7. Biodiversity Patterns in Relation to Climate
119
regions. For example, in a 496,000 ha region at the southern end of the
subpolar type (the Yakutat forelands) , only 350 taxa have been recorded
(Sheppard, USDA Forest Service Chatham Area, Sitka, Alaska, personal
communication). A maximum of 329 species are estimated to occur on
the recently glaciated coast from Yakutat north to Prince William Sound,
with more than two thirds found in rainforest habitats (Borchers &
Wertheim, 1992; Farr & Hard, 1987; Hulten, 1968). Within the perhumid
zone of southeast Alaska, approximately 1200 taxa are known to
occur (Hulten, 1968; Muller, 1977; USDA Forest Service, unpublished
manuscript). Species richness varies substantially by subregion (Farr &
Hard , 1987). One of the small northern subregions, Lynn Canal fjord,
has the richest flora (804 species) of any subregion, because of its linkage
to a wide range of interior climates in British Columbia and the Yukon (it
is close to both) and because of the pronounced rainshadow within the
fjord, allowing for the development in close proximity of boreal, firedominated, and rainforest communities as well as a broad range of alpine
and other nonforest communities. The other coastal subzones vary from
497 to 520 species, with no obvious latitudinal trend.
Species richness within forest habitats in the temperate rainforest zone
is generally a small fraction of that which occurs over the whole landscape .
In the perhumid zone , for example, only 205 species out of a flora of
more than 1000 species are considered forest species (Martin, 1989;
Muller, 1977). Surprisingly little variation occurs in the proportion of
plant species belonging to different structural groups that grow on forest
habitats. Whereas approximately 60% or more of the flora is composed of
woody species in tropical rainforests and Southern Hemisphere temperate
rainforests, in all the central to northern temperate rainforests on
the Pacific coast , 55% -75% of the species are herbaceous, with only
13%-17% of the species being woody (compare with Arroyo et aI.,
this volume). Graminoid species are particularly poorly represented in
rainforest regions in general and in forest habitats in particular. The slight
increase in proportion of graminoid species with latitude thus represents
not an increase in graminoid species (it is actually a substantial decrease)
but a substantial impoverishment of herbaceous species.
Within the perhumid zone , species diversity varies substantially by
forest type and productivity class. On a per plot basis (0.04 ha) , the
greatest species richness in the perhumid zone is attained on sites with the
lowest productivity (DeMeo et aI., 1992). This is primarily because the
least productive sites have the most open overstory canopy , so more solar
radiation is available for the most diverse vegetation strata-the shrubs,
herbs, and bryophytes. Owing to the diversity of microsites and hydrological conditions, peatland forests have the highest stand level species
diversity, including many forest, alpine, and riparian species as well as
endemics to this habitat. Unproductive forests are the next most diverse
on this basis, and productive forests are the least diverse (DeMeo et al.,
119
regions. For example, in a 496,000 ha region at the southern end of the
subpolar type (the Yakutat forelands) , only 350 taxa have been recorded
(Sheppard, USDA Forest Service Chatham Area, Sitka, Alaska, personal
communication). A maximum of 329 species are estimated to occur on
the recently glaciated coast from Yakutat north to Prince William Sound,
with more than two thirds found in rainforest habitats (Borchers &
Wertheim, 1992; Farr & Hard, 1987; Hulten, 1968). Within the perhumid
zone of southeast Alaska, approximately 1200 taxa are known to
occur (Hulten, 1968; Muller, 1977; USDA Forest Service, unpublished
manuscript). Species richness varies substantially by subregion (Farr &
Hard , 1987). One of the small northern subregions, Lynn Canal fjord,
has the richest flora (804 species) of any subregion, because of its linkage
to a wide range of interior climates in British Columbia and the Yukon (it
is close to both) and because of the pronounced rainshadow within the
fjord, allowing for the development in close proximity of boreal, firedominated, and rainforest communities as well as a broad range of alpine
and other nonforest communities. The other coastal subzones vary from
497 to 520 species, with no obvious latitudinal trend.
Species richness within forest habitats in the temperate rainforest zone
is generally a small fraction of that which occurs over the whole landscape .
In the perhumid zone , for example, only 205 species out of a flora of
more than 1000 species are considered forest species (Martin, 1989;
Muller, 1977). Surprisingly little variation occurs in the proportion of
plant species belonging to different structural groups that grow on forest
habitats. Whereas approximately 60% or more of the flora is composed of
woody species in tropical rainforests and Southern Hemisphere temperate
rainforests, in all the central to northern temperate rainforests on
the Pacific coast , 55% -75% of the species are herbaceous, with only
13%-17% of the species being woody (compare with Arroyo et aI.,
this volume). Graminoid species are particularly poorly represented in
rainforest regions in general and in forest habitats in particular. The slight
increase in proportion of graminoid species with latitude thus represents
not an increase in graminoid species (it is actually a substantial decrease)
but a substantial impoverishment of herbaceous species.
Within the perhumid zone , species diversity varies substantially by
forest type and productivity class. On a per plot basis (0.04 ha) , the
greatest species richness in the perhumid zone is attained on sites with the
lowest productivity (DeMeo et aI., 1992). This is primarily because the
least productive sites have the most open overstory canopy , so more solar
radiation is available for the most diverse vegetation strata-the shrubs,
herbs, and bryophytes. Owing to the diversity of microsites and hydrological conditions, peatland forests have the highest stand level species
diversity, including many forest, alpine, and riparian species as well as
endemics to this habitat. Unproductive forests are the next most diverse
on this basis, and productive forests are the least diverse (DeMeo et al.,