiration values estimated from sap flow during early hours were comparatively lower, which can be attributed to cooler air temperature and lower VPD. It has been previously shown that solar radiation and VPD have significant effects on instantaneous sap flow and total tree transpiration (Oguntunde et al., 2007; Pereira et al., 2006). Thus, a gradual increase in air temperature upregulates evaporative demands resulting in more intensive sap flow as the sun approaches the zenith.

The maximum flow rates observed at midday align with previous findings by Burgdorf (2006), who studied the sap flow of eight birch trees over multiple years. Gartner et al. (2009) found that when water availability was limited, with the area receiving only 58% of the long-term average precipitation, the maximum sap flow in spruce trees occurred at 10:30 hours, and flow rates decreased gradually after that. However, when water availability was not a constraint, their reported findings were similar to ours, with the maximum transpiration rates observed during midday between 14:00–15:00 hours. Our results are consistent with the findings of Deutscher et al. (2016), who investigated diurnal sap flow patterns in mixed forests and observed the peak flow occurrence at 14:00 hours. Furthermore, in a simulation study, Jacobs et al. (2020) showed that the average diurnal air temperature and relative humidity are inversely related, suggesting that air temperature reaches its maximum value during midday, and relative humidity values drop to their lowest point at midday. This observation corresponds to our findings, suggesting that the maximum average temperature recorded in the South sector (31^oC) during the study period could explain the higher transpiration rates observed in this sector.

The elevated transpiration rate during midday can be attributed to the direct intense solar radiation and subsequent increase in VPD when the sun is positioned at its zenith. The stomatal opening occurs in response to red and blue light exposure, facilitating gas exchange between mesophyll tissues in leaves and the surrounding environment (Matthews et al., 2020). Gas exchange is required not only for the inward diffusion of CO₂ and photosynthesis but also for water uptake by roots (Inoue and Kinoshita, 2017). As temperature increases, the mesophyll conductance improves as a result of decreasing water viscosity, enhancing water supply to evaporation sites (Cochard et al., 2000; Urban et al., 2017). However, according to Lawson et al. (2011), plants maintain a balance between transpiration and photosynthesis by regulating these processes in parallel. Additionally, high temperatures can lead to reduced photosynthesis due to decreased RubisCO activity (Crous et al., 2022), but some studies suggest that this relationship may not always hold under extreme temperatures, where gas exchange may remain high despite reduced assimilation (Ameye et al., 2012; von Caemmerer and Evans, 2015). Therefore, even at high temperatures during midday, assimilation may decrease without significantly reducing transpiration. Nevertheless, most trees demonstrate a uniform diurnal pattern of sap flow under adequate water conditions (Burgdorf, 2006; Gartner et al., 2009; Deutscher et al., 2016), albeit there exist species-specific temporal variations.

4.3 Sap flow relations to tree density

Our study demonstrates a discrete inverse correlation between sap flow and neighboring tree density. Consistent with our hypothesis, we observed that trees within a higher-density stand had lower transpiration rates, while those in sparser density exhibited higher transpiration rates. Although the categorical tree distribution shows a high significance of density parameters (p < 0.01), the regression model indicates the highest correlation in the West, followed by the South and East sectors, respectively (Fig. 4).

These results build on the existing evidence of competition effects by neighboring trees (Canham, 2004; Bhandari et al., 2021; Zabihi et al., 2023). As sessile organisms, trees cannot relocate to more favorable environments when they encounter stressful conditions. Consequently, as tree density rises, the competition for resources intensifies among individuals, both above and below ground. A previous study by Thorpe et al. (2010) suggests that the competition effects exhibited a significant decline as the distance between neighboring trees and the target tree increased, which confers with our finding (see Fig. 4). In this study, the observed decrease in sap flow in response to increasing tree density can be attributed to intensified competition for resources. Trees require a sufficient amount of light, water, and nutrients, among other resources, to optimize photosynthesis rate, which any of these factors can significantly limit, ultimately leading to a reduction in transpiration.

Generally, trees are protected against solar radiation either by their own branches (individual shading) or by the crowns of neighboring trees (collective shading) (Jakuš et al., 2011). It has been shown that collective shading buffers trees against direct solar radiation (Mezei et al., 2014b) and consequent abiotic and biotic stressors. However, according to Bertamini et al. (2006), spruce trees grown in the shade had different pigment compositions and lower activities of Photosystem (PS) 1 & Photosystem (PS) 2, two membrane-protein complexes absorbing longer and shorter light wavelengths, respectively. In addition, excessive shading is likely to induce changes on the donor site of PS2 in spruce needles, thereby abrupting the stomatal conductance and reducing photosynthate activity (Bertamini et al., 2006). Therefore, the reduction of sap flow among denser stands could be theoretically driven by altered stomatal conductance. In other words, the limited availability of light due to shading by neighboring trees may restrict or reduce the transpiration rate. However, due to the lack of gas-exchange measurements, the reasons for the observed decline of sap flow in denser stands are speculative but plausible. On the contrary, individual trees growing in open areas exhibit greater water usage driven by various factors, such as higher water availability per tree, increased exposure to radiation, and greater canopy roughness and ventilation in thinned forests (Raper, 1998)

Previous findings suggest that competition for below-ground resources is stronger than for above-ground resources (Casper and Jackson 1997). Despite the lack of water limitation in our study site, the reduced transpiration among denser stands could possibly be a consequence of morphological constraints of the root system. The shallow root system of spruce makes it susceptible to water stress, especially for trees grown in dense stands (Horgan et al., 2003; Caudullo et al., 2016); thus, mature trees need more space to access below-ground water for survival in drought conditions. Although trees possess a variety of mechanisms to tolerate or withstand prolonged drought and maintain their physiological and morphological functions (Brunner et al., 2015), the root architecture might be constrained by lack of space due to adjacent trees, thereby reducing the total area available for horizontal root distribution. Furthermore, the utilization of soil space is recognized as a fundamental aspect of underground competition (Casper and Jackson, 1997). The capability to occupy soil space varies from species to species and relies on several root attributes, such as fine root density, total surface area, biomass, and relative growth rate (Casper and Jackson, 1997). Even though the majority of roots are present in the upper 30 cm of soil, some roots can extend to greater depths providing more area to explore for resources. Since different species have different spatial and temporal rooting patterns (Casper and Jackson, 1997), mixed forests with diverse species composition may have better overall functioning than monoculture forests due to minimal overlap between neighboring root systems.

4.4 Disturbance processes and associated effects on spruce physiology

Previous studies suggest that trees growing in gaps or at the forest edges exhibit higher sap flow rates due to increased solar radiation exposure (Marešová et al. 2020, Özçelik et al. 2022). Higher transpiration rates are associated with better tree defense capability as transpiration is a part of constitutive and induced defenses (Özçelik et al. 2022). Reduction in living foliage volume reduces transpiration and photosynthetic rates, compromising resource demanding induced defense mechanism against bark beetles (Herms and Mattson 1992), which select hosts with more intensive total and primary crown defoliation compared to secondarily attacked and non-attacked spruces (Korolyova et al. 2022b). However, under water-limiting conditions, trees in fragmented stands are likely to downregulate transpiration. Water deficit, especially in hot summer periods, may result in reduced transpiration rates and an ensuing decline in the cooling effect of sap flow. Marešová et al. (2020) showed that despite high transpiration levels, trees growing at sun-exposed forest edges exhibit higher bark surface temperature and volatile compound (VOC) emissions than shaded trees located in stand interiors. Despite a substantial cooling effect of sap flow on cambium temperature, the maximum temperature difference between a tree with high sap flow and one without transpiration did not exceed 2.5°C (Hietz et al. 2005). These facts indicate that higher levels of transpiration in trees located in fragmented stands may not cause sufficient cooling to avoid colonization by insect herbivores. Elevated bark surface temperature and radiation may predispose trees to beetle-induced mortality (Kautz et al. 2013) through thermal effects on Ips typograpus life cycle and population development (Wermelinger and Seifert, 1999). Spruce survival probability during bark beetle outbreak declines substantially with increasing proportion of stem exposed to the sun (Korolyova et al., 2022b) and less extensive tree crown (Jakuš et al., 2011; Korolyova et al., 2022a).

The negative relationship between tree crowding and sap flow, based on our results, supports the previous findings suggesting that aggravated resource competition significantly reduces spruce survival potential during bark beetle disturbance (Buonanduci et al., 2020; Korolyova et al., 2022a). Light, nutrient, and water deficit impairs plant physiological functions and carbon assimilation processes. Depletion of energy pools available for the production of secondary metabolites may ensue tree life history trade-offs, suggesting that carbon allocation to maintain transpiration and photosynthesis is prioritized over synthesis of defensive compounds (Jones and Hartley, 1999).

This study employed two partial goals concerning the sap flow dynamics of Norway spruce during nine hot summer days in 2019. The first goal was to identify the diurnal transpiration pattern and its dynamic dependency on solar radiation. The findings of this study align with prior research indicating that transpiration is maximum during midday in June and July due to the high intensity of radiation and subsequent increases in VPD and ET₀. The second goal was to demonstrate the influence of density on tree transpiration, revealing that neighboring tree density significantly impacts tree transpiration. Our findings suggest that tree-level sap flow is relatively higher in sparser stands than in denser stands under uniform water availability. However, this relationship might not hold under drought conditions, and sparser stands could be more vulnerable to stress and subsequent pest infestations.

From a practical perspective, these findings can be helpful in forest management strategies by making informed decisions about thinning and other forest management practices to promote healthy tree growth and reduce the risk of drought stress and pest infestations. Further, mixtures could be a preferable plantation strategy over monocultures (Zhang et al., 2022). The variation in canopy length and size in trees of mixed forests may provide better coverage and reduce the incoming solar radiation to the forest floor, thus lowering evaporation from the ground. On the contrary, microclimate offsets are less prominent in monocultures (Zhang et al., 2022). Therefore, mixed forests may better adapt to changing environmental conditions due to the greater genetic diversity within the forest (Liu et al., 2018). Mixed forests are generally more resilient to windthrow events and pest infestations than monoculture forests and can provide a broader range of ecosystem services and functions (Liu et al., 2018).

Further follow-up studies in forests with varying species composition and environmental conditions are required to understand the relationship between stand density and its subsequent effect on tree-level transpiration. For instance, hilltops are likely to experience higher solar radiation and subsequently exhibit increased transpiration rates due to lower solar incidence angles compared to hillsides during the mid-day and early afternoon in the summer season. This could potentially lead to elevated water stress conditions on hilltops, rendering them more susceptible to pest infestations. Considering that tree density generally decreases with increasing elevation (Muñoz Mazón et al., 2020; Zabihi et al., 2023), it is recommended that future studies investigate the interplay between diurnal sun azimuth, transpiration rate, aspect, elevation, slope gradients, and tree density, and how these factors collectively influence forest resilience against pest infestations. Previous research by Mezei et al. (2014b) has indicated a positive correlation between tree mortality in all stages of bark beetle outbreaks and elevation gradients, further highlighting the importance of understanding the complex dynamics between these variables.

Funding

This work was funded by grant No. CZ.02.1.01/0.0/0.0/15_003/0000433, "EXTEMIT – K Project," Operational Program Research, Development and Education (OP RDE), grant QK1910480 "Development of integrated modern and innovative diagnostic and protection methods of spruce stands with the use of semiochemicals and methods of molecular biology," financed by the Ministry of Agriculture of Czech Republic, Slovak Research and Development Agency (APVV-19-0606), and Internal Grant Agency (IGA A_28_22, VIVEK VIKRAM SINGH) at the Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, Prague, Czech Republic.

Authors' contributions

Vivek Vikram Singh: Investigation, Validation, Data curation, Formal analysis, Visualization, Funding acquisition, Writing – original draft. Khodabakhsh Zabihi: Data curation, Investigation, Formal analysis, Writing – review & editing. Aleksei Trubin: Data curation, Writing – review & editing. Rastislav Jakuš: Supervision, Conceptualization, Funding acquisition, Data curation, Writing – review & editing. Pavel Cudlín: Data curation, Writing – review & editing. Nataliya Korolyova: Data curation, Writing – review & editing. Miroslav Blaženec: Conceptualization, Formal analysis, Visualization, Investigation, Writing – review & editing.

Acknowledgements

We thank Prof. Fredrik Schlyter, Prof. Hynek Burda, and Dr. Ewald Grosse-Wilde for their constructive comments on the initial manuscript draft. We also thank Aisha Naseer and Rajarajan Ramakrishnan for their continuous support and constructive comments.

Availability of data and material (data transparency)

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of interest/Competing interests

The authors declare that they have no conflict of interest.

Consent for publication

All authors gave their informed consent to this publication and its content

Reference to pre-print servers: http://dx.doi.org/10.2139/ssrn.4480120

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Effect of diurnal solar radiation regime and tree density on sap flow of Norway spruce (Picea abies [L.] Karst.) in fragmented stands

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Abstract

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1. INTRODUCTION

2. METHODS

2.1 Study Area

2.2 Climatic data and meteorological measurements

2.3 Tree buffer and neighboring tree density estimation

2.4 Sap flow, tree, and crown characteristics measurements

2.5 Data processing and analysis

3. RESULTS

3.1 Temporal dynamics of sap flow in response to solar radiation regime

3.2 Tree density and solar azimuthal angle effects on sap flow

4. DISCUSSION

4.1 Significance of data normalization relative to the crown length and defoliation

4.2 Temporal dynamics of sap flow in response to solar radiation regime

4.3 Sap flow relations to tree density

4.4 Disturbance processes and associated effects on spruce physiology

5. CONCLUSION

6. MANAGEMENT IMPLICATIONS

Declarations

References

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