During the last years, disturbances in the forest have received increasing attention, especially after the devastating storms of December 1999 and its unprecedented consequences. Nearly every summer the media reports about large wildfires, especially in the Mediterranean area and North America. Possible consequences of a changing climate on the frequency and impact of disturbances are also topic of debate. On the other hand, in numerous European countries there is a trend towards more nature oriented forest management, in which natural processes, including disturbances, have their place.
Disturbances such as storms and fire form an integral part of the forest ecosystem. They have always occurred in forests, either in natural or managed forests. Information, however, on the events of disturbances are very scattered and incomplete, although first records of disturbances do date back to several hundreds of years, even as far as 1449 (Schelhaas et al. 2001). Forest fires have received most attention in the recent past, and most European countries have collected comprehensive statistics on forest fires. The FAO has been compiling these national statistics already since the early 1980s. Information on other disturbances is scattered throughout literature in European countries, many of which is in the national language and therefore not very easily accessible. Alterra (Wageningen, The Netherlands) and the European Forest Institute (Joensuu, Finland) have co-operated in order to identify and compile this information as far as possible. The result of this work is a database on forest disturbance events in Europe including nearly 30,000 records. The entries range from small biotic damages to large scale windthrows and devastating forest fires.
In the compilation process, an important source of information were the International Forest Fire News (IFFN) Country Notes, very often containing detailed and extensive time series on forest fires. With the help of these time series, it was possible to compile a European wide time series on forest fires that extends further back than the FAO series. This article presents some results of the analysis of the database contents on forest fires at a European scale. An estimate is made on the development of the burnt area in the classes ‘forest land’ and ‘forest and other wooded land’ over the period 1961-2000, as well as an estimate on the number of fires over the period 1971-2000.
This study focuses on Europe excluding the Newly Independent States, including in total 31 countries. For all countries the burnt area was extracted from the database for the classes ‘forest land’, and ‘forest and other wooded land’. The same was done for the number of fires. Since it is often not clearly stated if data refer to ‘forest land’ or ‘forest and other wooded land’, the resulting figures had to be analysed on its consistency. In some cases the same figures were reported, but in one source as being based on ‘forest land’ and in another source on ‘forest and other wooded land’. From overlaps in time series for individual countries, these errors could be corrected.
Not for all countries a full time series was available, therefore it was necessary to estimate missing values. For each country the average ratio was calculated between the burnt area of the two classes over the years where data was available. For years where only one of the two classes provided data, an estimation was performed to produce the value of the missing class using this ratio and the known value.
For all 31 European countries, averages were calculated over all known years for the burnt area on ‘forest land, on ‘forest and other wooded land’ and the number of forest fires. These averages were aggregated to get a mean value for Europe for each of the required parameters. The contribution of each country to these mean values was then calculated for all countries subject to this study.
If for a particular year one or more countries lacked data for a required parameter, the total of the known values was calculated, as well as the average contribution of the corresponding countries in the European total. From these average contributions the total value for Europe could be estimated.
Example: Missing data on the forest fire area for a certain year in Spain.
The total forest fire area for all other European countries adds to 240,000 ha, and the average share of Spain in the European total is 20%. Therefore, the total forest fire area in Europe is estimated to be 240,000 / 0.8 = 300,000 ha for that year.
Over the period 1971-2000, the annual number of forest fires has increased visibly, from 40,000 on average in 1971-1980 to 95,000 on average in 1991-2000 (Figure 1, Table 1). Especially in the 1990s the increase is quite obvious. Several factors can play a role in this trend. Firstly, the detection methods and the quality of the databases have improved considerably, most likely resulting in the detection and reporting of more fires. Secondly, due to aggressive and effective fire suppression policies, fires are extinguished earlier, which may lead to more but smaller fires. This development is reflected in Figure 4, which shows a decrease of the average fire size over time.
Other factors are also important, but more difficult to assess. This accounts especially for various socio-economic factors, since most of the fires originate from human causes. One example is the increase of the recreational use of forests in the recent past that may contribute to an increase of forest fires.
The annual burnt area both on ‘forest land’ and ‘forest and other wooded land’ appears rather stable in the 1960s, after which it increases during the 1970s. This increase stops at the end of 1970s and in the second half of the 1990s the annual area burnt shows signs of a decrease. The inter-annual variability is quite high and is most visible in the period 1975-2000 with a number of peak years clearly visible (Figure 2 and 3). The observed increase in inter-annual variability could be due to more incomplete data coverage, less effective detection methods and reporting systems before 1975. What are the exact causes of the increase is quite difficult to identify. Certainly to be mentioned are socio-economic factors, e.g. (1) the decreased use of firewood, leading to higher fuel loads in the forest, (2) depopulation of the countryside, resulting in larger areas of closed forest and (3) the influence of climate change. Also forest management itself plays a role, e.g. by establishing large monoculture plantations.
In the period 1991-2000, the average annual burnt area of ‘forest land’ amounts to 227,000 ha, which is about 0.13% of the total forest area of the countries under consideration, using data from the Temporal and Boreal Forest Resource Assessment 2000 (UN-ECE/FAO, 2000). The average annual burnt area on ‘forest and other wooded land’ in the same period is about 0.23% of the total area of that class. These figures also vary considerably among countries and years as in the data from the DFDE database.
Mediterranean countries account to most of the fires. On average 94% of the annual burnt forest area occurs in the Mediterranean region, which is approx. 0.3% of ‘forest land’ in the time span 1991-2000. The average annual area burnt on ‘forest and other wooded land’ is higher for the same period (approx. 0.47%). The most affected country is Portugal. An average of 1.25% ‘forest land’ is annually affected by fires and 2.73% of forest and other wooded land.
Figure 1. The total reported and estimated number of forest fires in Europe 1970-1999.
Table 1. Averages of the estimated number of fires and the estimated fire area for the two classes ‘forest land’ and ‘forest and other wooded land for Europe’ (excluding NIS countries) for 10-year intervals.
Figure 3. The total reported and estimated area of fires on ‘forest and other wooded land’ in Europe 1961-1999.
Figure 4. The average burnt area/fire on ‘forest and other wooded land’ 1970-1998.
Sources often do not clearly state which land category the reported data refer to, and in cases they may even be reported apparently to the wrong class. By implementing comparisons of different sources these errors could mostly be corrected for. Often differences were found in data concerning the same year and class. Partly these differences accounted to the use of different definitions or the use of different classes (e.g. only state owned forest land). In the majority of cases differences between sources could not be explained, indicating that the reliability of the data remains uncertain. This became apparent when analysing data from different sources as a time series. For some countries, the ratio between burnt area on ‘forest land’ and ‘forest and other wooded land’ changed considerably between different time spans, indicating e.g. a possible change in definitions or methodology. Also the methods of fire detection have improved during the investigated time period, making the resulting databases more reliable in recent decades.
The method that is used for upscaling from the country level to European is quite rough, and it assumes that the area and number of fires in a particular year are correlated among the countries. To a certain extent this is true as neighbouring countries often face the similar weather conditions. This however holds only to a certain extent, since country specific factors may influence the fire frequency and intensity. Combined with the uncertainty within the data, one should be careful in interpreting the results. Despite these restrictions the collected data on forest fires certainly provides a satisfactory picture of the trends in forest fires over the last decades.
A database has been constructed from the data that was collected during 1999 and 2000. Using these data it was possible to construct a European wide overview of the number and area of forest fires. Although the uncertainty in the data most likely increased while analysing data back to the 1960s, as does as the uncertainty connected with the method used, a trend is visible. It indicates an increase in the number of fires and the annual burnt area, especially in the 1970s and partly the 1980s. The average size of the fires is apparently decreasing, which might be due to increased fire fighting efforts. The fire situation is most severe in the Mediterranean area, accounting for about 74% of all forest fires and more than 90% of the annual burnt forest area in the 31 investigated European countries.
The analysis performed in this study shows only one aspect of possibilities of the Database on Forest Disturbances in Europe (DFDE). The DFDE is currently still expanding, both with recent data and historical data. Also more detailed forest fire records are collected from individual countries or regions, thereby increasing the possibilities for more detailed analyses.
Contributions from IFFN readers are very welcome!
Schelhaas, M.J., Varis, S., Schuck, A., 2001. Database on Forest Disturbances in Europe (DFDE), European Forest Institute, Joensuu, Finland. http://www.efi.fi/projects/dfde/
UN-ECE/FAO, 2000. Forest Resources of Europe, CIS, North America, Australia, Japan and New Zealand. Main report. Geneva Timber and Forest Study Papers, No. 17. United Nations, New York and Geneva. 445 p.