Management Implications
Understanding dispersal distances and deposition patterns of Polygonum spp. (knotweed) is important for practitioners to implement site-specific restoration efforts on riparian vegetation, which can help facilitate effective control and removal efforts. We quantified the potential dispersal of Polygonum spp. stems in a second-order stream in New Hampshire, USA. Under generally low- to regular-flow conditions, Polygonum spp. dispersal was generally constrained to ∼500 m of release, with initial dispersal distances linked to stream sinuosity (greater sinuosity resulted in greater retention) and time (most movements occurred soon after release). However, higher-flow events appeared to increase Polygonum spp. stem dispersal, and an extreme high-flow event in 2021 resulted in all stems being flushed downstream of our study system, implying that long-distance dispersal is possible via floods. Practitioners should also recognize that we only examined the potential dispersal of stems, while rhizomes generally contribute more to Polygonum spp. spread. Predictive tools identifying areas of higher probability of hydrochory-based dispersal by integrating stream habitats, flows, and Polygonum spp. biology could help to focus removal efforts or to identify riparian habitats that are at highest risk for spread within watersheds. Dispersal distances can be paired with mapping of Polygonum spp. along riparian habitats to identify reaches greatest at risk for invasion and at-risk habitats susceptible to erosion and degraded native plant diversity. For example, these data may be valuable for managers developing monitoring plans near Polygonum spp. stands after high-disturbance events such as logging. Early detection and rapid response efforts should be focused on reaches with higher sinuosity, complex habitats that can retain stems or rhizomes, and interventions immediately after higher flows recede.
Introduction
Non-native invasive plant species are a major cause of ecosystem degradation and impairment of ecosystem service benefits in the United States (Ferreira et al. Reference Ferreira, Figueiredo, Graça, Marchante and Pereira2021; Greene Reference Greene2014; Lavoie Reference Lavoie2017). Forested riparian areas provide many ecosystem service benefits to humans and are vital to the water quality of streams and rivers (Naiman et al. Reference Naiman, Decamps and Pollock1993; Riis et al. Reference Riis, Kelly-Quinn, Aguiar, Manolaki, Bruno, Bejarano, Clerici, Fernandes, Franco, Pettit, Portela, Tammeorg, Tammeorg, Rodríguez-González and Dufour2020). Riparian vegetation is adapted to natural flow regimes and supports high biodiversity and several essential ecosystem services for adjacent fluvial and terrestrial ecosystems (e.g., nutrient cycling, reducing leaching of pollutants, mitigating soil erosion, and producing soil organic matter; Su et al. Reference Su, Wu, Lind, Cai and Zeng2022). However, riparian areas are at high risk of invasion by non-native plants because they are among the most human-disturbed ecosystems in the world (Allan and Flecker Reference Allan and Flecker1993; Greene Reference Greene2014; Hammer and Gunn Reference Hammer and Gunn2021; Lavoie Reference Lavoie2017; Liendo et al. Reference Liendo, Biurrun, Campos, Herrera, Loidi and García-Mijangos2015).
Riparian ecosystems are structurally unstable due to frequent disturbances from water fluctuations (Naiman et al. Reference Naiman, Decamps and Pollock1993; Riis et al. Reference Riis, Kelly-Quinn, Aguiar, Manolaki, Bruno, Bejarano, Clerici, Fernandes, Franco, Pettit, Portela, Tammeorg, Tammeorg, Rodríguez-González and Dufour2020; Seeney et al. Reference Seeney, Eastwood, Pattison, Willby and Bull2019; Wang et al. Reference Wang, Liu, Ma, Ding, Wu, Jia, Chen, Yi, Zhang, Li, Luo and Huang2022); fluctuations in water levels outside the range of natural variability can trigger fundamental changes in composition and structure of riparian plant communities, which provides opportunity for colonization by invasive non-native plant species (Richardson et al. Reference Richardson, Holmes, Esler, Galatowitsch, Stromberg, Kirkman, Pyšek and Hobbs2007; Wang et al. Reference Wang, Liu, Ma, Ding, Wu, Jia, Chen, Yi, Zhang, Li, Luo and Huang2022). Further, invasive plant species in northern latitudes frequently begin growth earlier in the spring than native species, reducing the resistance of native species (Wang et al. Reference Wang, Liu, Ma, Ding, Wu, Jia, Chen, Yi, Zhang, Li, Luo and Huang2022). In conjunction with predictions of more frequent and severe flooding events in the northeastern United States, there is concern of significant potential for the development of a positive-feedback loop between increased non-native invasive species populations, more common and severe climate-induced flood events, erosion, and the dispersal of viable non-native invasive species propagules, especially Polygonum spp. (Houtt.) (Colleran and Goodall Reference Colleran and Goodall2014).
Japanese knotweed [Polygonum cuspidatum (Houtt.) Ronse Decr.], also referred to as Itadori knotweed, is native to Asia and is one of the most invasive plants in the world. It is a rhizomatous perennial that grows rapidly in the spring and can form dense clonal patches (Rouleau et al. Reference Rouleau, Bouchard, Matte and Lavoie2023). In its introduced range, P. cuspidatum is particularly abundant along riparian corridors (Colleran and Goodall Reference Colleran and Goodall2014, Reference Colleran and Goodall2015; Rouleau et al. Reference Rouleau, Bouchard, Matte and Lavoie2023). In northern New England, USA, most Polygonum spp. plants are believed to be P. cuspidatum, although hybridization with giant knotweed [Polygonum sachalinense (F. Schmidt ex Maxim.) Ronse Decr.] produces the hybrid bohemian knotweed [Polygonum×bohemicum (J. Chrtek & Chrtková) Zika & Jacobson [cuspidatum × sachalinense]] (Gammon et al. Reference Gammon, Grimsby, Tsirelson and Kesseli2007; Gammon and Kesseli Reference Gammon and Kesseli2010). In this study, we refer to all invasive knotweed plants as Polygonum spp. to allow for the possibility that the study site contained any hybrid of P. cuspidatum and P. bohemica (Hammer and Gunn Reference Hammer and Gunn2021). Invasive Polygonum spp. colonies are often associated with degraded forest structure and reduced stream habitat quality (Fogelman et al. Reference Fogelman, Bilger, Holt and Matlaga2018; Gerber et al. Reference Gerber, Krebs, Murrell, Moretti, Rocklin and Schaffner2008; Lavoie Reference Lavoie2017; Lecerf Reference Lecerf, Patfield, Boiché, Riipinen, Chauvet and Dobson2007; Seeney et al. Reference Seeney, Eastwood, Pattison, Willby and Bull2019; Serniak et al. Reference Serniak, Corbin, Pitt and Rier2017; Urgenson Reference Urgenson2006). The threat goes beyond a significant negative impact on plant biodiversity and forest structure. In particular, Polygonum spp. can have strong negative effects on instream macroinvertebrate decomposers, gastropods, amphibians, and native fish habitat (Colleran and Goodall Reference Colleran and Goodall2015; Fogelman et al. Reference Fogelman, Bilger, Holt and Matlaga2018; Gerber et al. Reference Gerber, Krebs, Murrell, Moretti, Rocklin and Schaffner2008; Lavoie Reference Lavoie2017; Lecerf et al. Reference Lecerf, Patfield, Boiché, Riipinen, Chauvet and Dobson2007) and have also been found to reduce stream depths under low baseflow conditions (Vanderklein et al. Reference Vanderklein, Galster and Scherr2014). Polygonum spp. is also associated with erosion (Kaehler Reference Kaehler2023), likely by reducing ground cover (i.e., root structure of native plants) that holds soil in place during floods, which can inhibit the regeneration of native species that provide critical structural support to riverbanks (Colleran and Goodall Reference Colleran and Goodall2015). Once established, Polygonum spp. is extremely difficult to eradicate and may have severe ecological, economic, or infrastructure effects (Colleran and Goodall Reference Colleran and Goodall2015).
Although Polygonum spp. reproduces by seed, it spreads effectively through fragmentation once established, which most commonly takes place following flood events and mowing (Colleran and Goodall Reference Colleran and Goodall2014, Reference Colleran and Goodall2015). Across plants, hydrochory (i.e., dispersal by water) is the most prominent dispersal form in river systems, because many floating propagules are spread by water and deposit and establish on downstream riparian zones (Hyslop and Trowsdale Reference Hyslop and Trowsdale2012; Nilsson et al. Reference Nilsson, Brown, Jansson and Merritt2010; Su et al. Reference Su, Wu, Lind, Cai and Zeng2022). Polygonum spp. plant propagules (i.e., stem or rhizome fragments) are commonly washed downstream, because the brittle stems often extend over the stream and are easily broken off during high-flow storm events (Hammer Reference Hammer2019). Such transport can be the dominant vector of spread for this invasive plant along a river (Duquette et al. Reference Duquette, Compérot, Hayes, Pagola, Belzile, Dubé and Lavoie2016), with areas closer to rivers associated with Polygonum spp. (Martin et al. Reference Martin, Dommanget, Janssen, Spiegelberger, Viguier and Evette2019). Additionally, the poor ability of Polygonum spp. to stabilize banks can cause them to collapse into the stream, allowing individual plants, stems, or root and rhizome fragments to enter the stream channel (Arnold and Toran Reference Arnold and Toran2018; Hammer Reference Hammer2019; van Oorschot et al. Reference van Oorschot, Kleinhans, Geerling, Egger, Leuven and Middelkoop2017). Segments of stem or rhizome can sprout and successfully regenerate, even when the segment is exceptionally small, if the segment contains at least one node (Colleran and Goodall Reference Colleran and Goodall2014; Rouleau et al. Reference Rouleau, Bouchard, Matte and Lavoie2023). Yet spatial distribution of Polygonum spp. invasion along riparian corridors and factors that may determine distance of spread and likelihood of establishment are not well understood (Hammer and Gunn Reference Hammer and Gunn2021; Wang et al. Reference Wang, Liu, Ma, Ding, Wu, Jia, Chen, Yi, Zhang, Li, Luo and Huang2022).
Understanding potential dispersal distances and deposition patterns of Polygonum spp. stems is important for practitioners to implement site-specific restoration efforts on riparian vegetation. There is a need to discern how Polygonum spp. spreads so that effective proactive control and management measures can be prioritized and implemented. Identifying such factors can facilitate target control and removal efforts to minimize cost and maximize effectiveness. The purpose of our study was to quantify potential dispersal distances and deposition patterns of experimentally released Polygonum spp. stems and assess whether such metrics vary among stream reaches within a system with extensive Polygonum spp. patches. We used passive integrated transponder (PIT) tags combined with regular stream walks to assess downstream movements of Polygonum spp. stems. PIT tags are commonly used to assess movement of biota in stream systems (Bubb et al. Reference Bubb, Thom and Lucas2008; Zydlewski et al. Reference Zydlewski, Horton, Dubreuil, Letcher, Casey and Zydlewski2006) or even stream sediment transport (Arnaud et al. Reference Arnaud, Piégay, Béal, Collery, Vaudor and Rollet2017; Lamarre et al. Reference Lamarre, MacVicar and Roy2005), but for plants, PIT tags are generally only applied to seeds (Kempter et al. Reference Kempter, Nopp-Mayr, Hausleithner and Gratzer2018; Suselbeek et al. Reference Suselbeek, Jansen, Prins and Steele2013), and we are unaware of their use in quantifying potential hydrochory. Our study stream system, Garland Brook in northern New Hampshire, is a tributary to the Connecticut River, with its headwaters in the White Mountain National Forest. Garland Brook has well-known extensive Polygonum spp. patches (Hammer Reference Hammer2019). We hypothesized that: (1) total dispersal distances and movement rates of Polygonum spp. stems would vary among release sites, with higher stream sinuosity resulting in less Polygonum spp. movement; and (2) movement rates of Polygonum spp. stems would vary through time within the summer season.
Materials and Methods
Site Description
The headwaters of Garland Brook are in the Kilkenny Mountain Range in the White Mountain National Forest. Garland Book is located within Hydrologic Unit Code (HUC) 12-010801010804 (Jones et al. Reference Jones, Niknami, Buto and Decker2022; USGS 2023c). Several first-order headwater streams converge and flow in a single channel through an intact closed-canopy forest (Figure 1). The land surrounding Garland Brook is primarily agricultural with forested riparian buffers. Garland Brook contains a historic hydropowered logging mill (Garland Mill) that is heavily invaded by Polygonum spp. (Hammer Reference Hammer2019). Polygonum spp. is the dominant vegetation type on both sides of the mill access road, the mill property, and along the stream banks. The study site is located just downstream of the mill where Garland Brook begins to converge as it flows along both sides of a large pasture (Figure 1). Garland Brook continues through a matrix of forest, grazed pasture, and homesteads with an intermittent and narrow riparian buffer at the lower reaches before flowing into the Israel River (Hammer Reference Hammer2019; Figure 1). Stream surveys for terrestrial invasive plants conducted in the riparian areas along both sides of Garland Brook found 324 patches of invasive plants. Polygonum spp. is the most abundant invasive species in the study area (followed by Morrow’s honeysuckle (Lonicera morrowii A. Gray) and glossy buckthorn (Frangula alnus Mill.) (Hammer Reference Hammer2019).
Figure 1. Study site location with upstream, midstream, and downstream Polygonum spp. release sites (green squares) along Garland Brook, Lancaster, NH, USA. Habitat survey locations (yellow circles) denote where individual habitat units were identified and measured (see Table 1).
Field Methods
Stream habitat was assessed before experimental releases in 2021. Starting at the downstream end of our study site (Figure 1), the primary habitat type was identified (i.e., pool, glide, riffle, or cascade) based on stream gradient and depth. Within each habitat unit, a series of measurements were collected, including gradient (percent), wetted width (m), bankfull width (m), length (m), and stream depth (m; taken at 25%, 50%, and 75% distances along the wetted width; Table 1). Stream depths and widths were taken at five latitudinal transects across the stream, at approximately equal distances along the habitat unit’s length; these values were then averaged for each habitat unit and across habitat units within the section (Table 1). In addition, the dominant substrate type was noted, along with other metrics appropriate for fish habitat assessments being used in a separate study. The reach’s thalweg (i.e., line of lowest elevation within streambed) was recorded via handheld GPS in each habitat unit. These thalweg locations were used to generate a line shapefile of the system. From these surveys and digitization of the thalweg, experimental release sites were identified (Figure 1; Table 1) based on their habitat characteristics and channel sinuosity. Channel sinuosity was quantified by measuring the total distance of the thalweg that falls within a straight-line distance of 500 m from the release site (calculated in ArcMap, ESRI, Redlands CA, USA). The upstream release site was described as a meandering, slow-moving channel; the midstream release site was a straight channel with many large in-stream boulders; and the downstream release site was a complex, braided, gravel-filled channel.
Table 1. Habitat assessment summaries for the 500 m downstream of each of the three release sites a .
a Values shown for depth (m), wetted width (m), full width (m), and gradient (%), are mean values (±SD).
b Sinuosity = distance (stream)/500 m (straight-line distance).
On June 14, 2021, and August 4, 2022, we conducted experimental releases of Polygonum spp. stems to assess downstream movements. Each Polygonum spp. stem was cut from the top 1 m of a stem from an established Polygonum spp. patch at each of the three release sites (upstream, midstream, downstream; see Figure 1). Polygonum spp. is well established throughout the study site. We used the top portion of Polygonum spp. stems in our experimental releases rather than rhizomes because (1) it was easier to maintain a similar size “unit” among released stems, (2) we assumed that the top portion of the plant was most likely to break off and enter the stream (we observed this anecdotally in the system in prior years), and (3) we could easily sample stems without digging out rhizomes and potentially causing erosion of stream banks in the field site. We recognize that although stems can sprout and lead to spread (De Waal Reference De Waal2001), rhizomes are likely the dominant mechanism of spread (Colleran and Goodall Reference Colleran and Goodall2014; Gowton et al. Reference Gowton, Budsock and Matlaga2016). Each released stem (approximately 1 m in length) had several nodes but no attached roots. We released Polygonum spp. stems with PIT tags. PIT tags (Biomark APT12, 12.5-mm long, 2.03-mm diameter, full duplex FDX-B; Biomark, Boise, ID) were glued to the stems and wrapped in electrical tape. Each stem was painted orange around the location of the PIT tag to increase the likelihood of detecting it after release during stream walks. A total of 90 Polygonum spp. stems were released per field season (n = 180 between 2 yr), with 30 replicate stems released at each release site per field season. Stems were released at 30-s spacing intervals into the stream flow at river center. To relocate tagged stems, stream walks were conducted semi-regularly (described below), in addition to opportunistic searching during other related stream ecology studies in the system (unpublished data) in both field seasons. When tagged stems were found, their identities were recorded with a PIT tag reader (Biomark HPR Lite or HPR Plus). We also recorded any node shoots or sprouting, as this can occur rapidly (within weeks) from Polygonum spp. stems (De Waal Reference De Waal2001), and assessed whether any establishment occurred.
We assumed larger in-stream stem movements would happen in stages during higher flows; thus, we conducted stream walks to search for tagged stems after periods of elevated precipitation. We used the water gages for the nearby Ammonoosuc River to track rain events (https://waterdata.usgs.gov, site ID number 01130000 and 01137500; Figure 2) with follow-up monitoring within 5 d of large rain events (defined as > ∼12.7 to 25.4 mm of rain). There were also final monitoring visits at the end of each of the 2021 and 2022 field seasons to record ending locations of stems, if established, and to collect the stems released in the study.
Figure 2. Upper and Lower Ammonoosuc River gages (Upper = north of Garland Brook: gage site 01130000; USGS 2023b; Lower = south of Garland Brook: gage site 01137500; USGS 2023a) for 2021 and 2022 field seasons (source: https://waterdata.usgs.gov). Vertical dashed lines represent release date of experimental Polygonum spp. stems, with date labeled top left. Horizontal dashed lines represent mean discharge values for each USGS gage site for the dates shown between 1991 and 2023. Note the seasonal difference in dates along the x axis between the two panels.
Data Analysis
PIT tag detections and their associated latitudes and longitudes were brought into a geographic information system (GIS) using ArcMap v. 10.8.2 (ESRI). Garland Brook’s thalweg was digitized (Figure 3) from in-stream habitat assessments (Figure 1; Table 1) and aerial imagery (Figure 1). Habitat unit characteristics were considered among habitats within 500 m downstream of each release site (Table 1). The location of each tagged Polygonum spp. stem recapture was assigned a stream position (meters from release to the nearest location of the thalweg) using the Locate Features Along Routes tool in ArcMap (Figure 3). Any obvious false detections (i.e., detection upstream of the release site) were removed. Any small (<5-m) upstream “movements” of PIT tags were presumed to be GPS measurement error and were treated as no movement (given the same location as previous detection).
Figure 3. Polygonum spp. movement for 2021 and 2022 field seasons. Recapture events (white circles) are all via passive integrated transponder (PIT) tag detection; each Polygonum spp. stem could be detected multiple times.
Kruskal-Wallis tests were used to determine whether the maximum downstream distance moved by Polygonum spp. stems varied among the three released sites in each year. When these tests were significant, pairwise Wilcoxon tests with continuity correction were used to identify which release sites significantly differed in maximum downstream distance moved. To further analyze how downstream movements varied with time and among the three release sites in both years, we used a generalized additive mixed model (GAMM) framework. Movement rate (m d−1 between successive recaptures of individual Polygonum spp. stems) was the response variable, with the release site (upstream, midstream, or downstream) and duration of time since release (in decimal days) as candidate explanatory variables. The smoothed term (duration of time since release) was fit using a penalized thin-plate regression spline (Wood Reference Wood2003). Separate models were constructed for each release year. Due to the repeated captures of some Polygonum spp. stems, we included the Polygonum spp. identity as a random effect in all models. To reduce potential for overfitting to the continuous variable of time since release, we limited the number of effective degrees of freedom to three (k = 4). We used the Akaike information criterion (AIC) to rank models containing both or one of the two explanatory variables, with lower AIC values indicating more parsimonious models. We used the mgcv package (Wood Reference Wood2011, Reference Wood2017) in R v. 4.2.2 (R Core Team 2022) to construct GAMMs.
Results and Discussion
Polygonum spp. Stem Recaptures
In 2021, the 90 released stems of Polygonum spp. were recaptured 369 times, and in 2022 the 90 released Polygonum spp. stems were recaptured 317 times. Of the 90 released stems in 2021, 87 stems (∼97%) were detected at least once (maximum 7 times); 88 (∼98%) of 90 stems released in 2022 were detected (maximum 6 times). Maximum distances moved from individual release sites varied between 30 m and 875 m downstream in 2021 (mean = 155 m; SD = 142 m) and 13 to 1,233 m in 2022 (mean = 283 m, SD = 220 m; Figure 4). In both years, the maximum distances tracked of Polygonum spp. stems differed significantly among the three release sites (2021 Kruskal-Wallis test P < 0.0001; 2022 Kruskal-Wallis test P < 0.0001). In 2021, the mean maximum distance moved was highest for stems released at the upstream site (208 m, SD = 49 m), intermediate for the midstream site (161 m, SD = 175 m), and lowest for stems released at the downstream site (97 m, SD = 150 m; Figure 5). Pairwise Wilcoxon rank-sum tests indicated the maximum distances moved differed significantly among all pairs (downstream–midstream P = 0.017; downstream–upstream P < 0.0001; midstream–upstream P = 0.0002), even though the median maximum distances moved were quite similar between segments released downstream (76 m) and midstream (73 m). In 2022, significant differences were detected in the maximum downstream movements among the three release sites (Kruskal-Wallis test P < 0.0001), with stems released at the midstream site (468 m) traveling farther than those released upstream (240 m) or downstream (132 m). Similarly in 2022, stems released downstream experienced the least maximum downstream dispersal (mean = 132 m, SD = 62 m), but midstream-released stems experienced the highest downstream dispersal (mean = 468 m, SD = 203 m), and upstream-released stems dispersed an intermediate distance (mean = 240 m, SD = 202 m; Figure 5). All pairwise comparisons in the maximum dispersal distance among the three release sites were significantly different for each year (Wilcoxon rank-sum test for all 2021 pairwise comparisons P < 0.05; in 2022, all P < 0.0001). Thus, maximum distances moved varied among release sites, consistent with our original hypothesis.
Figure 4. Downstream dispersal distance through time of Polygonum spp. stems in 2021 (top) and 2022 (bottom).
Figure 5. Maximum dispersal distance of Polygonum spp. stems for each year and release site.
Polygonum spp. Stem Movement Rates
GAMMs examining variability in movement rates of Polygonum spp. stems identified both time since release and release site as important explanatory variables. For both years’ models, these two variables were retained in models with the lowest AIC values (Table 2). Time since release appeared to be the more important explanatory variable; the second-best model for both years retained this variable rather than release site and had higher percent deviance explained, following our original hypothesis and expectations (stems will travel upon release until they become entangled or beached, then movements will slow or stop until the stem is freed again). In 2021, stem movement rates experienced a nonlinear relationship with time since release; fastest movement rates occurred shortly after release, which then declined slowly over the first week post-release (Figure 6). Movement rates increased in mid-July 2021, coinciding with periods of increased in-stream flow. In 2022, this relationship was largely monotonic, with fastest movement rates experienced shortly after release and then declining in a near-linear fashion as the time since release increased. Coefficients for release sites (Table 3) indicated that movement rates were slower for downstream sites in both years (Figure 6); this was presumably driven by the lack of faster movements (no stems released downstream were ever observed moving >50 m d−1) rather than a clearly lower median or mean value. In parallel with our maximum-distance investigations, movement rates were more variable between midstream and upstream sections between the 2 yr. Polygonum spp. stems in the downstream release site consistently dispersed less than the Polygonum spp. stems in the midstream or upstream release sites (Figure 5). These results are consistent with our original hypothesis that movements of Polygonum spp. stem would be site dependent within the stream.
Table 2. Generalized additive mixed model (GAMM) results for modeling Polygonum spp. stepwise movement rates (m d−1 between successive recaptures).
a Models are ranked by Akaike information criterion (AIC).
Figure 6. Left, Generalized additive mixed model (GAMM) model fits showing smoothers between days since release and movement rates for 2021 (top) and 2022 (bottom). Right, Movement rates (m d−1 between successive relocations) between years and among release sites.
Table 3. Model coefficients for the top-ranked models (from Table 2) explaining variability in Polygonum spp. stem stepwise movement rates (m d−1 between successive recaptures) for each year a .
a Each generalized additive mixed model (GAMM) can include both parametric variables (release site) and continuous factors evaluated by a smoothing function (“Smooth”; time since release). For the smoothed variable, the effective number of degrees of freedom (edf) used by the smooth is stated.
Hydrochory of Polygonum spp. and Influences of Stream Characteristics
In general, Polygonum spp. stems across sites were recaptured within 500 m of their original release sites under generally low flows in 2021 and low to regular flows in 2022, with all recapture events occurring within 2 km of release locations, consistent with previous studies, where downstream dispersal distances of mimic propagules were limited to 3 km from release (Su et al. Reference Su, Wu, Lind, Cai and Zeng2022) and rapidly decreasing probability of recapturing released plant fragments with distance below point of release (Didier et al. Reference Didier, Borgniet, Le Bouteiller, Evette, Boyer and Dommanget2023; Riis and Sand-Jensen Reference Riis and Sand-Jensen2006). Generally, Polygonum spp. patches along rivers are found in near proximity, even if the spread was not specifically monitored (e.g., <100 m between patches; Hart et al. Reference Hart, Bailey, Hollingsworth and Watson1997). Thus, retention of Polygonum spp. stems was generally high at these flows, with the greatest potential for hydrochory to be impactful at the reach scale rather than among stream orders. Most new Polygonum spp. plants originate from rhizome fragments, with only ∼15% to 30% originating from stems (Colleran and Goodall Reference Colleran and Goodall2014; Gowton et al. Reference Gowton, Budsock and Matlaga2016); however, this is a higher rate of success than individual seeds (3%; Gowton et al. Reference Gowton, Budsock and Matlaga2016) and the combination of rhizomes and stems can be the dominant vector for spread in riverine systems (Duquette et al. Reference Duquette, Compérot, Hayes, Pagola, Belzile, Dubé and Lavoie2016). At the start of the growing season, Polygonum spp. dedicates more energy toward stem growth than rhizomes (Colleran and Goodall Reference Colleran and Goodall2015), highlighting that stem regrowth and establishment are likely still important contributors to dispersal.
Although not consistently significant across all analyses, there was evidence that Polygonum spp. stems released in the downstream habitat, characterized by higher sinuosity, traveled shorter distances and less quickly. Stream sinuosity is generally considered to promote retention of coarse organic matter (James and Henderson Reference James and Henderson2005), although broad-shaped or leaf-life matter is more easily retained than dowel- or rod-like shapes (James and Henderson Reference James and Henderson2005), and sinuosity is not always predictive of dispersal distances in hydrochory (Su et al. Reference Su, Wu, Lind, Cai and Zeng2022). Regardless, bank curvature has been positively correlated with presence of Polygonum spp. deposited on stream banks (Didier et al. Reference Didier, Borgniet, Le Bouteiller, Evette, Boyer and Dommanget2023), supporting the role of stream sinuosity in deposition of Polygonum spp. In addition to stream sinuosity, we observed potential differences in in-stream habitat that may have also contributed to decreased movements in stems released at the downstream site. Although not directly measured or characterized, the downstream site in particular had several overturned logs or other dense stems of vegetation within the bankfull width, resulting in opportunities for Polygonum spp. to be retained via snags at a given location for days to weeks. Such snags are more likely to retain coarse matter than boulders (James and Henderson Reference James and Henderson2005) that were observed more in the midstream reach. Increased contact among the streambed, stream bank, and vegetation (whether it be native or non-native) should increase retention of stems (Riis and Sand-Jensen Reference Riis and Sand-Jensen2006). Thus, identifying and quantifying potential areas where Polygonum spp. stems are likely to make contact may improve predictive frameworks for identifying habitats likely to retain Polygonum spp. dispersed via hydrochory. Nonetheless, many of these traits (presence of in-stream vegetation, large woody debris, or complex habitat and in-stream vegetation) that we associated with Polygonum spp. stem retention can be associated with more tortuous or sinuous streams, relative to straight channels (Diez et al. Reference Diez, Elosegi and Pozo2001; Nakamura and Swanson Reference Nakamura and Swanson1994).
Beyond stream geomorphology, flow conditions and regimes can affect hydrochory, with deposition more likely to occur on receding flow regimes (Hyslop and Trowsdale Reference Hyslop and Trowsdale2012; Merritt and Wohl Reference Merritt and Wohl2002; Su et al. Reference Su, Wu, Lind, Cai and Zeng2022; van Leeuwen et al Reference van Leeuwen, Sarneel, Paassen, Rip and Bakker2014). Although not specifically compared, in general, Polygonum spp. dispersed more in 2022, when flows were generally higher than in 2021 (with the exception of the flood event described below) and with more above-average flow events during our specific field season (Figure 2). Flows in 2021 were generally low, with Polygonum spp. dispersing generally only <250 m, but a very high-flow event in mid-July 2021 resulted in multiple stems being dislodged and further dispersed, followed by the eventual loss of all stems from the study area under extreme flows. Thus, our estimates of high retention and relatively low dispersal distances (within ∼500 m) of Polygonum spp. stems are specific to normal or low flows; with higher flows and floods likely resulting in greater potential dispersal, as observed with increased dispersal in generally higher flows in 2022 and the flood event in 2021 that flushed stems even farther, downstream of our study area. In summary, stream hydrology and fluvial geomorphology, along with the biology (Hyslop and Trowsdale Reference Hyslop and Trowsdale2012) of Polygonum spp., will likely dictate the success of hydrochory in allowing Polygonum spp. to establish downstream.
Considerations for Polygonum spp. Reestablishment
For hydrochory to be successful, the stem or propagule would need to be deposited successfully in suitable habitat and growth conditions, while the stem is still viable. Polygonum spp. stem viability does not extend past the second spring following its dispersal (Colleran and Goodall Reference Colleran and Goodall2015). Our work at Garland Brook supports this time frame, as stems recaptured in the 2022 field season from 2021 releases appeared to be dead and were not established into the riverbanks or riparian ecosystem; they were simply retained on or within the woody debris along the riverbank. Within a given study season, the majority of released Polygonum spp. stems developed node shoots, consistent with rapid bud and node shoot development (∼2.9 mm d−1) within weeks (De Waal Reference De Waal2001), but we never observed establishment at the end of each field season. Generally, fragments or seeds of plants are deposited along shallow slopes (Su et al. Reference Su, Wu, Lind, Cai and Zeng2022) or areas associated with still water (pools or eddies; Hyslop and Trowsdale Reference Hyslop and Trowsdale2012). Unvegetated shallow slopes along the riparian zone are likely to be ideal for seed or fragment deposition as well as regrowth or establishment (Su et al. Reference Su, Wu, Lind, Cai and Zeng2022) via increased light that can promote growth of shoots (Martin Reference Martin2019). Given we largely described dispersal of stems during normal flows, most stems we monitored were deposited within the banks of the channel, on snags and large woody debris. Deposition within the banks of the channel, versus higher up on banks, might not represent ideal habitat for reestablishment. Our study only quantifies potential dispersal patterns via hydrochory. Broadly, Polygonum spp. is most successful in disturbed habitats (Navratil et al. Reference Navratil, Brekenfeld, Puijalon, Sabastia, Boyer, Pella, Lejot and Piola2021; but see Didier et al. Reference Didier, Borgniet, Le Bouteiller, Evette, Boyer and Dommanget2023) with high light availability (Dommanget et al. Reference Dommanget, Spiegelberger, Cavaillé and Evette2013). More specifically, if the propagules deposit in the riparian zone as water levels recede (likely aided by floods or increased flows), they could have a higher chance to colonize due to ideal growing conditions (e.g., moist soil), and long-term submergence of habitat could reduce aboveground native vegetation (Su et al. Reference Su, Wu, Lind, Cai and Zeng2022), providing further opportunities for successful invasion.
Study Limitations
Our ability to describe the potential dispersal of Polygonum spp. is dependent on our ability to track individually marked stems. PIT telemetry is robust in that tags do not have batteries and are durable over time. However, their read range (the maximum distance between receiver and the tag that still allows for detection) is generally quite short (∼0.3 m), requiring us to individually locate and scan tags. In addition, tag collisions are possible when many tags are co-located; such aggregations occurred at retention “hotspots” such as log jams and snags where at times >10 tags were located at a single habitat unit for several days. We took efforts to slowly scan through the group and repeating this process over days likely minimized chances of missing tags completely. Our approach also required us to be able to identify and see stems to scan with hand units; future research that installed PIT arrays across the stream could more comprehensively quantify when individual stems reached certain distances downstream. Finally, we could not locate how far and how quickly any stems moved once they were downstream of our study reach (e.g., after the 2021 high-flow event). Regardless, we had high rates of detection of marked stems and thus are confident we adequately described their general dispersal distances.
Conservation and Management Implications
The ability of Polygonum spp. to disperse via water for hundreds of meters even under regular flows likely contributes to the ability to quickly invade and impact stream and riparian habitats. Identifying characteristics that contribute to Polygonum spp. propagules being released could help further build predictive frameworks for Polygonum spp. spread in stream ecosystems across settings (across flow variations, natural spread vs. after high-disturbance events such as logging, etc.). The root system of Polygonum spp. promotes erosion along stream banks (Arnold and Toran Reference Arnold and Toran2018; Colleran et al. Reference Colleran, Lacy and Retamal2020; Kaehler Reference Kaehler2023; Matte et al. Reference Matte, Boivin and Lavoie2022), which in turn can promote propagule dispersal, especially during high flows (Colleran et al. Reference Colleran, Lacy and Retamal2020), generating a potential reinforcing feedback loop via continued erosion and dispersal. In response, studies have suggested focusing control efforts on Polygonum spp. patches most susceptible to erosion and thus propagule dispersal (Colleran and Goodall Reference Colleran and Goodall2014, Reference Colleran and Goodall2015).
Complications in managing Polygonum spp. arise from its plasticity in environmental tolerance, resilience to disturbance, vegetative dispersal capabilities, and extensive energy storage in rhizomes (Gillies et al. Reference Gillies, Clements and Grenz2016; Hocking et al. Reference Hocking, Toop, Jones, Graham and Eastwood2023). Numerous control methods have been studied (e.g., covering, cutting, burning, digging, and encapsulation) and herbicidal control is considered the most effective (Hocking et al. Reference Hocking, Toop, Jones, Graham and Eastwood2023), but all methods are expensive (Rouleau et al. Reference Rouleau, Bouchard, Matte and Lavoie2023). However, the use of herbicides to control invasive species should be done with caution, as the use of chemicals has its own set of environmental consequences, especially in and around riparian areas. Continuing to improve frameworks for predicting dispersal and establishment dynamics of Polygonum spp. in riparian areas will help prioritize control efforts.
In conclusion, we provide estimates of dispersal potential in a small New England stream, with Polygonum spp. stems generally dispersing <500 m from release sites under low and regular flows, with higher flows increasing dispersal distance and extreme flow events having the capacity to push stems presumably several kilometers. These short-term dispersal distances are weakly correlated with stream characteristics, including sinuosity and presence of large woody debris or snags; although in-stream large woody debris may not be ideal for Polygonum spp. establishment, stream reaches with increased sinuosity may experience greater retention of Polygonum spp. stems and increased chances of spread. In addition to sinuosity, short-term dispersal distances were also negatively correlated with time since release, with only times of increased flow resulting in further movements shortly after release. Such information on stream channel morphology, habitats, and flow variation can be paired with mapping of Polygonum spp. along riparian habitats to identify reaches that are at greatest risk for invasion for potential early detection; subsequent impacts of Polygonum spp., such as erosion (Arnold and Toran Reference Arnold and Toran2018; Colleran et al. Reference Colleran, Lacy and Retamal2020; Kaehler Reference Kaehler2023; Matte et al. Reference Matte, Boivin and Lavoie2022); and negative impacts to local plants and diversity (Colleran and Goodall Reference Colleran and Goodall2015; Wilson et al. Reference Wilson, Freundlich and Martine2017).
Acknowledgments
We appreciate regional landowners who allowed us to access field sites via their properties. Additional field efforts were provided by N. Hermann and C. Pearson. C. Hammer’s MS thesis (University of New Hampshire) helped inspire this work.
Funding statement
This research was supported by the Dick George Fund provided to the Natural Resources and Environment Department at the University of New Hampshire. Furey was partially supported by the Class of 1937 Professorship in Marine Biology from the University of New Hampshire’s School of Marine Science and Ocean Engineering.
Competing interests
The authors declare no conflicts of interest.
