Switzerland: Fire Ecology and History Research (IFFN No. 15)

Fire Ecology and History Research
in the Southern Part of Switzerland

(IFFN No. 15 – September 1996, p. 13-21)


In Switzerland most forest fires occur in the southern part, a small region of 4000 km2 (9.8% of the total national area) with a forest cover of 44% (176,000 ha). An average of 740 hectares of woodland in southern Switzerland burn annually, generally during the dry winter period, but recently also during the summer seasons.

The southern part of Switzerland is situated in a small basin, closed toward the north and the west (Alps) and open toward the south and the east (Po Valley). This geographical situation with the monotonous barrier on the one hand and the open plain of the Po Valley on the other hand modifies the atmospheric currents of air. The western currents, which are normally humid and relatively warm, are often shunted to the north, the polar and subpolar air masses from the north have to pass over the Alps and arrive at the foot of the Alps in central and southern Ticino relatively dry and mild. The southwestern, southern, southeastern and eastern air masses can arrive in Ticino relatively directly, which means that the currents from the south often bring heavy rainfall and from the east cold air masses in winter and hot air masses in summer. The climate therefore is characterized by dry and sunny winters with periods of north-foehn (main time of forest fires) but also by occasionally strong snowfalls, by wet springs and autumns and by sunny summers with very heavy falls of rain (thunderstorms).

The typical vegetation under the climatic conditions in this region are chestnut forests on acid soils, deciduous broadleaved mixed forests on limestone and beech forests at altitudes between 800 and 1300 m a.s.l..

In 1992 the FNP Sottostazione Sud delle Alpi (FNP SdA), a branch station of the Swiss Federal Institute of Forest, Snow and Landscape Research, started its research on forest fires by creating a wildfire database including all information available on forest fires in southern Switzerland. Since that time several research topics about forest fires and their management have been under study at the FNP SdA.

In this first contribution about forest fire research in Switzerland we will report on the ongoing studies on fire ecology and fire history research in the southern part of the Swiss Alps which are being conducted at the FNP SdA in close collaboration with other Swiss research groups.

Forest Fire History of this Century in Southern Switzerland

In 1992 a forest fire research project (Conedera et al. 1995) was started within the scope of the Swiss National Research Programme 31 (NRP 31) “Climate Change and Natural Disasters”. The NRP 31 project made it possible to reconstruct on the basis of more than 5300 fire-events the pattern of forest fires in the present century and also to study the relationship between weather and forest fires.

The most notable aspect of the development of the fire regime in this century is the general increase in the occurrence of fires since the sixties (Fig.1), with a marked rise of fire occurrence in summer (May to November) since the 1970s (Fig.2). This change cannot be explained simply through the analysis of particular meteorological factors or the inclusion of the major anthropogenic causes. Rather it must be seen in relation to the rapid changes in the socio-economic conditions on the southern side of the Alps. These have lead to an acceleration of the increase in the area of forest, an abrupt cessation of litter utilisation, and other agricultural activities, as well as a drastic reduction in the exploitation of timber since the post-war years.

Consequently, the amount of fuel has increased. This in turn has reawakened fire to its role as a natural regulator of the biomass in the forest ecosystems, a role which will certainly become more influential in the future, when we regard the natural evolution already taking place in the forest ecosystems of the chestnut belt towards mixed deciduous broadleaved forests. Further, the probable consequences of a possible climatic warming with a concomitant shift of the vegetation zones at fairly high altitudes would render forest fire one of the most important factors in bioregulation.

Another remarkable aspect of forest fire behavior in southern Switzerland is that nearly all forest fires are surface fires. Crown fires occur very rarely. In connection with the increase in the occurrence of fires in summer since the seventies, ground fires as well as surface fires combined with ground fires have also increased (Fig.3).

Figure 4 shows the burned area per year in southern Switzerland. Although there is a relationship between the number of forest fires and the burned area, high frequency of forest fires does not always correspond to extensive burned areas. Vice versa, during years with relatively few forest fires extensive areas can burn (Figs.1 and 4, for example 1970, 1976, 1984, 1989). An annual average of 0.4% of the total forest cover burns, in extremely dry years up to 4.1% (1973) can be destroyed by forest fires.

These results show that studies on the ecological aspects of forest fires in southern Switzerland should be intensified, because the ecological effects, especially of winter fires (December to April), which are dominant in the southern part of Switzerland, are still little known.

Fig.1. Evolution of the annual number of forest fires in southern Switzerland
(will be added)

Fig.2. Evolution of the occurrence of forest fires during the winter and summer periods in southern Switzerland
(will be added)

Fig.3. Evolution of the distribution of types of forest fires in southern Switzerland
(will be added)

Fig.4. Evolution of the forested area annually burned in southern Switzerland
(will be added)

Forest Fire Paleohistory and Vegetation Paleoecology in Southern Switzerland

As shown by Clark et al. (1989) in southwestern Germany, and Odgaard (1992) in Denmark, the vegetation history in the temperate latitudes of Europe can be influenced by forest and heathland fire. Paleoecological methods reveal long-term tree and shrub succession following forest fire and may also answer the question as to whether past forest fires are anthropogenic, climatic, or both. Early human impact can be traced by the proportion of pollen of crop plants and weeds. On the other hand pollen and plant macrofossil records can also be used to reconstruct timberline fluctuations indicating temperature changes during the Holocene.

Because little is known of the fire paleohistory of the southern Alps, the University of Bern started a study of the lake sediments of Lago di Origlio in southern Ticino (416 m a.s.l.) in order to reconstruct past forest fires and their possible effects on vegetation (Tinner and Conedera 1995). Continuously deposited unlaminated silty gyttja sediments with a thickness of 13.15 m were cored in the deepest part of the lake at a water depth of 6 m. Terrestrial plant macrofossils and silty gyttja were radiocarbon dated by AMS-techniques by K.van der Borg (Utrecht) in order to provide a time scale. The radiocarbon ages in Figure 5 are presented as conventional uncalibrated radiocarbon years BP (Before Present). Charcoal analysis was carried out by means of image analysis and the areas of the charcoal particles (bigger than 75 m m2) in the pollen slides per sediment volume (106m m2/cm3) were calculated. These values were then compared with the results of pollen analysis (% of total sum or pollen grains/cm3).

Fig.5. Spores of Pteridium aquilinum and concentration ofcharcoal areas in the lake sediments of Lago di Origlio in southern Ticino.
(will be added)

The preliminary results of pollen analysis and radiocarbon dating suggest an undisturbed stratigraphy from the Younger Dryas to the present. The fire history of Lago di Origlio can be subdivided into four major periods. The first period probably corresponds to the end of the Paleolithic and to the Mesolithic and is characterized by low charcoal values. The second period covers the Neolithic and shows medium charcoal values. The highest charcoal values occur in the third period, probably during the Bronze and Iron Ages. Finally, the fourth, lasting from Roman to Modern Times, shows rapidly decreasing charcoal values. During this last period the charcoal curve reaches its Holocene minimum. The first big peak in the charcoal curve is dated shortly after 6000 BP (ca. 4950 BC dendrocalibrated) in the period of the Early Neolithic. The most intensive forest fires around Lago di Origlio during Holocene seem to have been anthropogenically induced. Strong indications for this assumption are regular findings of pollen grains of Plantago lanceolata, Cerealia and other plants of managed habitats since ca. 5000 BP (Tinner and Conedera 1995). Possibly the Neolithic, Bronze Age and Iron Age farmers used fires for clearing forest in southern Switzerland.

All marked peaks in the charcoal curve since the Neolithic correlate with decreases of tree pollen, indicating that trees around Lago di Origlio suffered great damage by forest fires. The damaged trees were mainly Abies alba, Fraxinus excelsior, Ulmus, and Tilia. In contrast, Pteridium aquilinum, Corylus, and since the Subboreal Calluna, were able to spread after fires (Fig. 5 and Tinner and Conedera, 1995). Around 2200 BP, at the end of the Iron Age, even the relatively fire-tolerant stands of Alnus glutinosa t. (t.=pollen type) were reduced, providing space for the introduction of Castanea sativa. With the spreading of Castanea (up to 46 % pollen) after 2000 BP the charcoal curve strongly decreases. Probably, forest fires were no longer useful to farmers but rather endangered the cultivation of Castanea sativa.

The vegetation changes since the Neolithic, caused most probably by anthropogenic forest fires, are considerable: the forests around Lago di Origlio became strongly thinned, and Abies alba disappeared from the surroundings of the lake between 5000 and 4000 BP. The strong reduction of Ulmus, Tilia, and Fraxinus excelsior t. at around 4000 BP led to a definitive transition from mixed-oak to oak forest.

Similar investigations are in progress in the central-alpine dry-climate region of Switzerland (the Valais). In order to compare the charcoal contents in sediments with the wildfire database (Conedera et al. 1993; Marcozzi et al. 1994), the uppermost 47 cm of the lacustrian deposits from a frozen core in Lago di Origlio will be continuously analyzed. Higher time resolution, more radiocarbon dates, and further coring sites will allow us to refine our investigations and to approach successional and spatial questions.

Forest fire consequences for the vegetation:
In 1990 the University of Lausanne studied the consequences of fires on the chestnut forest vegetation on south facing slopes in detail for the first time (Delarze et al. 1992).

In 1994 and 1995 we studied the consequences of fires on mixed deciduous forest vegetation on southern slopes, on chestnut forest vegetation on northern slopes and on beech grove vegetation. During these two years we worked together with the University of Lausanne (Prof. Dr. P. Hainard, Dr. R. Delarze), with Dr. M. Marchetti (Italeco S. p. A., Roma) and with Dr. G. Carraro (Dionea S. A., Locarno).

We used 100 m2 phytosociological plots according to Braun-Blanquet’s method, made in areas hit by various fire frequencies. At the same time dendrological measurements were recorded for each plot in order to calculate an index of fire damage from fire scars on tree barks. Moreover, these measurements allowed analysis to be made of the basal area (area in square metres of the cross-section of a tree stem at breast height and inclusive of bark) for some tree species under different fire regimes.

For appreciation of the site conditions we used ecological indices (ranging from 1 to 5) according to Landolt. In Switzerland almost every higher plant species is characterized by eight of these empirical indices, representing its needs for soil moisture (F) and basicity (R), soil nutrients (N) and humus contents (H), soil dispersion (D), average light (L) and temperature (T) during the growing season, and continentality (K). The effect of fire on these indices can be estimated through the species composition of plots from different zones. We analysed our data using statistical tests, mainly variance and factor analysis. No effects of fire on the total basal area was observed, but we noticed that some tree species were substituted by more fire resistant ones in the most frequently burned areas of the mixed deciduous forests. In this case, indices of fire damage are not adapted for a correct evaluation of the situation. But they are for the almost monospecific and fire tolerant chestnut forests.

The consequences of fire on the vegetation physiognomy are almost the same everywhere. Fire leads to a lessened tree cover, which results in more light reaching the ground, a steeper temperature gradient in the soil, and an increased continentality. Therefore, species diversity usually increases shortly after a forest fire. Some species are almost always favoured by fires (Pteridium aquilinum, Robinia pseudoacacia, Rubus fruticosus s.l., Galeopsis tetrahit) while others are almost always curbed (Hedera helix, Corylus avellana, Fraxinus spp. and Tilia cordata).

It was possible to represent most different post-fire reaction patterns (acidification, loss or enrichment of nutrients and humus content, increase or decrease of humidity of the sites, etc.) as a function of forest type, fire frequency, slope aspect and time elapsed since the last fire.

Some of these post-fire effects on the vegetation and on the soils will have to be discussed again in the light of the soil erosion study which is in our programme.

Soil erosion and runoff after forest fires:
After forest fires, which frequently break out in the steep colline-submontaneous belt between 200 and 1000 m a.s.l., the ground surface lies partly bare because most of the vegetation, the litter and also the organic matter on the ground surface have been at least partly burned.

Different ecological effects on the forest ecosystems can be observed depending on natural conditions (precipitation, soil, topography, etc.), the fire intensity, the fire frequency, the elapsed time since the last forest fire, the type of forest fire, the size of the burned area, and the moment of the fire outbreak during the vegetation season. Beside the vegetation and the organic matter the microorganisms of the soils and the mycorrhizal fungi, which can stabilize the soils, can also be destroyed while varying physical and chemical changes of soil parameters can lead to a destabilisation of the soils. Under these conditions the sandy and acid Haplic Podzols (after FAO classification) in the region of the chestnut belt, which are relatively poor in nutrients, are exposed to increased runoff, soil erosion, and nutrient loss.

A soil erosion project was started in November 1995 and will be realized in the next three years in close collaboration with the Faculty of Geography of the University of Basle.

The knowledge of sediment yield (quantity per unit area and time) and runoff rates is of primary interest for the valuation of soil degradation after the first and repeated forest fires. The soil erosion process therefore will be studied by using testplot (3m x 10m) measurements as well as a process-oriented geoecological methodology on representative chestnut forest land burned under natural conditions. By using sediment traps and tubes for catching eroded material, ash, and runoff it will be possible to quantify and also to qualify materials transported from burned slopes as well as from unburned slopes, where we will also install testplots. Thus we will be able to distinguish between natural (unburned plot) and fire-induced (burned plot) erosion.

The quantification of sediment yield rates, runoff and nutrient loss is being done for the first time on burned slopes in Switzerland and is therefore an important aspect of this project.

The landscape ecological approach being used considers the geoecological factors soil, georelief, climate, vegetation and also the anthropogenic influences, and therefore supplies quantitative data on larger burned areas. With the extrapolation of the data received at “plot scale” to “burned area scale”, by using soil erosion models we can create for example soil erosion risk maps, which will be an important instrument in fire management policies.

Another important aim of this soil erosion project is to see if we can detect a correlation of soil erosion and runoff rates with the meteorological parameters, other geoecological factors, and fire-induced parameters such as fire intensity or the elapsed time since the last forest fire. A further aspect is the comparison between actual and earlier soil erosion rates, which we hope to find out with the help of charcoal analysis of different horizons of soil profiles. With that it should also be possible to calculate tolerable soil loss rates, which is important with regard to the proposal for eventually necessary soil conservation practices.

First results of the testplot measurements on 65% to 90% steep chestnut coppices near Mte Brè s. Locarno at 900 m a.s.l. and near Tenero – Contra at 550 m a.s.l., where it was burning in November 1995 and in April 1996 respectively, show that there is a high runoff rate from the slopes at Mte Brè s. Locarno, where it burnt five times during the last 30 years. The runoff rate under very similar precipitation conditions is also higher in relation to the slopes at Tenero – Contra, where it burnt for the first time this century. With the runoff more nutrients leave the burned areas compared with the unburned areas. Plots on slopes with similar natural conditions but affected by different fire intensity show that higher fire intensity correlate with higher soil erosion rates and also higher nutrient loss.

The first soil erosion events, which could be observed and measured during May and June, are still low. This is not surprising because the really erosive precipitations in the southern part of Switzerland happen during summer. Higher amounts of sediment yield and runoff rates are therefore to be expected in the coming months.

Because this study only started last November it is too early to present final results, but it seems that fire intensity as well as fire frequency (probably also the elapsed time since the last forest fire) play a very important role in erosion processes as well as the erosivity of the precipitation, the erodibility of the soils, the topography, the vegetation cover, and the human activities (silviculture).

Perspectives for the future:
The study on the occurrence of forest fires together with the construction of an information system formed a decisive first step in the intensification of interdisciplinary studies on the ecological effects related to forest fires on the southern side of the Swiss Alps. In future it will also be necessary to intensify team-work on an international level, with regions and countries which have similar problems with forest fires, to seek common solutions.

For this year it is planned to start a project which will examine the influence of forest fires on invertebrate biodiversity in chestnut forests on the southern slopes of the Swiss Alps. The role of forest fires in regulating forest biomass and in initiating adaptation processes in plants and animals will be studied. The aim is to find out what effects single and repeated forest fires have on the faunistic biodiversity in the chestnut forest ecosystems in southern Switzerland.

We also plan to initiate studies on the effects of forest fires on other ecosystem components, such as mycorrhizal fungi or humus fractions in the near future.

Ecological fire research studies in other parts of the world show that forest fires are often a driving evolutionary force and sometimes even an important prerequisite for sustainable biodiversity. A closer look at the situation in Switzerland will show whether such circumstances are also valid for this region, where most fires take place during the dormant season.

Although forest fires in southern Switzerland seldom became a threat to the life and property of local residents and tourists, some problems can originate from forest fires in connection with the protective function of the forest, soil conservation or economic aspects of the timber industry. Therefore extensive and expensive measures are taken to prevent and fight fires under all circumstances.

With the ongoing studies about the ecology of forest fires we hope to elaborate a decisive instrument to support the authorities responsible for fire management and fire-brigades, because it is our intention to aim at a more differentiated fire management strategy for southern Switzerland.

References on Forest Fire Research in Switzerland

Clark, J.S., J. Merkt, and H.Müller. 1989. Post-glacial fire, vegetation, and human history on the northern alpine forelands, south-western Germany. J. Ecology 77, 897-925.

Conedera, M., M.Marcozzi, and B.Jud. 1993. Banque de données sur les incendies de forêt au Sud des Alpes suisses. Symposium “Contribution of European Engineers to Reduction of Natural Disasters”, 29-30 Septe,ber 1993, Lausanne, pp. 165-171.

Conedera, M., M.Marcozzi, B.Jud, D.Mandallaz, F.Chatelain, C.Frank, F.Kienast, P.Ambrosetti, and G.Corti. 1996. Incendi boschivi al Sud delle Alpi: passato, presente e possibili sviluppi futuri. Rapporto di lavoro PNR 31, vdf Hochschulverlag AG, Zürich, 143 pp.

Delarze, R., D.Caldelari, and P.Hainard. 1992. Effects of fires on forest dynamics in Southern Switzerland. Vegetation Science 3, 55-60.

Marcozzi, M., M.Conedera, and B.Jud. 1994: Forest fire research in Switzerland. Int. Forest Fire News No. 12, 32-33.

Odgaard, B. V. 1992. The fire history of Danish heathland areas as reflected by pollen and charred particles in lake sediments. TheHolocene 2-3, 218-226.

Tinner, W., and M.Conedera. 1995. Indagini paleobotaniche sulla storia della vegetazione e degli incendi forestali durante l’Olocene al Lago di Origlio (Ticino meridionale). Bollettino della Società Ticinese di Scienze Naturali 83, 91-106.

From: Marco Conedera, Peter Marxer and Willy Tinner and Brigitta Amman Claire Hofmann

FNP Sottostazione Sud delle Alpi
Geobotanisches Institut casella postale 2014 der Universität Bern via Pedotti 14 Altenbergrain 21 CH – 6501 Bellinzona CH – 3013 Bern Fax: ++41-91-8215565 Fax: ++41-31-3322059 Tel:  ++41-91-8215562/63 Tel:  ++41-31-6314922/11 e-mail: marco.conedera@wsl.ch e-mail: peter.marxer@wsl.ch

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