Acta Palaeobot. 41(1): 43–67, 2001 Development of bogs in a coast-inland transect in northern Norway KARL-DAG VORREN Department of Biology University of Tromsø, N-9037 Tromsø, Norway, e-mail: [email protected] Received 19 March 2001; accepted for publication 9 July 2001 ABSTRACT. Bio- and litho-stratigraphy of eight ombrotrophic bogs and one fen in a 76 km long transect from coast to the interior of the district of Troms were studied. The general mode of development of a bog was from an initial carr or swamp forest via a more or less open Cyperaceae-Amblystegiaceae fen to an open Ericaceae- Sphagnum bog with hummocks and hollows. The most marked litho- and biostratigraphical changes concentra- ted about 2750 14C-years BP (2855 cal. years BP). Transition to ombrotrophic environments normally occurred with some time lag after a climatically induced lithostratigraphical change. This lithostratigraphical transition most frequently occurred during the time interval 3355–2760 BP. The oldest date of the transition was 4465 BP and the youngest (interpolated) date 2400 BP. KEY WORDS: ombrotrophic mires, palaeoclimate, litho- and bio- stratigraphy, northern Norway INTRODUCTION The Målselv river catchment area, and the Woodlands and forest stands dominated by Malangen fjord dominate the area of central birch (Betula pubescens Ehrh.), grey alder Troms (Fig. 1). Mountains, the crests of which (Alnus incana (L.) Moench.) and pine (Pinus reach 1200–1700 m a.s.l., distinguish the sylvestris L.) characterize the morainic, glacio- areas between the river valleys. fluvial and marine deposits of the valleys. In S 1 Tromsø 2 Malangen III N II I IV 3 4 M 5 6,7 B 9 Målselv TROMS 8 Ø 10 D 0 50 km Fig. 1. Position of mire sites 1–10 and meteorological stations (S, M, B, Ø and D). I-IV represents four mire-complex sections along the oceanic-continental gradient 44 addition, different Salix species and aspen N. Lat. and 18o05’ to 18o50’ E. Long., and con- (Populus tremula L.) may play important roles stitutes a ca. 76 × 16 km large rectangle. in some of the forest types. The purpose of this investigation is to high- On these Quaternary deposits the humid light regional climatic signals in bio- and li- climate of the region induces extensive mire thostratigraphy, and to study the succes- and bog formation, even in areas with an an- sion/development towards ombrogenic envi- nual precipitation of about 500 mm and less. ronments. The climatically balanced climax stage of the lowland mires west of the section bound- ary II/III (Fig. 1) is an ombrotrophic bog (cf. THE INVESTIGATION AREA Eurola & Vorren 1980). East of this line there is a transitional section (III) with both ombro- trophic and minerotrophic mire complexes. CLIMATE There is also an altitudinal trend from the lowland bog complexes to mire complexes The maritime-continental climate gradient characterized by sloping fens in the subalpine from northwest to southeast is summarized in birch region. Table 2 (extracted from Aune 1993, Førland The present investigation concerns develop- 1993, and Bruun 1967). Typical features are: ment of mires and bogs in section II and III 1) Decrease in annual precipitation 940–659 along a northwest-southeast transect (Fig. 1). mm to 282 mm in Dividalen. 2) Decrease in The transect covers the coast at the mouth of annual mean temperature (3.9o–0.9oC). 3) In- the Malangen fjord (sites 1 and 2, Table 1), via creasing, or as an “arcuate” form, mean June- the fjord head of Malangen (sites 3 and 4), and August (summer) temperatures from coast to the lower part of the valley of Målselvdalen inland (10.7oC in the coastal region, 11.9oC in (sites 5–7), to the middle part of Målselvdalen the fjord region, to 11.8oC in the middle valley (sites 8–10). Sites 1–9 occur within the mire region to 11.3oC in Dividalen). 3) The annual section II (the sub-maritime Racomitrium-Cal- mean amplitude between the warmest and the luna bog section), whereas site 10 belongs to coldest month increases, or exhibits an “arcu- section III. Site 10 is also the only one repre- ate” form from coast to inland (13.8oC at the senting the sub-alpine birch region. The other coast, 19.1oC in the fjord region to 24.7oC in sites are situated in the “pre-alpine” vegeta- the middle part of the valley to 23.4oC in Divi- tion belt (cf. Elven & Vorren 1980). The tran- dalen). 4) Increasing amount of summer pre- sect is limited by the latitudes 68o50’ to 69o35’ cipitation as related to the annual precipita- Table 1. The investigated mire sites. All sites are positioned within the UTM grid 34W. Cores from all sites have been pollen-analyzed. For a limited number of sites some chemo-stratigraphical parameters, colorimetric humification and con- centration/influx of pollen and spores have been analyzed Core Humifi- length, UTM Con-cen- Site Mire area Altitude, cation, Site name com- position of tration/ Mg, Ca No. (da) m a.s.l. colori- pressed coring site influx data metric (cm) 1 Austeinmyra 10 168 CC847247 25 + 2 Brensholmyra 1 910 178 CC851229 11 + 3 Straumen 30 156 CB952979 12 4 Målsnes 5 285 DB036924 45 + 5 Aspmo 60 243 DB033774 25 + 6 Olsborg hummock 240 55 DB050739 33 + + + 7 Olsborgmyra 2 240 123 DB050739 33 + + + 8Smedrud 250 151 DB067611 75 + (Bjørndalsmyra) 9 Fossli 15 180 DB073617 100 10 Vardfjellmyra 10 225 DB115549 365 + + 45 Table 2. Some climate parameters from the meteorological stations Sommarøy (S), Meistervik (M), Bardufoss (B), Øver- bygd (Ø), Dividalen (D) (the 1961–90 normal). Positions in Fig. 1 Vegetation period Precipitation Mean temperature oC Site days mm t≥6oC* J F M A M J J A S O N D Yr Days Jun-S Yr S -1.9 -1.9 -1.0 1.7 5.5 8.9 11.9 11.4 8.1 4.5 1.0 -1.1 3..9 136 290 940 M -6.0 -5.5 -3.0 0.9 5.8 10.5 13.1 12.0 7.2 2.7 -2.0 -4.7 2.6 130** 284 860 B -10.4 -8.9 -5.4 -0.2 5.6 10.5 13.0 11.5 6.3 0.9 -5.5 -8.9 0.7 125 222 652 Ø -10.2 -8.7 -5.2 -0.2 5.5 10.7 13.2 11.5 6.4 1.4 -4.9 -8.7 0.9 125 241 659 D -9.4 -8.2 -5.5 -0.8 5.0 10.3 12.8 10.9 6.3 1.0 -4.8 -8.0 0.8 122 164 282 * period 1931–60 ** Data from a neighbouring station: Navaren (N) tion, along the coast-inland transect (30.8%, and during the period 10 000 – 9500 BP in the 34.0%, 36.6% up to 58.0% in Dividalen). fjord and valley district (G. Corner, T. Vorren, The oceanity of the climate is probably best pers. comm; Sollid & Torp 1984). The marine differentiated by means of the oceanity-con- limit reaches an altitude of about 27 m a.s.l. in tinentality index of Kotilainen (1933). This the northwest and (theoretically because of index includes the mean temperature of the blocking inland ice) 90 m a.s.l. in the southeast- warmest and the coldest month, the length of ern part of the investigation area. spring, summer and autumn as well as the an- nual precipitation. The maritime coast (Som- FLORA AND VEGETATION marøy) has an index value of ca. 117, the sub- maritime fjord head (Meistervik/Navaren) ca. The flora atlases of Benum (1958) and En- 50, the sub-maritime to sub-continental main gelskjøn and Skifte (1995) of the county of valley (Bardufoss and Øverbygd) ca. 30, and Troms give detailed information of the dis- the sub-continental lateral valley (Dividalen) tribution of vascular plants in the study area. ca. 13. Fremstad and Øvstedal (1978) and Spjelkavik The only station with a clearly sub-con- (1986) have described the forest vegetation in tinental climate, Dividalen, is situated outside the southern part of the district (the valley the present transect. The precipitation pat- and inner fjord region). Vorren and Johansen tern, with high autumn and winter precipita- (1990) have investigated the mire vegetation tion, characterizes the investigation area as in different altitudes of an area in the south, having a maritime or oceanic precipitation and Vorren et al. (1999) have described the re- regime, whereas the mean annual tempera- gional lowland mire vegetation of Troms and ture amplitude points to a strong thermal gra- the adjacent part of Nordland county. dient towards a sub-continental temperature climate regime. VEGETATION HISTORY GEOLOGY From the southern part of the study area some simplified pollen diagrams from both The geology of the area is characterized by lakes and bogs have been published, mainly the North-Norwegian Cambro-Silurian mica from higher altitudes (Alm et al. 1996, Vorren schist formation (Sigmond et al. 1984) on the et al. 1996, Vorren & Stavseth 1996), with the mainland (valley and fjord district) and acid purpose of reconstructing Holocene forest-line gneiss on the coastal part of the islands. Most displacements. The main features are: of the bogs in the valley and fjord region lie on Ca. 9500–9000 BP: Deglaciation and succes- Quaternary leached deposits of mainly sand sion of snowbed and heath vegetation, which and gravel. Below the sites on the island, the by 9200 BP was replaced by a Hippophaë – Be- mineral substrates are mainly marine de- tula pubescens vegetation and later by a posits, frequently coarse shell sand and gravel.