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Investigating Water Storage Dynamics in the Litter Layer: The Impact of Mixing and Decay of Pine Needles and Oak Leaves
Little is known about how the degree of mixing various forest-forming species affects forest floor hydrology. We evaluated the water storage capacity of the resulting litter layer by mixing the litterfall of Scots pine and sessile oak and studying their decomposition time. We prepared 90 artificial samples containing pure pine litter, pure oak litter, and mixed pine–oak litter with varying shares of pine needles. These samples were subjected to 15 months of decomposition in soil. After every three months of decay, some samples were removed from the soil, and their water storage capacity, bulk density, and C:N ratio were evaluated. Our findings indicate that samples with the greatest water storage capacity had a low C:N ratio and a predominant share of oak leaves. Conversely, samples with a high C:N ratio and a predominant share of pine needles had the lowest water storage capacity. After 12 and 15 months of decomposition, the water storage capacity increased by more than 52% compared to the initial water capacity of the samples. The highest increase in water storage capacity (>40%) was observed in samples with a predominant share of oak leaves, while the lowest (approximately 28%) was recorded in samples with 80 and 100% of pine needles. Our findings suggest that introducing mixed-species stands, with deciduous species as the predominant component, can yield several ecological benefits, such as an increased ability to store water in forest floor.
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.
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.