Branden i tyresta 1999 Dokumentation av effekterna ISBN 91-620 ...

they predominantly take favour of the fungi
that colonize the ground after a fire. Others,
especially insects of prey, take favour on the
low concurrence from other species. Often,
pyrophilous insects are able to locate fires
at a long distance. Many of these animals
have decreased during the 1900s due to a
more effective fire-fighting and are today
strongly threatened.
The study in Tyresta was conducted with
catching on chiefly six sites with different
fire intensity rate, ranging from site 0 outside
the burnt-off area and site 6 on intensively
burnt ground. Different types of traps
were mainly used, but at some occasions
small very smoky fires were lighted at strategic
places in order to attract pyrophilous
insects. A special study of gnaw-marks from
wood-living insects was also made.
In general, the least intensively burnt
parts of the area, typically the moister sections
of the forest, were most important for
the re-colonization of ground-living species,
working as centres from which the lower
animal life spread. The best catches were
in mildly burnt spruce forest. Some species,
on the other hand, were best represented in
the intensively burnt areas, like Melanophila
acuminata, a well-studied pyrophylous species.
Nutritious pools was another interesting
environment with a rich animal life.
C. 30 truly pyrophilous species were
recorded, chiefly beetles (19), mosquitos
and flies (7) and micro-moths (2). Many
small wildfires in the surroundings seem to
have been favourable for many species. At
the same time, many common wood-living
insects were caught in unexpectedly low
numbers.
In the large material there are not less
than 250 species that are new for Sweden
and, furthermore, a number of previously
undescribed species, among them a gall gnat
which has been given the scientific name
Tritozyga tyrestaensis.
Influences on the chemical
composition of water from
wildfire and fire-fighting
It is probably less recognised that a wildfire
affects not only the animal and plant life but
also the water quality in influenced streams
and lakes. Water chemical effects following
wildfires have been demonstrated mainly at
investigations in North America, but there
are few studies on this topic from Scandinavia,
why Tyresta is a very important
research object after the wildfire of 1999.
A number of questions arose, e.g. if the
use of brackish water from the Baltic Sea
would influence the chemical composition
of the influenced stream waters. Furthermore,
the streams and lakes of the Tyresta
forest are since decades included in several
research- and environmental monitoring
programs, which facilitates interpretations
and comparisons.
Four connected streams originating
within the burnt-off area were studied, as
well as the receiving Lake Stensjön. The samples
were collected monthly during February
to October 1999—2004. Water chemical
analyses were conducted and the frequency
and composition of species among plant- and
animal plankton were determined.
The results clearly indicate a considerable
influence on the water of the wildfire.
Some effects occurred immediately after the
fire and have since diminished; others have
been more long-term and were sometimes
still detectable in January 2004. pH dropped
dramatically after the fire and the streams
became very acid. This in turn affected pH
dependant elements such as inorganic aluminium,
which increased to toxic levels.
Later, pH slowly increased again but never
reached the pre-fire levels during the study
period. A number of trace metals showed
peak levels during the rainy period following
the fire. The composition of major
ions changed following the wildfire due to
19