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Paul B. Alaback
land area is considered, then riparian forests have far and away the
highest species richness , especially for nonvascular plant species. Peatland
and other forest types are then a distant second, with alpine having the
lowest diversity. Peatland and alpine habitats have the highest proportion
of circumpolar species, whereas riparian forests have the highest
endemism. A nearly opposite ranking of plant communities occurs if
species packing in small survey plots «0.25 ha) is measured, as discussed
previously.
Disturbance regime also plays a key role in regulating landscape
diversity. Recently disturbed sites , especially in the southern portion of
the coastal forest zone, and old-growth forests have the highest level of
diversity. Immediately following disturbance , graminoid and herbaceous
species often colonize sites, significantly elevating diversity. Within 5 to
10 years, these species are shaded out by woody shrubs and tree seedlings
(Alaback, 1982). In the perhumid and subpolar zones, these increases in
species richness are less significant than in the sea sonal or warm temperate
zones. Following the development of a dense conifer fore st canopy, most
understory plants are shaded out, especially on productive , well-drained
sites , leading to a substantial deterioration in wildlife habitat values
(Alaback, 1982; Wallmo & Schoen, 1980) .
In the perhumid zone, catastroptncally disturbed upland sites may take
as long as 300 to 400 years to attain the same level of diversity as oldgrowth forests. Riparian forests may take 500 years or more to attain
maximal diversity of vascular and particularly nonvascular plant species
following catastrophic disturbance (Alaback, in press). Because of the
tight coupling between hydrology, geomorphology, and vegetation processes, disturbance may also destabilize these ecosystems, leading to
shrub- or hardwood-dominated forest (Schrader, 1991). Some of the most
diverse landscapes have a long history of disturbance, including both
catastrophic and small-scale maintenance disturbance (Veblen & Alaback,
this volume ; Veblen , Schlegel, & Oltremari , 1983).
Across the gradient from warm temperate to subpolar rainforest,
landscape patchiness and the grain of the mosaic also vary substantially.
In the subpolar zone, forest occupies less than 14% of the land surface
and includes a relatively depauperate forest , usually on steep slopes.
Gentle terrain is dominated by wetland types such as peatland, bogs ,
fens , and alpine meadows. This results in an extremely fine-grained
mosaic with relatively stable natural edges, particularly between forest
and bog . Forest cover is a clear limiting factor for overwinter survival for
a variety of animal species (Eck, 1984; Parker, 1988). In the perhumid
zone, forest occupies approximately 30% of the land area but is mostly
composed of open unproductive forest. Only 4%-8% of the land area is
occupied by productive close-canopy forest , with high wildlife cover
values. In the seasonal and warm temperate forest zones, the great
majority of land area is capable of supporting forest , often in a highl y
Paul B. Alaback
land area is considered, then riparian forests have far and away the
highest species richness , especially for nonvascular plant species. Peatland
and other forest types are then a distant second, with alpine having the
lowest diversity. Peatland and alpine habitats have the highest proportion
of circumpolar species, whereas riparian forests have the highest
endemism. A nearly opposite ranking of plant communities occurs if
species packing in small survey plots «0.25 ha) is measured, as discussed
previously.
Disturbance regime also plays a key role in regulating landscape
diversity. Recently disturbed sites , especially in the southern portion of
the coastal forest zone, and old-growth forests have the highest level of
diversity. Immediately following disturbance , graminoid and herbaceous
species often colonize sites, significantly elevating diversity. Within 5 to
10 years, these species are shaded out by woody shrubs and tree seedlings
(Alaback, 1982). In the perhumid and subpolar zones, these increases in
species richness are less significant than in the sea sonal or warm temperate
zones. Following the development of a dense conifer fore st canopy, most
understory plants are shaded out, especially on productive , well-drained
sites , leading to a substantial deterioration in wildlife habitat values
(Alaback, 1982; Wallmo & Schoen, 1980) .
In the perhumid zone, catastroptncally disturbed upland sites may take
as long as 300 to 400 years to attain the same level of diversity as oldgrowth forests. Riparian forests may take 500 years or more to attain
maximal diversity of vascular and particularly nonvascular plant species
following catastrophic disturbance (Alaback, in press). Because of the
tight coupling between hydrology, geomorphology, and vegetation processes, disturbance may also destabilize these ecosystems, leading to
shrub- or hardwood-dominated forest (Schrader, 1991). Some of the most
diverse landscapes have a long history of disturbance, including both
catastrophic and small-scale maintenance disturbance (Veblen & Alaback,
this volume ; Veblen , Schlegel, & Oltremari , 1983).
Across the gradient from warm temperate to subpolar rainforest,
landscape patchiness and the grain of the mosaic also vary substantially.
In the subpolar zone, forest occupies less than 14% of the land surface
and includes a relatively depauperate forest , usually on steep slopes.
Gentle terrain is dominated by wetland types such as peatland, bogs ,
fens , and alpine meadows. This results in an extremely fine-grained
mosaic with relatively stable natural edges, particularly between forest
and bog . Forest cover is a clear limiting factor for overwinter survival for
a variety of animal species (Eck, 1984; Parker, 1988). In the perhumid
zone, forest occupies approximately 30% of the land area but is mostly
composed of open unproductive forest. Only 4%-8% of the land area is
occupied by productive close-canopy forest , with high wildlife cover
values. In the seasonal and warm temperate forest zones, the great
majority of land area is capable of supporting forest , often in a highl y