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Paul B. Alaback
data on low levels of genetic diversity of trees from the Southern
Hemisphere are generally representative of these forests, then the
Southern Hemisphere would be expected to have a higher sensitivity to
climate change than the Northern Hemisphere. High levels of endemism
and widespread use of fire following forest clearing (but a low natural
frequency of fire) suggest a high level of impact to Southern Hemisphere
forests both from climate change and human impact . In the Northern
Hemisphere, by contrast, regional endemism is low, but sensitivity to
changes in disturbance regime, climatic constraints, and landscape structure is more complex, leading to significant landscape-level changes in
biodiversity patterns with both climate change and human disturbance.
Implications for Resource Management
Basic patterns of biodiversity of forest vegetation in the rainforest zone of
North America suggest that considerable opportunity should exist for
resource development activity without substantially compromising regional plant species richness. In the temperate rainforest region, the
primary source of stress and change in forest ecosystems in the future
should come from a combination of climate change and human disturbance
or management (Alaback & McClellan, 1992). The greatest challenges
for management of biodiversity will be maintaining an adequate area and
distribution of specialized habitats. Historical cutting patterns suggest a
strong impact on highly productive, low-elevation old-growth forests
initially and rapid landscape fragmentation (e.g., Harris, 1984). Since the
proportion of land in highly productive soil types rapidly declines with
latitude, this implies greater sensitivity of high-latitude sites to landscapelevel modification by human disturbance. Species that are specially
adapted to these forest types will therefore require special attention
to avoid significant population declines . Of particular concern are
nonvascular plant species, which tend to be habitat specialists and are
closely associated with large organic debris (logs and snags, or standing
dead trees) (During, 1992; Harmon et aI., 1986). Forest management for
wood fiber production generally results in an eventual elimination of
coarse woody debris, since trees do not grow for as long a period as
they would under natural-disturbance regimes, and mortality is often
minimized through thinning and by maintaining high levels of stand vigor.
If current management practices are continued into the future, reduction
of this key structural component to forest diversity may have direct
constraints on nonvascular plant diversity, as has already been demonstrated in boreal forests (e.g., Soderstrom, 1988). Structural diversity also
appears to playa key role in maintaining habitat for many animal populations (e.g., Hansen, Spies, Swanson, & Ohmann, 1991).
Paul B. Alaback
data on low levels of genetic diversity of trees from the Southern
Hemisphere are generally representative of these forests, then the
Southern Hemisphere would be expected to have a higher sensitivity to
climate change than the Northern Hemisphere. High levels of endemism
and widespread use of fire following forest clearing (but a low natural
frequency of fire) suggest a high level of impact to Southern Hemisphere
forests both from climate change and human impact . In the Northern
Hemisphere, by contrast, regional endemism is low, but sensitivity to
changes in disturbance regime, climatic constraints, and landscape structure is more complex, leading to significant landscape-level changes in
biodiversity patterns with both climate change and human disturbance.
Implications for Resource Management
Basic patterns of biodiversity of forest vegetation in the rainforest zone of
North America suggest that considerable opportunity should exist for
resource development activity without substantially compromising regional plant species richness. In the temperate rainforest region, the
primary source of stress and change in forest ecosystems in the future
should come from a combination of climate change and human disturbance
or management (Alaback & McClellan, 1992). The greatest challenges
for management of biodiversity will be maintaining an adequate area and
distribution of specialized habitats. Historical cutting patterns suggest a
strong impact on highly productive, low-elevation old-growth forests
initially and rapid landscape fragmentation (e.g., Harris, 1984). Since the
proportion of land in highly productive soil types rapidly declines with
latitude, this implies greater sensitivity of high-latitude sites to landscapelevel modification by human disturbance. Species that are specially
adapted to these forest types will therefore require special attention
to avoid significant population declines . Of particular concern are
nonvascular plant species, which tend to be habitat specialists and are
closely associated with large organic debris (logs and snags, or standing
dead trees) (During, 1992; Harmon et aI., 1986). Forest management for
wood fiber production generally results in an eventual elimination of
coarse woody debris, since trees do not grow for as long a period as
they would under natural-disturbance regimes, and mortality is often
minimized through thinning and by maintaining high levels of stand vigor.
If current management practices are continued into the future, reduction
of this key structural component to forest diversity may have direct
constraints on nonvascular plant diversity, as has already been demonstrated in boreal forests (e.g., Soderstrom, 1988). Structural diversity also
appears to playa key role in maintaining habitat for many animal populations (e.g., Hansen, Spies, Swanson, & Ohmann, 1991).