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Patterns of Mortality in a Wild Population of White-footed Mice
Christopher R. Collins and Roland W Kays

Northeastern Naturalist, Volume 21, Issue 2 (2014): 323–336

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Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 303 2014 NORTHEASTERN NATURALIST 21(2):303–322 Natural Plant Establishment along an Urban Stream, Onondaga Creek, New York Catherine L. Landis1,* and Donald J. Leopold1 Abstract - Urbanization results in a suite of harmful effects to streams, including removal or degradation of riparian vegetation. Many stream-restoration programs address this by adding plants, with limited quantitative knowledge about vegetation dynamics already occurring within the stream corridor. This project examined natural plant establishment along an urbanized stream channel in Syracuse, NY. It had three objectives: first, to relate plant establishment along an urban stream gradient to substrate condition; second, to quantify seeds dormant in the soil at those same sites; and third, to indicate what passive revegetation responses might occur to various treatments along a rural-to-urban gradient. Three sites were selected along such a gradient on Onondaga Creek, near Syracuse, NY. Vegetation plots were established at each site to assess plant germination and establishment under substrate conditions designed to mimic restoration interventions. We also conducted a seedbank study using soil cores collected from these sites. Plant communities were dominated by grasses and forbs. Numbers of alien species increased from 34% at more rural sites to 51% at more urban sites. Seedlings of native riparian trees nonetheless germinated at all three sites along the gradient. Recruitment of native riparian trees (especially Populus deltoides [Eastern Cottonwood], Fraxinus pennsylvanica [Green Ash], and Acer negundo [Boxelder]) exceeded non-native and invasive ones. The riparian seedbank showed disproportionate dominance by herbaceous plants (95.5% of individuals) at all locations surveyed, and invasive species were common (about 25% of all germinants). This study shows some potential for natural regeneration of native trees, but also found a significant source of invasive plants in the soil seedbank that could reduce restoration success. Notably, the study recorded the presence of 16 bryophyte taxa, and the common ones were those typically associated with disturbances. Introduction Urban streams are among the most extensively disturbed and degraded aquatic systems in North America (Hession et al. 2000). In particular, riparian deforestation associated with urbanization reduces food availability for wildlife, affects stream temperature, and disrupts sediment, nutrient, and toxin uptake from surface runoff (Paul and Meyer 2001). Resulting changes in hydrology create “hydrologic drought” by lowering water tables, which in turn alters soil, vegetation, and microbial processes in urban riparian zones (Groffman et al. 2003). There is growing interest in restoring function to urban waterways, including that of the riparian vegetation that plays so many important roles along streams (Riley 1998). These efforts often involve adding plants to the site, but this step is costly 1Department of Environmental and Forest Biology, 241 Illick Hall, State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210. *Corresponding author - cllandis@syr.edu. Manuscript Editor: M.A. Leck Northeastern Naturalist 304 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 and labor intensive. Passive restoration, which relies on propagules already in the system to recover riparian vegetation, appeals due to its low cost and simplicity (Bernhardt and Palmer 2007, White and Stromberg 2005). For example, streams could transport seeds and plant parts from more upstream, natural sections to enhance the flora of downstream, urbanized reaches of the stream. A second source of seeds lies in the urban streamside soil seedbank. Few studies have examined these sources—existing upstream propagules and the riparian seed bank—to assess their role in urban stream restoration. Many riparian plant species are disturbance tolerant, early successional, and fast growing, traits that could allow them to compete well even in anthropogenic settings. Moreover, in a system where upstream reaches of the watershed remain relatively natural, those areas could provide a pool of propagules of native species to replenish depauperate areas downstream. On the other hand, streams can also facilitate dispersal of less-desirable species such as Polygonum cuspidatum Siebold and Zucc. (Japanese Knotweed), Rhamnus cathartica (European Buckthorn), and Phalaris arundinacea (Reed Canary Grass) (Tickner et al. 2001). Thus, research on natural regeneration is important to inform stream-restoration strategies, including the design of seeding and planting schemes (Gurnell et al. 2006). The purpose of this study was to investigate these existing pools of plant material along an urban stream, in order to assess their potential contribution to restoration efforts. In some cases, especially where site conditions are harsh, added vegetation can facilitate the arrival and establishment of other plants (Padilla and Pugnaire 2006). Success of restoration projects in these instances can be enhanced by the addition of such facilitators or “nurse plants”. Along streams, adding vegetation to barren areas can directly add a propagule source. Also, by increasing bank roughness, these added plants can increase seed capture from hydrochory. Streamside plants could “nurse” any newly established seedlings by mitigating high-velocity erosive flows, moderating soil moisture (via shading), and protecting against intense herbivory. In this study, we added small trees (Ulmus americana L. [American Elm]) and Salix nigra Marsh. (Black Willow) stakes (cuttings) to investigate facilitation effects. We also conducted a seedbank study to assess the nature of this propagule resource, with special focus on its potential role in restoration. Further, we collected hydrological data to permit the assessment of stream stage relative to plot elevation (e.g., recording frequency of inundation). The first objective was to relate plant establishment along an urban stream to site or substrate condition; here, the study focused especially on the establishment of woody plants that could form a floodplain forest. The second objective was to quantify seeds dormant in the soil. A third objective was to begin to relate plot treatment as well as seedbank results to potential differences in vegetation restoration along the urbanization gradient. We hypothesized that plant colonization would be highest in the scarified plots due to the removal of all existing vegetation, which resulted in reduced competition for soil, light, and water resources in these areas. Colonization in the willow and Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 305 elm treatments was expected to follow, since these plots also experienced reduced competition, in this case resulting from the herbicide treatment of existing vegetation before the trees were added. We added four willow stakes, and one elm seedling (≈50 cm), and expected differences in colonization here based on stem numbers and plant architectural form. The “no mow” and “mow” plots were expected to have the lowest levels of colonization because the established vegetation was not removed or killed. We further hypothesized that numbers of native riparian species would decrease with increasing urbanization, and that seedbanks in urban areas would harbor fewer native plant seeds relative to rural areas. Methods Field-site description Onondaga Creek, flowing through the heart of Syracuse, NY, represents an urban stream for the final 14.5 km of its course (Fig. 1). From headwaters along the Valley Heads Moraine near Tully, NY, to its mouth along the shores of Onondaga Lake, Onondaga Creek flows about 43.5 km. Historically, Onondaga Creek meandered through floodplain forests, cedar swamps and, near its outlet at Onondaga Lake, inland salt marshes. Like many urban streams, Onondaga Creek was straightened and deepened, while associated wetlands were drained and filled in f or development. Study sites. Experimental units were set up at three sites along a rural-to-urban gradient, from where stream channelization began in Nedrow, NY, to Franklin Square in the city of Syracuse. Franklin Square is about 1 km south of the mouth of Onondaga Creek in the Inner Harbor of Onondaga Lake (Fig. 2). The three sites also represent various engineering treatments along Onondaga Creek. Site descriptions are found in Table 1. The study took place along a grass-lined channel that was mowed 1 to 3 times per growing season, depending on the site. All three sites had a grass-lined “shelf” or mini-floodplain along the stream where the study plots were located. The most downstream site, Franklin, was unique in having concrete walls and hard rock lining to the creek bottom in addition the vegetated streamside shelf. At all sites, woody species had established in spite of the mowing regime. Due to the “coppiced” growth form of these trees, seed production was minimal in the immediate Table 1. Location and other data for study sites along Onondaga Creek. Sub-basin area and land-cover figures are based on data prepared by M. Hall (SUNY ESF, Syracuse, NY) using NLCD data (USGS). “Urban land cover” is described in the text. Sites Nedrow Seneca Franklin Latitude 42.972188N 43.004248N 43.055651N Longitude 76.150713W 76.149318W 76.158262W Upstream contributing area (km2) 233.7 248.3 321.9 % upstream contributing area in urban land cover 6.84 32.7 60.5 Mowing regime 1x/year (June) 1–2x/year 2–3x/year Channel lining Grass Grass Rock Northeastern Naturalist 306 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 riparian area. Riparian soil texture at all three sites was almost uniformly a sandy loam. Overflow frequency affects seed delivery via hydrochory, so we estimated the number of times the channel overtopped the stream banks at each site. At least some portion of the plots was inundated at Nedrow on 17 days, at Seneca on 11 days, and at Franklin on 57 days for a total of 3.3%, 2.1%, and 11.0%, respectively, of the 520-day study period. Figure 1. Onondaga Creek watershed. Map © Onondaga Environmental Institute (OEI). Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 307 At Nedrow, floodplain vegetation consisted of grasses, forbs, and woody plants such as Fraxinus pennsylvanica (Green Ash) and Platanus occidentalis (American Sycamore). At Seneca, Lolium arundinaceum (Schreb.) S.J. Darbyshire (Tall Fescue) dominated the herbaceous layer. Cornus amomum (Silky Dogwood), Green Ash, and Juglans nigra (Black Walnut) were among the woody plants present at Figure 2. The three study sites (green triangles) along Onondaga Creek. Northeastern Naturalist 308 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 this site. Tall Fescue, Japanese Knotweed, Salix fragilis L. (Crack Willow), and Eastern Cottonwood all occurred near the stream at the Franklin site. Urban-to-rural gradient. To quantify the rural-to-urban gradient, we estimated percent urban cover. Land-cover analyses were conducted using ArcGIS for all subwatersheds of the creek. We used these data to help meet study objective 3, relating results to increasing urbanization. Urban cover values for areas included in this study are shown in Table 1. Urban cover is defined according to the NLCD 1992/2001 Retrofit Change Product, using modified Anderson Level 1 class codes and descriptions (MRLC 2011). Urban cover includes developed open spaces as well as areas occupied by single-family housing units, multifamily housing units, and retail, commercial, and industrial uses. The study sites follow a gradient of urbanization. However, while we use “site” effects as an indication of what might happen along an urbanization gradient, our results cannot confirm an urbanization effect without true replication. We use our “site” effects to highlight site differences, but caution that they indicate only a potential pattern of differences along an urbanization gradient. Plot sampling: treatment plots Statistical design. The study was set up as split plots in a completely randomized design (CRD), with 3 whole-plot treatments per site, each consisting of a collection of 1-m2 subplot treatments (factorial treatment design). Intially 5 different substrate treatments were applied, each one selected to mimic, on a smaller scale, conditions that could occur in a restoration project. Treatments were as follows: • Scarify soil. We removed turf to expose bare mineral soil. Preparation of 1-m2 scarified plots took place in mid-May 2006 mimicking the timing of sand- and gravel-bar formation that follows recession of spring floods. A second set of scarified plots was prepared in mid-May 2007. • No mow. These plots were left unmowed for the duration of the study. • Add plants: American Elm. A single elm sapling, approximately 50 cm tall, was added to the center of the plot after application of herbicide. • Add plants: Black Willow. Five 20” dormant Black Willow stakes (cuttings) were added per plot after application of herbicide. Because greenwood stakes did not root in 2006, dormant stakes were used to replace them in 2007. • Control: mow. The 1-m2 control plots were mowed according to the local municipal maintenance schedule, but by the first author to avoid erroneous removal of plant cover. Mowing was the default management treatment and so was considered as the background or control for these sites. Collectively, one of each type of the 1-m2 treatment plots were considered to make a “whole plot” for the purpose of statistical design; the whole plots were replicated three times at each site (Fig. 3). As a result, each site had 3 “whole plots” each made up of 5 individual 1-m2 (small) treatment plots. In the 1-m2 plots where trees (willow and elm) were added, existing vegetation was cleared by application of Roundup herbicide (Monsanto Co., St. Louis, MO) Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 309 before planting. A new set of scarified 1-m2 plots was added at each site during the second field season, raising the number of 1-m2 treatment plots from 5 to 6 per whole plot in 2007. To compare treatments, we used a plant species’ aerial cover as the response variable, i.e., a modified Daubenmire cover-class system (Daubenmire 1959). Plant names follow Mitchell and Tucker (1997). For woody plant recruitment, density of seedlings of individual species became the response variable. Seedbank study. The seedbank study used the same split-plot array as described above, except that the collection of soil samples occurred at only 2 subplots, representing the two elevations, high and low. Five soil cores, each 10 cm in diameter, were collected from each elevation on either side of the stream per whole plot for a total of 60 cores per site (Fig. 3). Where possible, soil was collected from two elevations, high and low. Low elevations were those subject to periodic overflows, as identified by driftlines of debris or actual observations of water levels in the stream. High elevations were not subject to the same frequency of stream overflows and occurred above driftlines in a drier zone. Sampling depth was limited to the upper 5 cm of soil because this is the location of most of the seedbank available for germination (Wilson et al. 1993). The resulting volume per core was approximately 400 cm3 (392.75 cm3), and the volume per sample (i.e., composite of all ten cores representing one elevation in a whole-plot) totaled ≈3930 cm3. All soil sampling occurred 15–22 May 2007 in order to focus Figure 3. Schematic of plot layout at a study site. Northeastern Naturalist 310 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 collection on the persistent seedbank. Soil samples were kept refrigerated and moist until 11 June 2007 when they could be placed in the greenhouse. We used the seedling-emergence method to estimate numbers of viable seeds. Each soil sample was sifted through 1.27-cm (0.5-inch) mesh to remove rocks, rhizomes, and root pieces, then spread out to a depth of 1 cm on 4 cm of sterile planting medium before being set out in the greenhouse. Seedlings were counted for a 6-month period between 11 June and 11 December 2007. Soil in trays was kept moist by manual watering as needed. Greenhouse temperatures varied according to season and time of day, with a range of approximately 16–30. °C, and nearly all light was solar (i.e., we did not add artificial grow lights). Plants were removed upon identification in order to reduce competition in trays. Unknown seedlings were potted separately and allowed to flower or otherwise reach a stage of development at which they could be identified. Unknown species were identified using Gleason and Cronquist (1991) with supplemental assistance from Newcomb (1977) and Uva et al. (1997). Two control trays containing only sterilized soil were used to detect contaminant seeds. Parietaria pensylvanica Muhl. ex Willd. (Pennsylvania Pellitory), Cardamine pensylvanica Muhl. ex Willd. (Pennsylvania Bittercress), Oxalis spp. (Woodsorrel), Populus spp., and Salix spp. were identified as green house “weeds” in this way and disqualified from data analyses. Data analysis To provide a measure of dominance and the relative contribution of a plant to the structure of the community, we used relative cover as a metric. Further comparisons between treatments, such as woody seedling density, were made using analysis of variance (ANOVA) for split plots in a CRD. We analyzed these data using SAS software, version 9.1.3 (SAS 2002–2003). Where necessary (as with much of the woody seedling data), we square root transformed data as a remedy for non-normality and heterogeneous variance (Kuehl 2000). We made post-hoc comparisons using least square means test; alpha value for all statistical tests was 0.05. We used the restricted maximum likelihood (REML) algorithm as the default variance-component–estimation procedure used for mixed models in SAS (Littell et al. 1996). We used species frequency, the number of plots in which a species appears (Wilson et al. 1993), to indicate which plants were most common throughout the areas sampled, in contrast to those species seldom encountered. We also recorded community indicators for plants such as wetland-indicator status (based on USDA 2008), species origin (alien or native; USDA 2008), and whether or not the plant was considered invasive (Invasive.org 2009). Results Treatment plots Post-treatment, a total of 86 species was found in the 1-m2 field plots during the two seasons of the study at the three sites along Onondaga Creek (Supplemental Appendix 1, available online at https://www.eaglehill.us/NENAonline/suppl-files/ Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 311 N21-2-983-Landis-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/ N983.s1). Numbers of alien (non-native) species (45, or about 52%) slightly exceeded those of natives (37, or 43%). Percentage of alien species increased from 34% to 40% to 51% of total species richness at the three sites along the rural–urban gradient (Table 2). Taken altogether, the three sites sampled were dominated by herbaceous plants, especially grasses—not a surprising result for an area maintained as a grass-lined channel. This result was true of the pre-treatment transects as well as the treated plots. Woody plants, however, appeared in the treatment plots at frequencies—if not abundances— comparable to the herbaceous ones. The eleven most common plants are listed in Table 3. The list includes two native riparian trees, Green Ash and Acer negundo (Boxelder). Woody plant recruitment. The seedlings included in these counts were limited to first-year germinants for either 2006 or 2007. For nearly all woody species, the data were marked by considerable variability from year to year. Among the most common woody seedlings were native riparian species: Green Ash = 113, Eastern Cottonwood = 53, and Boxelder = 55 seedlings (Table 4). Eleven seedlings of the nonnative Ailanthus altissima (Tree of Heaven) appeared but only in plots at the most urban site, Franklin, over 2006–2007. European Buckthorn, a calciphilic species that has invaded entire sections of Onondaga Creek’s historic floodplain, did germinate in the streamside plots Table 3. Plant species frequency for 2006–2007, with data combined for treatment plots along Onondaga Creek. Total plots = 98. Native riparian trees are highlighted with *. Plant Frequency Relative frequency (%) Tall Fescue 57 58.16 Purple Loosestrife 57 58.16 Bedstraw 26 26.53 Narrowleaf Plantain 26 26.53 Green Ash* 25 25.51 Clasping-leaved Dogbane 24 24.49 Unknown grass 23 23.47 Queen Anne’s Lace 22 22.45 Scouring Rush 21 21.43 Yellow Foxtail 19 19.39 Boxelder* 16 16.33 Table 2. Community indicators for treatment plot study 2006–2007. Nedrow is most rural, Franklin most urban. Wetland-indicator status based on USDA (2008). Obligate wetland species (OBL) almost always occur in wetlands. Facultative wetland (FACW) species usually occur in wetlands but may appear in non-wetlands. Species are categorized as invasive based on Swearingen (2008) and alien based on USDA (2008). Site Richness FACW/OBL Invasive Alien % Alien Nedrow 47 18 3 16 34.0 Seneca 48 13 5 19 39.6 Franklin 51 9 3 26 51.0 Northeastern Naturalist 312 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 but at numbers (12 individuals total) well below most of the native woody species. The only shrub represented was Cornus sp. (dogwood) with 6 new individuals. No Salix sp., American Sycamore, or Acer saccharinum L. (Silver Maple) appeared in the plots, although these species are present at least sporadically throughout the stream corridor. For all woody species combined (Fig. 4a), woody plant recruitment declined from the rural to urban sites. Mean seedling density varied significantly across sites (P = 0.025), with the Franklin and Nedrow sites, representing opposite ends of the urban–rural spectrum, accounting for that difference (P = 0.021). Seedling densities at Seneca, the more centrally located site, did not vary significantly from those at either Nedrow or Franklin (Appendix 1). Mean seedling densities did vary significantly with treatment (P = 0.021), a result probably driven by the fact that cottonwoods only germinated on the scarified plots. No interaction effects between site and treatment were detected (P = 0.221). Eastern Cottonwood (Fig. 4b) appeared to germinate independently of site (P = 0.170), but was highly dependent on treatment as mentioned above, establishing only on exposed soils. Results also revealed site x treatment interaction for cottonwood. (P = 0.045). Treatment effects of scarification were magnified at the more Table 4. Total number of woody seedlings germinating in treatment plots in 2006 and 2007. The same set of plots was used in both years, with the addition of a second set of scarified plots in 2007. Plots were observed near the end of the growing season (August) for each year, and new germinants only were counted. Species Nedrow Seneca Franklin 2006 Boxelder 2 0 5 Norway Maple 0 0 0 Tree of Heaven 0 0 8 Dogwood (Cornus spp.) 1 0 0 Green Ash 14 2 0 Eastern Cottonwood 19 14 6 Cherry (Prunus spp.) 0 0 0 European Buckthorn 3 0 0 Poison Ivy 0 0 0 Unknown woody seedling 0 0 0 Total 39 16 19 2007 Boxelder 30 18 0 Norway Maple 0 2 0 Tree of Heaven 0 0 3 Dogwood (Cornus spp.) 2 3 0 Green Ash 99 19 1 Eastern Cottonwood 11 3 0 Cherry (Prunus spp.) 0 2 0 European Buckthorn 7 2 0 Poison Ivy 3 0 0 Unknown woody seedling 0 1 0 Total 152 50 4 Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 313 rural site. In contrast to cottonwood, mean seedling densities for Green Ash did not vary with treatment. Total woody seedling establishment in the plots did vary significantly by year for the two field seasons 2006 and 2007 (P = 0.030). Numbers of cottonwood seedlings showed no significant difference from year to year while mean seedling density of ash did increase (P = 0.005) from 2006 to 2007. Values for Boxelder (Table 3) also increased significantly from 2006 to 2007 ( P = 0.003). Figure 4. Mean seedling density (± standard error) of plants germinated in treatment plots, 2006 and 2007. (a) Results for all species combined. (b) Eastern Cottonwood. This tree germinated only on scarified plots, regardless of site. (c) Green Ash. Ash showed site effect, but no treatment effect. All data were analyzed using Least Square Means test for mixed models at α = 0.05. Northeastern Naturalist 314 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 For all woody species combined, recruitment in the scarified plots was significantly higher than the no-mow and mow treatments (Fig. 5). Remaining treatment differences were not statistically significant. Mean woody seedling density was actually highest in the elm plots at 5.55 seedlings/m2, but this result was due largely to a single elm plot in which 74 elm seedlings were counted in September 2007. Addition of an elm or a willow seemed to have no significant effect on plant colonization in these plots. Seedbank A total of 2355 seedlings emerged during the 6-month duration of the seedbank study. Mean seedling density was 199.9 seedlings/m2 across all sites, and ranged from a low of 20.4 seedlings/m2 at an upper elevation plot at Seneca, to 863.3 seedlings/ m2 at an upper-elevation plot at Nedrow. Over one hundred vascular plant species emerged from the soil samples collected along Onondaga Creek (Supplemental Appendix 2, available online at https://www.eaglehill.us/NENAonline/suppl-files/N21-2-983-Landis-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/N983.s1). Most were identified to the species level, except one Juncus, one Cyperus, and seven of unknown identity. Of the unknowns, most could at least be assigned to a family. Of the 102 species, 45 (44%) were native to North America, 55 (54%) were introduced species, while 2 were classified as both (NI; USDA 2008). The number of wetland-indicator species (FACW and OBL combined) was 28. Seven woody species emerged. When examined by site, the seedbank data revealed a pattern similar to that observed in the field-treatment plots along the rural-to-urban gradient. Percentage of alien species increased from 44.8 to 64.7, while numbers of wetland-indicator species dropped substantially, from 22 at Nedrow to 3 at Franklin. The ten most abundant species represented a mixture of wetland, oldfield, and generalist species. Lythrum salicaria (Purple Loosestrife) outnumbered all other Figure 5. Recruitment (± standard error) by treatment for all woody plant germinants across all three sites in the study. Means having the same letter are not significantly different. Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 315 species; in fact, its seedlings made up nearly one quarter (564/2355 = 23.95%) of all germinants and was the most frequently encountered species, germinating in 25/30 soil samples. Of the 177 Juncus articulatus (Jointleaf Rush) that emerged, 151 were found in a single sample (from Nedrow, WP 1, low elevation). Only 11 seedlings of 7 trees and shrubs species appeared. Of those 11 plants, 6 were Rubus occidentalis (Black Raspberry). There were no significant differences detected between seedling density at low versus high elevations (least significant means test: P = 0.846; Appendix 2). Significant differences did appear, however, among seedling densities at sites, with Nedrow being higher than both Franklin and Seneca (P = 0.015; Fig. 6). In addition to vascular plants, a number of bryophytes appeared during the course of this study (Supplemental Appendix 3, available online at http://www. eaglehill.us/NENAonline/suppl-files/N21-2-983-Landis-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/N983.s1). Since none of these mosses appeared in the control trays, these were probably not spore contaminants, but rather from the soil samples collected along Onondaga Creek. The moss flora included mainly disturbance-tolerant species such as Ephemerum crassinervum, Funaria flavicans, and Physcomitrium pyriforme. Physcomitrella patens, a typical floodplain bryophyte, also appeared. Some of these mosses could have emerged from an actual persistent diaspore bank (Leyer 2006), and others from spores shed in early spring the same year (2007) that soils were collected (soil samples were taken in mid-May). Discussion Plant community: Treatment effects We hypothesized that colonization by both herbaceous and woody species would be highest in scarified plots, a projection borne out by the data. Remaining Figure 6. Seedling density (per m2; (± standard error) by site and elevation for all species that germinated in seed bank study. Means having the same letter (a, b) are not significantly different. Northeastern Naturalist 316 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 treatment differences were not significantly different from the mowed control. Results also supported the hypothesis that numbers of native riparian species decrease with increasing urbanization, as other studies have shown (e.g., Burton et al. 2005, Kowarik 1995). Facilitation. Addition of an elm or a willow sapling or stake seemed to have no significant effect on plant colonization in these plots. These non-significant results, however, should be regarded as inconclusive rather than negative, for several reasons. First, the Black Willow greenwood cuttings did not develop roots and grow during the first year, and the second-year planting did not have sufficient time to influence establishment of other species. Second, the herbicide used in the facilitation treatment plots was applied in early spring before all herbaceous plants had emerged. Roundup is only effective on actively growing plants, and not effective as a preemergence herbicide. As a result, living vegetations was not eliminated from some plots, making it difficult to assess effects of adding willows or an elm on a “new” plant assemblage. Finally, facilitation effects resulting from addition of woody plants could require a longer time period than the two field seasons available for this study. Woody plant recruitment In the course of this study, Eastern Cottonwood, Green Ash, and Boxelder germinated in streamside plots under substrate conditions suitable to each species. Recruitment occurred even under the current hydraulic conditions at all points surveyed along Onondaga Creek. However although a critical first step, these seedlings would take many years to form a mature riparian forest (Grubb 1977). In this study, Green Ash readily colonized diverse riparian substrates except at the most-urban site. Green Ash appears to be a versatile generalist colonizer in a range of riparian zones from rural to urban, germinating in far greater numbers than Silver Maple, for example, or American Sycamore in this system. Mortality as a result of feeding by Agrilus planipennis Fairmaire (Emerald Ash Borer), however, may soon eliminate Green Ash as a viable member of the floodplain forest (Herms et al. 2004, Knight et al. 2007). Woody plant recruitment and level of urbanization. In general, woody plant recruitment decreased along the rural-to-urban gradient. This decline could result from reduced seed sources in urban areas or it could be related to the more erosive flows typical of urban stream hydrology. One exception to this pattern, Eastern Cottonwood, appeared to recruit equally well regardless of site. In the case of cottonwood, seed source was present within 50 m of the most-urban site, and these trees are likely candidates for parentage of the cottonwood seedlings appearing in the study plots. In addition, cottonwood seeds may disperse long distances by wind or, especially, water (Van Haverbeke 2008). Cottonwoods also produce copious amounts of seed, and good seed crops are the rule. Estimates of annual seed production of a single open-grown tree have been as high as 48 million seeds (Van Haverbeke 2008). Such fecundity and wind dispersal of seeds can translate into seeds reaching more unoccupied gaps compared to other species (Thompson et al. 2002). Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 317 Numbers of invasive woody plants recruited during the course of this study were relatively few—11 individuals of Tree of Heaven, 2 Norway Maple, and 12 European Buckthorn—compared to native woody species. These low numbers are in contrast to the herbaceous flora, of which invasives, or at least aliens, appear to make up a greater proportion (based on both aerial cover and seedbank data). Few individuals of European Buckthorn germinated in the treatment plots, despite the fact that this invasive plant has colonized entire segments of the historic floodplain along Onondaga Creek. Seedbank Few studies have examined seedbanks along an urban stream. It is doubtful that we would find an “ancient” seedbank still present, previously deposited during the accretion of the natural floodplain (Goodson et al. 2002), because channelization has interrupted accretion and literally moved the stream to another position in the valley. Channel construction along Onondaga Creek also involved the addition of much fill transported from other locales (R. Peiffer, retired engineer who worked on the channelization project, pers. comm.). Nonetheless these manipulated soils may have accumulated seeds and spores that could be relevant to restoration and/ or different management schemes. Fluvial processes of deposition, transport, and erosion continue even in a channelized stream. Overflows onto the instream shelf built into flood-control channels such as Onondaga Creek deposited seeds that were incorporated into the riparian seedbank. Results revealed a seedbank heavily dominated by herbaceous species at urban as well as more rural sites. These included important invasives such as Purple Loosestrife. The presence of such weedy species suggests that the seedbank may not be a reliable source of plants for restoration and may actually have adverse economic or environmental impact (Pysek et al. 2004). Herbaceous weedy species could interfere with survival of native trees. However, in the case of Purple Loosestrife, Galerucella calmariensis L. (Loosestrife Leaf Beetle) was observed feeding on it throughout the Onondaga Creek corridor, and could provide some check on growth and reproduction of this prolific seedbank invasive. Only six tree seedlings appeared among the >2000 plants that germinated in the seedbank study. This small number may reflect an absence of viable tree and shrub propagules in soils along Onondaga Creek. Because canopy trees along these mowed reaches have been removed, the numbers of seeds may be reduced compared to intact riparian forests. Also, due to the dynamic conditions of the floodplain environment, the persistent-seedbank strategy might not be employed by riparian plants. Silver Maple, cottonwood, and willows produce seeds with dispersal and short-term viability that coincide with the recession of spring high flows (Van Haverbeke 2008). The seedbank did generate a remarkable diversity of plants, including wetland indicators (18 FACW or OBL species appeared at Nedrow, the most rural site). Unfortunately, few of these wetland species appeared to be incorporated into the riparian seedbank at downstream, more-urban sites. Only 3 wetland indicator Northeastern Naturalist 318 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 species—Eupatorium maculatum (Spotted Joe-pye Weed), Spergularia salina (Saltmarsh Sand-spurrey), and Purple Loosestrife—were found at the most-urban site. Due to their prolific output of tiny seeds (Grime et al. 1988) and because rivers are known to move seeds over considerable distances (Goodson et al. 2002), we might expect rushes (Juncus spp.), for example, that occur in the vegetation to colonize riparian areas. However, although Jointleaf Rush appeared in 11/30 of the seedbank samples, including urban ones, indicating transport, no Jointleaf Rush plants were observed growing at the more downstream sites. Low numbers of wetland plants found in the urban seedbank samples could also reflect “flashy” flows experienced by city streams. Both the channel morphology and vegetation of the riparian zone play important roles in controlling fluvial deposition of seeds and sediment, and thus of seedbank characteristics. If these conditions are not favorable to deposition of flood-borne debris, any viable seeds could simply continue downstream with the flow. In addition, variation in the seedling-emergence patterns could have been influenced by experimental methods. Seedbanks may vary seasonally (Thompson and Grime 1979), so results would have differed had we collected the soil samples earlier to document the transient component, e.g., Impatiens capensis (Jewelweed). The seedling-emergence method can underestimate seedbank composition because germination requirements may not be met for all plant species present (Brown 1992). The six-month emergence period may have been too short for some plants, such as those requiring a double dormancy. Also, the seedbank composition reflects the location sampled, and it would be interesting to sample a lower elevation zone, even closer to the stream than the shelf chosen for the “low” samples. During very low flows, Eleocharis (spikerush), Bidens (beggar-ticks), Juncus (rushes), and other wetland herbs appeared at this basal elevation within the grass-lined reaches of channel (C.L. Landis, pers. observ.). Soils at these elevations may have greater interaction with fluvial dynamics and therefore different seedbank composition. Management implications Plans are already underway to implement green infrastructure in the Onondaga Creek urban watershed as a means of reducing stormwater runoff, thus reducing combined sewer overflow (CSO) discharge to Onondaga Creek and Onondaga Lake (Onondaga County 2001–2013; OEI 2009). In designing these projects, and in species selection, data suggest species that could function as propagule sources for local riparian restoration. Depending on site hydrology, these could include wetland plants such as rushes, sedges, spikerushes, and Eupatorium spp., as well as selected woody plants including Eastern Cottonwood, Green Ash, and Boxelder. Species could be chosen not only for their fit to the site but also for aesthetics and for a potential role in passive restoration of riparian vegetation both along the stream’s main corridor as well as tributaries. To replenish a depauperate urban seedbank, seed sources should be provided in urban and suburban areas via plantings and gardens, with an eye to re-connecting Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 319 systems via tributaries and reducing dispersal distances along the main channel. As much as possible, physical processes and morphologies that support plant community development should be restored. This approach requires rethinking the form of urban design to work with natural processes and the regenerative capacity of soils and ecosystems (Ferguson et al. 2001). Acknowledgments Thanks to Ted Endreny, Karin Limburg, John Stella, Karen Missell, and Steve Stehman for extended discussions of this research and for their review of this manuscript. Funding was provided by the US EPA (to T. Endreny and D. Leopold) and The Edna Bailey Sussman Foundation. We also thank the Onondaga Environmental Institute for assistance. The detailed comments of independent reviewers much improved this paper, and to them we extend our thanks. Keith Bowman helped identify bryophytes. Literature Cited Bernhardt, E.S., and M.A. Palmer. 2007. Restoring streams in an urbanizing world. Freshwater Biology 52:738–751. Brown, D. 1992. Estimating the composition of a forest seedbank: A comparison of the seed-extraction and seedling-emergence methods. Canadian Journal of Botany 70:1602–1612. Burton, M.L., L.J. Samuelson, and S. Pan. 2005. Riparian woody plant diversity and forest structure along an urban–rural gradient. Urban Ecosystems 8:93–106. Daubenmire, R. 1959. A canopy-coverage method of vegetational analyses. Northwest Science 33:43–64. Ferguson, B., R. Pinkham, and T. Collins. 2001. Restorative redevelopment: The Nine Mile Run model. Stormwater July/August 2001. Available online at www.stormh2o.com. Accessed 3 January 2007. Gleason, H.A., and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. 2nd Edition. The New York Botanical Garden, Bronx, NY. 910 pp. Goodson, J.M., A.M. Gurnell, P.G. Angold, and I.P. Morrissey. 2002. Riparian seedbanks along the lower River Dove, UK: Their structure and ecological implications. Geomorphology 47:45–60. Groffman, P.M., D.J. Bain, L.E. Band, K.T. Belt, G.S. Brush, J.M. Grove, R.V. Pouyat, I.C. Yesilonis, and W.C. Zipperer. 2003. Down by the riverside: Urban riparian ecology. Frontiers in Ecology and the Environment 1:315–321. Grime, J.P., J.G. Hodgson, and R. Hunt. 1988. Comparative Plant Ecology: A Functional Approach to Common British Species. Unwin Hyman, London, UK. Grubb, P.J. 1977. The maintenance of species-richness in plant communities: The importance of the regeneration niche. Biological Reviews 52:107–145. Gurnell, A.M., A.J. Boitsidis, K. Thompson, and N.J. Clifford. 2006. Seedbank, seed dispersal, and vegetation cover: Colonization along a newly created river channel. Journal of Vegetation Science 17:665–674. Herms, D.A., A.K. Stone, and J.A. Chatfield. 2004. Emerald Ash Borer: The beginning of the end of ash in North America? Special Circular No. 193. Ohio Agricultural Research and Development Center,Wooster, OH. Pp. 62–71. Northeastern Naturalist 320 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 Hession, W.C., T.E. Johnson, D.F. Charles, D.D. Hart, R.J. Horwitz, D.A. Kreeger, J.E. Pizzuto, D.J. Velinsky, J.D. Newbold, C. Cianfrani, T. Clason, A.M. Compton, N. Coulter, L. Fuselier, B.D. Marshall, and J. Reed. 2000. Ecological benefits of riparian reforestation in urban watersheds: Study design and preliminary results. Environmental Monitoring and Assessment 63:211–222. Invasive.org. 2009. WeedUS: Database of Plants Invading Natural Areas in the United States. Center for Invasive Species and Ecosystem Health, University of Georgia.Available online at http://www.invasive.org/species/list.cfm?id=76. Accessed 4 March 2007. Knight, K.S., R.P. Long, A. Smith, K. Gandhi, and D.A. Herms. 2007. How fast will the trees die? A transition matrix model of ash decline in forest stands infested by the Emerald Ash Borer. Pp. 29–30, In V. Mastro, R. Reardon, and G. Parra (Eds.). Proceedings of the Emerald Ash Borer Research and Technology Development Meeting; 23–24 October 2007; Pittsburgh, PA. FHTET 2008-07. USDA Forest Service, Forest Health Technology Enterprise Team Morgantown, WV, . Kowarik, I. 1995. On the role of alien species in urban flora and vegetation. Pp. 85–103, In P. Pysek, K. Prach, M. Rejmanek, and P.M. Wade (Eds.). Plant Invasion—General Aspects and Special Problems. SPB Academic, Amsterdam, Netherlands. Kuehl, R.O. 2000. Design of Experiments: Statistical Principles of Research Design and Analysis. Duxbury Press, Pacific Grove, CA. Leyer, I. 2006. Dispersal, diversity, and distribution patterns in pioneer vegetation: The role of river-floodplain connectivity. Journal of Vegetation Science 17:407–416. Littell, R.C., G.A. Milliken, W.W. Stroup, and R.D. Wolfinger. 1996. SAS System for Mixed Models. SAS Institute Inc., Cary, NC. 633 pp. Mitchell, R.S., and G.C. Tucker. 1997. Revised checklist of New York State plants. New York State Museum Bulletin No. 490. State Education Deptartment, Albany, NY. Multi-Resolution Land Characteristics Consortium (MRLC). 2011. National Land Cover Database (NLCD) 1992/2001 Retrofit Change Product, using modified Anderson Level 1 class codes and descriptions. Available online at http://www.mrlc.gov/faq.php. Accessed 31 October 2012. Newcomb, L. 1977. Newcomb’s Wildflower Guide. Little, Brown, and Co., New York, NY. Onondaga Environmental Institute (OEI). 2009. Onondaga Creek conceptual revitalization plan draft report. Available online at http://www.oei2.org/OEIResources_OCRPDRAFT. html. Accessed 13 January 2010. Onondaga County. 2001–2013. Save the Rain program. Available online at http://www. ongov.net/sustainability/water/str.html. Accessed 10 December 2010. Padilla, F.M., and F.I. Pugnaire. 2006. The role of nurse plants in the restoration of degraded environments. Frontiers in Ecology and the Environment 4:196–202. Paul, M.J., and J.L. Meyer. 2001. Streams in the urban landscape. Annual Review of Ecology and Systematics 32:333–365. Pysek, P., D.M. Richardson, M. Rejmanek, G.L. Webster, M. Williamson, and J. Kirschner. 2004. Alien plants in checklists and floras: Towards better communication between taxonomists and ecologists. Taxon 53:131–143. Riley, A. 1998. Restoring Streams in Cities. Island Press, Washington, DC. SAS Institute. 2002–2003. SAS version 9.1.3. SAS Institute, Cary, NC. Thompson, K., and J.P. Grime. 1979. Seasonal variation in the seedbanks of herbaceous species in ten contrasting habitats. Journal of Ecology 67:893–921. Thompson, K., L.C. Rickard, D.J. Hodkinson, and M. Rees. 2002. Seed dispersal: The search for trade-offs. Pp. 155–172, In J.M. Bullock, R.E. Kenward, and R.S. Hails (Eds.). Dispersal Ecology. Blackwell, Oxford, UK. Northeastern Naturalist Vol. 21, No. 2 C.L. Landis and D.J. Leopold 2014 321 Tickner, D.P., P.G. Angold, A.M. Gurnell, and J.O. Mountford. 2001. Riparian plant invasions: Hydrogeomorphological control and ecological impacts. Progress in Physical Geography 25:22–52. US Department of Agriculture (USDA). 2008. Plants database. Available online at http:// plants.usda.gov. Accessed numerous times. Uva, R.H., J.C. Neal, and J.M. DiTomaso. 1997. Weeds of the Northeast. Cornell University Press, Ithaca, NY. Van Haverbeke, D.F. 2008. Populus deltoides Bartr. ex Marsh., Eastern Cottonwood. Silvics of North America, volume 2. Available online at http://www.na.fs.fed.us/spfo/pubs/ silvics_manual/volume_2/populus/deltoides.htm. Accessed 25 January 2010. White, J., and J.C. Stromberg. 2005. Opportunities for passive restoration of the Salt River riparian corridor. University of Arizona Water Resources Research Center publications. Available online at http://www.ag.arizona.edu/AZWATER/research/2005/Stromberg. pdf. Accessed 11 March 2007. Wilson, S.D., D.R.J. Moore, and P.A. Keddy. 1993. Relationships of marsh seedbanks to vegetation patterns along environmental gradients. Freshwater Biology 29:361–370. Northeastern Naturalist 322 C.L. Landis and D.J. Leopold 2014 Vol. 21, No. 2 Appendix 1. ANOVA results for split plots, field treatment plots (woody plant recruitment) study: all woody seedlings (10 species) combined, Eastern Cottonwood, and Green Ash. Dependent variable = seedling density. Covariance parameter Effect Num df Den df F value Pr > F estimates All woody seedlings (10 species) Site 2 6 7.28 0.0248 0.0502 Treatment 5 75 2.84 0.0212 1.2110 Site*treatment 10 75 1.35 0.2211 1.2110 Eastern Cottonwood Site 2 6 2.41 0.1702 0.0000 Treatment 5 75 15.00 less than 0.0001 0.2588 Site*treatment 10 75 2.00 0.0451 0.2588 Green Ash Site 2 6 4.60 0.0616 0.1052 Treatment 5 75 0.64 0.6666 0.7441 Site*treatment 10 75 1.70 0.0972 0.7441 Appendix 2. ANOVA results for seedbank densities examining site, elevation and interactions. Dependent variable = seedling density. Covariance Effect Num df Den df F value Pr > F parameter estimates Site 2 6 9.27 0.0146 0 Elevation 1 15 0.04 0.8459 20705 Site*elevation 1 15 1.50 0.2398 20705