Altitude and Stem Height Position as Determinants of the Hydrological Properties of Norway Spruce Bark

Abstract

Tree bark plays a crucial role in the distribution of rainfall within forest ecosystems, particularly through its impact on stemflow. To gain a comprehensive understanding of how bark controls stemflow, it is essential to identify all factors affecting bark water storage capacity, as this determines the onset of stemflow during rainfall events. Our study analyzed how the position of bark on the stem and the altitude above sea level impact bulk density, water storage capacity, and the time required for bark saturation. We conducted research on Norway spruce bark collected at four altitudes: 400, 550, 700, and 1150 m asl. Our findings revealed that bark from the 400 m altitude had a bulk density that was approximately 24.5% greater than that from higher altitudes. Additionally, the water absorption time for bark from 1150 m was over 68% longer than that for bark from other altitudes. The longest absorption time (about 6.4 days) was observed in the bottom part of the trees, while the shortest (about 4.4 days) was in the top part of the trees. We also observed that the bark water storage capacity increased from the base to the top of the trees and with increasing altitudes. Specifically, the water storage capacity of bark taken from 400 m was approximately 33% lower than that from 1150 m. These findings highlight the significance of stem height position and altitude as key determinants of bark water storage capacity.

1. Introduction

Bark properties play a crucial role in rainfall partitioning within forest ecosystems, influencing interception, throughfall, and stemflow [1,2]. Specifically, factors such as bark thickness, texture, density, and moisture content affect the water storage capacity of bark, which in turn impacts stemflow production. For example, species with smooth bark tend to produce higher stemflow than those with rough bark [3,4]. Additionally, bark wettability is strongly correlated with stemflow yield, with highly wettable species exhibiting different stemflow dynamics than non-wettable species [5]. Bark modifies the chemistry and composition of water draining over its surface during precipitation, affecting sub-canopy precipitation quality. It accumulates, transports, and reacts with various materials, influencing the fate of rainwater in forest ecosystems [6,7]. The anatomical features of bark, such as the thickness of the rhytidome and the presence of cellular structures, control the leaching rates of macronutrient ions in stemflow, impacting nutrient cycling through the bark [8]. Species with furrowed textures, greater depth, and more furrows demonstrate pronounced differences in ion enrichment in their stemflow [9].

Although tree bark plays a pivotal role in the partitioning of rainfall, it has received limited focus in ecohydrological studies. Unlike other components of the forest canopy, the specific functions and contributions of bark to these processes remain underexplored. Understanding the factors that influence bark’s water storage capacity is particularly important, as the point at which bark becomes saturated marks the initiation of stemflow during rainfall events [10]. Bark water storage capacity is required for determining the amount of rainfall interception and understanding throughfall and stemflow processes in forest ecosystems [11].

The water storage capacity of bark varies among tree species, with certain species exhibiting higher capacity than others [12]. For instance, hickory demonstrated higher hygroscopicity than oaks in a comparative study of tree species, indicating distinct species-specific differences in water storage mechanisms [13]. These differences arise from the varying physical properties of bark and the hygroscopicity of different tree species. Studies conducted on several European tree species showed that the water storage capacity of tree bark was related to bark and tree thickness [14]. Thin bark had a higher water storage capacity than thick bark, and bark taken from thinner trees exhibited a higher water storage capacity than bark obtained from thicker trees. Some studies suggest that the physical properties of bark can vary along tree stems [15]. Bark thickness generally decreases with height, with notable differences above and below breast height for some species [16]. In Scots pine, a significant change in bark structure along the stem was observed, transitioning from thick and rough to thin and smooth [17]. An analysis of four broadleaved species in the Western Carpathian Mountains revealed that bark thickness and specific surface mass increased with stem diameter and decreased with distance from the base of the stem [18]. The study also emphasized variations in bark properties among species, with common aspen showing the thickest bark and sycamore the thinnest. Bark thickness varies regionally, with differences being attributed to latitude, crown position, and genetic identity [16,18]. Moreover, a study on fir saplings showed that those grown at low altitudes had a greater bark proportion. In contrast, those at high altitudes had thicker tracheid walls, suggesting an influence of altitude on bark properties [19]. A study in France detected a small but significant altitude effect on bark thickness at breast height for certain tree species, indicating species-specific responses to altitude [20].

Although the influence of various factors, including environmental conditions, on the physical properties of bark (especially bark thickness) is well documented in the literature, few studies have explored how vertical variability in bark properties along tree stems affects their water storage capacity [21,22]. Furthermore, to the best of our knowledge, no studies have yet been conducted on how elevation above sea level and the resulting variability of climatic conditions along the altitude gradient might shape the water storage capacity of tree bark. Thus, the primary objective of this study was to analyze how the location of Norway spruce (Picea abies [L.] Karst) bark on the stem and the altitude above sea level at which the tree grows influence three bark properties: bulk density, water storage capacity, and water absorption time. We hypothesized that (1) the water storage capacity of tree bark varies significantly with its position on the stem, with bark from different stem heights exhibiting different capacities and absorption times; (2) the altitude above sea level has a measurable impact on the physical and hydrological properties of tree bark, including bulk density, water storage capacity, and saturation time, with higher altitudes leading to bark with greater water storage capacities and longer absorption times.

2. Materials and Methods

2.1. Study Site and Bark Samples Collection

Bark samples from Norway spruce trees were collected from four designated sites within the Regional Directorate of State Forests in Katowice, southern Poland (

Table 1. Characteristics of the sample collection sites and Norway spruce trees selected for bark sampling.

Altitude (m asl)	Location	Average Annual Temperature 1 [°C]	Annual Precipitation 1 [mm]	Number of Trees Felled	Tree Height Range [m]	Diameter at Breast Height Range [cm]
1150	49.6045° N	4.0	1200	3	21–24	32–36
	18.9967° E
700	49.5977° N	5.5	1075	3	25–28	32–35
	18.9422° E
550	49.5616° N	6.0	1000	3	28–32	31–33
	18.8568° E
400	49.7638° N	8.0	850	3	30–32	33–35
	18.8730° E

¹ based on https://www.bdl.lasy.gov.pl/portal/mapy-en, accessed on 28 November 2024.

). The region is characterized by a temperate climate, featuring moderate temperatures and distinct seasonal variations. The site selection criteria included two key factors: (1) altitude, represented by approximately 400, 550, 700, and 1150 m above sea level, and (2) a consistent diameter at breast height (DBH) of the trees, specifically in the range of 30–36 cm.

At each site, three representative trees were carefully selected, ensuring that trees with lichens or evidence of beetle damage were excluded to preserve the integrity of bark hydrology during subsequent laboratory analyses. Each selected tree was systematically felled and divided into 11 evenly spaced sections along its height: 0.0 (base), DBH, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 (tree top). Rectangular bark samples were then extracted from each section using appropriate tools, including a chisel, knife, and hammer, for further analysis.

2.2. Laboratory Tests

Upon transporting the fresh bark samples to the laboratory, they were cut into uniform pieces approximately 1 cm wide using a band saw. The prepared samples were then dried in an oven at 35 °C to remove excess moisture. Once fully dried, the samples underwent a series of analyses to measure water absorption time, bark water storage capacity, and bulk density.

For the water absorption tests, the dried bark samples were completely submerged in beakers filled with water. To ensure consistent hydration conditions and minimize evaporation, the samples floating on the surface of the water were covered with a layer of cotton fabric. This setup ensured that the floating samples remained in contact with the water throughout the experiment. The water absorption time was recorded from the moment of immersion until the samples sank to the bottom of the beaker, which indicated that their density had exceeded 1 g cm⁻³, consistent with the criteria established in previous studies [22,23].

Samples that sank to the bottom were retrieved immediately upon sinking but not less frequently than once every 12 h. For each retrieved sample, the wet mass was measured, and the volume was determined using the water displacement method in a graduated cylinder. Finally, the samples were dried in an oven at 105 °C to a constant weight and their dry mass was recorded.

The bark water storage capacity (%) and bulk density (g cm⁻³) of each sample were calculated using the formulas [13,22]:

$$\mathrm{B}\mathrm{a}\mathrm{r}\mathrm{k} \mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{S}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e} \mathrm{C}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}=\left({\displaystyle \frac{\mathrm{W}\mathrm{e}\mathrm{t} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}-\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}}}\right)·100$$

(1)

$$\mathrm{B}\mathrm{u}\mathrm{l}\mathrm{k} \mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}/\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}$$

(2)

2.3. Statistical Analysis

The statistical analyses and corresponding graphical representations were conducted using Statistica 13.3 PL (StatSoft Inc., Tulsa, OK, USA). To evaluate the differences in bark water storage capacity, bulk density, and water absorption time across various tree heights and altitudes, we utilized the non-parametric Kruskal–Wallis test. This test was selected after verifying the absence of normality with the Shapiro–Wilk test and confirming the lack of homogeneity of variances using Levene’s test. Additionally, to further investigate the effects of tree location and altitude on bark hydrology and bulk density, we employed a general linear model (GLM). All statistical tests were performed at a significance level of 0.05.

3. Results

The bulk density of Norway spruce bark ranged from 0.209 to 0.508 g cm⁻³, with an average of 0.293 ± 0.002 g cm⁻³. The bark density decreased with increasing altitudes above sea level (Figure 1A). Bark from the lowest altitude (400 m) had, on average, 24.5% higher density compared to bark from the higher altitudes (p < 0.001), with the greatest difference in bark density (28.6%) being observed between 400 m and 1150 m asl (p < 0.001). Regardless of altitude, the average bark density varied with stem height, rapidly decreasing from the base (0.0) to 30%–40% of the tree height and then remaining relatively consistent through the middle and upper portions of the stem (

Table 2. Summary of the average and standard error of the bulk density, water absorption time, and water storage capacity of Norway spruce bark depending on the stem height position. Different letters within a vertical column indicate statistical differences in bark density, water absorption time, and bark water storage capacity between stem height positions (Kruskal–Wallis test, p < 0.05).

Stem Height Position	Bulk Density [g cm−3]		Water Absorption Time [Days]		Bark Water Storage Capacity [%]
	Mean	SE	Mean	SE	Mean	SE
0.0	0.414 a	0.008	6.2 a	0.3	153.3 a	5.0
DBH	0.317 b	0.007	6.4 a	0.3	237.3 b	7.6
0.2	0.291 bd	0.007	6.6 a	0.4	266.2 bc	7.5
0.3	0.282 cde	0.006	5.7 ab	0.3	272.2 bc	6.7
0.4	0.285 bef	0.006	5.0 ad	0.2	270.9 bc	6.9
0.5	0.279 cdf	0.006	6.0 ae	0.4	276.1 bc	6.0
0.6	0.277 cdf	0.005	5.7 af	0.4	283.3 c	5.1
0.7	0.277 cdf	0.007	4.4 b-g	0.2	283.9 c	7.6
0.8	0.270 cdf	0.005	4.0 cfh	0.3	290.5 c	6.4
0.9	0.266 cdf	0.005	4.9 aghi	0.3	292.6 c	6.6
1.0	0.273 cdf	0.007	4.2 bcdhi	0.3	291.8 c	10.6

DBH—diameter at breast height, SE—standard error.

). Bark density at the base was ~31% higher than at diameter at breast height (DBH, p = 0.001) and, on average, ~49% higher than in the remaining parts of the trees (p < 0.001). The GLM analysis confirmed that both altitude and stem height position significantly influence the bulk density of Norway spruce bark (

Table 3. General linear model analysis for bark characteristics.

Bark Properties	Altitude		Stem Height Position		Altitude × Stem Height Position
	F	p	F	p	F	p
Bulk density	98.1	0.000	50.9	0.000	3.4	0.000
Water absorption time	159.3	0.000	18.4	0.000	5.8	0.000
Bark water storage capacity	175.7	0.000	46.7	0.000	4.9	0.000

The significance effect (p < 0.05) is shown in bold.

).

The water absorption time of Norway spruce bark ranged from 1.1 to 12.6 days, with an average of 5.4 ± 2.4 days. While the average water absorption time for bark at altitudes of 400, 550, and 700 m above sea level was similar (4.3 ± 1.7 days), the time required to saturate bark collected from an altitude of 1150 m was over 68% longer than that for bark from the other altitudes (Figure 1B). Interestingly, the water absorption time decreased with the tree height. We observed that the bark from the lower parts of the trees (up to approximately 20% of their height) had the longest water absorption time, averaging around 6.4 days. In contrast, the bark from the top part of the trees (above 70% of their height) had the shortest water absorption time, averaging 4.4 days. Between 30% and 70% of the tree height, the average water absorption time was 5.6 days.

The water storage capacity of spruce bark varied from 87.7 to 416.2%, with an average of 265.8 ± 2.6%, and increased systematically with altitude (Figure 1C). The water storage capacity of bark obtained from trees at an altitude of 1150 m above sea level (asl) was significantly higher than that of bark collected at altitudes of 400, 550, and 700 m asl (p < 0.001) by an average of 32.7%, 15.7%, and 8.6%, respectively. The water storage capacity of bark exhibited variation along the vertical gradient of the tree: in the upper sections, it was nearly twice as high as in the basal part of the tree (Table 2). We found that the water storage capacity of bark was dependent on its bulk density, i.e., the higher the density, the lower the water storage capacity (Figure 2). The GLM analysis confirmed that both altitude and stem height position significantly influence the water storage capacity and the water absorption time of Norway spruce bark (Table 3).

4. Discussion

This study examined the impact of altitude above sea level and stem height position on the bulk density and bark hydrology of Norway spruce, the most common and important forest tree in Europe [24]. Our study demonstrates that bark properties, including bulk density, water storage capacity, and water absorption time, are significantly influenced by both the altitude at which trees grow and the vertical position of bark along the stem.

Some studies showed Norway spruce trees’ climate-growth relations between high- and low-elevation sites [25,26]. Thus, the noted enhancement in bark water storage ability with altitude suggests that trees may modify their bark characteristics to maximize water retention in response to different environmental conditions. More extreme climate factors in elevated areas, usually lower temperatures and higher precipitation [27,28], may promote the evolution of bark with greater water storage capacities. Similarly, the gradient of water storage capacity from the base to the top of the stem suggests a functional adaptation, possibly related to differences in exposure to rainfall. Levia and Wubbena [21] demonstrated that the bark in the lower sections of the stem of eastern white pine (Pinus strobus L.) retains much more water than the bark in the upper sections, revealing a pronounced vertical gradient in bark water storage capacity. In our study, which supports the findings of Ilek et al. [22], we found that Norway spruce bark has the opposite trend. This difference suggests that vertical variation in bark hydrology may also be a species-specific trait influenced by natural distribution ranges and vertical gradients, requiring further and more complex research.

Our findings showed that the bulk density of Norway spruce bark decreases along the vertical gradient of the tree stem, with higher values being observed at the base. This trend is consistent with observations by Quilhó and Pereira [15], who reported a slight reduction in bark density along the stems of Eucalyptus globulus trees. In contrast, Bhat [29] recorded a different pattern in two birch species, observing a slight increase in bark density from the lower parts to the upper sections of the stem. The decrease in bark density with altitude in our study may suggest that trees growing at higher elevations develop less dense bark as an adaptive response to colder temperatures and increased precipitation. Considering the established relationship between bark morphology and moisture absorption [30], this potential adaptive response could significantly impact the hydrological cycle of watersheds, highlighting it as a critical area for further investigation. This perspective is supported by the findings of Nie et al. [31], which demonstrated that spatial variations in bark thickness are significantly influenced by many factors, such as annual mean temperature, mean diurnal temperature range, and altitude. Additionally, MacFarlane’s research [32] underscores that tree bark density exhibits considerable variability, reinforcing its adaptive significance in shielding trees from environmental stressors.

The variation in bark density of Norway spruce at different altitudes can be attributed to differences in the composition and relative proportions of the outer and inner bark layers. Meyer et al. [33] reported that the outer bark generally has a higher density due to its compact structure, while the inner bark is characterized by a lower density and higher moisture content [34,35,36]. This distinction may explain the increased water storage capacity observed in samples collected from an altitude of 1150 m above sea level. Furthermore, prior research has established a strong correlation between bark density and porosity [13], indicating that lower density is associated with higher porosity. Increased porosity creates more void space within the bark tissues for water retention, which could contribute to the high water storage capacity seen in bark from higher altitudes. The methodology used in this study to determine bark water storage capacity further supports these findings. Specifically, the moment a bark sample sank to the bottom of the beaker—indicating that its wet density exceeded 1 g cm⁻³—was considered the threshold for full water saturation. Bark with a lower density and higher porosity requires a larger volume of water to reach this threshold, which likely explains the longer water absorption times observed in samples from 1150 m above sea level (Figure 1B). Additionally, the observed low bark density at elevated altitudes, along with prolonged water absorption times and substantial water storage capacities, suggests that the bark of trees in these regions plays a critical hydrological role. Specifically, it may enhance water retention during extended rainfall events and contribute to the mitigation of surface runoff.

The variation in bark water storage capacity along the stem, particularly between the lower and upper regions, indicates that this capacity is influenced not only by bark density but also by the contributions from both the outer and inner bark. Research has established that bark thickness generally decreases with distance from the base of the tree, as observed in spruce trees [22] and others [17,35]. This trend correlates with changes in bark age along the vertical stem profile, where older bark is located at the lower sections and younger bark is found at higher sections. Additionally, inner and outer bark proportions usually vary along the stem height [18,37,38]. The observed reduction in water absorption time from the base to the upper parts of the tree and the increased bark water storage capacity indicates that the younger bark in the upper regions becomes saturated more quickly during rainfall. It is interesting to note that, regardless of the bark’s position—whether at different heights or locations along the stem—the water absorption rate in spruce bark varied between 1 and 13 days. This variation underscores the complexity of the absorption process and suggests that a single rainfall event is not enough to thoroughly saturate the internal tissues of the bark [14].

5. Conclusions

This study investigated how the stem height position and altitude above sea level influence the physical and hydrological properties of Norway spruce bark, providing an initial exploration of how these factors affect bark characteristics. The key findings of this study are as follows:

(1)

The bulk density of spruce bark decreases with both the height along the tree stem and increasing altitude.

(2)

The time required for the bark to absorb water is inversely related to the vertical stem gradient. Specifically, bark collected from the highest altitude (~1150 m asl) took over 68% longer to saturate compared to bark from lower altitudes.

(3)

There is a negative correlation between bulk density and bark water storage capacity: as bulk density increases, the bark water storage capacity decreases.

(4)

Bark water storage capacity consistently increases with both stem height and altitude, with bark from the top of the tree and from high altitudes showing the greatest water storage capacity.