Fire-related Plant Traits - Fire in Mediterranean Ecosystems - page 78

against extreme heat or cold. There is one case though where fire would seem to
have been the primary selective force. In the genus Pinus all species from fire-
prone habitats have very thick bark, whereas the non-fire-prone desertic pinyon
pines and the timberline white pines, exposed to the hottest and coldest conditions
respectively, have the thinnest bark (Keeley & Zedler 1998 ). A selective basis for
thick bark is also supported by its extensive development in savanna oaks com-
pared with the rather thin bark in scrub oaks adapted to crown fire regimes where
aboveground survivorship is unlikely (Zedler 1995a ).
Regeneration by the dominant species in closed-canopy woodlands and forests
with surface fire or mixed fire regimes is commonly restricted to open sites with
bare mineral soil created by fires. Regeneration in these species is dependent on
the survival of parent trees in proximity to fire-created gaps. When cones or fruits
are present at the time of fire, recruitment may occur in the first growing season
after fire, but often times it is delayed until subsequent fruiting cycles. Many trees
in surface fire regimes have masting cycles of reproduction (Keeley & van Man-
tgem 2008 ). If a fire coincides with a mast year there will be abundant seedling
recruitment in the first postfire year but following other fires it may be delayed
and result in substantially lower recruitment ( Fig. 3.6 ). Thus, there is a certain
level of serendipity involved in fires coinciding with seed production. The longer
the interval between fire and a mast year the greater the competitive inhibition
by understory vegetation, which may dictate very different successional

trajectories.

Postfire Colonization

Some fire-sensitive species drastically reduced by fire or species that are transitory
on sites are generally absent from recently burned sites and ultimately enter those
communities only through colonization. The role of colonization is quite variable
between different vegetation types. For example, in California chaparral and in
Mediterranean Basin shrublands all dominant shrubs recover endogenously and
colonization during early succession typically accounts for less than 10% of the
cover (Keeley et al. 2005a ). In contrast, in semi-arid parts of western North
America the dominants in non-MTC woodlands, pinyon ( Pinus spp.), juniper
( Juniperus spp.) and sagebrush ( Artemisia tridentata ) all recover slowly from crown
fires by recolonization, often from unburned patches resulting from uneven
patterns of burning.

Fire-independent Recruitment

In crown fire regimes obligate resprouters, as the name implies, do not recruit
seedlings after fires. Comparisons of these postfire obligate resprouters with post-
fire seeders often contrast these species as representing vegetative vs. sexual