Hypothesis and theory: Do trees “release the tension” in rainwater? Surface tension reduction in throughfall and stemflow from urban trees

Knowledge of the processes and impacts associated with the canopy’s partitioning of rainfall into stemflow (water that drains to the base of tree stems) and throughfall (water that drips through gaps and from canopy surfaces) has expanded in recent years. However, the effect of canopy interactions on the fundamental physical properties of rainwater as it travels through the canopy to the soil, particularly surface tension, remains understudied. To discuss specific hypotheses within this context and their relevance to ecohydrological theory, the surface tension of rainwater samples was examined directly. Over a period of 9 months, open rainwater, throughfall and stemflow samples were collected during 20 storms from 12 study trees located in Secrest Arboretum (about 2.5 km outside Wooster, Ohio). Study trees were selected to highlight a range of canopy characteristics, with each tree being from a unique deciduous species. Surface tension was measured using pendant drop goniometry, and measurements were analyzed for variation across study trees and correlation with event air temperature and rain intensity. In general, surface tension was reduced in throughfall and stemflow compared to measurements made for event rainwater, with median surface tension changes of −0.446 mN m−1 and −0.595 mN m−1 for throughfall and stemflow, respectively. The extent of this reduction varied among study trees (with changes as great as −6.5 to −5.5 mN m−1), and storm event characteristics were directly and indirectly correlated with surface tension changes in select cases. Hypothetically, a number of mechanisms may account for the observed reduction (and variation in this reduction) in surface tension, including differences in tree surface properties, canopy microenvironments, and microbiomes, and each warrant further research. Testing these hypotheses may advance broader ecohydrological theory as surface tension changes will influence wetting, absorption, and solute exchange processes within the canopy which, in turn, may affect related surface processes

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

Tree Bark: A Surprising and Diverse Reservoir for Water

Bark is the outside layer of wood that all trees have. Bark protects trees from harsh environmental conditions including weather, pests, disease, and damage from hungry animals. Just like leaves, bark is different across species. Some trees have thick, rough bark while others have thin, smooth bark. When it rains, bark acts like a sponge and absorbs water. Some trees have bark with large pore spaces that make it easy to absorb rain water quickly. Other trees have bark with smaller pore spaces, which absorb water slowly. Each tree species has a maximum storage capacity of water that can be held in the bark. In fact, some mature trees can store more than 100 L of water in their bark—that is about as much water as you would use in a 10-min shower! In this way, bark influences the water cycle of individual trees and entire forests.</jats:p

Seasonal changes in water absorbability of some litterfall compoSeasonal changes in water absorbability of some litterfall components in Scots pine stands differing in agenents in Scots pine stands differing in age

Understanding the water-holding capacity of the litter layer is of interest when constructing forest hydrology models, where the presence of litter affects soil moisture content and fire behavior. However, to understand the process of water storage in the litter layer it is not only important to know (i) how much water the litter layer can store, but also (ii) how much water particular litter components can store. Little is known about the role of organic matter chemistry in water absorption and saturation of its internal capillarity. We hypothesized that water absorption of freshly fallen organic matter changes with stand age and during the year, i.e. the term when organic matter falls (month of the year or season) affects its water absorbability. Thus, we determined seasonal changes in water absorption time, carbon and nitrogen contents, and the C/N ratio of bark and needles taken from Scots pine stands of different ages during laboratory tests. Pine needles and bark were collected every month for one year in five stands in north-western Poland. The time of water absorption for bark was about 30% shorter than that of needles. The age of the stand did not affect the time of water absorption in the litterfall components. We observed that the term when litter falls (month of the year or season) significantly affected the water absorption time. It indicates that organic matter reaching the forest floor and forming the litter layer is characterized by different output properties affecting the water storage capacity of the litter layer.</jats:p

Hygroscopic contributions to bark water storage and controls exerted by internal bark structure over water vapor absorption

Abstract Key message Hygroscopicity is a crucial element of bark water storage and can reach &gt;60% of water holding capacity of bark depending on tree species Abstract Bark forms the outer layer of woody plants, and it is directly exposed to wetting during rainfall and reacts to changes in relative humidity, i.e., it may exchange water with the atmosphere through absorption and desorption of water vapor. A current paradigm of bark hydrology suggests that the maximum water storage of bark empties between precipitation events and is principally controlled by bark thickness and roughness. We hypothesize that (1) the ability of bark to absorb water vapor during non-rainfall periods (i.e., hygroscopicity) leads to partial saturation of bark tissues during dry periods that may alter the rate of bark saturation during rainfall, and (2) the degree of bark saturation through hygroscopic water is a function of internal bark structure, including porosity and density, that varies among species. To address these questions, we conducted laboratory experiments to measure interspecific differences in bark physical structure as it relates to water storage mechanisms among common tree species (hickory (Carya spp.), oak (Quercus spp.), sweetgum (Liquidambar styraciflua), and loblolly pine (Pinus taeda)) in the southeastern United States. Furthermore, we considered how these properties changed across total bark, outer bark, and inner bark. We found a distinct difference between hickory and oak, whereby hickory had 5.6% lower specific density, 31.1% higher bulk density, and 22.4% lower total porosity of outer bark resulting in higher hygroscopicity compared to oaks. For all species, hygroscopicity increased linearly with bulk density (R2 = 0.65–0.81) and decreased linearly with total porosity (R2 = 0.64–0.88). Overall, bark hygroscopicity may constitute an average of 30% of total bark water storage capacity. Therefore, in humid climates like those of the southeastern USA, the proportion of bark that remains saturated during non-storm conditions should not be considered negligible. </jats:sec