Vertical Variability in Bark Hydrology for Two Coniferous Tree Species

Anna Ilek,Jarosław Kucza,John T Van Stan,Karolina Morkisz

doi:10.3389/ffgc.2021.687907

Abstract

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.

Highlights

IntroductionWhen rain falls over forests, a hydrologically and ecologically relevant portion of that water (up to several mm event−1, depending on storm and canopy conditions) is retained on the canopy’s leaves, epiphytes, and bark (Klamerus-Iwan et al, 2020)
When rain falls over forests, a hydrologically and ecologically relevant portion of that water is retained on the canopy’s leaves, epiphytes, and bark (Klamerus-Iwan et al, 2020)
The inner bark is usually connected with wood, has a high moisture content (Reifsnyder et al, 1967; Kain et al, 2020), and its role in rainwater absorption is probably much smaller than the role of outer bark in this process. These results reveal difficulties in comparing past research results on bark– water relations due to the lack of uniform methodology, i.e., determination of bark water storage capacity is usually by entirely submerged of bark samples in water for variable amounts of time, from 3 days (Levia and Herwitz, 2005; Valovà and Bieleszovà, 2008), 4 days (Levia and Wubbena, 2006), 7 days (Ilek et al, 2017b, 2021), 1 month (Harmon and Sexton, 1995), or until the bark sample mass remained steady for three consecutive measurements, and further immersion does not increase the bark mass by more than 5% (Van Stan et al, 2016)

Summary

Introduction

When rain falls over forests, a hydrologically and ecologically relevant portion of that water (up to several mm event−1, depending on storm and canopy conditions) is retained on the canopy’s leaves, epiphytes, and bark (Klamerus-Iwan et al, 2020). This canopy water storage fuels a major part of the Earth’s terrestrial evaporation flux by returning rainwater back to the atmosphere as canopy interception (Porada et al, 2018). The dynamics of inner (living portion of) bark water storage have been linked to a suite of plant functions (Rosell et al, 2015; Wolfe and Kursar, 2015; Loram-Lourenco et al, 2020). Research on the outer (non-living) bark’s capacity and filling-and-emptying dynamics is comparatively under-represented

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