FIRE ECOLOGY
SPECIES: Tilia americana | Basswood 



FIRE ECOLOGY OR ADAPTATIONS : 
Basswood is rated as a fire sensitive species; the thin bark is easily
damaged by fire, as are the shallow roots [16].  However, basswood that
has been top-killed by fire will sprout vigorously from the root crown [71].

Basswood is most common in forests with long fire-free intervals.  Fire
suppression has apparently encouraged its increase.  In the absence of
fire, paper birch (Betula papyrifera)-eastern white pine-bur oak forests
are being replaced in by basswood, balsam fir (Abies balsamea), and
black ash (Fraxinus nigra) in the north-central states [1].  Basswood
and sugar maple usually replace postfire aspen stands (Populus
tremuloides and P. grandidentata) on rich, moist sites in Minnesota
[33].  The presence of sugar maple-basswood stands is correlated with
longer fire-free intervals in Minnesota; pines (including white pine,
red pine [Pinus resinosa] and jack pine [P. banksiana]) and aspens
occur on sites that burn more frequently [14].  In jack pine or red
pine-white pine stands which experience frequent fire, basswood is rare
[36].  Basswood occurs in moist canyons in Nebraska that appear to be
refuges from fire [35].  Also in Nebraska, basswood and other hardwoods
are more common than they were 130 years ago, and it is thought that
this is related to a decrease in the frequency and severity of fires
[60].  In New England northern hardwood forests that include basswood
typically have fire-free periods on the order of 800 to more than 2,000
years [25].

Many authors report that hardwood forests including basswood are
encroaching onto former grasslands since fires have been suppressed
[1,3,38,60,72].  In Iowa oak savanna is replaced by dense sugar
maple-basswood forest process in approximately 200 years without fire
[56].  In Wisconsin basswood are invading mixed oak stands from which
they had formerly been excluded by wildfire [13].  However, Auclair and
Cottam [5] stated that this is not a general trend; succession to sugar
maple-basswood is confined to specific sites largely because of
fragmentation of forests and the resultant lack of seed sources.  They
do concur that red oak stands may represent sugar maple-basswood sites
that have either been retrogressed to oak by fire, or from which sugar
maple and basswood have been excluded by fire [5].

A stand of basswood and eastern hophornbeam in Minnesota contained
scattered open-grown, large bur oaks.  The basswood and eastern
hophornbeam were even-aged.  Sugar maple was not present in the dominant
layer even though its presence would normally be expected.  The forest
structure was explained by Daubenmire [19] as a sugar maple-basswood
stand that had experienced a brief period of burning.  Daubenmire
proposed the following relationship of sugar maple-basswood forests to
fire:  1) sugar maple is eliminated by single fires; 2) repeated fires
eliminate elms and red oak and leave basswood and eastern hophornbeam as
sprout thickets; 3) continued fires can eliminate basswood and eastern
hophornbeam thickets, which are replaced by bur oak and grasses; 4)
severe fires will eventually eliminate bur oak, leaving prairie.  The
structure of the forest described above was explained as the result of
sugar maple-basswood-eastern hophornbeam invasion of a bur oak savanna
(possibly stage 3).  The sugar maple forest developed to maturity
without experiencing fire.  This forest then experienced a short period
of burning (perhaps only one fire) and had reached stage 2 when burning
ceased, leaving the basswood and eastern hophornbeam to grow up into an
even-aged canopy.  Daubenmire concluded that the climatic limits of
sugar maple-basswood are further west than the actual limits due to fire
[19].  In Iowa oak savanna is replaced by dense sugar maple-basswood
forest, the process occurring over the course of approximately 200 years
without fire [56].

Mature sugar maple-basswood forests are very resistant to burning.
Decomposition of potential fuels is rapid, particularly on base-rich
mull soils.  Dense shade reduces the numbers and cover of shrubs and
herbaceous species, and therefore very little fuel exists at ground
level.  The tree trunks are not very flammable, and the open crowns do
not carry fire well.  Low solar radiation, high relative humidity, and
low wind speeds enhance the moisture retention of ground-level fuels,
thereby inhibiting ignition and/or spread of fire.  Ordinarily, only the
leaf litter ever reaches a flammable state, creating conditions
conducive only to patchy, creeping surface fires [32].

The Minnesota Big Woods (sugar maple-basswood cover type) is often
described as an old, stable, climax forest.  New evidence suggests that
in fact, these woods are of comparatively recent origin [65].  Climate
factors (increased moisture and lower temperatures) contributed to a
natural decrease in the frequency and intensity of fire.  Once
established, sugar maple-basswood forests do not burn easily [32],
creating conditions in which these forests were able to survive climatic
warming and increases in fire frequency.  The extent of these woods has
been greatly reduced by human activities, but they have also experienced
lower danger from fire since humans have stopped setting fires and have
been active in suppressing fires [65].

Basswood occurs in aspen forests which are of postfire origin and
fire-maintained.  The fire regime in these woods usually consists of
short fire intervals (on the order of 10 years) with low intensity
surface fires.  Where these forests are protected from fire, closed
canopies and higher species diversity develop [37].


POSTFIRE REGENERATION STRATEGY : 
   Tree with adventitious-bud root crown/soboliferous species root sucker

Related categories for Species: Tilia americana | Basswood