Trees concentrate rainfall to near-stem soils via stemflow. When canopy structures are organized appropriately, stemflow can even induce preferential flow through soils, transporting nutrients to ...biogeochemically active areas. Bark structure significantly affects stemflow, yet bark-stemflow studies are primarily qualitative. We used a LaserBark to compute bark microrelief (MR), ridge-to-furrow amplitude (R) and slope (S) metrics per American Society of Mechanical Engineering standards (ASME-B46.1-2009) for two morphologically contrasting species (Fagus sylvatica L. (European beech), Quercus robur L. (pendunculate oak)) under storm conditions with strong bark water storage capacity (BWSC) influence in central Germany. Smaller R and S for F. sylvatica significantly lowered BWSC, which strongly and inversely correlated to maximum funnelling ratios and permitted stemflow generation at lower rain magnitudes. Larger R and S values in Q. robur reduced funnelling, diminishing stemflow drainage for larger storms. Quercus robur funnelling and stemflow was more reliant on intermediate rain intensities and intermittency to maintain bark channel-dependent drainage pathways. Shelter provided by Q. robur's ridged bark also appears to protect entrained water, lengthening mean intrastorm dry periods necessary to affect stemflow. Storm conditions where BWSC plays a major role in stemflow accounted for much of 2013's rainfall at the nearest meteorological station (Wulferstedt).
Editor M.C. Acreman; Associate editor not assigned
Snowpack accumulation in forested watersheds depends on the amount of snow intercepted in the canopy and its partitioning into sublimation, unloading, and melt. A lack of canopy snow measurements ...limits our ability to evaluate models that simulate canopy processes and predict snowpack. We tested whether monitoring changes in wind‐induced tree sway is a viable technique for detecting snow interception and quantifying canopy snow water equivalent (SWE). Over a 6 year period in Colorado, we monitored hourly sway of two conifers, each instrumented with an accelerometer sampling at 12 Hz. We developed an approach to distinguish changes in sway frequency due to thermal effects on tree rigidity versus intercepted snow mass. Over 60% of days with canopy snow had a sway signal that could not be distinguished from thermal effects. However, larger changes in tree sway could not generally be attributed to thermal effects, and canopy snow was present 93%–95% of the time, as confirmed with classified PhenoCam imagery. Using sway tests, we converted changes in sway to canopy SWE, which were correlated with total snowstorm amounts from a nearby SNOTEL site (Spearman r = 0.72 to 0.80, p < 0.001). Greater canopy SWE was associated with storm temperatures between −7°C and 0°C and wind speeds less than 4 m s−1. Lower canopy SWE prevailed in storms with lower temperatures and higher wind speeds. Monitoring tree sway is a viable approach for quantifying canopy SWE, but challenges remain in converting changes in sway to mass and distinguishing thermal and snow mass effects on tree sway.
Key Points
Six years of tree sway data from accelerometers on two conifers revealed changes in sway frequency at sub‐daily to seasonal scales
After accounting for thaw‐freeze cycles, changes in tree sway due to snow interception were detected and checked with time‐lapse images
Sway data yielded canopy snow mass estimates consistent with snowfall data and storm conditions
Intense rain events have become more frequent in some regions due to climate change, and this trend is particularly concerning in dryland regions where the ecological and geomorphological impacts of ...rainfall are intimately tied to its intensity. The interception of rainfall by vegetation is a critical process in the water balance of drylands; thus, this study estimated the canopy interception capacity and interception rates as well as stemflow of three typical Mediterranean shrub species (Rosmarinus officinalis, Thymus vulgaris, and Macrochloa tenacissima) of three size classes in Spain under a simulated extreme rainfall rate (~8 mm min−1, historical return period of >100 years). Given that these plants' canopy structures markedly differ from taller woody plants (i.e., trees), a novel method was developed to assess the stemflow fraction. Results showed significant differences in interception amount, rates, and storage capacity among the shrub species, with variations in plant morphology, such as shrub height and canopy diameter, being the key factors determining interception capacity. R. officinalis had the highest interception fraction per unit canopy area, or ‘specific interception,’ (18.4%). In contrast, the lowest specific interception fraction was measured for M. tenacissima (6.5%). Thymus vulgaris was characterized by the highest stemflow yields per unit canopy area (4.85 mm) and fraction (up to 29.6% of rainfall), which was the lowest for M. tenacissima (1.09 mm, ~1%–4%). Strong linear correlations were found between canopy interception and shrub canopy diameter (|r| > −0.51, max = −0.90), when observations were grouped for size class. These linear correlations between shrub morphology and partitioning enabled multiple‐regression linear models to be developed that predicted canopy interception and stemflow with good accuracy (r2 > 0.64, with a maximum of 0.82) from shrub height, canopy diameter, dry biomass, size class, and species. Despite these measurements being conducted under one extreme storm depth and intensity, the results provide: (i) values of rainfall partitioning for important shrub species in Mediterranean dryland environments; and (ii) a simple but reliable model that may be further developed (e.g., embedding variable rainfall values as weather input or incorporating other morphological parameters) and may be integrated into complex hydrological models.
A novel method is developed to assess interception and stemflow fraction at the local scale. Rosmarinus officinalis shows the highest interception fraction per unit canopy area (18.4% of rainfall), while Macrochloa tenacissima the lowest (6.5%). Thymus vulgaris yields the highest stemflow fraction (up to 30%), and Macrochla tenacissima the lowest (~1%–4%). Multi‐regression linear models predict with good accuracy (r2 > 0.64) rainfall partitioning fractions from shrub height, canopy diameter, and dry biomass, as well as plant species and size class.
•We studied the canopy hydrometerological responses to tree dieback phenomenon•Data from multiple time scales, crown dieback states, and DBH were collected•We observed significant stemflow ...variability among canopy dieback states•Oak decline results in a substantial alteration of canopy ecohydrological parameters•Our study provides fundamental information for water and forest managers
The potential for sudden tree dieback exists when there is significant variation in perturbation frequency and intensity, which can alter canopy-atmosphere interactions, like canopy rainfall partitioning. In the context of close-to-nature silviculture practices, dieback trees can endure for several decades in specific environments. Therefore, it is warranted to explore the interaction between tree dieback and rainfall partitioning, including throughfall, stemflow, and rainfall interception. The primary aims of this study were to (a) measure the partitioning of incident rainfall into throughfall, stemflow, and rainfall interception beneath Quercs brantii (Lindl; Brant's oak) canopies in various dieback states in a Mediterranean region of Iran, and (b) quantify canopy and trunk ecohydrological parameters in the various treatments. To do this, we randomly selected 30 trees in three crown dieback states (i.e., healthy, moderate dieback, and severe dieback classes), with two diameters at breast height (DBH) classes (small and medium DBH). The rainfall partitioning, as well as the underlying canopy ecohydrological parameters, differed meaningfully across the oak decline classes. This is most likely driven by changes in forest canopy structure as oak dieback progresses. The greatest changes included the increase in throughfall and a decrease in interception with leaf mortality. Regarding stemflow input, we observed significant stemflow variability among canopy dieback states—with stemflow percentage comprising 7.6% of rainfall for healthy small DBH oaks, decreasing by half in severely declined ones—and it was notable that the precipitation thresholds required for stemflow initiation increased from healthy to severely declined trees. However, it should be noted that while decreased intercepted water by declined oak trees may occur, this does not necessarily translate to a significant increase in the proportion of rainfall transformed into stemflow. After the decline of oak trees, there were significant alterations to the ecohydrological parameters of both the canopy and trunk. Notably, canopy ecohydrological parameters (e.g., crown saturation points, and canopy storage capacity), which are integral to the interception and storage of rainwater, experienced a marked decrease. In contrast, trunk ecohydrological parameters had not a constant trend among treatments, and it would require further study. These findings can help clarify rainfall partitioning and subsequent ecohydrological parameters dynamics and driving factors across crown dieback treatments, in addition to that offer parameters for hydrological simulations specifically tailored to Mediterranean forests. Studying how rainfall is distributed and the resulting changes in ecohydrological parameters across various crown dieback states, phenological scales, and DBH classes can enhance our comprehension and predictive abilities concerning the contribution of forests to hydrological recharge processes.
Pollen shedding can produce rapid, abundant exchanges of nutrient-rich biomass from plant canopies to the surface. When pollen deposits onto understory plants, it can be washed off during storms via ...throughfall (a drip flux) and stemflow (a flux down plant stems). Pollen deposition may also alter the organismal community on plant surfaces, changing other biological particulates transported by throughfall and stemflow. We report concentrations and fluxes of pollen and other biological particulates (flagellate cells, nematodes, rotifers, mites and hexapodans) in throughfall and stemflow from an understory forb,
Eupatorium capillifolium
(Lam. dogfennel), during a
Pinus palustris
(Mill. longleaf pine) pollen shedding event, then compare these results to observations collected when pollen was absent. Pollen flux was 95.6 × 10
6
grains ha
−1
season
−1
from dogfennel canopies (63% and 37% transported by throughfall and stemflow, respectively), representing 0.1–3.2 g ha
−1
. Median concentrations in flagellates, nematodes and rotifers for throughfall and stemflow were higher during pollen shedding; however, mites and hexapodan concentrations were similar regardless of pollen presence. This is the first report of flagellate and hexapodan concentrations in canopy drainage waters. Flagellate concentrations were higher than for other organisms—being similar to those reported for streams, 10
5
–10
7
cells L
−1
—and hexapodan fluxes were ~ 50 individuals m
−2
per 1 cm of rainfall. These results indicate that throughfall and stemflow can (i) transport ecologically relevant amounts of pollen and organisms from the phyllosphere to the surface, and (ii) that the composition and flux of biological particulates can change markedly during pollen shedding.
Litter layers develop across a diverse array of vegetated ecosystems and undergo significant temporal compositional changes due to canopy phenological phases and disturbances. Past research on ...temporal dynamics of litter interception has focused primarily on litter thickness and leaf fall, yet forest phenophases can change many more litter attributes (e.g., woody debris, bark shedding, and release of reproductive materials). In this study, weekly changes in litter composition over 1 year were used to estimate litter water storage dynamics and model event‐based litter interception. Litter interception substantially reduced throughfall (6–43%), and litter water storage capacity ranged from 1 to 3 mm, peaking when megastrobili release and liana leaf senescence occurred simultaneously during fall 2015. Tropical storm disturbances occurred during the sampling period, allowing evaluation of how meteorological disturbances altered litter interception. High wind speeds and intense rainfall from 2 tropical storms increased litter interception by introducing new woody debris, which, in this study, stored more water than the pre‐existing woody debris. After 2 extreme weather events, a third (Hurricane Hermine) did not increase woody debris (or litter interception), suggesting that the canopy pool of branches susceptible to breakage had been largely depleted. Needle and bark shedding had minor effects on litter interception. Results suggest that the release of reproductive materials and meteorological disturbances appear to be the major compositional drivers of litter interception beyond their obvious contribution to litter thickness.
Over geologic timescales, forests have intercepted precipitation and thereby modified the intensity, duration, and spatial patterns of water fluxes to forest soils. Across a range of environmental ...conditions, persistent focused water flows can dissolve carbonate substrates, and form conical dissolution features—termed “dissolution cones.” These cones generally fill with soils, becoming localized soil (and water) reservoirs occupied by vegetation. A myriad of mechanisms are hypothesized to have formed dissolution cones. Prior work has sought to explain co-located palm trees and modern dissolution cones in tropical unconsolidated carbonates as the result of the chemical action of weakly acidic stemflow funneled by palm canopies down their stems, and into the substrate. Using a geochemical modeling program, PHREEQC, we find that for a range of environmental conditions and favorable assumptions, stemflow is unable to dissolve a benchmark volume of carbonate substrate that typifies tropical dissolution cones. Therefore, dissolution cone formation by abiotic dissolution from stemflow funneling is unlikely to be the chief geomorphic process. Further hypotheses to be tested are discussed.
For one-third of Earth’s land surface, precipitation passes through tree canopies (as throughfall or stemflow) before entering watersheds. Over a century of research has described fluxes of water and ...solutes along these “hydrologic highways”, yet little is known about their “traffic” –that is, the organisms and nonliving particulates frequently discarded from water samples after filtration in the lab. A comprehensive understanding of the composition of sub-canopy precipitation is necessary to estimate the total nutrient and pollutant inputs to watersheds for redistribution downstream, as well as to systematically investigate precipitation effects on organismal exchanges along the atmosphere–plant–soil continuum. Here, we review current concepts and research showing that the hydrologic highways from tree canopies to soil carry ecologically relevant quantities of biologic (viruses, microbes, microfauna, and meiofauna) and abiotic particulates. Their fate may have important consequences for the biogeochemistry and biodiversity of terrestrial systems.
Arboreal epiphytes, plants that grow on trees, can significantly increase rainwater storage and evaporation (i.e., “interception”) within canopies. Drought conditions may affect this hydrological ...role, as epiphytes' physiological responses change leaf properties that affect water retention. Drought-induced changes in epiphyte water storage capacity could substantially alter canopy hydrology, but have not been studied. We tested the effects of drought on the water storage capacity (Smax) of leaves and leaf properties of two epiphytes with distinct ecohydrological traits: resurrection fern (Pleopeltis polypodioides), and Spanish moss (Tillandsia usneoides). Both species are common in maritime forests of the Southeastern USA, where climate change is expected to decrease precipitation in spring and summer. To simulate drought, we dried leaves to 75 %, 50 %, and ~25 % of fresh weight, and quantified their Smax in fog chambers. We measured relevant leaf properties: hydrophobicity, minimum leaf conductance (gmin; a measure of water loss under drought), and Normalized Difference Vegetative Index (NDVI). We found that drought significantly reduced Smax and increased leaf hydrophobicity for both species, indicating that lower Smax may be due to shedding of droplets. While the overall reduction in Smax did not differ between the two species, they exhibited distinct drought responses. Dehydrated T. usneoides leaves had lower gmin, demonstrating the ability to limit water loss under drought. P. polypodioides increased gmin when dehydrated, consistent with its extraordinary ability to withstand water loss. NDVI decreased with dehydration in T. usneoides but not P. polypodioides. Our results suggest that increased drought may have a dramatic effect on canopy water cycling by reducing the Smax of epiphytes. Reduced rainfall interception and storage in forest canopies could have widespread effects on hydrological cycling, thus understanding the potential feedbacks of plant drought response on hydrology is crucial. This study highlights the importance of connecting foliar-scale plant response with broader hydrological processes.
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•Epiphytes growing in tree canopies capture and store rainwater, but this role may be vulnerable to increasing drought.•We tested the effects of drought (leaf desiccation) on water storage capacity (Smax) and leaf properties of two epiphytes.•Drought significantly reduced Smax and increased leaf hydrophobicity for both species.•Plants’ responses to drought may reduce rainfall storage in forest canopies, potentially altering hydrological cycling.
The first contact between precipitation and the land surface is often a plant canopy. The resulting precipitation partitioning by vegetation returns water back to the atmosphere (evaporation of ...intercepted precipitation) and redistributes water to the subcanopy surface as a “drip” flux (throughfall) and water that drains down plant stems (stemflow). Prior to the first benchmark publication of the field by Horton in 1919, European observatories and experimental stations had been observing precipitation partitioning since the mid-19th century. In this paper, we describe these early monitoring networks and studies of precipitation partitioning and show the impressive level of detail. Next to a description of the early studies, results included in this synthesis have been digitized and analyzed to compare them to recent studies. Although many early studies lack modern statistical analyses and monitoring tools that have become standard today, they had many strengths (not necessarily shared by every study, of course), including: A rigorous level of detail regarding stand characteristics (which is often lacking in modern ecohydrological studies); high-resolution spatiotemporal throughfall experiments; and chronosequential data collection and analysis. Moreover, these early studies reveal the roots of interest in precipitation partitioning processes and represent a generally forgotten piece of history shared by the hydrology, meteorology, forestry, and agricultural scientific communities. These studies are therefore relevant today and we hope modern scientists interested in plant-precipitation interactions will find new inspiration in our synthesis and evaluation of this literature.
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