Agricultural and Forest Meteorology 102 2000 51–67
A frost assessment method for mountainous areas
L. Lindkvist
a,∗
, T. Gustavsson
a
, J. Bogren
b
a
Forestry Board, West Sweden, Box 343, SE-503 11 Borås, Sweden
b
Göteborg University, Department of Earth Sciences, Physical Geography, Laboratory of Climatology, Guldhedsgatan 5C, SE-413 81 Göteborg, Sweden
Received 15 October 1997; received in revised form 26 July 1999; accepted 3 August 1999
Abstract
The present paper describes a model for frost assessment in mountainous areas in relation to forest management. Data were collected at 38 locations within a 625 km
2
region, which is characterised by a diverse topography and vegetation cover. Air temperature measurements were performed during the peak of the growing season. July–August 1996 in the southern
Swedish mountains at elevations between 500 and 1200 m a.s.l. The variation in nighttime minimum temperatures was analysed in relation to the prevailing weather and terrain type of the measuring sites.
From the analysis of the temperature and weather conditions, i.e., wind and radiation, it was concluded that more than 90 of the frost situations, occurring during the study period, were of the radiation type. It was further concluded that the variation
among the studied stations was closely related to the terrain type during these situations. Frost occurred most frequently in narrow valleys, then in concave and flat locations. Elevated and convex areas were found to have very few situations of
radiation frost.
Local terrain information was used together with a calculated frost index for assessment of the spatial variation in frost risk. Furthermore, a grid net was applied to the study area and the pixels were given a terrain form of the type convex, slope,
flat, wide concave or narrow concave according to the dominating terrain curvature. For each pixel a frost index value was estimated from the recorded temperatures at the field stations. A cluster analysis was used to group the terrain types according
to the index, whereby six obvious clusters were obtained each with clearly differentiated frost intensity. The analysis showed that this kind of treatment is a suitable method for assessing the spatial variation in frost risk. © 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Growing season; Radiation frost; High elevation; Topoclimatology; Cluster analysis; Frost risk assessment
1. Introduction
Assessment of the local climate variability is an im- portant issue in sound forest management. It is a partic-
ularly relevant component in areas with elevated boreal
∗
Corresponding author. Tel.: +46-33-177331; fax: +46-33-177389.
E-mail address: lars.lindkvistsvsvg.svo.se L. Lindkvist
forests, due to its exposed environment near the limit of physical tolerance for tree vegetation. Thus, moun-
tain forests represent a thermally marginal ecosystem which is believed to show an early response to varia-
tions in climate e.g., Schlesinger and Mitchell, 1989; Mitchell et al., 1990.
The variability of low summer night temperatures is of crucial concern for the survival and progress of
young trees as shown, for example, by Christersson
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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
