52 L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67
1971 and Burke et al. 1976 and the implications of frost during the peak of the growing season is of
specific importance for the establishment and devel- opment of conifer saplings Li and Sakai, 1981; Sakai
and Larcher, 1987. It should also be mentioned that the risk of injury might be high during the spring when
buds are flushing and the apical shoots develop.
Furthermore, survival and yield of coniferous veg- etation is related in a broad sense to variations in
altitude Persson and Ståhl 1990. In this context, local terrain complexity plays an important role, i.e.,
marked variations in the topography leading to terrain constrictions with implications on the establishment
of frost. Therefore, methods that point out hazardous areas with regards to clear-felling and succeeding
plant regeneration are useful planning tools in the lo- cal management of elevated forests. Furthermore, it is
important that such methods are general in their appli- cability and yet accurate in their usage across different
areas, due to the strong variations in local climate that can be expected between different terrain types.
Information on the temperature distribution in different terrain, different latitudes and different el-
evation is commonly used in indices and for classi- fications related to, vegetation distribution and tree
growth, e.g., Thornthwaite, 1931; Thompson, 1969; Ahti, 1970. Several authors have concerned them-
selves with mapping of the bioclimate in Scandinavia the temperature climate in particular, as it is related
to the distribution of various types of forest ecosys- tems on both a regional and national scale e.g., Ahti
et al., 1968; Hustich, 1979; Tuhkanen, 1980; Odin et al., 1983. Often, such studies make use of variables
which are closely related to temperature e.g., coldness sum, frost sum, heat sum, degree-days d.d., growth
units and length of growing season. However, stud- ies on the temperature climate or related expressions
that are applicable to a local mountainous terrain are less frequent. There are several obvious reasons to
this, for example, the network of synoptical meteo- rological stations becomes less dense in mountainous
regions and consequently does not support local cli- mate applications. Also, the mountainous areas are
often vast, have low accessibility and are, therefore, more difficult to assess manually.
Early works concerning frost investigations dealt primarily with empirical formulae for specific areas
such as orchards, where anticipated low temperatures and the duration of temperatures at certain levels are
predicted. The frost risk is often calculated from an analysis of the energy balance and heat transfer pro-
cesses near the surface. Several of the empirical mod- els are derived from the Brunt equation Brunt, 1944.
Several authors have performed mapping of tem- perature variations and variation in local frost risk.
Lomas et al. 1989 have analysed a large number of temperature recordings in order to produce local-frost
risk maps. These maps show the number of occasions expected for selected sub-regions in relation to long
term data. More sophisticated models have also been developed such as the three-dimensional local scale
numerical models for simulation of the microclimate near the ground surface in complex terrain Avissar
and Mahrer, 1988.
The present project is focused upon the establish- ment of minimum temperature variation during differ-
ent weather situations in relation to terrain types. The purpose is to develop a method for a terrain division
that accounts for a high proportion of the local vari- ability of summer frost in elevated, complex terrain.
Such an empirical model could further be used for other areas, i.e. the method should be based on general
parameters which could be determined for new areas with little or no help from measurements.
2. Characteristics of the study area
The topography of the study region is characterised by isolated mountain outcrops reaching over 1200 m
a.s.l. resulting in an undulating topography including valley systems and lakes. The Scandinavian mountain
range ‘the Scandes’ extends in a north–south direction along the borderline between Norway and Sweden,
Fig. 1 shows the location of the study area in relation to areas more than 600 m a.s.l. in Sweden. Towards the
east in this zone, the isolation of hilltops becomes more obvious whereas towards the west the elevated areas
form a massif into which river valleys penetrate. This gives rise to a clear drainage pattern extending from
the west and northwest towards the east and southeast. Several of the valleys have narrow and elongated lakes
e.g., the study area includes L. Messlingen 680 m, L. Särvsjön 645–660 m and L. Grundsjön 630–650 m.
The relative relief of the region is characterised by strong local variations and feature elevations between
L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67 53
Fig. 1. The location of the study area in the southern part 62–64
◦
N of the Scandinavian mountain range Scandes. The location of areas above 600 m a.s.l. in the Scandes are included
for the Swedish side.
500 and 1400 m a.s.l. A detailed map of the study area showing relief contours, lakes and mountain peaks is
provided in Fig. 2. The weather of the study area can be characterised
as an alternation of low pressure systems frequenting the region most frequently from the west and periods
2–5 days in between with anticyclone conditions. Periods of blocking highs are also relatively frequent.
During such longer clear periods frosts are experi- enced. This has important implications for plant injury
and mortality of conifers during the peak of the grow- ing season July–August and in spring at the time
when flushing and the development of apical shoots takes place.
During the most recent standard period, 1961–1990 SMHI, 1991 the nearby synoptic weather station
at Fjällnäs, located 30 km west of the study area at 780 m showed an annual average temperature of
− 0.4
◦
C. The mean temperatures for July and August are 10.5 and 9.4
◦
C, respectively. At mean height of the study area 750 m the annual average tempera-
ture was −0.2
◦
C and the monthly averages for July and August were 10.9 and 9.5
◦
C, respectively, for the period 1994–1996.
The growing season normally starts at the beginning of June and extends to mid-September. Based on the
measuring period 1961 to 1976 the average length of the growing season for the region has been established
by Odin et al., 1983 to 120–140 days and the tem- perature sum to 550–650 d.d., threshold value +5
◦
C. These numbers are reduced to less than 110 days and
530 d.d. at mean height in the study area. Mountain birch Betula pubescens subspecies ssp.
tortuosa forms the tree-line at approximate elevations between 850 and 1000 m above sea-level and are gen-
erally frequent above 800 m. However, in the south- ern part of the region, conifers occasionally form the
tree-line without the presence of any birch. At heights between 750 and 800 m, poorly developed and widely
spaced conifers Picea abies ssp. and Pinus silvestris usually dominate the forests. Birch intermingles to a
various degree with pine and spruce in this zone of transition.
The distribution of trees is obviously associated with altitude. The coniferous vegetation tends to show
a gradual improvement in progress as height above sea-level decreases, however the complexity of the ter-
rain has a strong influence on the distribution. Thus, in local areas like valleys and low level terrain down
to 600 m, trees may be completely absent, i.e. inverted tree-line, or show a highly reduced development. In-
clined ground, particularly south and southwest facing slopes exhibit the best environments for tree growth
in the study area.
The frost risk levels used in this study correspond to a limited time period of the growing season. The
use of this particular period is considered due to the fact that plants show a very high susceptibility to frost
injury during the time when they are fully dehardened at the peak of the growing season, cf. Weiser, 1970;
Li and Sakai, 1981; Sakai and Larcher, 1987. Conse- quently, frosts during this period have important im-
plications for the establishment and development of conifer saplings. However, it should be pointed out
that the risk may be equally as high at shorter time periods during the spring when flushing and the early
development of apical shoots takes place.
54 L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67
Fig. 2. Topographical map over the study area, the temperature sampling locations are shown.
3. Methods and materials
