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Horsetail Spike-rush

Background

1.1   Description

Horsetail spike-rush is an aquatic, perennial plant in the sedge family (Cyperaceae). It has unbranched, green aerial stems (“culms”) 50–100 cm in height and 3–5 mm in diameter, which are round in cross-section. Horsetail spike-rush is, in part, distinguished from other spike-rushes within its range by relatively stout culms that are at least the same thickness as its mature terminal spikelets. Another distinctive characteristic is the presence of internal, cross-partitioning membranes inside the culm. The plant does not have leaf blades, but instead has narrow, papery scales at the base of the culm. Horsetail spike-rush begins flowering in late spring and fruits from July to October. Fruits are held within a terminal spikelet of 15–40 mm in length, which is covered in light brown scales dotted purple on the inside. The small fruits are oval-shaped and flattened to triangular, with a long protrusion extending from one end. Horsetail spike-rush also reproduces clonally by underground rhizomes. Technical descriptions and illustrations are available in Voss (1972), Gleason and Cronquist (1991), and Holmgren (1998).

1.2   Populations and Distribution

Horsetail spike-rush is primarily a species of the southern Atlantic and Gulf coastal plains, also occurring locally at the northern extent of its range along the southern Great Lakes.It is considered globally apparently secure (G4) (NatureServe 2006). It has been recorded in 23 American states, but is considered rare (S1–S3) in 13 of these and historical (SH) or extirpated (SX) in another five (Appendix 1).

In Canada, horsetail spike-rush is known from a single occurrence at Long Point National Wildlife Area in southwestern Ontario (Reznicek and Catling 1989; Sutherland 2000). It is considered critically imperiled (S1) in Ontario (Natural Heritage Information Centre 2005) and this occurrence represents less than 1% of the species’ global population (Sutherland 2000). Long-term trends within this single population are unknown owing to an absence of data from earliest accounts. While the number of culms and areal extent of the population of this plant fluctuated somewhat between surveys in 1988 and 2004 (Table 1), this may indicate annual variation only, due in part to natural changes in water level. The recent (2004) drop in the number of culms is not considered to be an indicator of serious decline, as the areal extent had not changed greatly, and water levels were unusually high that year (M. Oldham, pers. comm. 2005). Owing to the clonal nature of this species, it is also possible that the occurrence represents only a single plant.

Table 1. Population Data for horsetail spike-rush on Long Point, Ontario
YearCollector/observerPopulation/plant numbersArea (m2)
1953LandonNo estimateNo estimate
1956Soper & DaleNo estimateNo estimate
1960LandonNo estimateNo estimate
1988A.A.Reznicek et al.Approximately 12 culms1
1993D. Sutherland151 culms (91 fruiting)6
1999A.A. Reznicek & M. OldhamApproximately 100 culms (10 fruiting)8–10
2004M. Oldham, A.A. Reznicek, & J. Robinson19 culms (9 fruiting)5–10

Sources: Sutherland (2000); Oldham (2004).

In 1999, a possible new location was reported at Turkey Point Provincial Park but no specimen or photograph exists to document this finding. The plant was not located again in spite of considerable searching at the reported location by botanists familiar with the species (M. Oldham, pers. comm. 2005). The nearest recorded populations of horsetail spike-rush are at least 200 km east of Long Point in Monroe County, New York, and over 300 km west in Washtenaw County, Michigan (Young and Weldy 2005; Michigan Natural Features Inventory 2006).

Extant Canadian population of horsetail spike-rush

Figure 1 Extant Canadian population of horsetail spike-rush

1.3    Needs of Horsetail Spike-rush

1.3.1   Habitat and biological needs

In Ontario, horsetail spike-rush on Long Point grows in sandy organic muck along the south-facing shoreline of an inland pond (Sutherland 2000). The pond lies between two stabilized dune ridges near the tip of Long Point and is affected by natural processes. Horsetail spike-rush is considered an aquatic species, growing in water between 4 and 35 cm deep. Like many other Atlantic coastal plain species, it may be tolerant of periodic water level fluctuations (Schneider 1994), although its specific tolerance limits have not been studied.

The shoreline community at this location is dominated by buttonbush (Cephalanthus occidentalis) and red-osier dogwood (Cornus stolonifera). Other species observed growing in association with horsetail spike-rush include water bulrush (Scirpus subterminalis), Small’s spike-rush (Eleocharis smallii), grass-leaved pondweed (Potamogeton gramineus), long-leaved pondweed (Potamogeton nodosus), common coontail (Ceratophyllum demersum), slender naiad (Najas flexilis), bulhead pond-lily (Nuphar variegatum), northern wild rice (Zizania palustris), and slender sedge (Carex lasiocarpa). A complete list of vascular plants recorded for this pond site can be found in Sutherland (2000).

Very little is known about the specific biological requirements of horsetail spike-rush, beyond a general identification of its current habitat. Horsetail spike-rush occurs rarely throughout northeastern North America and is presumably of limited distribution in Canada because of climate. The possibility that this occurrence represents very few or perhaps even a single plant suggests that the genetic diversity of this population may be very low, which may limit its long-term viability.

Like other members of the genus Eleocharis, horsetail spike-rush is presumably wind-pollinated. European studies have demonstrated that seeds of Eleocharis species can be dispersed in the fecal matter of migratory waterbirds (Charalambidou and Santamaria 2005). Propagules of a number of common wetland plants (including Scirpus spp., Eleocharis spp., and Chenopodium spp.) can also remain viable following waterbird gut passage (De Vlaming and Proctor 1968; Holt-Mueller and van der Valk 2002). Waterbirds may therefore be responsible for transporting propagules of this species.

1.3.2   Ecological role

The specific role that the species may play within the ecosystem (e.g., providing food or shelter to other organisms) has not been identified.

1.3.3   Limiting factors

It is not known whether reproductive factors or life history is biologically limiting in any way.

1.4    Threats

Currently, there are few threats that affect horsetail spike-rush and its habitat, as the only Canadian occurrence of the species is within an infrequently accessed area inside a national wildlife area. The habitat is considered secure, and the surrounding area is subject mainly to natural processes. Property adjacent to the national wildlife area and relatively close to the species’ occurrence has recently been purchased by the Nature Conservancy of Canada (H. Arnold, pers. comm. 2005), but the lands are still used by the various joint landowners for outdoor recreational pursuits. This may cause some disturbance to any potential habitat present on these adjacent lands. However, it does not directly impact the known occurrence or its current habitat.

Five potential threats to the species and its habitat have been identified; however, none of these has currently been demonstrated as having a negative impact on the species in Canada. Threats are listed in estimated order of probability and significance.

1.4.1   Invasion by common reed (Phragmites australis

The invasive common reed (Phragmites australis) is expanding on Long Point National Wildlife Area, as it has in the entire Great Lakes region. It has been recently observed in the pond where horsetail spike-rush occurs (M. Oldham, pers. comm.2005). It has not been reported from this site in the past, in spite of its distinctive appearance (e.g., Sutherland 2000). The dense stand is within several hundred metres of the location of horsetail spike-rush (M. Oldham, pers. comm. 2005). Common reed may reduce a site’s species richness, due to its ability to form dense, monospecific stands that crowd out other plant species and reduce structural habitat diversity for wetland birds (Mal and Narine 2004). Its further spread probably poses the most serious threat to horsetail spike-rush habitat. 

1.4.2          Susceptibility to stochastic impacts  

The threats due to environmental stochasticity have not been studied and are virtually impossible to quantify. In general, evidence indicates that stochastic impacts may increase as population size decreases (Lande 1993). With only one Canadian occurrence, the very small areal extent of this occurrence (5–10 m2) and number of individuals may make it extremely vulnerable to loss, even in a single, unpredictable chance event (e.g., disease, storm, flood). The rare occurrence of the species throughout northeastern North America also means that recolonization is unlikely. Stochastic processes that would affect this population are natural processes. For example, the extirpation during the Holocene (5700–2800 BP) of localized populations of horsetail spike-rush in Indiana, which has been demonstrated via the pollen record, was likely due to rapid climate drying (Jackson and Singer 1997).

1.4.3          Herbivory

High populations of white-tailed deer (Odocoileus virginianus), in the absence of natural levels of predators, have been a problem at Long Point in the past; however, ongoing deer management has successfully reduced the population on the Point (J. Robinson, pers. comm. 2005). Evidence of direct deer herbivory on horsetail spike-rush at this site has not been observed to date (J. Robinson, D. Sutherland, and M. Oldham, pers. comm. 2005), and the population persisted even when deer numbers were high. On Long Point, the horsetail spike-rush population is buffered from the shoreline by dense shrubs and may be inaccessible to deer. However, deer herbivory has been observed on other species of Eleocharis (D. Sutherland, pers. comm. 2005), and it may be considered a possible threat. Herbivory by beavers (Castor canadensis) and muskrats (Ondatra zibethicus) is also possible, but has not been observed. 

1.4.4          Water level elevation or stabilization 

Beaver activity has been reported within the pond in the past and may cause short- to medium-term changes in water levels. Like other shoreline emergent species that are relicts of Atlantic coastal plains (see Reznicek 1994), horsetail spike-rush is probably adapted to fluctuating water levels. It is possible that prolonged stabilization of water levels may threaten this species. However, while many annual Atlantic coastal plain species require water level fluctuations in order to germinate (Keddy and Reznicek 1982), it is also possible that horsetail spike-rush’s perennial rootstocks may buffer it against such changes to the hydroperiod.

1.4.5          Loss of genetic diversity 

Reductions in population size can reduce the number of alleles in a population and decrease the overall genetic variability of populations over time, thus lowering individual fitness and population viability. Immigration of pollen or seeds from nearby populations can reduce the likelihood of allele loss, introducing variation to a small population (Young et al. 1996). The Long Point population of horsetail spike-rush may be vulnerable to genetic loss, owing to its small size and the fact that pollen transfer and seed immigration from distant populations are likely very low. It is unknown whether one small patch of culms will be sufficient to maintain individual fitness and population viability over the long term. There is no information on this threat, and therefore it is speculative.

1.4.6          Climate change 

Climate change may threaten horsetail spike-rush if its current, narrow habitat preference in Ontario becomes unsuitable. The effects of climate change may lead to an alteration in the moisture regime of the area or a shift in winter temperatures, which may impact the species’ survival.

1.5    Actions Already Completed or Under way

The tip of Long Point is subject to natural processes, and active management for horsetail spike-rush has not occurred. Ongoing management of deer populations should continue to maintain a sustainable deer population while protecting biodiversity.

1.6    Knowledge Gaps

Research important to the Canadian population is identified in Table 2 (see section 2.5). However, because of the almost complete lack of ecological research on this species in North America, any basic ecological research (reproduction, seed viability, etc.) would be helpful in managing this species. Below are significant areas where further information is required and research questions that would help refine the goals and objectives:

·       Threat identification: What are the real versus perceived threats at this site and at others in its range? How does population vary with water level? Is horsetail spike-rush vulnerable to herbivory? Do muskrats pose a threat to the survival of the species?

·       Ecology: What is the range of tolerance of horsetail spike-rush to water level fluctuations? What biotic and abiotic factors explain the habitat preferences of this species? How long do seeds persist in the seed bank, and what are their germination requirements? How far is pollen able to travel? Is this species able to self-pollinate? What is the role of the species in the ecosystem, e.g., as a food source or host?

·       Genetics: What is the genetic variability within this occurrence? Is there evidence of gene flow between this population and others in the northeastern United States?

·       Recruitment and demographics: What are the recruitment rate and demographic structure of the Ontario population? What is the probable demographic viability of this population? How many individuals of this species are required to ensure long-term viability, and how much habitat do they require?