The Dynamics of Island Populations of Platanthera Bifolia in the Biebrza National Park (NE Poland)
Total Page：16
File Type：pdf, Size：1020Kb
Ann. Bot. Fennici 40: 243–253 ISSN 0003-3847 Helsinki 29 August 2003 © Finnish Zoological and Botanical Publishing Board 2003 The dynamics of island populations of Platanthera bifolia in the Biebrza National Park (NE Poland) Emilia Brzosko University in Białystok, Institute of Biology, ul. Swierkowa 20B, PL-15-950 Białystok, Poland (e-mail: [email protected]) Received 20 Aug. 2002, revised version received 20 Nov. 2002, accepted 22 Jan. 2003 Brzosko, E. 2003: The dynamics of island populations of Platanthera bifolia in the Biebrza National Park (NE Poland). — Ann. Bot. Fennici 40: 243–253. Two isolated populations of Platanthera bifolia (Orchidaceae) were monitored for six years in the Biebrza National Park. Platanthera bifolia populations seem to be affected by habitat conditions. The populations differed in size, proportion of fl owering indi- viduals and the effectiveness of reproduction. The natural fruiting rate of P. bifolia is high (over 90%), due to high levels of autogamy (almost 60%). Some costs of repro- duction (in terms of reduction of leaf size) were observed. Leaves were largest before fl owering and in subsequent years their size decreased. Dormancy affected population dynamics, varying from 22.4% in one population to 16.3% in the other. Both P. bifolia populations appeared to be healthy and are not threatened with extinction at the two study sites. Key words: autogamy, cost of reproduction, dormancy, fl owering, fruiting, orchids, Platanthera bifolia, population dynamics Introduction authors, because according to Ackerman (1998) “populations are the basic unit of species con- Information on biology and demography of servation”. In most cases monitoring of orchid orchid species is still limited given the size of the populations follows year-to-year changes in num- Orchidaceae, reaching 20 000 species (Atwood bers of individuals, their fl owering and fruiting 1986). Orchidaceae is one of most interesting rates, the frequency of dormancy and the costs plant families because of a large variation of of reproduction (Wells & Willems 1991, Waite life history strategies. Among members of this et al. 1991). Such studies, however, usually do family are short- and long-lived species, clonal not report important information on mortality and and non-clonal species, and a wide range of pol- reproduction rates — ecological processes shap- lination mechanisms (Tremblay 1992). Generally, ing population dynamics. Specifi city of orchid life however, there has been a trend toward decreas- histories, for example long time of below-ground ing numbers of orchid species on all continents, growth or phenomenon of dormancy, makes anal- mainly due to human activity. Therefore, the yses of dynamics of orchid populations diffi cult. necessity of monitoring of changes in orchid This paper is one of the reports on the populations has been recently stressed by many dynamics of island populations of orchid species 244 Brzosko • ANN. BOT. FENNICI Vol. 40 BIEBRZA NATIONAL PARK 0 10 km ZAB FORT flowering vegetative Calamagrostis epigejos shrubs trees other species 1m 10 m Fig. 1. Localities of the Platanthera bifolia populations and the distribution of individuals in the year of their maxi- mum abundance, 1999. in the Biebrza National Park, Poland (Brzosko fl owers producing nectar and attracting noctuid 2002a, 2002b). Here, I present the results of long-tongued moths (Sphingidae, Noctuidae) have multi-year observations of two Platanthera bifo- a strong, sweet scent (Nilsson 1992). Flowers open lia populations. The objective of this project was sequentially, and after two–three weeks all the to describe the dynamics of the two populations fl owers are open. Only a few fl owers in the lower studied, and to examine some aspects of biology part of the spike may wither before that. A mature of this species with respect to habitat conditions. capsule contains up to 8000 seeds (Maad 2000). Bog orchids, Platanthera, are found in moist habitats in temperate and subtropical regions (Patt et al. 1989). Platanthera bifolia is a common ter- Material and methods restrial orchid in Poland, where it occurs in a vari- ety of forest and meadow habitats. The species is Platanthera bifolia populations were studied a polycarpic perennial. Typically, it has two ovoid in the Biebrza National Park, NE Poland. The leaves at the base of a single fl owering stem. The Zabudnik population (ZAB) was located on a infl orescence spike displays white fl owers. The typical island dune, with ridges or fused para- ANN. BOT. FENNICI Vol. 40 • The dynamics of island populations of Platanthera bifolia 245 bolic forms, within a mosaic of widespread peat 100 ZAB FORT 500 Total number of plants bogs (Żurek 1975). The dune covers a subpeaty 80 ZAB 400 60 300 surface and the soils are fl uvial of eolian and FORT river origin. The island is about 1.2 m above the 40 200 plants (%) surrounding peat and its total area is 5500 m2. 20 100 Proportion of flowering 0 0 A very small percentage of the island supports 1996 1997 1998 1999 2000 2001 an oak–linden–hornbeam forest. The remaining Years parts are covered by small individuals of Betula Fig. 2. Numbers of Platanthera bifolia individuals in the pubescens, Rhamnus cathartica and Viburnum Biebrza National Park (lines) and proportion of fl ower- opulus. Calamagrostis epigejos forms large ing individuals (bars). Total population is the sum of patches. The second population (FORT) is situ- above-ground plants. ated on the periphery of an old fortifi cation from the First World War, about twelve kilometres from the ZAB population. Vegetation around number of fruits per plant and percent of fruit-set. the fortifi cation is dominated by pine (Pinus Each year (in June and July) the study sites were sylvestris) forest. A P. bifolia population was searched for new plants. In 2001 a pollination found on the banks of a water-fi lled moat. The experiment was carried out. Five infl orescences soil moisture was higher in that location than in were covered by cotton net in the ZAB popula- the neighbouring pine forest, because the orchid tion to evaluate seed production without pollina- site was adjacent to the moat. Vegetation cover tors (autogamy). These infl orescences were cov- was sparser than in the pine forest; from 10% ered before any fl owers were open and the cotton to about 50% of the soil was covered by herbs, net remained in place until fruiting time. with the following dominating: Fragaria vesca, The infl uence of light on fl owering and fruit- Melampyrum pratense, Leontodon hispidus, ing was evaluated using 1998 and 1999 data for Convallaria majalis, Anthriscus sylvestris and the ZAB population. This was tested in the ZAB Maianthemum bifolia. Populus tremula domi- population only because almost all individuals in nated the tree layer and leaves of this species the FORT population grew in shade. Reproduc- covered the soil at the study site. ing plants growing in full light were compared FORT population covered about 100 m2. with fl owering plants in the shade for the length Some of the individuals were found among the of infl orescence, number of fl owers and fruits concrete debris of old fortifi cations. per fl owering stalk. The cost of reproduction was Observations were carried out over six years calculated as the reduction of leaf size in years (1996–2001) in the ZAB population and during following fl owering. fi ve years (1997–2001) in the FORT population. Plants were individually marked and mapped (Fig. 1). This allowed to monitor individual plants Results in subsequent years, as well as to detect and ana- lyse the phenomenon of dormancy (the absence Population size of above-ground shoots). Those analyses included only individual plants with above-ground shoots Individuals of Platanthera bifolia grew in aggre- present in the fi rst and in the last year of monitor- gations of different sizes at both sites (Fig. 1). ing. Each year the observations were carried out The number of plants in the FORT population in late June, when the fl owers were fully open. (Fig. 2) varied from 205 (2000) to 292 (1999). Fruiting took place in July and August. For each In the ZAB population the number of individu- plant the following parameters were recorded: (1) als, except 1996 and 1997, when only a frag- reproductive status: fl owering, non-fl owering, (2) ment of population was observed, ranged from length and width of the largest leaf, (3) height of 352 (1998) to 458 (2000) (Fig. 2). The average fl owering stem and the length of infl orescence; density of plants in year of maximum abundance and (4) three measures of reproductive suc- varied from 0.6/10 m2 in the ZAB population to cess were applied: number of fl owers per plant, 19.4/10 m2 in the FORT population. 246 Brzosko • ANN. BOT. FENNICI Vol. 40 60 ZAB FORT 50 50 40 40 30 30 20 20 Frequency (%) Number of plants 10 10 0 0 0 12345 0 1234 Successive years of flowering Time between flowering Fig. 3. Frequency of Platanthera bifolia plants that fl ow- Fig. 4. The number of years between fl owering events ered between 0 and 5 times during the study period. for individuals of Platanthera bifolia. Fig. 5. The mean size (± 1 standard error) of fl owering and non-fl owering individuals of Platanthera bifolia in the two populations studied. Flowering or three times was twenty. A few individuals fl owered four or fi ve times (Fig. 3). In the FORT Among the one hundred individuals in the ZAB population the frequency of fl owering was lower population that were present in 1996, twenty-fi ve and almost half of them (49.3%) never fl owered. never fl owered.