Large trees weakened by fire provide ideal conditions for fungal colonisation. On the southern side of the Alps the Research Institute WSL is investigating how beech, oak, and chestnut trees react to forest fires and which fungi colonise the trees. This knowledge is important for providing optimal forest management in freshly burnt areas.
Fig. 1 - The burn area in Cugnasco one year later. Many of the sweet
chestnut trees have numerous young vegative shoots.
Photo: Marco Conedera (WSL)
Human development started with the control of fire. Fire, along with storms and volcanic eruptions, is a major disturbance in a natural eco-system. Thanks to fire ecology we now know that in many forest eco-systems, such as the boreal pine forests, the Californian Chaparral forest or the African Savannah, that fire plays an important regulatory role.
Many plants and animal species have adapted themselves to forest fires. At times fire is even a requirement for them to be able to permanently survive (e.g. pine cones whose seeds are released only after extreme heat). Fire can also have negative effects, especially when it strikes mountain forests and damages its protection function. In such cases foresters are expected to take well timed silvicultural and technical measures in order to prevent follow on damage.
How much a tree is weakened by fire depends on many factors: on the intensity of the fire (height of flames, intensity and duration of heat radiation), on the trees heat resistance capacity (insulating rough bark, protection due to neighbouring trees), on the regeneration capacity of damaged tissue (overgrowth of open wounds, the production of substitutive shoots) and on infestation by tree weakening parasites and insects.
Fig. 2 - Most of the trees affected by the fire in
Cugnasco looked like this.
Photo: Marco Conedera (WSL)
In the United States there has been plenty of research into the reaction of trees after forest fires. Species specific criteria have been developed. These are used to estimate the survival chances of single trees depending on the injuries caused by the fire and to plan expedient silvicultural measures needed for the treatment of such stands. In Europe only isolated studies have been carried out on this subject, mainly in the Mediterranean area on pine species such as Pinus pinaster.
The EU Research Project "Fire Paradox", which started in 2006, permitted such observations to be carried out in deciduous forests. The Federal Research Institute WSL is closely observing three tree species; beech (Fagus sylvatica), oak (Quercus petraea and Q. robur) and the sweet chestnut (Castanea sativa). In selected forest fire areas in southern Switzerland it is examining the following three aspects:
Using these parameters the state of health and the reaction ability were recorded over many years for every fire and every tree species.
The forest fire near Cugnasco
Between the 3rd and 4th April 2006, during a strong north föhn wind, 55 ha of forest burnt on the southern slope above Sasso Fenduto (Cugnasco, Tessin). This area, which extends from 450 m to 850 m above sea level and which according to the forest data base had never had a fire in the past 100 years, was stocked with the remains of a sweet chestnut forest mixed with common and sessile oaks. Higher up there was also a beech stand.
As the traces of fire on the trees and the white ash clearly show (Fig 2.) the combination of drought, wind, degree of slope (an average of 60%), and an accumulation of fire material increased the fire intensity. The bark of many trees (especially the beech trees) was split open by the heat. The rough bark of the oak trees - which usually protects them from fire - drew the flames upwards.
By July 2007 some trees had recovered but some had died (Fig.1). Such stands are valuable for research purposes, as very little is known about the development of central European forests after fire.
The weakening of large trees due to fire provides ideal conditions for a massive colonisation by various fungi. The following fungi appeared most numerously in the second year after the fire in the burn area near Cugnasco (see box).
|Fig. 3 - The milk white toothed polypore (Irpex lacteus), a typical white rot pathogen.||Fig. 4 - The hairy curtain crust fungus (Stereum hirsutum), a fungus found world wide on deciduous trees.|
|Fig. 5 - The common porecrust (Schizophyllum commune), a typical representative of the "sunburn community".||Fig. 6 - The sac fungus Cryphonectria parasitica, which is responsible for the development of chestnut bark blight.|
|Fig. 7 - Infestation of Cyphonectria parasitica caused by the Trichoderma sp. mould||Fig. 8 - Crampballs (Daldinia sp.) on a beech tree|
|Photos: Lara Lucini und Ottmar Holdenrieder|
It is exciting to observe how fungal flora develops on damaged trees. What is astonishing is that we know very little about the effect that these fungi have on trees. Natural environments are difficult to reproduce in experiments. For instance, the effect fungi have on a tree depends on the size of the mycelia hidden within the tree. Also to be taken into consideration is that various fungi can have an affect on each other. That is why careful observations in natural environments are essential. They not only help predict future forest development, but they could also provide useful tips for nature conservation (occurrence of rare fungi species, colonisation of infected wood due to wood peckers, etc.).