7. Biodiversity Patterns in Relation to Climate
123
productive close-canopied ecosystem (Alaback & McClellan, 1992). The
mosaic in these regions is a function more of fire history (natural or
human caused) rather than intrusion of wetlands or other edaphic types
(Agee, 1991). In these forest types , cover is not considered as serious a
constraint for wildlife as it is in more northern forests.
Structural Diversity
Old-growth forests are a distinctive feature of the natural vegetation of
the entire Pacific forest region (Alaback & Juday, 1989; Franklin et al. ,
1981; Spies & Franklin, 1991). These forests typically develop for 2 to 5
centuries after a catastrophic disturbance, or they may be essentially
ageless and are maintained by periodic windthrow or other forms of
small-scale mortality or disturbance. Although old-growth forests do not
necessarily contain the highest level of species diversity in any given
region, they always contain the highest level of structural diversity and
are a key component of regional diversity (Spies , 1991). This diversity
comes from the development of multiple canopy layers, the presence of a
wide range of sizes and decay classes of snags and logs , and a complex
and highly structured understory layer . Overstory canopies tend to be
patchy and uneven in height, allowing for the development of a variety
of understory species , each responding to small-scale disturbances of
different intensity, frequency , or phenology and to local micro site conditions. Small clonal herbs and many shrubs seem to thrive particularly in
old-growth forests where canopy gaps are frequently created by disease or
wind (Tappeiner & Alaback, 1989).
Although it is often implied that secondary forests are inherently less
diverse than old-growth forests because of the age of dominant trees,
secondary forests with high structural diversity are usually able to provide
habitat for a wide range of forest species. Naturally created secondary
forests may maintain a high level of diversity, depending on the nature of
the disturbance. Fire-origin forests , for example, often display a large
carryover from the previous stand, including residual dominant trees,
snags, and logs (Juday, i976; Spies & Franklin, 1991; Spies , Franklin, &
Thomas, 1988). Understory propagules are often associated with residual
trees as well. Multiple canopies can also develop in these younger forests
if the original disturbance was patchy. Managed forests or anthropogenic
forests in general are much simpler structurally (Alaback & Herman,
1988; Alaback & McClellan, 1992). Snags and logs are much less common
and the overstory canopy is more uniform, leading to a more uniform
depauperate understory (Alaback, 1982; Spies, 1991). What limited data
are available also suggest that insects are much less diverse in these
simplified managed ecosystems (Schowalter, 1989). There have not been
any studies in either the mesophytic or temperate rainforest regions in
123
productive close-canopied ecosystem (Alaback & McClellan, 1992). The
mosaic in these regions is a function more of fire history (natural or
human caused) rather than intrusion of wetlands or other edaphic types
(Agee, 1991). In these forest types , cover is not considered as serious a
constraint for wildlife as it is in more northern forests.
Structural Diversity
Old-growth forests are a distinctive feature of the natural vegetation of
the entire Pacific forest region (Alaback & Juday, 1989; Franklin et al. ,
1981; Spies & Franklin, 1991). These forests typically develop for 2 to 5
centuries after a catastrophic disturbance, or they may be essentially
ageless and are maintained by periodic windthrow or other forms of
small-scale mortality or disturbance. Although old-growth forests do not
necessarily contain the highest level of species diversity in any given
region, they always contain the highest level of structural diversity and
are a key component of regional diversity (Spies , 1991). This diversity
comes from the development of multiple canopy layers, the presence of a
wide range of sizes and decay classes of snags and logs , and a complex
and highly structured understory layer . Overstory canopies tend to be
patchy and uneven in height, allowing for the development of a variety
of understory species , each responding to small-scale disturbances of
different intensity, frequency , or phenology and to local micro site conditions. Small clonal herbs and many shrubs seem to thrive particularly in
old-growth forests where canopy gaps are frequently created by disease or
wind (Tappeiner & Alaback, 1989).
Although it is often implied that secondary forests are inherently less
diverse than old-growth forests because of the age of dominant trees,
secondary forests with high structural diversity are usually able to provide
habitat for a wide range of forest species. Naturally created secondary
forests may maintain a high level of diversity, depending on the nature of
the disturbance. Fire-origin forests , for example, often display a large
carryover from the previous stand, including residual dominant trees,
snags, and logs (Juday, i976; Spies & Franklin, 1991; Spies , Franklin, &
Thomas, 1988). Understory propagules are often associated with residual
trees as well. Multiple canopies can also develop in these younger forests
if the original disturbance was patchy. Managed forests or anthropogenic
forests in general are much simpler structurally (Alaback & Herman,
1988; Alaback & McClellan, 1992). Snags and logs are much less common
and the overstory canopy is more uniform, leading to a more uniform
depauperate understory (Alaback, 1982; Spies, 1991). What limited data
are available also suggest that insects are much less diverse in these
simplified managed ecosystems (Schowalter, 1989). There have not been
any studies in either the mesophytic or temperate rainforest regions in