Annals af Glacialagy 5 1984 © International Glaciological Society MEAN SUMMER TEMPERATURES AND CIRCULATION IN A SOUTH-WEST NORWEGIAN MOUNTAIN AREA DURING THE ATLANTIC PERIOD, BASED UPON CHANGES OF THE ALPINE PINE-FOREST LIMIT by L. Seising and E. Wish man (Arkeologisk museum i Stavanger, Boks 478, N-4001 Stavanger, Norway) ABSTRACT hAW Height of climatic alpine pine-forest limit Radiocarbon dates on 16 pine stumps and trunks during the Atlantic period, west side revealed the position of the alpine pine-forest hp Mean height of present clima tic alpine pine­ limit for the biostratigraphic (i.e. pollen)-defined forest limit Atlantic period (8.2 to 5.0 ka BP) in the coastal hPE Mean height of present climatic alpine pine­ mountains of south-west Norway. Based on the present forest limit, east side topographic mean summer temperature gradients hpW Mean height of present climatic alpine pine- (mean temperature from June to September), and the forest limit, west side difference in elevation between the alpine pine- hS Height of watershed forest limits of the present and the Atlantic hE Height, east side periods, mean summer temperature differences between hW Height, west side these ~eriods in the forest limit zone are calculated PAE Point on mean summer temperature curve for as 0.7 C to the west and 1.0·C in the inner valley Atlantic period, east side (Fig.6) districts to the east of the watershed. On the eastern PAW Point on mean summer temperature curve for side the mean summer temperature difference becomes Atlantic period, west side (Fig.6) greater towards lower altitudes. The alpine pine- ~ hA - hp, difference between Atlantic period forest limits of both the present and the Atlantic and present climatic alpine pine-forest periods are characterized by a marked west-to-east el evation inclination. This indicates pronounced westerly ~E hAE - hpE, difference between Atlantic winds during the Atlantic period, similar to those period and present climatic alpine pine­ today, characterized by more cloudy weather, higher forest elevation, east side precipitation and less sunshine to the west than to ~W hAW - hpW' difference between Atlantic the east of the watershed. period and present climatic alpine pine­ forest elevation, west side T Mean summer temperature, i.e.· the mean telnp­ erature for June, July, August and September LIST OF SYMBOLS (UtetratermU in Norwegian) APFL Climatic alpine pine-forest limit, Atlantic TS Mean summer temperature at the watershed period Tp Mean sumner temperature at optimal cl imatic PPFL Climatic alpine pine-forest limit, present alpine pine-forest limit gA Topographic gradient of mean summer temperature TPE Mean summer temperature at optimal climatic in atmosphere of Atlantic period alpine pine-forest limit, east side gAE Topographic gradient of mean summer temperature TPW Mean summer temperature at optimal climatic in atmosphere of Atlantic period, east side alpine pine-forest limit, west side gAW Topographic gradient of mean summer temperature IHA Difference in mean summer temperature of east in atmosphere of Atlantic period, west side and west sides, Atlantic period gp Topographic gradient of mean summer temperature lItp Difference in mean summer temperature of east in present atmosphere and west sides, present gPE Topographic gradient of mean summer temperature 6TE Difference between mean summer temperature of in present atmosphere, east side the Atlantic period and present, east side gpw Topographic gradient of mean summer temperature 6TW Difference between mean summer temperatureof in present atmosphere, west side the Atlantic period and present, west side hA Height of cl imatic alpine pine-forest 1 imit lITS Difference between mean summer temperature of during the Atlantic period the Atlantic period and present, at watershed hAE Height of climatic alpine pine-forest limit TE Mean summer temperature, east side during the Atlantic period, eilst side TW Mean summer temperature, we st side 127 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 06:15:21, subject to the Cambridge Core terms of use. Se Lsing and wi shman: c:Lim::r.t;e roconst;1'Uct;ion from pine- forost; Limit; 1. nlTRODUCTION 2. THE PRESENT OPTIMAL CLIMATIC ALPINE PINE-FOREST The post-glacial climatic optimum is we11- LIMIT documented " in Scandinavi~. Evidence of this warm Mark and Heiberg (1937) define the alpine forest period in western Norway is based mainly upon studies limit as the zone between the forested and the non­ of vegetation history by pollen analysis (F~gri 1940, forested areas, where the distance between the trees, Moe 1977). Climatic interpretation in terms of mean which are at least 3 m high, is greater than 30 m; temperatures of the Atlantic period usually refer to we have adopted this definition in the present work. lowland areas, and yearly and summer mean temperatures By means of detailed field studies (Fig.2l, we have. of 1 to 2°C above those of the present are frequently determined the alpine pine-forest limit close to mentioned (see, for example, Lamb 1977). systematically collected subfossi1 trunks and stumps. In our study we present some quantitative esti­ mates of mean summer temperatures in the mountain area between Suldal and Setesdal in south-west NO ,rway lEGE NO (Fig.l). These estimates are based upon the interpre­ • Locahtles w,th roolO­ ca'bon dall!(! potJe tation of the presence of old pine stumps and trunks stumw'Irunk Fig.2. Locations of radiocarbon-dated pine stumps and trunks found above PPFL. PPFL was determined in the enclosed areas. The highest alpine forest limit was found on south­ west exposed slopes, confirming the findings of Aas (1969), and the optimal climatic present pine-forest limit (PPFL) is defined by a line drawn through these points (Fig.3). This line is unambiguous and allows chronological and geographical correlations to be made. The PPFL generally rises eastwards (Fig.3) and is lowest at the westernmost localities at 630 m a.s.l. It rises to 750 m a.s.l. west of the water­ shed, and reaches 930 m a.s.l. east of the watershed . The mean elevations were estimated to be hpW = 700 m a.s.1. and I1PE = 930 m a.s.1. on the Su1da1 and Setesda1 sides, respectively. 3. PRESENT SUMMER CLI MATE The mean summer temperatures for the "normal" 1931-60 period change with elevation on the west and Fig.l. Map showing locations of meteorological stations. east sides of the watershed (Fig.4). The mean summer temperature curves are linear regression curves based on temperature records from meteorological stations (Fig.1) for seven years reduced to a 1931-60 above the present pine-forest limit, which, we be­ standard. Stations to the west of the watershed are lieve, represents a higher pine-forest limit. We Mo, Fjellberg, Sandsa and HIIJga10ft, at 58, 381, 630, assume that moisture has always been sufficient in and 1 095 m'a.s.l., respectively, and stations to this area and that possible moisture variations have the east of the watershed are Byglandsfjord, Brokke not affected the pine-forest limit. Therefore the and Bj~en, at 212,443 and 920 m a.s.l., respectively. alpine pine-forest limit is mainly determined by the The linear regression curves for the areas west mean amount of heat available during the summer sea­ and east of the watershed are given by the equations son. As an indicator for the summer heat, we use the m~an summer temperature (T), i.e. the mean temperature hW = 2 400 - 173TW for June, July, August and September (or "tetraterm" and in Norwegian). hE = 2 250 - 14BTE The area investigated (Fig.l) lies between Suldal to the west and the north-south oriented valley of respectively, hand T being corresponding elevation/ Setesdal to the east. The lowland in Sulda1 is at mean summer temperature values. Each area is sea-level, while the valley floor of Setesda1 is at characterized by a specific temperature gradient, 500 to 900 m a.s.1. The mountainous area lies between which is 0.58°C per 100 m (gpwl for the western area, 1 000 and 1 200 m a.s.1., although in the northern and 0.68°C per 100 m (9PE) for the eastern area. part some summits reach 1 600 m a.s.1. The mountain­ The mean summer temperature curves are con­ ous area is above the tree line belonging to the low sidered as mean curves for the Suldal and Setesdal and middle alpine zone. It forms a topographic and areas. 1he curves intersect at a point hS = 1 350 m climatological divide between the districts to the a.s.l. and TS = 6.1°C. Below the intersection point, east and the west of it. the mean summer temperature difference between the 128 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 06:15:21, subject to the Cambridge Core terms of use. SeL8ing and Wi8lvnan: CUrrat e PeconBtT'uction fmm pine-foPe8t Umit ma.s.1. mas.i. 1300 W E 1300 Suldal Upper Setesdal 1200 . T~7.0 1200 1100 o 1100 0... 1000 --.- 1000 900 G'h---- 900 800 - .- 0 - 0 800 o_._ ·{> ·-o (; - - - 700 ._ 0- - --'- . .7-10.0 700 600 600 500 500 Vertical scale 25 x exaggeration 400 400 o Subfossil pine s tump or trunk from the Atlanti c period 300 o 2 subfossil pine stumps or trunks from the Atlanti c period 300 --- t; Present optimal climatic alpine pine forest I imi t (PPFL) 200 200 Alpine pine forest limit based upon pine s tumps and trunks from the Atlanti c period 100 ::T Mean wamrseason terrperatur e (mean terrperature June-Septerti:ler 1931-60) 100 Seising & Wishman 1983 OL-------------------------------------------------------------------------~ o Fig.3.