Norm C. Kenkel
Department of Botany, University of Manitoba
Winnipeg, Manitoba, Canada
E-mail: kenkel@umanitoba.ca
|
Demography of clonal ostrich fern (Matteucia struthiopteris).
I. Year one of a long-term study
Norm C. Kenkel Department of Botany, University of Manitoba Winnipeg, Manitoba, Canada E-mail: kenkel@umanitoba.ca |
|
Plant population ecology involves the study of the number of individuals of a plant species, and how and why a population size changes over time. Plant demography is the study of such populations changes, and the causes of change, throughout the life cycle of the species (Silvertown 1987). Demographic information can be obtained either by following the fate of individuals over time (practical for relatively short-lived species), or by estimating age-specific mortality probabilities from the age structure of a population at a given time (commonly used for forest trees).
In the majority of plant species, recruitment through the production of vegetative offshoots (clonal growth) predominates over recruitment by diaspore germination. Clonal growth can be defined as the "horizontal extension of a plant by the addition of ramets which develop their own roots" (Silvertown 1987: 108). In established clonal populations, there may be controlling mechanisms to ensure that the number of ramets produced does not exceed the carrying capacity of the habitat (Kenkel 1993). Once a clonal plant has become established and has reached the carrying capacity of the habitat, there is a regular annual pattern of ramet birth and deaths. Clonal plants typically have a pool of dormant buds (analogous to a pool of dormant seeds), which can be 'recruited' following disturbance (Noble et al. 1979).
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. However, it has also been argued that individual ramets are largely independent even if they remain physically attached (Kenkel 1993).
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). Turkington and Harper (1979) have demonstrated that 'fine-scale biotic differentiation' may occur, with different genets occurring in different micro-environments within the same general habitat.
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 long-term study, I use a spatial approach to examine the 'fate' of individual ramets of the clonal ostrich fern (Matteucia struthiopteris). Specifically, I will relate individual ramet productivity (ramet size), reproduction (production of fertile fronds), and longevity to spatial configuration of the stand (i.e. the size and proximity of ramet 'neighbours').
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. 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. Vegetative fronds die back in the fall, while fertile fronds persist for at least 2 years.
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 2 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 pensylvanica). 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 assininboinensis, Rhus radicans, Osmorhiza longistylis, Actaea rubra and Rudbeckia laciniata. Löve (1959) characterizes most of these species as having 'eastern' floristic affinities.
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.
The location of the 5 x 5 m study plot was based on the following considerations: (a) dense, monodominant population of ostrich fern with very few other species present; (b) perceived uniformity of abiotic conditions, including soils and degree of shading by trees in the area; (c) absence of trees within the plot. The study plot selected is near the 6 x 12 m plot described in Kenkel (1991, 1992).
All ramet rootstocks within the 5 x 5m study plot were numbered with a small red 'flag' and mapped. Mapping was accomplished by measuring the distance (± 1 cm) to each of the four corner posts of the study plot. The law of cosines was then used to obtain the spatial coordinates of each rootstock. For each rootstock, the size (three classes: 'full', 'half' and 'small' size) and number of vegetative fronds were recorded. For rootstocks with fertile fronds, the number of current (this year's) and older (previous years) fertile fronds were also recorded. Mapping and recording of rootstocks was performed in early August, 1993. A total of 235 rootstocks were mapped, corresponding to a density of 94,000 rootstocks/ha.
A map of the study plot was obtained, showing the locations of all sterile and fertile rootstocks. In addition, simple summary statistics were computed, and the distribution of the number of sterile fronds per rootstock was plotted.
A map of rootstock locations within the study plot is shown in Fig. 1. The spatial pattern of the rootstock distribution has not been examined in detail, but it appears to be random (a complete analysis is planned; note that Kenkel (1992) found that the pattern was random for a nearby 6 x 12 study plot). There is some suggestion of a lower density of rootstocks on the western side of the study plot, but this has not been confirmed statistically. Rootstocks with one or more fertile fronds are distributed relatively evenly throughout the study plot.
There were a total of 1033 fronds (from 235 rootstocks) in the study plot, of which only 31 (3.00%) were fertile. The number of vegetative fronds per rootstock ranged from 1 - 8 (mean = 4.264). The distribution of number of vegetative fronds is approximately normal (Fig. 2), though this has not been statistically tested. The majority of the rootstocks (171, or 72.77%) were classified as being 'full-sized'. Of the remaining, 55 were classified as 'half-sized', and 27 as 'small-sized'. Most 'small-sized' rootstocks had a fewer than average number of fronds (range 1 - 6, mean = 2.704), and these fronds were often brown-coloured and unhealthy looking.
Only 21 of the 235 rootstocks (8.94%) had one or more fertile fronds. Of these, 15 had both older and current fertile fronds, indicating that they have remained fertile for at least two years. An additional 7 rootstocks had older fertile fronds, but did not produce any fertile fronds in 1993. All rootstocks producing fertile fronds were large, and generally also produced a large number of vegetative fronds (range 4 - 8, mean = 6.43). An average of 1.476 fertile fronds were produced per fertile rootstock (distribution: 13 produced one fertile frond, 6 produced two, and 2 produced three). These results suggest that the production of fertile fronds may be energetically costly, with only the most robust rootstocks producing fertile fronds, and even then in small numbers.
Thanks to Krista Copeland for her help in setting up the plot, mapping the rootstocks, and collecting the baseline information. Cooperation and assistance from the staff of the University Field Station is greatly appreciated. This study is supported by Natural Sciences and Engineering Research Council individual operating grant A-3140 to N. C. Kenkel.
Kenkel, N. C. 1991. Spatial competition models for plant populations. In Feoli, E. and Orlóci, L.(eds.). Computer assisted vegetation analysis, pp. 387-397. Kluwer Academic, Dordrecht, Netherlands.
Kenkel, N. C. 1992. Neighbour interactions in the clonal fern Matteucia struthiopteris (ostrich fern). University of Manitoba Field Station (Delta Marsh) Annual Report 27: 61-64.
Kenkel, N. C. 1993. Modeling Markovian dependence in populations of Aralia nudicaulis. Ecology 74: 1700-1706.
Langer, R. H. M., Ryle, S. M. and Jewiss, O. R. 1964. The changing plant and tiller populations of timothy and meadow fescue swards. I. Plant survival and the pattern of tillering. J. Appl. Ecol. 1: 197-208.
Löve, D. 1959. The postglacial development of the flora of Manitoba: a discussion. Can. J. Bot. 37: 547-585.
Noble, J. C., Bell, A. D. and Harper, J. L. 1979. The population biology of plants with clonal growth. I. The morphology and structural demography of Carex arenaria. J. Ecol. 67: 983-1008.
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.
Silvertown, J. W. 1987. Introduction to plant population ecology. 2nd edition. Longman Scientific, New York. 229 pages.
Turkington, R. A. and Harper, J. L. 1979. The growth, distribution and neighbour relationships of Trifolium repens in a permanent pasture. 4. Fine-scale biotic differentiation. Can. J. Bot. 57: 245-254.