Hydrol. Earth Syst. Sci., 24, 4587–4599, 2020https://doi.org/10.5194/hess-24-4587-2020© Author(s) 2020. This work is distributed underthe Creative Commons Attribution 4.0 License.

Rainfall interception and redistribution by a commonNorth American understory and pasture forb,Eupatorium capillifolium (Lam. dogfennel)D. Alex R. Gordon1,6, Miriam Coenders-Gerrits2, Brent A. Sellers3,4, S. M. Moein Sadeghi5, and John T. Van Stan II6

1Geology and Geography, Georgia Southern University, Statesboro, GA, USA2Delft University of Technology, Water Resources Section, Delft, the Netherlands3Agronomy Department, University of Florida, Gainesville, FL, USA4Range Cattle Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences,Gainesville, FL, USA5Department of Forestry and Forest Economics, University of Tehran, Karaj, Iran6Applied Coastal Research Lab, Georgia Southern University, Savannah, GA, USA

Correspondence: Miriam Coenders-Gerrits (a.m.j.coenders@tudelft.nl)

Received: 25 October 2019 – Discussion started: 5 November 2019Revised: 13 July 2020 – Accepted: 14 August 2020 – Published: 22 September 2020

Abstract. In vegetated landscapes, rain must pass throughplant canopies and litter to enter soils. As a result, some rain-water is returned to the atmosphere (i.e., interception, I ) andthe remainder is partitioned into a canopy (and gap) drip flux(i.e., throughfall) or drained down the stem (i.e., stemflow).Current theoretical and numerical modeling frameworks forthis process are almost exclusively based on data from woodyoverstory plants. However, herbaceous plants often popu-late the understory and are the primary cover for importantecosystems (e.g., grasslands and croplands). This study in-vestigates how overstory throughfall (PT,o) is partitioned intounderstory I , throughfall (PT) and stemflow (PS) by a dom-inant forb in disturbed urban forests (as well as grasslandsand pasturelands), Eupatorium capillifolium (Lam., dogfen-nel). Dogfennel density at the site was 56 770 stems ha−1,enabling water storage capacities for leaves and stems of0.90±0.04 and 0.43±0.02 mm, respectively. As direct mea-surement of PT,o (using methods such as tipping buckets orbottles) would remove PT,o or disturb the understory par-titioning of PT,o, overstory throughfall was modeled (P ′T,o)using on-site observations of PT,o from a previous field cam-paign. Relying on modeled P ′T,o, rather than on observationsof PT,o directly above individual plants means that signifi-cant uncertainty remains with respect to (i) small-scale rela-tive values of PT and PS and (ii) factors driving PS variabil-

ity among individual dogfennel plants. Indeed, PS data fromindividual plants were highly skewed, where the mean PS :

P ′T,o per plant was 36.8 %, but the median was 7.6 % (2.8 %–27.2 % interquartile range) and the total over the study pe-riod was 7.9 %. PS variability (n= 30 plants) was high(CV > 200 %) and may hypothetically be explained by fine-scale spatiotemporal patterns in actual overstory throughfall(as no plant structural factors explained the variability). Thetotal PT : P

′T,o was 71 % (median PT : P

′T,o per gauge was

72 %, with a 59 %–91 % interquartile range). Occult precipi-tation (mixed dew and light rain events) occurred during thestudy period, revealing that dogfennel can capture and draindew to their stem base as PS. Dew-induced PS may help ex-plain dogfennel’s improved invasion efficacy during droughts(as it tends to be one of the most problematic weeds in theimproved grazing systems in the southeastern US). Over-all, dogfennel’s precipitation partitioning differed markedlyfrom the site’s overstory trees (Pinus palustris), and a discus-sion of the limited literature suggests that these differencesmay exist across vegetated ecosystems. Thus, more researchon herbaceous plant canopy interactions with precipitation ismerited.

Published by Copernicus Publications on behalf of the European Geosciences Union.

4588 D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory

1 Introduction

Precipitation (Pg) across most of the global land surface willinteract with plant canopies. Precipitation–canopy interac-tions during storms result in three general hydrologic pro-cesses; one which returns water to the atmosphere (intercep-tion) and two others that route water to the surface (through-fall and stemflow). Interception is the evaporation of dropletssplashing against (Dunkerley, 2009) or stored on canopy sur-faces, like leaves (Pereira et al., 2016), bark (Van Stan etal., 2017a) and epiphytes (Porada et al., 2018). Dependingon the vegetation and storm conditions, interception can besmall per unit area (David et al., 2006) or return half the an-nual precipitation to the atmosphere (Alavi et al., 2001). Inthis way, canopy interception can evaporatively cool regions(Davies-Barnard et al., 2014), recycle moisture to generatenearby storms (van der Ent et al., 2014) and reduce stormwa-ter runoff to save millions of dollars (US) in stormwater in-frastructure costs (Nowak et al., 2020). Throughfall is the wa-ter that drips to the surface through gaps or from canopy sur-faces, whereas stemflow is the water that drains down plantstems. The portion of precipitation that drains as throughfallversus stemflow is also highly variable depending on vege-tation and storm conditions: ranging annually from 10 % to90 % for throughfall and from < 1 % to 60 % for stemflow(Sadeghi et al., 2020). As throughfall and stemflow reachthe surface at different locations, they differentially inter-act with subsurface hydrological and biogeochemical pro-cesses – having been implicated in fine-scale patterns in soilphysicochemistry (Gersper and Holowaychuk, 1971), micro-bial community composition (Rosier et al., 2015, 2016), N-cycling functional genes (Moore et al., 2016) and metazoancommunity composition (Ptatscheck et al., 2018). Accurateaccounting for each of these precipitation partitioning fluxesis, therefore, necessary for the accurate prediction of atmo-spheric and surface hydro-biogeochemical processes.

Current theoretical and numerical modeling frameworksfor canopy precipitation partitioning (see review by Muzyloet al., 2009) are almost exclusively based on observations be-neath woody plants, like forests and shrublands (Sadeghi etal., 2020). In forests, the past 150 years of research has pri-marily targeted dominant overstory trees (Ebermayer, 1873;Van Stan and Gordon, 2018). However, herbaceous plantscommonly dominate forest understories and can be abun-dant beneath shrublands (Jiménez-Rodríguez et al., 2020;Lajtha and Schlesinger, 1986; Specht and Moll, 1983). Asa result, our current understanding of “net” precipitation (asmeasured beneath woody overstory canopies) is not repre-sentative of the actual precipitation that reaches the surface(or litter layer; Gerrits and Savenije, 2011) beneath the under-story. Herbaceous canopies are relevant to precipitation par-titioning in more than the one-third of the global land surfacerepresented by forests; they also cover 27 % and 11 % of theglobal land surface in grasslands and croplands, respectively(Alexandratos and Bruinsma, 2012; Suttie et al., 2005). It is

Figure 1. Partitioning of gross rainfall by the overstory (light blue)and the understory (dark blue). Overstory throughfall (PT,o), theinput to the understory canopy, was estimated from past work atthe site. In this study, overstory throughfall was modeled (P ′T,o, seeSect. 2.2.2), and maximum understory water storage capacity (SU),throughfall (PT) and stemflow (PS) were measured.

unlikely that current knowledge on precipitation partitioningbased on woody vegetation is applicable to herbaceous veg-etation, as they differ in many hydrologically relevant mor-phological features: smaller height, the lack of bark structureand the presence of other stem features (like trichome hairsor desiccated leaves), among others. This raises unansweredand under-researched, questions that must be addressed to in-corporate herbaceous plants in precipitation partitioning the-ory, including the following:

– How do these significant morphological differences af-fect canopy and stem water storage capacities?

– Do herbaceous plants also favor throughfall generation,like woody plants, or do they more efficiently drain pre-cipitation to their stem bases (and, thereafter, their shal-low roots)?

In fact, several long-standing (and hitherto unanswered) callsfor greater research on the precipitation partitioning of non-woody plants (rooted in detailed observations) have beenmade (Price et al., 1997; Price and Watters, 1989; Verryand Timmons, 1977; Yarie, 1980). These are general ques-tions identified by the community; however, in this study wefocus on the following research question: how is overstorythroughfall (PT,o: Fig. 1) partitioned into understory inter-ception, throughfall (PT: Fig. 1) and stemflow (PS: Fig. 1)by a dominant forb in disturbed urban forest understories(as well as grasslands and pasturelands), Eupatorium capilli-folium (Lam., dogfennel)?

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D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory 4589

Very little is known about how understory plants parti-tion PT,o into understory PT and PS (Fig. 1). Overstory stem-flow is currently assumed to bypass the understory and litterlayers (Carlyle-Moses et al., 2018); however, this assump-tion, particularly regarding the bypass of litter, has rarelybeen tested (Friesen, 2020), and overstory stemflow has beenobserved to runoff for long distances away from the stem(Cattan et al., 2009; Keen et al., 2010). We do not investi-gate interactions between the understory and overstory stem-flow in this study, because stemflow from this study site isnegligible (< 0.2 %: Yankine et al., 2017). Most observa-tions of precipitation partitioning beneath any plant besidesoverstory woody plants have been done on maize (Zhenget al., 2019, and references therein) and other cash crops(Drastig et al., 2019, and references therein), which leaveplants of forest understories, grasslands or pasturelands rela-tively unstudied. Even the few studies on forest understoryinterception, PT, and PS overwhelmingly focus, again, onwoody plants (González-Martínez et al., 2017; Price andWatters, 1989), limiting net precipitation observations be-neath understory herbaceous plants to ferns (Verry and Tim-mons, 1977) and nonvascular plants (Price et al., 1997).These scant observations, however, indicate that precipitationpartitioning by nonwoody understory plants is hydrologicallyrelevant, as they can store as much water as woody plants(Klamerus-Iwan et al., 2020), evaporate significant portionsof PT,o (Coenders-Gerrits et al., 2020) and redistribute 7 %–90 % of event PT,o as PS (Sadeghi et al., 2020). For ourstudy on dogfennel, we hypothesized that, compared withpast research on woody plants, dogfennel stems and leaves(i) can store a hydrologically relevant amount of rainwater(i.e., within the range of water storage capacities reported forwoody plants; Klamerus-Iwan et al., 2020), (ii) significantlyreduce net rainfall flux to the surface (i.e., PT+PS� PT,o)and (iii) redistribute a substantial portion of PT,o to the sur-face via PS (i.e., PS will often “funnel” more rainwater perstorm to the soils surrounding stems than PT, PT,o or Pg overthe same area). To test these hypotheses, PT,o was modeledfrom past on-site observations (P ′T,o) as monitoring PT,o, PSand PT simultaneously was not possible without disruptingor removing PT,o. We explicitly acknowledge that the deci-sion to rely on modeled P ′T,o leaves a nontrivial uncertaintyregarding the influence of actual overstory throughfall spa-tiotemporal patterns on small-scale values of PT and individ-ual plants’ PS.

2 Materials and methods

2.1 Study site and study plant description

The study site, the Charles H. Herty Pines Nature Preserve,is a forest fragment in Statesboro, Georgia, USA (Fig. 2a),at Georgia Southern University’s main campus (32.430◦ N,81.784◦W; 65 m a.s.l.). The climate is subtropical (Köp-

Figure 2. (a) Location of the studied Pinus palustris (longleaf pine)forest fragment, Charles H. Herty Pines Nature Preserve, on theStatesboro, Georgia (USA), campus of Georgia Southern Univer-sity, where Eupatorium capillifolium (dogfennel) is a dominant un-derstory plant. (b) Dogfennel can dominate pastures as well, asshown by the photograph (credit: Brent A. Sellers). The map lay-ers were sourced from state and county boundaries and aerial im-agery ©Esri, TomTom North America, Inc. The land use layer wasderived from the National Land Cover Database 2011 (full meta-data and data access link: https://gdg.sc.egov.usda.gov/Catalog/ProductDescription/NLCD.html, last access: 22 July 2019).

pen Cfa) with mean monthly temperatures (1925–2014) inJuly that range from 21 to 33 ◦C and generally mild wintermonths, i.e., the lowest mean January temperature is 3.5 ◦C(University of Georgia, 2019). Mean annual precipitation is1170 mm yr−1, and precipitation occurs almost exclusivelyas rain, which is relatively evenly spread over the year. Theoverstory is dominated by Pinus palustris (longleaf pine),and overstory rainfall partitioning for this site has been re-ported (Mesta et al., 2017; Van Stan et al., 2018; Yankineet al., 2017). The trunk diameter at breast height (DBH)was relatively consistent across all trees in the study plot:49.7 cm (mean) with an interquartile range of 36.2–55.7 cm.The mean tree height was 30.4± 4.5 m and was derivedfrom terrestrial lidar (terrestrial lidar methods identical toVan Stan et al., 2017a). The stand density was 223 trees ha−1

with 50.4 m2 ha−1 of basal area. Dogfennel, our study plant,was particularly dominant along the forest edge. Dogfen-

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4590 D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory

nel is a forb of the Asteraceae family that is native to (andwidespread across) North America (Van Deelen, 1991; Wun-derlin and Hansen, 2003). Although dogfennel behaves as anannual plant throughout much of its North American range, itcan behave as a perennial in the southern US by overwinter-ing as a rosette, typically from January to March, before re-growing from a taproot in the spring, typically in April (Mac-donald et al., 1992, 1994). Dogfennel can be abundant in dis-turbed forest understories, particularly pine forests (Brock-way et al., 1998) and pastures (Fig. 2b). In the study pineforest, the dogfennel stem density was 56 770 stems ha−1

along the stand edge. In pasturelands, dogfennel can reachthis stem density within a single season and, if left unman-aged, dogfennel densities have been measured as high as74 stems m−2, or ∼ 740000 stems ha−1 (Dias et al., 2018).The growth habit of dogfennel results in “clumps” of stems.The dogfennel density was estimated in ten 10 m×10 mplots by counting the stems per clump for three randomlyselected clumps in each plot. For each plot, the mean stemsper clump were multiplied by the number of clumps per plot.Finally, all stems per plot were summed and scaled to 1 ha.Three dogfennel clumps were randomly selected for through-fall and stemflow monitoring. Within these three clumps,30 individual dogfennel stems were randomly selected forstemflow monitoring. Individual plant attributes – canopy ra-dius (cm), stem radius (cm), leaf angle at the stem (degreesfrom vertical) at various canopy heights (1.00, 1.25, 1.50,1.75, 2.00 m), and the relative location within the clump,interior (I), middle (M) or exterior (E) – were measuredfor each stemflow-instrumented plant (Table 1). Canopy andstem radii were determined manually with a tape measure,where canopy radii were the mean of measurements fromeight directions (N, NE, E, SE, S, SW, W and NW) and stemradius was determined by a single manual measurement atthe stem base. The leaf angle at the stem was determinedfor two leaves at each height using the Protractor™ app foriPhone (2013, Phoenix Solutions) which logs an angle afterthe leveling of the iPhone camera (see Fig. S1 in the Supple-ment for an example).

2.2 Hydrometeorological monitoring

2.2.1 Rainfall measurements

Rainfall amount, duration and intensity for discrete rainevents were automatically logged every 5 min by a weatherstation installed above the canopy (on the rooftop of nearbyBrannen Hall at a height of ∼ 40 m), which is located 100 mfrom Charles H. Herty Pines Nature Preserve. Rainfall ob-servations were recorded by three tipping bucket gauges(TE525MM, Texas Electronics, Dallas, TX, USA) inter-faced with a CR1000 datalogger (Campbell Scientific, Lo-gan, Utah, USA). This weather station logged a suite of othermeteorological variables; however, as these data do not rep-resent the meteorological conditions experienced by the un-

Table 1. Descriptive event statistics for rainfall (observed), over-story throughfall (estimated per Fig. 3) and measured individualplant traits. When minimum overstory throughfall was zero, dewoccurred – as verified by air temperatures equalling dew point tem-peratures.

Parameter (units) Mean Median ±SD Min. Max.

Rainfall (mm) 16.5 6.6 25.8 0.1 101.3Overstory throughfall (mm) 11.0 3.5 18.7 0.0 72.2Canopy radius (cm) 18.3 18.4 4.5 12.2 26.2Stem radius (cm) 0.5 0.6 0.1 0.3 0.7Canopy : stem radii 36.3 36.1 7.4 24.1 50.0

Leaf angle at the stem (degrees from vertical)

1.00 m height 54.0 54.0 2.0 50.5 59.01.25 m height 45.9 46.5 3.1 40.5 50.51.50 m height 39.6 39.5 1.8 36.0 43.01.75 m height 34.0 34.5 2.3 30.0 39.02.00 m height 31.9 32.0 2.8 25.0 36.5

derstory, they are not reported or examined here. A discreteevent was defined as any atmospheric moisture (rainfall ordew) that resulted in a measurable quantity of throughfall andstemflow (more than a few milliliters) that occurred after aminimum inter-storm dry period of 8 h. Few events consistedof early morning dew contributions (visually observed dur-ing sampling and verified by air temperatures equalling dewpoint temperatures), and these occurred after low-magnitudenighttime rainfall. When dew was present in the understory,there was no response from above-canopy rain gauges; thus,a post hoc estimate of occult dew contribution to PT,o wasmade by assuming the dew contribution was equal to the un-derstory canopy water storage capacity (1.33 mm – methodsdescribed later). An important limitation to this dew estimateis that it represents the maximum possible dew contribution.Rain events without dewfall required at least∼ 4 mm of rain-fall for generation of PT or PS from the monitored dogfennelcanopies.

2.2.2 Overstory throughfall estimation

As observing PT,o directly would prevent direct observationof PT and PS beneath dogfennel plants, PT,o was estimatedfrom previous field measurements at the site (Fig. 3). Auto-mated PT,o monitoring was performed from September 2016to September 2017 using ten 3.048 m long and 10.16 cmdiameter PVC troughs oriented at a moderate slope, witha 5.08 cm slot cut lengthwise for collection and drainageof PT,o to a Texas Electronics (Dallas, Texas, USA) TR-525I tipping bucket gauge, resulting in a 1.65 m2 collec-tion area. Tipping bucket gauges and their associated troughswere randomly placed within a 0.25 ha plot and recorded ev-ery 5 min by a CR1000 datalogger. All trough angles weremeasured with a digital clinometer to correct computationsof the trough area receiving PT,o. Trough and tipping bucketassemblies were field tested to ensure accuracy (±5 %) un-

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D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory 4591

Figure 3. Observed relative overstory throughfall (PT,o) in relationto above-canopy rainfall at the study site.

der storm conditions typical for the region (Van Stan et al.,2016). These PT,o data were reported by Mesta et al. (2017).To estimate overstory throughfall, P ′T,o, a regression modelwas generated from the association between PT,o (% of rain-fall) measured on site and storm size, and R (millimeters perstorm) using the “Aston” curve (Aston, 1979):

P ′T,o = a(

1− e−bR), (1)

where a and b are regression coefficients. This model andits fit statistics are provided in Fig. 3. We assume that thepast observed rainfall relationship with PT,o at the site wassimilar during our study period. Although we are unable toassess if and the degree to which there is a difference betweenthese observation periods, the canopy is mature and there hasbeen no known or noticeable disturbance or change in canopystructure since the previous observation period.

2.2.3 Understory throughfall and stemflowmeasurements

Throughfall gauges consisted of nine randomly placed fun-nels (506.7 cm2 collection area each), three per dogfennelclump (1520.1 cm2 total collection area per clump), con-nected to high-density polyethylene (HDPE) bottles thatwere manually measured with graduated cylinders immedi-ately after a storm ended (within 4 h). The total canopy areaof dogfennel plants at this site rarely exceed 2000 cm2; thus,the total throughfall gauge area per clump generally rep-resented > 75 % of canopy area, which is a comparativelymuch larger gauge-to-canopy area than most past throughfallstudies on forest canopies (Van Stan et al., 2020).

Standard stemflow measurement methods developed forwoody plants (use of flexible tubing wrapped around awoody stem; Sadeghi et al., 2020) are not suitable for dogfen-

nel; moreover, no standard stemflow collection devices ex-ist for herbaceous plants. Thus, stemflow collars were con-structed from aluminum foil, 15 mm inner-diameter flexiblepolyethylene tubing, electrical tape and silicon (see Fig. S2).The aluminum foil was folded over itself several times tostrengthen the collar (typically a ∼ 160 mm length of foilwas folded to∼ 40 mm) and connected to plastic tubing withstainless steel staples. The aluminum collar was then foldedaround the lower stem of the dog fennel and secured withelectrical tape. To seal the aluminum foil, staple connections,and the interstices between the foil, tubing and stem, siliconwas thinned with hydro-treated light (95 %–100 %) naphtha(VM&P Naphtha, Klean-Strip, Memphis, TN, USA), allow-ing for it to completely fill the aluminum cone up to the tubeopening and make a watertight seal. While naphtha-thinnedsilicon was poured into collars, the tube opening was cov-ered. An additional benefit of naphtha-thinned silicon wasthat, due to the evaporation of naphtha, the silicon shrinks,pulling the collar taut and stiffening and strengthening thestemflow collection device and extending the lifespan of thecollar. Stemflow was measured with a graduated pipette (with1 mL graduations) from 500 mL plastic bottles connected tothe tubing base.

2.3 Water storage capacity estimation

Maximum water storage capacity, Su (mm), was estimatedfor the dogfennel canopy and stem, both as volume (L) perunit surface area (m2). All field leaf and stem samples werecollected during an inter-storm dry period (> 24 h after anyrainfall). For the canopy, 50 leaves representing the mediansize of the site dogfennel plants were sampled (broken offat the base of the leaf), taken back to the lab, their “field-dry” mass (g) was determined on a bench scale and then thebroken ends of their leaf-stems were sealed with silicon toprevent water exchange from an area that was not previouslyexposed in its natural state. Sampling for the stems was sim-ilar; however, as dogfennel heights reach (and can exceed)2 m, the stems were cut into 5 cm sections. Just as with theleaves, 50 representative samples of these stem sections wereweighed in the lab and then sealed with silicon on both ends.Next, all leaf samples and stem sections were submerged inwater for 3 d until maximum saturation was achieved (perVan Stan et al., 2015), whereupon the maximum saturationmass (g) was recorded. For comparison with the field-drymass, all samples were oven-dried until their mass no longerchanged (mass recorded every 3 h), whereupon the oven-dried mass (g) was recorded. No leaf or stem samples wereoven dried longer than 15 h. The gravity convection oven(Isotemp, Fisher Scientific) was set to 40 ◦C (confirmed witha standard thermometer). The maximum volume of all sam-ples’ water storage capacity is the difference between the sat-uration and oven-dried masses. The oven-dried leaves andstems did not visually appear to be damaged (aside from thesampling cuts, obviously), and care was taken to ensure the

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4592 D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory

plant samples were not damaged. It is likely that internal (notexternally intercepted) water was exchanged during this pro-cess; however, this is not entirely problematic as plant sur-faces are known to permit interaction between externally in-tercepted water and internal water (Berry et al., 2019). More-over, we explicitly acknowledge that although these submer-sion methods are commonly used, they produce the “maxi-mum” possible water storage capacity (hence, our objectiveto estimate maximum water storage capacity), as multiple in-trinsic and extrinsic factors of plant surfaces could reducethe available water storage capacity in situ (Klamerus-Iwanet al., 2020).

Specific water storage capacity, SL (mL cm−2), for theleaves and stems was determined by dividing the lab-derivedmaximum volume (mL) by the samples’ surface area (cm2).For leaves, after sampling, leveled photos of each sam-ple were taken on a grid system (every block represent-ing 2.5 cm× 2.5 cm for scale), and the leaf images werethen vectorized and processed for 2-D projected surface areausing the “Measure Path” extension in Inkscape (v. 0.92,Inkscape.org). An example vectorized image of leaf areais provided in the Supplement (Fig. S3). Error in thisvector-based leaf surface area estimate was estimated byrepeating the process five times for each leaf. Stem sur-face area for all samples was estimated from their radii andheight. SL estimates for the stem (0.436 mL cm−2) and leaves(0.195 mL cm−2) were then scaled to SU (mm as L m−2) us-ing stem and leaf surface area estimates per plant (A= 171.9and 807.5 cm2 per plant, respectively), and multiplied by thesite plant density (D = 5.68 plants m−2) before being di-vided by 1000:

SU =(SLstem ×Astem×D

)/1000+

(SLleaf ×Aleaf×D

)/1000. (2)

Plant stem and leaf surface area estimates were determinedfrom five representative plants that were cut from the siteand separated into leaves and stems, and the sums of theleaf and stem areas (determined as mentioned earlier in theparagraph) were then divided by 5. Total leaf surface areacompares well to values reported from ∼ 1 m tall dogfennelplants (212 cm2 per plant; Carlisle et al., 1980) consideringthat our plants were much taller (∼ 2 m).

2.4 Data analysis

Descriptive statistics were compiled for all variables pre-sented and regression analyses were performed to relateplant canopy and hydrologic variables. All statistical analy-ses were done using Statistica 12 (StatSoft, Tulsa, OK, USA).Throughfall volumes (L) from all gauges were summed andconverted to yields (mm) by dividing by the total gaugearea (m2). Stemflow yield (mm) for an individual plantwas determined by dividing its volume (L) by the projectedcanopy area (m2). To compare stemflow production acrossplants, two metrics were computed per plant for each storm:

normalized stemflow, P S,i (–), and the funneling ratio, F (–).P S,i was computed per Keim et al. (2005):

P S,i =

(PS,i −P S

)sS

, (3)

where PS,i is stemflow volume (mL) from each individualplant in a single storm, P S is the mean stemflow for all plantsin a single storm and sS is the standard deviation of stemflowfor all plants in a single storm. F values for individual plantsin each storm were computed per (Herwitz, 1986):

F =PS,i

BiP, (4)

where Bi is the basal area (cm2) at the base of an individualplant, and P will be either Pg or P ′T,o (this will be explicitlyindicated in the results). There are an increasing number ofF metrics (Carlyle-Moses et al., 2018; Levia and Germer,2015); however, the selected method is the most commonF metric applied to stemflow data to date. Moreover, in situobservations of non-collared dogfennel plants during rainfallconfirmed that dogfennel PS rates did not produce visiblerunoff areas.

3 Results

3.1 Storm and plant structural conditions

Discrete rain events, as measured above the forest canopy,ranged in magnitude from 0.1 mm (during dewfall) to101.3 mm (Table 1). The distribution of storm magnitudeswas skewed, such that the mean, 16.5 mm, was many timesgreater than the median, 6.6 mm (Table 1). Estimated over-story throughfall (P ′T,o), as shown in Fig. 3, ranged from 0(again, during dewfall) to 72.2 mm, with a median of 3.5 mm(Table 1). Thirty of the plants in the selected dogfennel clus-ters – those being monitored for stemflow – had an av-erage canopy radius of 18.3 cm (±4.5 cm standard devia-tion), which was nearly identical to the median canopy radius(Table 1). The stem radii of all measured dogfennel plantsranged from 0.1 to 0.7 cm, with a mean radius of 0.6 cm (Ta-ble 1). The resulting ratio of canopy : stem radii was also nor-mally distributed, with a mean and median of ∼ 36 (dimen-sionless), but ranged from 24 to 50 (Table 1). For all plants,the mean leaf angle decreased from 54 to 32◦ from verticalwith increasing canopy height, i.e., the higher in the dogfen-nel canopy, the closer the leaf angle was to vertical (Table 1).This trend appears consistent across each individual studyplant regardless of which clump the plants’ were located in,as the standard deviation across all elevations are low, 1.8–3.1◦ from vertical, and do not overlap (Table 1).

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Table 2. Descriptive statistics of relative throughfall (PT) and stemflow (PS) yield from dogfennel plants expressed as a proportion of grossrainfall (Pg) and modeled overstory throughfall (P ′T,o). Coefficients of variation (CV) and quartile variation (CQV) are also provided. Forstorms where dew occurred in the understory, dew was not measured by above-canopy Pg gauges but was included in the P ′T,o estimate byassuming that dew represented at least an additional 1.33 mm (i.e., Su).

Parameter Mean (SD) Median Q1 Q3 Max CV CQV

Rain storms

PT : Pg (%) 43.6 (15.2) 44.9 34.3 52.4 101.7 34.9 20.9PS : Pg (%) 18.8 (47.3) 4.1 1.7 13.8 434.3 251.6 78.1PT : P

′T,o (%) 76.6 (29.3) 72.0 58.5 91.1 190.6 38.3 21.8

PS : P′T,o (%) 36.8 (93.5) 7.6 2.8 27.2 900.3 254.1 81.3

Mixed storms∗

PT : Pg (%) 70.3 (43.7) 58.0 39.5 102.9 149.4 62.2 44.5PS : Pg (%) 32.7 (45.2) 14.7 5.2 39.7 198.0 138.2 76.8PT : P

′T,o (%) 72.0 (30.2) 69.1 53.2 86.9 191.6 41.9 24.1

PS : P′T,o (%) 33.4 (86.2) 8.1 3.0 24.3 900.3 257.4 78.0

∗ Storms with occult precipitation.

3.2 Partitioning into water storage, throughfall andstemflow

Note that P ′T,o is an event-scale estimate derived from pastobservations, limiting its utility in examining fine-scale PTand individual-plant scale PS. The sum of data from allstorms throughout the study period resulted in PT, PS and I

of 71 %, 8 % and 21 % as a portion of P ′T,o, respectively,beneath dogfennel plants at our site. Water storage capacityachieved by dogfennel leaves in the lab was 0.90±0.04 mm,whereas dogfennel stems stored a capacity of 0.43±0.02 mm(Fig. 4). This resulted in the total SU of dogfennel plantsin the understory of this study site being approximately1.3 mm. This SU estimate agrees with the reductions of P ′T,obelow dogfennel plants; for example, mean PT : P

′T,o was

76.6 % for rain-only storms (Table 2), or a mean yield ofPT = 12.9 mm which exceeds a 1.3 mm reduction (due to SUand evaporation) in the estimated mean P ′T,o yield, 16.5 mm(from Table 1). A large portion of the rainwater capturedon dogfennel canopies was able to overcome the stem waterstorage capacity and generate PS. Dogfennel PS data werehighly skewed, producing a mean relative PS (PS : P

′T,o) of

36.8 % but a median of 7.6 % within a narrow interquar-tile range, 2.8 %–27.2 % (Table 2). For events including oc-cult precipitation, both maximum PS : P

′T,o and PT : P

′T,o ex-

ceeded 100 %: PT : P′T,o during mixed storms reached a max-

imum at 192 %, whereas the maximum for PS : P′T,o was just

over 900 % (Table 2). Note that dew in the understory was notmeasured by the above-canopy rainfall gauges, and P ′T,o wasonly increased by an assumed maximum dew contributionequal to SU (1.33 mm); thus, dew accumulation allows PTand PS to exceed 100 % of Pg and P ′T,o (Table 2). Whencompared to rainfall above the overstory (Pg), the mediansare much smaller: PT : Pg values are 45 % and 58 % for rain-

Figure 4. Water storage capacity (standard error) for the (a) canopyand (b) stem of Eupatorium capillifolium (dogfennel) per lab-basedsubmersion tests on samples collected from the Charles H. HertyPines Nature Preserve understory.

only storms and mixed storms, respectively, and PS : Pg val-ues are 4.1 % and 14.7 %, respectively (Table 2).

Yield values (mm) were estimated for dogfennel PTand PS across storms, and both event-level PT and PSyields linearly correlated with estimated event-level P ′T,o(Fig. 5a, b). Regarding PT, as the catchment area (canopyarea above the gauge) is equal to the input area (soil area be-low the gauge), the PT yield from the canopy and the PT sup-ply to the surface are equal; therefore, the term “yield” willbe applied for both. Median PT yield beneath dogfennel forthe measured storms was 4.4 mm with an interquartile rangeof 1.1 to 11.3 mm (Fig. 5c). The maximum PT yield ap-proached 50 mm during a large-magnitude rain storm (wherePg = 101.3 mm). As the canopy area that generates stem-flow is many times greater than the surface area around plant

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4594 D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory

stems that receive stemflow (see Table 1), PS yield and F willdiffer. F values are typically used to represent PS supply tosoils. Yields of PS from dogfennel were as high as 24 mm,but the median was 0.4 mm and the interquartile range wasnarrow, 0.1–1.3 mm (Fig. 5c).

3.3 Stemflow and throughfall variability

Coefficients of variability (CV) and quartile variabil-ity (CQV) were computed for both PS and PT, relative to Pgand P ′T,o (Table 2), and storm-normalized temporal stabil-ity plots were generated for PS yield only (Fig. 6). Storm-normalized temporal stability plots were not generated for PTyields because the experimental design accounts for the spa-tial variability of PT via the deployment of large gauge areas(compared with the dogfennel canopy area); this permits esti-mates of variability across a few large-area gauges (Table 2),but it limits the observable variability. CV and CQV for rela-tive PT ranged from 22 % to 90 % and were generally lowerfor rain-only storms, < 40 %, than for mixed storms, > 60 %(Table 2). Variability in relative PS across study plants, rang-ing from 77 % to 257 %, was always greater than observedfor relative PT for the monitored storms (Table 2). Due tothe greater skew in the relative PS data compared with rela-tive PT, the CV was many times greater than CQV for rela-tive PS (Table 2). CV and CQV for PS : P

′T,o was similar for

rain and the mixed storms; however, the CV for PS : Pg wasgreater for rain-only storms compared with mixed storms.

Temporal stability of normalized stemflow, P S,i (Fig. 6)indicates that there were only a few plants that capturedmost of the PT,o drained as stemflow (three plants’ meanP S,i � 1). Thus, most of the studied dogfennel plants cap-tured similar amounts of P ′T,o as stemflow – having P S,i

between −1 and 1 (y = 0 represents the central tendencyof P S,i data). Funneling ratios (F based on P ′T,o) show thatall plants concentrated PS yields to the surface around theirstem bases (Fig. 6). Mean F across all plants was 87, andfor the 27 plants whose mean P S,i fell between −1 and 1,median F ranged from 18 to 200 (Fig. 6). However, forthe three plants with the highest P S,i , their mean F valueswere 287, 476 and 484 (Fig. 6). These voluminous stemflow-generating plants single-handedly account for one-third oftotal PS volume (8734 from 27 870 mL). To evaluate pos-sible canopy structural influences on PS variability, variousdirectly measured structural metrics were compared: radii ofcanopies and stems and the vertical variability in leaf angle(see Fig. S4). No clear visible or statistical correlations orcorrespondences were found between these structural vari-ables and P S,i across plants (Fig. S4). In fact, variabilityin the measured canopy structural variables was low (Ta-ble 1) compared with the variability observed for dogfen-nel PS and P S,i (Fig. 6).

4 Discussion

4.1 Overstory throughfall partitioning by dogfennel

Partitioning of overstory throughfall by this example dom-inant understory and pasture forb resulted in hydrologicallyrelevant losses of rainwater to the surface at our site (Table 2).As the maximum water storage capacity is a major driver ofrainfall interception (Klaassen et al., 1998), the magnitudeof dogfennel’s overstory throughfall interception may be at-tributed to its canopy being able to store a sizable magni-tude of rainwater per unit area, 1.33 mm (Fig. 4). Althoughmass changes of dried and submerged vegetation samples arediscrepant from the processes and temporal scales of natu-ral rainfall interception, it is a common method with well-known and long-discussed limitations that was selected toestimate water storage capacity as more direct water storagecapacity estimation methods are still currently under devel-opment – see discussions in reviews by Friesen et al. (2015)and Klamerus-Iwan et al. (2020). Methodological limitationswithstanding, the SU estimates in this study fit within therange of water storage capacities of other herbaceous plantssynthesized by Breuer et al. (2003). This synthesis is fo-cused on the leaves of herbaceous plants (alongside otherplant types) (Breuer et al., 2003), but less research has es-timated the stem component (or reported a total includ-ing the stem component) of the water storage capacity forshort vegetation (Bradley et al., 2003; Wang et al., 2016;Wohlfahrt et al., 2006; Yu et al., 2012). The stems of herba-ceous plants, even thick smooth stems (> 1 cm in diame-ter) can store nearly 0.5 mm, e.g., Taraxacum officinale (dan-delion) (Wohlfahrt et al., 2006). Even thin (< 1 cm radius)herbaceous stems with epidermal outgrowths, like hairs, canstore large amounts of rainwater, e.g., 0.25 mm for Achil-lea millefolium (yarrow) and 0.20 mm for Trifolium pretense(red clover) (Wohlfahrt et al., 2006). In the case of dogfen-nel stem water storage capacity at our site, the 0.43 mm esti-mate is within this range, and its magnitude is likely a re-sult of two principal factors: (1) dense stem coverage bydesiccated leaves (photo in Fig. 4) and (2) the fact that thisspecies can achieve large densities, up to 700 000 stems ha−1

(Dias et al., 2018) – 56 770 stems ha−1 at our study site. Wenote that, to our knowledge, stem water storage capacities forherbaceous plants with spines, thorns and other such physicalstructures have not been evaluated.

Overstory throughfall was also redistributed into a highlyspatially variable (Table 2) but temporally persistent patternbeneath dogfennel canopies (where CV or CQV was approx-imately 20 %–40 % for PT and 80 %–250 % for PS; Table 2),despite all measured canopy structures – such as branch an-gle, stem size and canopy size – being similar (Table 1).As our sampling plan measured PT over a large area of thedogfennel canopy (rather than at numerous localized points),this discussion point will focus on the intraspecific PS obser-vations. The high spatial variability and temporal persistence

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D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory 4595

Figure 5. Scatter plots showing the response of Eupatorium capillifolium (dogfennel) (a) throughfall (PT) and (b) stemflow (PS) yieldsacross all rainfall events (without occult precipitation). (c) Boxplot showing yields from individual PT gauges and plants’ PS. The line andbox represent the median and interquartile range, respectively, and the whiskers represent the non-outlier range; other symbols representoutliers and extreme values.

Figure 6. Mean and standard deviation (SD) of normalized stem-flow yield per plant and the associated funneling ratio per Her-witz (1986) and using modeled overstory throughfall (P ′T,o) in or-der of rank per mean normalized stemflow yield. Plant locationswithin clusters are indicated as follows: E denotes external; M de-notes middle, between the interior and exterior; and I denotes inte-rior.

of PS across plants despite canopy structural similarity raisesthe following question: what caused the intraspecific PS pat-terns observed in this study? A likely explanation may bethat, in this case, access to precipitation for stemflow pro-duction is related to overstory throughfall patterns (which,we reiterate, were not able to be measured without remov-ing or disrupting PT and PS). Overstory throughfall patternsare known to be spatially variable but temporally persistentacross forest types (Van Stan et al., 2020). Specifically, indi-vidual dogfennel plants that persistently generated greater PSthan other plants may have just received greater overstorythroughfall from persistent overstory drip points. If the over-story throughfall pattern is a major driver of intraspecificvariability in PS in this study, then the funneling ratios com-puted from mean overstory throughfall (per Fig. 3) would beincorrect (in Fig. 6). In this case, funneling ratios (computedfrom the localized overstory throughfall above each plant)could be similar across the monitored dogfennel plants. Test-ing this hypothesized relationship between dogfennel PS pat-

terns and overstory throughfall patterns was not possible inthe field, as sampling overstory throughfall would prevent PSfrom being generated by the plant. Future work to test thishypothesis could, however, make use of rainfall simulators.

The large diversion of rainwater and dew to their stem basemay be partially responsible for dogfennel survival duringextended periods of drought (or improved invasion efficacyduring droughts; Loveless, 1959; Forthman, 1973), and mayalso explain why this species tends to be one of the mostproblematic in improved grazing systems located in Florida(Sellers et al., 2009). Rainfall patterns in central and southFlorida may also intersect with dogfennel’s canopy waterbalance to “tip the scales” in its favor. Specifically, rainfall inour study region is often limited from January through May,with the bulk of the rainfall occurring from June through Oc-tober, and the water storage capacity of burgeoning dogfen-nel plants during early spring may enhance the chances of in-dividual plant survival (resulting in large infestations as ref-erenced previously).

4.2 Overstory (woody) and understory (herbaceous)canopies may partition rainfall differently

The dominant understory plant at our study site, dogfennel,intercepted similar amounts of modeled overstory through-fall, with an interquartile range of 11 %–59 % per storm (Ta-ble 2), compared to the gross rainfall interception by theiroverstory pine canopy, which had an interquartile range of19 %–60 % per storm (Van Stan et al., 2017b). Similar rain-water interception between dogfennel and the pine overstorymay be due to dogfennel’s maximum water storage capac-ity comparing favorably to that of overstory tree species,0.07–4.30 mm (Klamerus-Iwan et al., 2020). Even the max-imum stem water storage capacity is of a similar magni-tude to values reported by past work on woody plants, 0.2–5.9 mm (Klamerus-Iwan et al., 2020), albeit at the lower endof the range. Most current research on stem water storage

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4596 D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory

has focused on intrinsic factors of woody plant stems, likebark thickness, porosity, microrelief or roughness (Ilek et al.,2017; Levia and Herwitz, 2005; Levia and Wubbena, 2006;Sioma et al., 2018; Van Stan et al., 2016; Van Stan andLevia, 2010); however, other stem structures besides barkmay be capable of storing substantial water, e.g., the desic-cated leaves of our study plant.

There were differences in how gross rainfall was redis-tributed by the overstory canopy compared with how mod-eled overstory throughfall was redistributed by the dogfen-nel understory. Stemflow from the overstory, P. palustris wasnegligible at this site, 0.2 % of gross rainfall (Yankine etal., 2017), but median dogfennel PS was 7.6 % of modeledoverstory throughfall (with an interquartile range of 2.8 %–27.2 %) (Table 2). Annual relative PS (and PT) estimatesfrom trees and herbaceous plants reported by previous workindicates that herbaceous plants are generally greater stem-flow producers than woody plants (Sadeghi et al., 2020). Al-though relative PT beneath dogfennel was similar to obser-vations of relative overstory throughfall beneath P. palustrisat this site (Mesta et al., 2017), throughfall has been found tobe generally lower beneath herbaceous plant canopies thanfor woody plant canopies (Sadeghi et al., 2020). This seemsreasonable because if interception is similar between herba-ceous plants and woody plants, an increase in relative stem-flow would necessitate a decrease in relative throughfall. Theresults of this study support statements by several past studiessuggesting that plants in the understory and overstory interactdifferently with rainfall. Thus, we repeat the long-standingcalls for increased research on understory precipitation par-titioning, particularly stemflow (Price et al., 1997; Price andWatters, 1989; Verry and Timmons, 1977; Yarie, 1980).

4.3 A brief discussion on dew-generated throughfalland stemflow

For a few storms (n= 5), dew contributed significantly to PTand PS by the studied dogfennel plants. The median PTgenerated from dew beneath dogfennel plants at our sitewas 0.74 mm per plant with an interquartile range of 0.47–0.99 mm per plant, resulting in a total dew-related contri-bution to PT of 17.1 mm over the study period. Volumes ofstemflow under dewfall totaled 558 mL for all study plants,with individuals supplementing the dew-related PT withup to 61 mL per plant (yielding an additional ∼ 0.6 mm).Dew contributions to net precipitation below plant canopieshave rarely been studied. The earliest quantity for dewdrainage was 0.08 mm from a single event on a single treein Johanniskreuz, Germany (Ney, 1893). Since then, to ourknowledge, only one other study has examined dew-relateddrainage from plants, focusing on stemflow from the herba-ceous Ambrosia artemisiifolia, or common ragweed (Shureand Lewis, 1973). They estimated that the drainage of dewvia PS resulted in an additional input of 1.1 L per month dur-ing the growing season and hypothesized that this process

may “play a vital role in governing the density, diversity,and distribution of plant species within field ecosystems”(Shure and Lewis, 1973). Dew drainage from plant canopiesand down stems may, in addition to being a valuable wa-ter source, influence plant–soil interactions by transportingleached or dry-deposited materials to the soils – somethingalso discussed by Shure and Lewis (1973). Globally, dewcontributes a small percentage to the annual precipitation(Baier, 1966); however, in semiarid and arid (Baier, 1966;Hao et al., 2012), as well as summer-dry climates (Tullerand Chilton, 1973), dew can form a significant water input.In such ecologic settings as these, it is, therefore, reason-able to suppose that any factor that doubles the frequencyof plant-moisture availability, even though the amounts besmall, must materially affect the plant growing condition.Thus, further research is needed to assess dew (and mixedstorm) drainage in arid and semiarid climates, with days onwhich dew occurs being ≥ 70 % yr−1 (Hao et al., 2012). Theglobal importance of occult precipitation and resulting wetcanopy conditions has recently been reviewed and describedas a critical future research direction for plant sciences (Daw-son and Goldsmith, 2018). Given these scant but ecologicallyrelevant findings, further research on the influence of conden-sation events on plant–soil interactions via throughfall andstemflow may be merited.

5 Conclusions

Eupatorium capillifolium (Lam., dogfennel) in the under-story of an urban forest fragment intercepted 21 % of mod-eled overstory throughfall from Pinus palustris (Mill.). Theremaining 71 % and 8 % of modeled overstory throughfallreached the surface beneath dogfennel plants as understorythroughfall and stemflow, respectively. At the stand scale, thepartitioning of modeled overstory throughfall by this under-story forb differs considerably from the rainfall partitioningof the woody overstory, especially regarding stemflow (7.9 %versus < 0.2 %). During a few storms that occurred in tan-dem with dewfall, dogfennel plants were able to augmentstemflow (and throughfall) production by capturing dew.These processes may help explain how dogfennel survivesextended droughts and even shows improved invasion effi-cacy during droughts, making it one of the most problematicweeds in southeastern US grazing systems. Stemflow vari-ability among individual plants was very high (CV∼ 250 %),but no dogfennel canopy structures measured in this studyprovided statistically significant insights into this stemflowvariability. Future work will assess the extent to which actualoverstory throughfall variability drives understory stemflowvariability for plants, like dogfennel, of similar intraspecificcanopy structure. The inability to measure fine-scale over-story throughfall patterns without disturbing understory rain-fall partitioning in the field is a nontrivial limitation of thisstudy – a limitation that future work may overcome with rain-

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D. A. R. Gordon et al.: Rainfall interception and redistribution by a common North American understory 4597

fall simulations. Still, in forests, overstory throughfall is notthe final frontier for determining net rainfall, and investiga-tions on how it is intercepted and redistributed by herbaceousplants is needed to improve our understanding of exactly howmuch (and in what pattern) rainfall reaches the surface. Forother vegetated ecosystems where herbaceous plants are theoverstory (grasslands and croplands), precipitation partition-ing research is also needed.

Code and data availability. Data are permanently archived athttps://digitalcommons.georgiasouthern.edu/ (last access: Septem-ber 2020) (Georgia Southern University, 2020) and freely available.

Supplement. The supplement related to this article is available on-line at: https://doi.org/10.5194/hess-24-4587-2020-supplement.

Author contributions. DARG conceived and designed the study inconsultation with JTVS and AMJCG. DARG designed field collec-tion devices in consultation with JTVS and AMJCG, deployed thedevices, collected data, performed the data analysis, and drafted theinitial article with input from all authors. BAS contributed exper-tise regarding relevant rangeland and pastureland topics and assistedwith data analysis and interpretation. SMMS performed a literaturesynthesis for discussions comparing herbaceous and woody plants’rainfall partitioning and used this synthesis to assist in paper writ-ing. JTVS was the principal undergraduate research supervisor forDARG. All authors contributed to revisions of the paper.

Competing interests. The authors declare that they have no conflictof interest.

Special issue statement. This article is part of the special issue“Water, isotope and solute fluxes in the soil–plant–atmosphere in-terface: investigations from the canopy to the root zone”. It is notassociated with a conference.

Acknowledgements. The authors thank Georgia Southern Univer-sity’s Division of Facilities Services for study site access, mainte-nance and security. We also gratefully acknowledge the rigorous,thoughtful and helpful comments of the reviewers.

Financial support. This research has been supported by theU.S. Department of Education (Ronald E. McNair Postbaccalau-reate Achievement Program) and the Nederlandse Organisatie voorWetenschappelijk Onderzoek (grant no. 863.12.022).

Review statement. This paper was edited by Natalie Orlowski andreviewed by two anonymous referees.

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https://doi.org/10.5194/hess-24-4587-2020 Hydrol. Earth Syst. Sci., 24, 4587–4599, 2020