Seedbank of Bingham cienega, a spring-fed marsh in southeastern Arizona.
RESUMEN--Tierra fue recaudada entre 2002-2003 de una cienega en el sudeste de Arizona para evaluar especies presentes como semillas en el banco de semillas de tierra. En 2002, la vegetacion dominante en la cienega era junco (Schoenoplectus pungens). Debido a una sequia en el pantano y por otros cambios hidrologicos, la vegetacion en el sitio del estudio ha cambiado a monocultivo de girasol (Helianthus annuus). En experimentos de invernadero, un total de 20 especies germinaron en el banco de semillas de tierra. Los germinantes mas abundantes fueron junco (S. pungens), espadana (Typha domingensis) y Eleocharis macrostachya. La umbela acuatica de Huachuca (Lilaeopsis schaffneriana ssp. recurva), que esta en peligro de extincion, germino en varias muestras y mas semillas estuvieron presentes en la tierra. Durante el curso del estudio ocurrio una disminucion en la densidad total de las semillas, probablemente atribuible a la desaparicion de especies de pantano. La composicion de especies del banco de semillas y la vegetacion presente no estuvieron correlacionadas, lo cual es un resultado comun en estudios de bancos de semillas. Si la hidrologia es restablecida, predecimos que muchas de las especies de pantano van a germinar y recolonizar la zona, aunque esto se limita a la duracion de la vialidad de las semillas. Por lo tanto, el manejo del pantano debe incluir, tan pronto sea posible, el examen de la condicion hidrologica actual si el restablecimiento de especies de pantano, particularmente la umbela acuatica de Huachuca, es deseado.
Seedbanks include all viable seeds present on or in the soil and litter, and they are vital to maintenance of many plant communities (Simpson et al., 1989; Thompson et al., 1996). The importance of seedbanks to wetland communities is well documented (e.g., Leck, 1989). High densities of seeds have been reported from many herbaceous freshwater wetlands (van der Valk and Davis, 1978; Keddy and Reznicek, 1982; McGraw, 1987). Seedbanks can determine successional trends and they can buffer against disturbance (van der Valk and Davis 1978; Leck, 1989; Tsuyuzaki, 1989, 1991). In some communities, assessment of soil seedbanks can predict later stages of succession (Grime, 1989). However, if plant communities are determined largely by initial floristic composition (sensu Egler, 1954), which can be based substantially on chance events (Harper, 1977), then current composition of seedbanks might play only a minor role in determining future patterns of dominance (van der Valk and Davis, 1978; Grime, 1989).
At early stages of succession, composition of species present in both standing vegetation and seedbank might be similar, with both lacking species characteristic of later stages of succession due to seed-dispersal limitations. Therefore, a strong correlation between vegetation and seedbank could indicate that the vegetation is in an early stage of development rather than in a climax stage (Fenner, 1985; Tu et al., 1998). The correlation between vegetation and seedbank might be poor in later stages of succession because pioneer species with persistent seeds remain dominant in the seedbank (Fenner, 1985; Willems, 1988; Unger and Woodell, 1993; Thompson et al., 1994). However, if pioneer species are absent from the seedbank, the correlation might be high (Willems, 1995). Alternatively, the seedbank could better reflect recent deposition of seeds, whereas the standing vegetation reflects establishment.
The term cienega refers to mid-elevation wetlands or marshes in southwestern desert regions. Once relatively widespread, these habitats have been fragmented and greatly reduced over the past century (Hastings, 1959). Hendrickson and Minckley (1984) asserted that community associations in cienegas progress from a complex association of factors, but once achieved, are stable and persistent aquatic-climax communities. An understanding of seedbanks is critical to elucidate establishment of species and the means to conserve and restore wetland systems. Seedbanks of cienegas are poorly understood, and we know of no published study addressing the topic.
The purpose of our study was to examine species composition of the seedbank of a cienega undergoing severe drought and to elucidate the correlation between the seedbank and standing vegetation. In addition, we examined whether an endangered wetland plant present in the study area was contributing to the seedbank.
Bingham Cienega Preserve (855 m elevation) is a 115-ha tract within the floodplain of the lower San Pedro River just north of the confluences of Redfield Canyon, which drains the Galiuro Mountains to the east, and Edgar and Buehman canyons, which drain the Santa Catalina Mountains to the west. Climate is semiarid, with an annual precipitation of ca. 35 cm, most of which falls during winter and summer (National Oceanic and Atmospheric Administration, http://cdo.ncdc.noaa.gov/climatenormals/ clim60/states/Clim_AZ_01.pdf). The preserve is owned by Pima County Flood Control District and managed by The Nature Conservancy under a long-term contract. The site was used for farming and cattle grazing for over a century, and wetlands were ditched and bermed to partially drain portions of the land for agriculture. In 1989, Pima County Flood Control District purchased the property and began efforts at hydrologic restoration by breaching the berm and planting native riparian trees and big sacaton (Sporobolus wrightii) in portions of the former agricultural fields. Soils at the preserve were characterized as Glendale silt-loam (Natural Resources Conservation Service, http:// websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey. aspx). The Glendale Series consists of deep soils formed in mixed alluvium on stream terraces and alluvial fans. Our study site was along the western edge of the former agricultural fields, where spring flow supported a cattail-bulrush (Typha-Schoenoplectus) marsh. A spring on the preserve provided supplementary water that supported riparian-deciduous forest, mesquite (Prosopis) bosque, and extensive sacaton grasslands and cienega wetlands. In September 2002, the cienega wetlands were dominated by bulrush (Schoenoplectus pungens) and cattail (Typha domingensis). Due to prolonged drought and perhaps due to other hydrological changes, such as withdrawals of groundwater, the entire cienega, including the study plots, experienced a dramatic change in aboveground vegetation over the course of the study. Ultimately (2005-2006), much of the cienega was devoid of vegetation and some areas were dominated by monospecific stands of the upland sunflower, Helianthus annuus.
In May 2001, Huachuca water umbel (Lilaeopsis schaffneriana ssp. recurva; Apiaceae) was discovered in Bingham Cienega (P. J. Titus, in litt.). It is a rare, herbaceous, semi-aquatic to fully aquatic perennial plant endemic to southeastern Arizona and northern Sonora, Mexico. The plant is clonal and produces weak, spreading, underground rhizomes. Fruits are relatively large (2 by 2.5 mm), corky, and buoyant (Affolter, 1985) and, for this reason, hydrochory is expected to be the main dispersal mechanism. The taxon was federally listed as endangered on 6 January 1997 (United States Fish and Wildlife Service, 1997). The listing was considered necessary because of threats posed by degradation and loss of wetlands in the region.
During summer 2003, a fire burned in and around Bingham Cienega. It consumed much of the vegetation on nearby slopes and dried bulrush and cattail stems and thatch in the marsh. Subsequent rains washed a large quantity of upland sediment into the cienega. Depth of sediment in study plots was 4.6 [+ or -] 2.7 cm (mean [+ or -] SD, n = 12) on 11 September 2003.
Percent cover of each species in the standing vegetation was determined by visual estimation on 12 September 2002, 30 January 2003, 11 September 2003, 15 March 2005, and 6 January 2006 in each of six 1-[m.sup.2] plots in the northwestern part of the preserve, ca. 6-8 m from the wetland-upland boundary. These plots were established in August 2001 for a monitoring study of Huachuca water umbel (P. J. Titus, unpubl. data).
Seedbanks were estimated by using the emergence method (van der Valk and Davis, 1978; Benoit et al., 1989; Titus, 1991). From each of the six plots, four 7.2-cm-diameter cylinders 0-6.5 cm deep (volume = 260 mL) of soil including the litter layer were obtained on 12 September 2002, 30 January 2003, and 11 September 2003. Soil samples were passed through a 4-mm sieve to remove living rhizomes. The four samples were amalgamated and split in half to form two subsamples for each plot (12 subsamples for the six plots).
Collection of soil on 11 September 2003 occurred after the post-fire deposition of sediment. To determine which species might have entered the soil seedbank in this sediment, samples of both sediment and wetland soil below the sediment were collected separately.
Four holes 2 mm in diameter were drilled in the bottom of 12 surface-sterilized (1 h in 10% bleach) plastic trays 32 by 16 by 8 cm high. A layer of autoclaved potting soil 1.5 cm deep was placed in each tray and a 0.5-cm layer of autoclaved sand was added to the top of the potting soil. Soil samples were spread on top of the sand layer. Trays were placed in a greenhouse at Columbia University's Biosphere 2 Campus, Oracle, Arizona. Three control trays with potting soil and sand were placed with the seedbank trays. The seedbank collection on 12 September 2002 was maintained until 14 December 2002; the seedbank collection on 30 January 2003 was maintained until 8 May 2003; and the seedbank collection on 11 September 2003 was maintained until 8 December 2003. Trays were watered daily to maintain wet soil. Trays were rotated every 3 weeks, and after 7 weeks, the soil was disturbed to stimulate germination. Seedlings were counted and removed as soon as identifiable. Seedlings were checked to ensure they did not arise from rhizomes that might have passed through the sieve. Seedlings of all species were grown in the greenhouse to confirm identifications; however, a few seedlings (<1%) died before a definitive identification was possible. Nomenclature was based on the Integrated Taxonomic Information System (www.itis.gov).
Soil samples were checked for ungerminatedviable seeds by first passing soil samples through 2 and 0.212-mm sieves to remove large particles (ter Heerdt et al., 1996). Material remaining in sieves was searched with the aid of a stereomicroscope for viable seeds, as determined by a cross-section showing white and apparently healthy endosperm (ter Heerdt et al., 1996).
The emergence method has limitations. Germination accounts for an unknown proportion of seeds in the seedbank because the specific methods used might not provide requirements for germination of all species and searching though soils afterward could miss small-seeded species, such as Typha (Thompson and Grime, 1979; Jerling, 1983; ter Heerdt et al., 1996). Thus, we use the term seedbank to refer to readily germinated seedlings that emerged in the greenhouse and viable seeds that were collected in the soil.
In addition to the seedbank trials, soil from each plot was searched for the large and distinctive seeds of Huachuca water umbel in October 2001 and February 2002. Five 10-mL soil samples were collected from each of the six plots and thoroughly searched on sieves under a stereomicroscope for the seeds.
The mean number of seeds for each species from each plot was determined by averaging the two subsamples for each plot. Because the surface-collection area for each subsample was 1/ 122.8 [m.sup.2] (i.e., the area of one-half of the sample or two cans), each value was multiplied by 122.8 to approximate the number of seeds in a square meter. The mean [+ or -] SD was calculated for each species by averaging together the results from each of the six plots and determining the deviation. Thus, if only a single seedling was counted in only one seedbank tray, this would yield a density of 10 [+ or -] 25 seeds/[m.sup.2] for the species at that sampling date. The non-parametric Spearman's rank correlation was used to determine correlations between standing vegetation and composition of seedbank (SPSS 12.0, Apache Software Foundation, Forest Hill, Maryland).
Vegetation at Bingham Cienega at the first sampling date of September 2002 was dense and dominated by the obligate wetland species S. pungens (syn. Scirpus americanus; Table 1). In 2001, there was standing water in many of the plots. In September 2002, soils were moist below the surface, with no standing water present in the area. By January 2003, soils were dry, the ground surface was completely covered by thatch of S. pungens, and the only live aboveground vegetation consisted of a few small individuals of Polygonum punctatum. It is possible that the lack of vegetation was due to winter senescence. However, by September 2003, all aboveground vegetation was dead and fire had completely burned the thatch of S. pungens. Soils remained dry since that time, and plots were dominated by monospecific stands of H. annuus, which colonized the area in spring 2005.
In all of the seedbanks, the wetland species T. domingensis, Eleocharis macrostachya, and S. pungens were common (Table 2). Presence of these species in the sediment samples indicates that either soil-mixing occurred, which is likely during major erosional events, or seeds dispersed long distances from other wetlands along the San Pedro River. The upland species Medicago sativa and Eclipta prostrata were more abundant in sediment samples than in wetland samples. It is possible that a few individuals identified as M. sativa that never flowered might have been other species of legumes with similar leaves. The only species to have a large number of ungerminated but viable seeds was Cyperus odoratus, particularly in the seedbank for September 2002.
An overall decline in density of germinated seeds of wetland species from September 2002 to September 2003 would be expected due to loss of these species from the aboveground vegetation during the severe drought. Thus, viable seeds of wetland species in the seedbank in January 2003 and September 2003 were likely due to long-term persistence or long-distance dispersal.
Spearman's correlation tests between species composition of the seedbank and the standing vegetation were nonsignificant. Thus, species composition of the seedbank and the standing vegetation were not correlated. This result is common in studies of seedbanks (e.g., Tu et al., 1998; Valbuena and Trabaud, 2001). Seedbanks often contain seeds of species from nearby environments and different successional stages that are not represented in the standing vegetation. Furthermore, some species that are present in standing vegetation might not contribute to the seedbank. More species were evident in the seedbank than in the standing vegetation (20 versus 7). This is likely due to deposition of seeds from nearby vegetation. Bulrush and cattail might prevent establishment of these other species in the cienega due to deep thatch and dense roots. However, with drydown of the cienega and consequent absence of wetland species, H. annuus dominated the plots (Table 1).
Huachuca water umbel was present in vegetation sampling on September 2002, and several germinants emerged from the seedbank. In addition, 10 Huachuca water umbel seeds were found in soil samples for October 2001 (density 5 0.033 seeds/mL soil). No seed was detected in the soil collected in February 2002. The fact that seeds of water umbel were found in the soil and seedlings germinated in the seedbank implies that this species might persist until the hydrology is restored, because observations indicate that Huachuca water umbel seeds might remain viable for <10 years (P. J. Titus, unpubl. data). Germination of seeds of Huachuca water umbel of recent origin (<1 year old) in the greenhouse is >90% (Titus and Titus, 2008).
If hydrology of the cienega is restored to the study site, it is important to note that revegetation by long-distance dispersal is slower than by locally viable seeds already present in the system (Wood and del Moral, 1988). For this reason, plants whose seeds have more limited abilities to disperse and distant source populations, such as Huachuca water umbel, might not return to the site by natural means. Thus, it is important that the hydrology of Bingham Cienega is examined and restored to the extent necessary to support desirable native species, including species of conservation concern.
We thank M. Falk, J. Fonseca, and The Nature Conservancy for access to the study site and logistical assistance. Biosphere 2 students (S. Scott, K. Sovenyhazy, A. Zuhlke, M. Clinton, E. Robertson, K. St. Clair, P. Gallagher, A. Khishchenko, A. Reist, H. Garrett, L. Kirn, M. Evette, M. Hunter, E. Hoffman, A. Swetek, J. Bommorito, K. Bronson, A. Tweedell, J. Clancy, and J. Brenner) assisted in collections of soil and maintenance of the seedbank. Thanks to I. Vassoler for the Spanish translation.
Submitted 4 December 2006. Accepted 28 January 2008. Editor was Mark E. Eberle.
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JONATHAN H. TITUS * AND PRISCILLA J. TITUS
Biology Department, SUNY-Fredonia, Fredonia, NY 14063 (JHT, PJT)
* Correspondent: firstname.lastname@example.org
TABLE 1--Estimated percent plant cover in six 1-[m.sup.2] plots at Bingham Cienega, Pima Co., Arizona, during the three seedbank-sampling periods in 2002 and 2003 (mean [+ or -] SD) and two subsequent dates in 2005 and 2006. Soil moisture for each sampling date also is provided. Date Species September 2002 January 2003 Schoenoplectus pungens 99 [+ or -] 0 -- Eleocharis macrostachya 0.02 [+ or -] 0.04 -- Helianthus annuus - -- Lilaeopsis schaffneriana 0.05 [+ or -] 0.05 -- ssp. recuma Polygonum punctatum 0.02 [+ or -] 0.04 0.05 [+ or -] 0.05 Rumex crispus 0.05 [+ or -] 0.05 -- (nonnative species) Medicago sativa 0.02 [+ or -] 0.04 -- (nonnative species) Sonchus asper 0.03 [+ or -] 0.05 -- (nonnative species) Sisymbrium irio -- -- (nonnative species) Erodium cicutarium -- -- (nonnative species) Unknown seedlings 0.02 [+ or -] 0.04 -- Schoenoplectus thatch 83 [+ or -] 8 100 [+ or -] 0 Helianthus litter and -- -- standing dead Bare ground -- 1 [+ or -] 0 Soil moisture moist dry Date Species September 2003 March 2005 Schoenoplectus pungens 0.25 [+ or -] 0.37 0.2 [+ or -] 0.3 Eleocharis macrostachya -- - Helianthus annuus -- 9 [+ or -] 8 Lilaeopsis schaffneriana -- - ssp. recuma Polygonum punctatum -- - Rumex crispus -- - (nonnative species) Medicago sativa -- 0.04 [+ or -] 0.05 (nonnative species) Sonchus asper -- -- (nonnative species) Sisymbrium irio -- 0.6 [+ or -] 1.4 (nonnative species) Erodium cicutarium -- 0.1 [+ or -] 0.4 (nonnative species) Unknown seedlings 0.02 [+ or -] 0.04 0.4 [+ or -] 0.7 Schoenoplectus thatch 0.08 [+ or -] 0.04 3 [+ or -] 2 Helianthus litter and -- 31 [+ or -] 31 standing dead Bare ground 100 [+ or -] 0 69 [+ or -] 14 Soil moisture dry dry Date Species January 2006 Schoenoplectus pungens -- Eleocharis macrostachya -- Helianthus annuus -- Lilaeopsis schaffneriana -- ssp. recuma Polygonum punctatum -- Rumex crispus -- (nonnative species) Medicago sativa -- (nonnative species) Sonchus asper -- (nonnative species) Sisymbrium irio -- (nonnative species) Erodium cicutarium -- (nonnative species) Unknown seedlings -- Schoenoplectus thatch -- Helianthus litter and 83 [+ or -] 16 standing dead Bare ground 17 [+ or -] 16 Soil moisture dry TABLE 2--Number of emergent seedlings in 1 [m.sup.2] in the soil seedbank at Bingham Cienega, Pima Co., Arizona, during three sampling periods and in deposited sediments at the last sampling period (mean [+ or -] SD). Date Species September 2002 January 2003 Aster subulatus var. 154 [+ or -] 185 -- ligulatus Berula erecta 41 [+ or -] 50 -- Calibrachoa parviflora 2,794 [+ or -] 5,985 20 [+ or -] 50 Celtis pallida, -- 10 [+ or -] 25 Cirsium neomexicana 20 [+ or -] 32 -- Cyperus odoratus 1,003 [+ or -] 773 338 [+ or -] 177 Eclipta prostrata 41 [+ or -] 63 -- Eleocharis acicularis -- 123 [+ or -] 301 E. macrostachya 2,998 [+ or -] 1,952 1,791 [+ or -] 1,447 Helianthus annuus -- 20 [+ or -] 32 Juncus bufonius -- 41 [+ or -] 74 Lilaeopsis schafneriana -- 10 [+ or -] 25 spp. recurva Medicago sativa -- -- (nonnative species) Melilotus indica 389 [+ or -] 582 41 [+ or -] 100 (nonnative species) Nasturtium officinale 20 [+ or -] 32 -- (nonnative species) Polygonum punctatum 184 [+ or -] 174 20 [+ or -] 50 Polypogon monospeliensis 287 [+ or -] 672 -- (nonnative species) Schoenoplectus pungens 2,210 [+ or -] 2,210 1,382 [+ or -] 1,268 Typha domingensis 4,585 [+ or -] 2,721 348 [+ or -] 357 Veronica americana 399 [+ or -] 480 - Unknown 51 [+ or -] 46 20 [+ or -] 50 Date Sediment Species September 2003 September 2003 Aster subulatus var. -- -- ligulatus Berula erecta -- -- Calibrachoa parviflora 92 [+ or -] 101 82 [+ or -] 84 Celtis pallida, -- -- Cirsium neomexicana -- 20 [+ or -] 32 Cyperus odoratus 114 [+ or -] 72 72 [+ or -] 72 Eclipta prostrata -- 778 [+ or -] 1,451 Eleocharis acicularis 72 [+ or -] 119 31 [+ or -] 51 E. macrostachya 2,405 [+ or -] 1,161 461 [+ or -] 489 Helianthus annuus -- - Juncus bufonius 266 [+ or -] 456 82 [+ or -] 201 Lilaeopsis schafneriana 51 [+ or -] 72 -- spp. recurva Medicago sativa 78 [+ or -] 92 348 [+ or -] 582 (nonnative species) Melilotus indica 72 [+ or -] 90 348 [+ or -] 582 (nonnative species) Nasturtium officinale 164 [+ or -] 284 10 [+ or -] 25 (nonnative species) Polygonum punctatum 20 [+ or -] 32 -- Polypogon monospeliensis 450 [+ or -] 960 154 [+ or -] 232 (nonnative species) Schoenoplectus pungens 962 [+ or -] 1,202 82 [+ or -] 92 Typha domingensis 2,088 [+ or -] 469 1,463 [+ or -] 834 Veronica americana 123 [+ or -] 78 92 [+ or -] 121 Unknown 51 [+ or -] 60 31[+ or -] 51
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|Author:||Titus, Jonathan H.; Titus, Priscilla J.|
|Date:||Sep 1, 2008|
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