Geoscience Reference
In-Depth Information
Fig. 11.15. on the left, serotinous cones on a lodgepole pine
branch that has recently fallen to the ground. the diameter of
the branch indicates that the cones have remained closed for
many years, accumulating seed on the tree. one of the cones
is beginning to open, most likely because of higher tempera-
tures at the soil surface than in the tree's canopy. cones that
remain on the tree typically open with the heat of a fire, as
shown in the center photo. the pine needles have burned,
but only the exterior of the cone scales was blackened. in
contrast, on the right, the green leaves around the open cone
indicate that the heat of a fire was not required for seed dis-
persal from the cones of this tree.
higher elevations, where the last major disturbance was
a windstorm, insect outbreak, or other nonfire distur-
trolled and reflects long-term differences in selection
pressures in different environments. Because fire inter-
vals generally become longer with increasing elevation,
reaching 200-300 or more years, trees growing at high
elevations are more likely to reproduce successfully from
seeds dispersed into canopy gaps from nonserotinous
cones. in contrast, at lower elevations, where typical fire
intervals are less than 200 years, canopy gaps are less
common and serotinous cones may be a more successful
adaptation.
As described previously, selection for serotinous
and nonserotinous cones also may be influenced by
considerably more seed is produced per year in non-
serotinous cones than in serotinous cones, a pattern
that can be attributed to the need for allocating more
energy to protective tissues in the serotinous cones
both cone types in a stand seems adaptive for lodgepole
pine, as it ensures a seed source regardless of the type
of disturbance. nonserotinous cones also enable the
establishment of lodgepole pine in nonforest habitats,
such as sagebrush-dominated shrublands. Some have
suggested that forest management, in both wildlands
and timber production areas, should allow for both fire
and nonfire disturbances, so that both genotypes can
be sustained, thereby maintaining biological diversity.
Seeds in both serotinous and nonserotinous cones
can be burned by intense fire, but rarely does fire kill
all seeds of either type. Because the size of the can-
opy seed bank varies across the landscape—primarily
because of variation in the proportion of serotinous
trees and because of seed mortality during some fires—
the density of lodgepole pine seedlings after a fire varies
greatly. Seedling densities after the 1988 Yellowstone
fires ranged from less than 5 per acre to greater than
100,000 per acre. the fewest seedlings were found
where the burned canopy was a mix of lodgepole pine
and other species, essentially all lodgepole pine were
nonserotinous, and the fire burned through the canopy
at high intensity. the most seedlings occurred where
the burned canopy was essentially all lodgepole pine,
most trees were serotinous, and the fire scorched but did
As the seedlings grow into saplings and small trees,
the very dense post-fire stands become difficult to walk
through. Such dense pine forests are commonly referred
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