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Vertical Variability in Bark Hydrology for Two Coniferous Tree Species
As the outermost layer of stems and branches, bark is exposed to the influence of atmospheric conditions, i.e., to changes in the air’s relative humidity and wetting during storms. The bark is involved in water interception by tree canopies and stemflow generation, but bark–water relations are often overlooked in ecohydrological research and insufficiently understood. Relative to other canopy ecohydrological processes, little is known about vertical variation in bark properties and their effect on bark hydrology. Thus, the objective of this study was to analyze changes in physical properties (thickness, outer to total bark thickness ratio, density, and porosity) and hydrology (bark absorbability, bark water storage capacity, and hygroscopicity) vertically along stems of Norway spruce [Picea abies (L.) Karst.] and silver fir (Abies alba Mill.) trees. Our null hypotheses were that bark hydrology is constant both with tree height and across measured physical bark properties. We found that bark thickness and the ratio of outer-to-total bark thickness decreased with tree height for both species, and this was accompanied by an increase in the bark water storage capacity. In contrast, the bark’s density, porosity, and hygroscopicity remained relatively constant along stems. These results inform ecohydrological theory on water storage capacity, stemflow initiation, and the connection between the canopy water balance and organisms that colonize bark surfaces.
Altitude and Stem Height Position as Determinants of the Hydrological Properties of Norway Spruce Bark
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.