Norm C. Kenkel
Department of Botany, University of Manitoba
Winnipeg, Manitoba, Canada
E-mail: kenkel@umanitoba.ca
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Demography of clonal Ostrich Fern (Matteucia struthiopteris). II. Year two of a long-term study
Norm C. Kenkel Department of Botany, University of Manitoba Winnipeg, Manitoba, Canada E-mail: kenkel@umanitoba.ca |
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Plant demography is the study of population dynamics and its underlying causes (Silvertown and Lovett Doust 1993). Demographic information can be obtained either by following the fate of individuals over time, or by estimating age-specific mortality probabilities from the age structure of a population at a given time.
In many plant species, recruitment through the production of vegetative offshoots (clonal growth) is an important method of establishment and spread. A number of clonal plant populations have been studied from a demographic standpoint. Clonal ramets, like individual plants (genets), have individual demographic profiles (birth, death, size, reproductive capacity). Ramets differ in that they often remain attached to one another (via rhizomes or roots), and may therefore remain physiologically interconnected. Intraspecific competition in clonal species may be extremely important. It has been found that clonal growth often leads to large, dense stands of a single genotype dominating a population, even though there may initially have been many genets (Langer et al. 1964). This appears to be the case in the clonal bracken fern (Pteridium aquilinum), which forms large genetically uniform stands in burned areas in Finland (Oinonen 1967).
While a number of studies have demonstrated that the size and proximity of neighbours can affect growth rates of individuals in a population (Kenkel 1991), few studies have related spatial interactions and demographic processes. In this contribution, I report the second year results of a study established to examine long-term dynamics of a clonal Ostrich Fern (Matteucia struthiopteris) stand in Oxbow Woods, Delta, Manitoba. The biology of the species is well known, but age-specific mortality rates and longevity have not been studied (Prange and von Aderkas 1985).
The objective of the study is to relate individual ramet productivity (ramet size), reproduction (production of fertile fronds), and longevity to the size and proximity of ramet 'neighbours' in the stand. The first year results are reported in Kenkel (1994).
This fern species is a member of the Polypodiaceae. It is commonly known as the 'ostrich fern', or more generally as 'fiddle heads' after the edible frond shoots produced in the spring. A large clonal species, it occurs throughout much of northern North America and Eurasia. It often forms extensive, monodominant stands in moist deciduous forest, but it also occurs in the southern boreal forest. The species prefers rich alluvial sites, and is particularly common on river plain fluvial deposits. It is a good indicator of soil moisture conditions, preferring moist by well-drained soils (Mueller-Dombois 1964). In Manitoba, vegetative fronds have a stipe up to 40 cm in length and a blade to about 1 m in length. Individual ramets are erect rootstocks with a projecting crown of one or (usually) more fronds, and are connected by a stout, persistent runner. Some ramets, usually the largest, produce separate and distinctive fertile fronds that have a nutrient depletion effect on the vegetative fronds (Prange and von Aderkas 1985). In Manitoba, vegetative fronds complete their elongation in 2 to 3 weeks in late May and begin dying back in early to mid-August. Low moisture and high light reduce frond height and dry mass, and the species is considered to be 'shade-adapted' (Prange and von Aderkas 1985). Fertile fronds are persistent but not common. In New Brunswick, Prange and von Aderkas (1985) found that under shaded conditions only 1% of ramets produced fertile fronds, but that "in conditions of direct sunlight, a much higher percentage of plants (sic) develop fertile fronds".
The population studied occurs in a gallery forest (known locally as Oxbow Woods) on the property of the University of Manitoba Field Station (Delta Marsh), at 50°11'N, 98°23'W, approximately 3 km south of Lake Manitoba along a former oxbow of the Assiniboine River. The study plot was located within an extensive monodominant stand of ostrich fern located near Inkster Farm, Oxbow Woods (Kenkel 1992). The forest in this area is dominated by mature bur oak (Quercus macrocarpa) and green ash (Fraxinus pennsylvanica). Younger individuals of Manitoba maple (Acer negundo) occur at low abundance. The understory is locally variable and patchy. Few other species were found within the study plot, but in adjacent areas (where ostrich fern is not present) conspicuous understory species include Aralia nudicaulis, Carex assiniboinensis, Rhus radicans, Osmorhiza longistylis, Actaea rubra and Rudbeckia laciniata. Löve (1959) characterizes most of these species as having an 'eastern' floristic affinity.
The climate of the area is humid sub-continental, with short warm summers and long cold winters. Mean annual temperature is 1.5°C. July is the warmest month (mean of 19.1°C), and January the coldest (-19.8°C). Mean annual precipitation is 49.9 cm, approximately 75% of which falls as rain.
Soils in the Oxbow Woods are rich clay-loams, with approximately 20% organic matter content and a near-neutral pH.
There was little change in the number of rootstocks between 1993 and 1994, and rootstock turnover was very low. Of the 235 rootstocks in 1993, only 13 had died by 1994. In 1994, 12 new rootstocks were mapped (for a total of 234). The number of vegetative fronds was somewhat reduced in 1994, however. In 1993, 1002 vegetative fronds were counted (mean of 4.264 per rootstock), versus 902 in 1994 (mean of 3.855 per rootstock). These mean values are somewhat lower than the value of 6.75 reported by Prange and von Aderkas (1985) in New Brunswick populations. Most of the rootstocks that died between 1993 and 1994 were classed as 'tiny' (much smaller than the majority) in the 1993 survey, and most had a single vegetative frond (mean of 1.692 per rootstock). There was a statistically significant correlation between the number of fronds produced by rootstocks between 1993 and 1994 (sterile rootstocks only, n = 192, r = 0.612, p < 0.001); see Fig. 1.
There were 26 fertile rootstocks (11.11% fertility) in 1994, up from 21 (8.94% fertility) in 1993. These values are considerably higher than the 1% value reported for shaded populations in New Brunswick (Prange and von Aderkas 1985). The total number of fertile fronds was also higher in 1994, increasing from 31 (mean of 1.476 per fertile rootstock) in 1993 to 42 (mean of 1.615 per fertile rootstock). Of these, 17 were also fertile in 1993, and 9 were fertile in 1994 but not in 1993. Of these 9, two had persistent 'old' fertile fronds, indicating that they had been fertile at some time prior to 1993. An additional 4 ramets were fertile in 1993 but not in 1994. Of the 17 rootstocks that produced fertile fronds in both years, 8 produced the same number of fertile fronds in both years, while 5 produced more and 4 less than in 1993.
I thank Krista Coupland for help in plot setup and mapping in Year I (1993), and Phil Northover for similar assistance in Year II (1994). Assistance from the staff of the University Field Station is greatly appreciated. This study is supported by NSERC individual operating grant A-3140 to N.C. Kenkel.
Kenkel, N.C. 1991. Spatial competition models for plant populations. Pages 387-397 In Feoli, E. and L. Orlóci (eds.). Computer assisted vegetation analysis. Kluwer Academic, Dordrecht, Netherlands.
Kenkel, N.C. 1992. Neighbour interactions in the clonal fern Matteucia struthiopteris (ostrich fern). University Field Station (Delta Marsh) Annual Report 27: 61-64.
Kenkel, N.C. 1994. Demography of clonal Ostrich Fern (Matteucia struthiopteris). I. Year one of a long-term study. University Field Station (Delta Marsh) Annual Report 28: 47-49.
Langer, R.H.M., S.M. Ryle and O.R. Jewiss. 1964. The changing plant and tiller populations of timothy and meadow fescue swards. I. Plant survival and the pattern of tillering. Journal of Applied Ecology 1: 197-208.
Löve, D. 1959. The postglacial development of the flora of Manitoba: a discussion. Canadian Journal of Botany 37: 547-585.
Mueller-Dombois, D. 1964. The forest habitat types of southeastern Manitoba and their application to forest management. Can. J. Bot. 42: 1417-1444.
Oinonen, E. 1967. The correlation between the size of Finnish bracken (Pteridium aquilinum (L.) Kuhn) clones and certain periods of site history. Acta For. Fenn. 83: 1-51.
Prange, R.K. and P. von Aderkas. 1985. The biological flora of Canada. 6. Matteuccia struthiopteris (L.) Todari, ostrich fern. Can. Field-Naturalist 99: 517-532.
Silvertown, J.W. and J. Lovett Doust. 1993. Introduction to plant population biology. Blackwell Scientific, London. 210 pages.