Round-leaved greenbrier (Smilax rotundifolia) COSEWIC assessment and status report: chapter 6

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Biology

Life cycle, reproduction, and population biology

Round-leaved greenbrier is dioecious, meaning that individual plants are either functionally male or functionally female. This species flowers from late May to mid-June in southern Ontario. Fruit matures in the fall, but typically some is retained over the winter to the next spring.

In their survey of seven Ontario round-leaved greenbrier populations, Kevan et al. (1991) found two to contain both male and female plants, two to be unisexually pistillate, two to be unisexually staminate, and one to be “probably unisexual” (no fruit was seen, but the observers were unable to determine the sex of the plants present). The 2006 field surveys occurred too late to observe flowering status, but, of the six populations located in 2006, only one had fruiting plants (Table 2). Population 11, seen in flower in 2007 (Table 2) was potentially all male.

Round-leaved greenbrier is also clonal. Large single-sex, contiguous populations (such as those typifying the larger Ontario populations) are likely the result of clonal spread of one individual by means of the plant's stolons and rhizomes. The pattern of dense contiguous patches is typical of this species, and not restricted to its Ontario range. For example, in some areas in southwest Michigan, populations of round-leaved greenbrier have reached very high local densities, taking over acres, especially in more open habitats (Brewer et al. 1973).

The unisexually pistillate populations are unable to set fruit, although they will do so if exposed to pollen from another site. Even the mixed-sex populations are pollen limited: artificial pollination of pistillate plants in mixed-sex populations significantly increases fruit set (Kevan et al. 1991). This situation is likely due to a paucity of pollinators. Round-leaved greenbrier pollen grains are linked to each other by viscin threads, which prevent wind dispersal, reducing this species to dependence on insect pollinators. Kevan et al. (1991) suggest that mosquitoes are the most likely candidate pollinators for the Ontario populations, although small flies, small bees, and bumble bees are also possible. This breadth of candidate species does not necessarily indicate extensive pollinator activity--in their 50 person-hours of observations Kevan et al. (1991) witnessed only a single Bombus and two mosquitoes visit round-leaved greenbrier flowers.

No seedlings were observed during fieldwork for the original report (Ambrose 1994) or this one. Young plants that were investigated were found to be attached to a stolon or rhizome.

Field experiments in Arkansas (Shelton and Cain 2002) show that round-leaved greenbrier seeds can remain in the seedbank for at least three years with little to no effect on their germinability. Given suitable conditions, however, the seeds start to germinate rapidly, with little or no lag time, regardless of whether they had been in the seed bank for one, two, or three years (Shelton and Cain 2002). Under controlled conditions, the seeds can last for considerably longer: after being stored for five years at between two and seven degrees Celsius (and approximately two per cent moisture), the seeds were still highly viable (Pogge and Bearce 1989 in Carey 1994). Harvested seeds will germinate after a cold stratification pretreatment of undetermined duration (H. Kock, pers. comm. cited in Ambrose (1994)).

Herbivory

All species of Smilax are browsed by wildlife and the rhizomes are sometimes dug and eaten by domestic stock (Holmes 2002). Smilax species were among the most heavily grazed of 73 browse species in a study of an east Texas hardwood forest (Goodrum 1977). The tender shoots are very palatable, and the leaves are eaten all year round. In areas where this species is a considerable component of the understorey vegetation, it can be among the most important browse species (Smith 1974).

Rabbit browse has been cited as a stressor in a 1974 study by Niering and Goodwing, but Goodrum (1977) notes that species of Smilax can withstand heavy grazing by herbivores such as deer. Rhizomes usually produce new canes annually and if the tips are nipped off, new branches will form at the remaining nodes. Fifty to sixty per cent of the annual growth of Smilax species may be eaten without killing the roots (Goodrum 1977).

Physiology

In their Maryland study of three non-native vines (Pueraria lobata, Lonicera japonica, Hedera helix) and five native ones (Rhus radicans, Clematis virginiana, Smilax rotundifolia, Vitis vulpina, Parthenocissus quinquefolia) Carter and Teramura (1988) found that round-leaved greenbrier was physiologically strongly plastic in its responses to sunlight, being able to function under low-light conditions, while also being able to rapidly achieve relatively high photosynthetic rates under bright sunshine. This ability distinguished it from taxa like Pueraria, but was shared with Parthenocissus and Vitis, and suggests that, although it is well-adapted to understorey conditions, it is better able to flourish in openings and edge situations.

Dispersal/migration

Seeds are dispersed primarily by birds and mammals that eat the fruit. Ruffed Grouse, Northern Bobwhite, Wild Turkey, and at least 38 species of songbirds are known to eat round-leaved greenbrier fruit, as well as mammals such as White-tailed Deer, Black Bear, Opossum, Raccoon, squirrels and rats (Goodrum 1977). The fruits are an especially important late winter and early spring food for wintering birds (Baird 1980 in Carey 1994). While animals are the primary dispersers, seeds have also been reported to be dispersed by water (Newling 1990 in Carey 1994).

Interspecific interactions and related management issues

Round-leaved greenbrier is included in Richburg et al.’s (2001) list of 19 “invasive woody species in the Northeast and Mid-Atlantic states.”  They note that it can be a problematic competitor of small trees and shrubs, and can also act as a “ladder fuel,” increasing a forest’s susceptibility to fire.

Their general conclusions are echoed by Brockway et al. (2003), who describe the development of dense midstorey vegetation (including round-leaved greenbrier) in southern (USA) pine forests during periods of fire suppression, with a concomitant reduction in understorey plant diversity and an increase in risk of high-intensity wild fires (the fire ladder effect). In Ontario, however, round-leaved greenbrier populations and distribution are too limited for it to be a significant factor in this regard, a position which is supported by Smith (1974) who notes that, “in the northeast, [round-leaved greenbrier] rarely overburdens the supporting trees, and it seldom interferes seriously with tree or shrub regeneration.”

In a Kentucky study (Arthur et al. 1998), round-leaved greenbrier appeared to be negatively impacted by a single hot fire, and by the combination of a cooler fire and a hot fire, two years apart, as measured by percent cover two years after the second burn. The authors argue that the history of fire suppression in the northeastern United States has favoured round-leaved greenbrier, in terms of both population size, and plant size (height). In pine forests in the southern USA, Brockway et al. (2003) reached slightly different conclusions. In their system, round-leaved greenbrier responded favourably to mechanical management of the midstorey followed by prescribed burning. And Smith (1974) reports that Smilax responds favourably to fires of any intensity, and, in fact, the hotter the better. Taking the opposite focus (effect of Smilax on fire, instead of the effect of fire on Smilax), Ohman (2006) notes that old fields invaded by round-leaved greenbrier burn less frequently, but with greater intensity.

As mentioned under the “Habitat” section of this report, disturbance regimes that open up the canopy generally promote Smilax growth (e.g. Blair 1960 in Carey 1994). Smilax species are resistant to most herbicides, including glyphosate (Wendel and Kochenderfer 1982; Bovey 1977, both cited in Carey 1994).

A study of rights-of-way in Connecticut (Niering and Goodwin 1974) provides some interesting evidence of round-leaved greenbrier’s competitive abilities. In this example, open habitats (rights-of-way and abandoned pastureland) were selectively treated to eliminate tree taxa. The resulting community, comprised largely of clones of Gaylussacia baccata, round-leaved greenbrier, and Vaccinium vacillans, was relatively stable, and resistant to invasion by tree species, for at least 15 years.

For additional information on interspecific interactions, refer to the “Life cycle,” “Herbivory,” and “Dispersal” sections of this report.

Adaptability

Although round-leaved greenbrier reproduces asexually and is relatively long-lived, it is limited in Ontario by the few sites that have plants of both sexes present. No genetic studies have been published, but the limited capability for producing seeds potentially could lead to reduced natural variability, genetic diversity and therefore adaptability of the Ontario populations. It is evident, however, that some populations/clones, such as population 11, are clearly growing vigorously.