188
T.T. Veblen and P.B . Alaback
the western subpolar rainforest zone. Only .at Kruzof Island near Sitka
and Blue Lake in northern Misty Fjords National Monument are there
effects of recent (post-Pleistocene) volcanic eruptions within the perhumid
zone in Alaska (Brew, 1988). At sites on volcanic deposits, forest productivity appears to be low and the rate of stand development appears
slow as compared to adjacent, older terrains. Volcanic terrains also have
a strong tendency towards bog or fen formation (Klinger, 1988).
The most intensive work on post volcanic succession was done following
the 1980 eruption of Mount St. Helens in the southern end of the seasonal
rainforest zone (del Moral & Bliss, 1993). Despite the severity of the
eruption, preexisting vegetation played a significant role in the regeneration process (Franklin, Modmation, Swanson, & Sedill, 1985). In high
elevation locations snow diminished the effects of ash on plants (Antos &
Zobel, 1985). In other cases, rhizomatous plants were able to grow up
through the ash. Several species were able to germinate and establish on
surfaces with 10cm of ash or less. On sites with heavier ash deposition,
water stress, low nutrient status, and poor substrate stability limited
regeneration success. Early colonizing species mostly consisted of wind
dispersed small seeded species, such as Anaphalis and Epilobium. Lupinus
was the principal nitrogen-fixing species, which was an early colonist of
ash surfaces. In contrast to glacial succession, bryophytes and cyanobacteria played a very limited role and were not notable components of
the succession until 6 to 10 years after the eruption. Most of the oldgrowth forests and many plant communities on Mount St. Helens originated after the last major eruption (Kruckeberg, 1987). In contrast to
South America it does not appear that the eruption will result in a major
change in tree species composition . The same species which would
dominate after stand replacing wildfire (Pseudotsuga, Abies) appear to
dominate after ash deposition.
As with glacial succession, it appears that the early stages of postvolcanic succession are slow and sensitive to stochastic factors. During dry
summers, for example, colonization is limited and mortality may also
include established plants from previous years. Succession is slowest on
isolated sites and most rapid where intact mature communities are located
nearby . Limited summer rainfall combined with unstable surfaces has led
to limited plant colonization on Mount St. Helens (generally less than
10% plant cover 11 years after the eruption) . Alpine areas were particularly slow to recover because of poor dispersal of taxa there , but also
because of heavy unstable ash deposits. Lahars and mudflows appear to
revegetate in part more quickly than the upland areas because of the
reduced impacts of ash, but often are extremely heterogeneous due to
textural differences in exposed surfaces and differences in water movement and substrate stability. In many cases early pioneer species are not
facilitators directly but appear to limit colonization of climax species (del
Moral & Bliss, 1993). Lupinus appeared to be a major nitrogen-fixing
T.T. Veblen and P.B . Alaback
the western subpolar rainforest zone. Only .at Kruzof Island near Sitka
and Blue Lake in northern Misty Fjords National Monument are there
effects of recent (post-Pleistocene) volcanic eruptions within the perhumid
zone in Alaska (Brew, 1988). At sites on volcanic deposits, forest productivity appears to be low and the rate of stand development appears
slow as compared to adjacent, older terrains. Volcanic terrains also have
a strong tendency towards bog or fen formation (Klinger, 1988).
The most intensive work on post volcanic succession was done following
the 1980 eruption of Mount St. Helens in the southern end of the seasonal
rainforest zone (del Moral & Bliss, 1993). Despite the severity of the
eruption, preexisting vegetation played a significant role in the regeneration process (Franklin, Modmation, Swanson, & Sedill, 1985). In high
elevation locations snow diminished the effects of ash on plants (Antos &
Zobel, 1985). In other cases, rhizomatous plants were able to grow up
through the ash. Several species were able to germinate and establish on
surfaces with 10cm of ash or less. On sites with heavier ash deposition,
water stress, low nutrient status, and poor substrate stability limited
regeneration success. Early colonizing species mostly consisted of wind
dispersed small seeded species, such as Anaphalis and Epilobium. Lupinus
was the principal nitrogen-fixing species, which was an early colonist of
ash surfaces. In contrast to glacial succession, bryophytes and cyanobacteria played a very limited role and were not notable components of
the succession until 6 to 10 years after the eruption. Most of the oldgrowth forests and many plant communities on Mount St. Helens originated after the last major eruption (Kruckeberg, 1987). In contrast to
South America it does not appear that the eruption will result in a major
change in tree species composition . The same species which would
dominate after stand replacing wildfire (Pseudotsuga, Abies) appear to
dominate after ash deposition.
As with glacial succession, it appears that the early stages of postvolcanic succession are slow and sensitive to stochastic factors. During dry
summers, for example, colonization is limited and mortality may also
include established plants from previous years. Succession is slowest on
isolated sites and most rapid where intact mature communities are located
nearby . Limited summer rainfall combined with unstable surfaces has led
to limited plant colonization on Mount St. Helens (generally less than
10% plant cover 11 years after the eruption) . Alpine areas were particularly slow to recover because of poor dispersal of taxa there , but also
because of heavy unstable ash deposits. Lahars and mudflows appear to
revegetate in part more quickly than the upland areas because of the
reduced impacts of ash, but often are extremely heterogeneous due to
textural differences in exposed surfaces and differences in water movement and substrate stability. In many cases early pioneer species are not
facilitators directly but appear to limit colonization of climax species (del
Moral & Bliss, 1993). Lupinus appeared to be a major nitrogen-fixing