Dynamics of the Shore Vegetation of a North Swedish Hydro-Electric Reservoir ' During a 5-Year Period
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ACTA PHYTOGEOGRAPHICA SUECICA 69 EDIDIT SVENSKA VA XTGEOGRAFISKA SALLSKAPET Christer Nilsson Dynamics of the shore vegetation of a North Swedish hydro-electric reservoir ' during a 5-year period UPPSALA 1981 ACTA PHYTOGEOGRAPHICA SUECICA 69 EDIDIT SVENSKA VAXTGEOGRAFISKA SALLSKAPET Christer Nilsson Dynamics of the shore vegetation of a. North Swedish hydro-electric reservoir during a 5-year period Almqvist & Wiksell International, Stockholm UPPSALA 1981 Suggested citation: Nilsson, C. 1981, Dynamics of the shore vegetation of a North Swedish hydro-electric reservoir during a 5-year period. Acta Phytogeogr. Suec. 69. Uppsala. ISBN 91-721 0-069-9 (paperback) ISBN 91 -721 0-469-9 (cloth) ISSN 0084-591 4 Doctoral thesis at Umea University 1981 . Christer Nilsson 1981 © Svenska Vaxtgeografiska Sallskapet Box 559, S-751 22 Uppsala Editor: Erik Sjogren Technical editor: Gunnel Sj ors Phototypesetting by Textgruppen i Uppsala AB Printed in Sweden 1981 by Borgstroms Tryckeri AB, Motala Acta Phytogeogr. Suec. 69 Contents Introduction 5 2 The investigated area 8 2. 1 Physiography 8 2.2 Geology 8 2.3 Climate 9 2.4 Water-level variations 12 2.5 Previous botanical investigations 15 2.6 Shore formation processes 16 2. 7 In situ remnants of the former vegetation 17 3 Sites investigated 20 3. 1 Selection of sites fo r investigation 20 3.2 Description of the investigated sites 21 Forest vegetation 21, Vegetation of the cleared belt 21, Individual sites 22 4 Terminology and methods 26 4. 1 Shore terminology 26 Regulated shores 26, Unregulated shores 26 4.2 Substrate classificationand analysis 27 4.3 Vegetational analysis 27 Layers 27, Cover (C) 27, Number of individuals or shoots (N) 28, Number of species (n5) 28, Difference and changeability quotients 28 4.4 Determination of buried seed content, seed production and seed weight 29 5 Vegetation dynamics 30 5.1 A strongly-exposed shore 30 5.2 Moderately-exposed shores 31 5.3 Weakly-exposed shores 34 5.4 A sheltered shore 37 5.5 The vegetational development at Site 5 in 1980 38 6 Site habitats and dy namics of vegetation 40 6. 1 Degree of exposure 40 6.2 Substrate type 42 Shore-level 42, Different substrates 45 6.3 Water-level fluctuations 47 Sites 1-5 4 7, Site 6 50, Discussion 50 7 Survival strategies of the most frequent species 53 7. 1 Selection of species 53 7.2 Bryophytes 53 Bryophyte dynamics 53, Bryophyte survival strategies 53 7.3 Vascular plants 54 Survival strategies of vascular plants 54, Discussion 62 Acta Phytogeogr. Suec. 69 8 Synthesis 63 8.1 Vegetation dynamics 63 8.2 Degree of exposure, substrate type and water-level fluctuations 63 8.3 Survival strategies of the most fr equent species 64 8.4 Dynamics of the annual shuttle species 64 8.5 Concluding remarks 65 9 References 66 Appendix 69 Table 5 69, Table 6 71, Table 7 73, Tabl e 8 76, Table 9 80, Table 0 86, I Table ll 87 Nilsson, 1981, Dynamics of the shore vegetation of a North Swedish hydro-electric C. reservoir during a 5-year period. Acta Phytogeogr. Suec. 69, Uppsala. 96 pp. The study deals with the dynamics of the nearly annually inundated vegetation along the edges of the Gardiken hydro-electric reservoir during the period 1976-1980. This long term regulated reservoir was constructed in 1961 by damming-up a series of lakes. It has a vertical amplitude of 20 m, the water-level rhythm being reversed, from very low in spring and early summer to maximal in late summer and/or autumn. The new shorelines, which for most of the system were cut into formerly terrestrial vegetation now destroyed, remain scantily colonized, only within the upper levels, where, however, the timing and duration of the submersion have varied considerably. Determination of percentage cover and number of individuals or shoots was made twice a year within sixty-six permanent 1 m2 quadrats, located in formerly terrestrial areas, now subject to different degrees of wave action and other environmental exposure. The higher the degree of exposure, the coarser was usually the shore substrate. The mean changeability quotients in flora and vegetation cover were insignificant on uneroded patches of the original humus layer lying close to the maximum damming water level, but considerable on eroded parts of the humus layer and on all substrates below that level and now subject to periodic flooding. On the original humus layer near the damm ing-level, the vegetational changes were mainly unidirectional, while on the substrates below the erosion scarp created in the humus layer, cyclic changes in both the pattern of abun dance and in species diversity were observed. These changes were correlated with varia tions in (I) degree of exposure, (2) substrate type and (3) seasonal water-levels. Factors (I) and (2) were important for vegetational distribution while factor (3) chiefly governed the cyclic variations in plant cover, the richness in vascular plant species being related to between-year changes in the artificial fluctuations in water-level. Survival strategies of some common bryophytes and phanerogams, most of the latter weeds, are discussed with reference to During (1979). Most turned out to be annual shuttle strategists, fit for habitat conditions existing only for short periods. Christer Nilsson, Department of Ecological Botany, Umea University, S- Umea, Sweden 901 87 Acta Phytogeogr. Suec. 69 1 Introduction On the basis of his investigations on the Lenin reser Nilsson 1961 ; Runnstrom 1964 ), normally ceases voir Svirenko (1 932, 1934 [cited in Mel'nikov 1970]) after a few years (Runnstrom 1964; Aass 1973; identified three phases in the biological development N.-A. Nilsson 1973). Thereafter, judging from e.g. of reservoirs, viz. (1) a breakdown of the original Runnstrom (1 964; cf. Wright 1967), production may plant and animal communities and a recolonization decrease to a lower level than that before the by different species; (2) a stage of "trophic ex damming-up occurred and limnological conditions plosion", i.e. of an increased primary and secondary once again become less unstable (cf. Frey 1967; Bar production, mainly of plankton, and the establish dach & Dussart 1973). The above-mentioned stage ment of temporary associations; (3) a period of of relative stabilization is reached. However, no relative stabilization. Vogt (1978) fo und a similar detailed information about the biotopic development development in a number of polyhumic reservoirs in of North Swedish reservoirs during this stage of Finland. He termed Svirenko's (op. cit.) first two development is yet available; nor is it possible to phases the "beginning phase" and his third one the judge whether this stage is ever likely to lead to any "erosion phase" (cf. Dussart et al. 1972). longer periods of stability, i.e. with the constituent Svirenko's (op. cit.) three phases are all relevant to populations showing little fluctuation (cf. Krebs North Swedish hydro-electric reservoirs, where the 1972). course of construction involved building a dam and In the North Swedish reservoirs, an increase in fl ooding vast areas of fo rmer dry-land. The greatest primary production during the second stage of destruction occurs along the new shores, where the development is mainly recorded fo r the phyto original communities are already being destroyed plankton (see Rodhe 1964), while the macrophytes during the initial damming-up phase. With only fe w are of very little importance in this respect, simply exceptions the original plants along the new shore because most of them are soon killed-ofT. However, lines are entirely killed-ofT (e.g. Runnstrom 1962). parallel to the first two stages of development a Only the vegetation of floating 'islands' of peat is ex recolonization of the new shores by different cepted and may persist more or less intact for a long macrophytes may take place, with the formation of time afterwards (cf. Ruuhijarvi et al. 1976 [cited in new plant communities, different from those which Vogt 1978]). If the regulated, vertical water-level occupied the fo rmer littorals. According to Ekzertsev amplitude exceeds 7-8 m, even though the water (1979), during the first two stages, the success of transparency reaches or exceeds that value, the recolonization and of the fo rmation of new vegetation fo rmer sublittoral vascular plant communities below types in the Volga reservoirs mainly depended on the the regulated drawdown level are killed-ofT too, as the supply of diaspores, on the morphological features of hydrostatic pressure will become too great (see C. the reservoirs, and on the water-level regime. During Nilsson 1978). After being killed-ofT, decomposition the later stages, the decisive factors are the trophic of the fo rmer vegetation cover and soils sets in and conditions, interrelations between plant species and the reservoir water becomes enriched with nutrients plant communities, and the water-level regime. Ex (Rodhe 1964; L. Arnborg 1972; cf. Rzoska 1966). posure and erosion should be added, according to ex The final decomposition of the dead plant debris perience from Fennoscandian reservoirs. takes a long time (e.g. Lindstrom 1973; N.-A. It is thus obvious that the water-level regime is of Nilsson 1973), but provided the trees and shrubs fundamental importance fo r the fo rmation of new were cut down and removed, prior to damming, the vegetation types in both the initial and the later stages major portion of the organic material, including the of reservoir development. Some authors (e.g. plant litter and humus, seems to decompose within Bogacev 1952) consider it to be the most important the course of only a fe w years. In any case, the stage factor. Judging fr om the views of e.g.