Fire in California - Fire in Mediterranean Ecosystems - page 139

P. torreyana , Pseudotsuga macrocarpa and Torreya californica in the Pinaceae
family and several cypress species in the genus Hesperocyparis (formerly Cupres-
sus ) in the Cupressaceae family (Vogl et al. 1977 ; Minnich 1978 ; Zedler 1986 ;
Barbour 2007 ). Nearly all of these are endemic to the California Floristic
Province. Those in the latter family seldom exceed 10 m height whereas the others
range from 20 m to 40 m in height. All occur in relatively small pure populations
distributed amongst a mosaic of chaparral, grassland, woodland and forest.
The range of responses to fire is similar to those in chaparral. Some taxa are
obligate resprouters and others seeders. Torreya californica persists by resprouting
from the base but does not have postfire seedling recruitment, whereas most of the
pine and cypress species are non-resprouting serotinous trees. These serotinous
species have many attributes for persistence in a crown fire regime. Due to their
low stature, branching near the ground and limited self-pruning, they form dense
localized populations that burn in high-intensity crown fires, factors leading to the
evolution of serotiny (Keeley & Zedler 1998 ). Cones typically remain closed on the
tree for periods from a few years to a few decades, depending on the species. Soon
after fire, cones open and disperse seeds (see Fig. 3.5 ) . Seedling recruitment is
restricted to a single postfire pulse of seedlings, and in this respect they resemble
the crown fire chaparral response. Axelrod ( 1980 ), who assumed fire was largely a
recent anthropogenic phenomenon, hypothesized that serotiny in these pines was
not an adaptation to fire but rather to drought. However, we believe fire is the
selective force for the origin and maintenance of serotiny in these species. Fire has
been a potential selective force throughout the evolution of land plants, and life
history analysis would not predict drought to select for delayed reproduction to a
single postfire year (see Chapter 9 ).
A few tree species, such as Pinus sabiniana and Pseudotsuga macrocarpa , have a
unique fire response that involves metapopulations with dynamic fire-related
fluctuations (Keeley 2006a ; Schwilk & Keeley 2006 ). Both are often embedded
in chaparral and subject to periodic crown fires. Neither is serotinous nor has a
persistent soil seedbank. Although the latter species resprouts epicormically, on
sites with heavy fuels both species are vulnerable to local extirpation. However,
these trees persist in refugia, such as open alluvial plains, grasslands, rock out-
crops or extremely steep slopes with limited fuel loads, where they persist in the
face of high-intensity crown fires in the surrounding landscape. Under long fire-
free periods they spread into chaparral sites and persist until a high-intensity
crown fire once again restricts them to a more limited number of safe sites.
Coast Redwood and Other Mesic Forests
The northwest region of the California Floristic Province is moist with moderate
winters and the main vegetation is dense forests dominated by Pseudotsuga
menziesii (Pinaceae), Lithocarpus densiflora (Fagaceae) and localized populations
of Sequoia sempervirens (Taxodiaceae). There is a long history of high-intensity
crown fires in these forests (Veirs 1979 , 1982 ; Stuart 1987 ), but fire-scar
dendrochronology studies show many of these forests have a history of frequent