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Volume XXIX - 1998
Pumice Fields and a Sense of Landscape Wonder
By Ron Mastrogiuseppe
Coniferous forest which surrounds the caldera embracing Crater Lake
is broken by curious openings called pumice fields, especially around
the rim. Many of the pumice fields are spacious and provide grand vistas
of the Cascade Range, but they are also windows into the landscape's
past. The climactic eruption 7,700 years ago instantly erased biota
firmly rooted upon the slopes of ancient Mount Mazama. Pioneering
lifeforms of the pre-Mazama biota surviving in neighboring refugia
eventually migrated upslope and colonized suitable habitats, but the
eruption's power restricted the availability of these habitats.
Hypothesized appearance of Mazama at the beginning of
its climactic eruption. Drawing by Walter Rives, 1948.
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Despite dry and sunny summer weather enjoyed by human visitors, the
growing season of the pumice fields is abbreviated by extremes of
temperature and moisture. Topographic features create frost pockets in
swales, along lower slopes, and in crater depressions atop cinder cones.
This allows snow to drift and accumulate, so that it sometimes persists
through the growing season. Upland habitats, especially those with rocky
outcrops, are devoid of snowpack early in the summer but are subject to
dry winds -- as are south- and west-facing slopes. Surface temperatures
are normally intensified by high summer sun, but reduced by reflective
qualities of the light-colored pumice. Air temperatures near the ground
may change significantly over a 24 hour period, with a chance for frost
even in summer. Pumice soils, having been pulverized by volcanic
eruption, are very porous. This allows large quantities of water to
infiltrate and percolate deeply, but largely robs the surface of
moisture.
The volcanic landscape of Mount Mazama represents a mosaic of
habitat types, and the pumice fields have resisted encroachment by
individual trees and shrubs for centuries. The tree species best suited
for pioneering this seemingly inhospitable habitat appears to be
whitebark pine (Pinus albicaulis). This species even assumes the
role of pioneer in the fractured obsidian flow of Paulina Peak at
Newberry Caldera southeast of Bend. Similarly, much of the caldera's rim
edge around Crater Lake is fringed with whitebark pine as though planted
in a row single file. It is this edge habitat, which is swept free of
deep snow, that harbors a longer growing season than adjacent pumice
fields extending downslope from the rim. This is also where the Clark's
nutcracker (Nucifraga columbiana) caches seeds from whitebark
pines, thereby insuring some regeneration.
A pumice field near Union Peak in 1936 (top) and
forty years later (bottom). Photos by Homer Marion, 1936 (top) and Ron
Mastrogiuseppe (bottom).
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There are some "tree islands" within the pumice fields generally
featuring a single whitebark pine perhaps established a few centuries
earlier. These trees may be situated on churned soil where a Mazama
pocket gopher, Thomoys mazama, created a mound within the shelter
of a rock. During the growth of a "mother tree," local microclimate
changes to favor the establishment of herbaceous and shrub species in
addition to subalpine fir (Abies lasiocarpa) or mountain hemlock
(Tsuga mertensiana) sheltered by the whitebark's canopy. It is
common to find that the original "mother tree" has died, but still is
surrounded by younger trees which prevent the snag from falling. In an
environment marked by harsh growing conditions, chances for survival are
enhanced by development of individual saplings in aggregations or clumps
with grafted roots.1
Given the potential number of species available for colonizing
pumice fields, however, it is a mystery why additional lifeforms have
not yet invaded. Each year, and especially when there is a prolific seed
cone crop, the pumice fields are recipients of myriad seeds and
propagules. Yet this annual "rain" of seed seems unable to bring
additional numbers of species. The total number of plant species found
within the Pumice Desert, for example, is only fourteen.2
Other pumice fields are likely similar in species diversity. When
compared to the park total of some 700, this gives meaning to the term
depauperate flora -- one lacking in species richness.
The shape of pumice fields can change over time when surrounding
forest borders (usually dominated by lodgepole pine, Pinus
contorta) respond to favorable conditions. Conifer encroachment, as
documented in repeat photography, began during the drought episode of
the 1930s. During that decade growing seasons expanded by roughly one
month as the result of warmer temperatures and reduced
snowpack.3 This pattern of encroachment is not unique to the
Crater Lake region, having been observed throughout the subalpine zone
of the Cascade Range. Perhaps similar phenomena will be kickstarted by
the years of drought (which extended from 1976-77 until the early 1990s)
since these changes are believed to be driven by climatic, rather than
climactic, events.
Whitebark pine on the rim of Crater
Lake.
Notes
1 Fire seems to have little effect on this habitat.
Lightning may strike individual stems within the tree islands, but fuels
beyond them are too scant to carry a surface fire for any distance. It
should not be a surprise, then, that evidence of historic fire is almost
always absent from pumice fields.
2 See Ruth Monical and Stephen P. Cross. Mammals of
the Pumice Desert, Nature Notes from Crater Lake 23 (1992).
pp. 17-18.
3 The exception to this interpretation was the winter of
1932-33, when a record of more than 700 inches occurred.
Ron Mastrogiuseppe is a forest ecologist who has monitored
pumice fields and other phenomena in Crater Lake National Park since
1964.
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