108
Paul B. Alaback
summer. The analogous temperate rainforest types in southern Chile are
closely matched with those in North America, except that their climate is
even more equitable year-round and they are warmer , particularly during
the winter.
One of the key gradients within the temperate rainforest zones of
North America is precipitation, which steadily increases northward both
in equability of distribution and in yearly accumulation (Alaback, 1991;
Alaback & McClellan , 1992). Along this transect, moisture changes from
a scarce resource limiting plant productivity to an overabundant resource
which is associated with declines in soils fertility (Ugolini & Mann , 1979).
In the warm temperate and seasonal rainforests, precipitation primarily
occurs in the winter months. In the warm temperate zone, rainfall over
1400mm occurs only within 30 km of the ocean or at elevations greater
than 500m. The Klamath Mountain region provides a striking anomaly,
in that harsh dry climates persist almost to the coast because of the
unique physiography of the region. This geographic area also corresponds
with an extremely rich and diverse flora. This diversity is due not only to
the diversity in climates that occur there but also to a long history and
diverse geology (Whittaker, 1960). While the whole Klamath Mountain
region is sometimes considered to be in the Mediterranean zone , the
wetter coastal portions of this region are included here , due to their
floristic and climatic similarity to seasonal rainforests to the north.
Within the northern portions of the temperate rainforest region , the
most ecologically important effects of precipitation relate more to its
distribution than to its magnitude (Alaback, 1991). Throughout the
perhumid and subpolar rainforest regions , semipermanent low-pressure
systems bring high annual precipitation during most of the year . Upwelling
off the Pacific Ocean also increases moisture by producing fog during late
spring and early summer. Fog-drip interception is an important yet seldom
quantified element of the hydrological budget (Harr, 1982) and is also of
key importance in defining the geographic limit of the most productive
forest types, especially along the coastline of northern California and the
Pacific Northwest. Fog is also a key climatic factor on the outer coast of
British Columbia and Alaska.
Dry interior valleys extend their climatic influence on coastal vegetation
patterns by allowing local domination by fire- or drought-resistant species.
This pattern is most significant in the warm temperate rainforest, in
particular the Klamath Mountain region , but also occurs intermittently in
the seasonal rainforest zone and in a few large valleys in the perhumid
zone. The only significant occurrence of Pseudotsuga in the northern
seasonal zone or perhumid rainforest, for example, is in these major
interior-influenced valleys (e.g. , Nimkish, Bella Coola in British
Columbia).
An equally significant but more consistent gradient in temperature also
characterizes the north-south transect. The temperature gradient is closely
Paul B. Alaback
summer. The analogous temperate rainforest types in southern Chile are
closely matched with those in North America, except that their climate is
even more equitable year-round and they are warmer , particularly during
the winter.
One of the key gradients within the temperate rainforest zones of
North America is precipitation, which steadily increases northward both
in equability of distribution and in yearly accumulation (Alaback, 1991;
Alaback & McClellan , 1992). Along this transect, moisture changes from
a scarce resource limiting plant productivity to an overabundant resource
which is associated with declines in soils fertility (Ugolini & Mann , 1979).
In the warm temperate and seasonal rainforests, precipitation primarily
occurs in the winter months. In the warm temperate zone, rainfall over
1400mm occurs only within 30 km of the ocean or at elevations greater
than 500m. The Klamath Mountain region provides a striking anomaly,
in that harsh dry climates persist almost to the coast because of the
unique physiography of the region. This geographic area also corresponds
with an extremely rich and diverse flora. This diversity is due not only to
the diversity in climates that occur there but also to a long history and
diverse geology (Whittaker, 1960). While the whole Klamath Mountain
region is sometimes considered to be in the Mediterranean zone , the
wetter coastal portions of this region are included here , due to their
floristic and climatic similarity to seasonal rainforests to the north.
Within the northern portions of the temperate rainforest region , the
most ecologically important effects of precipitation relate more to its
distribution than to its magnitude (Alaback, 1991). Throughout the
perhumid and subpolar rainforest regions , semipermanent low-pressure
systems bring high annual precipitation during most of the year . Upwelling
off the Pacific Ocean also increases moisture by producing fog during late
spring and early summer. Fog-drip interception is an important yet seldom
quantified element of the hydrological budget (Harr, 1982) and is also of
key importance in defining the geographic limit of the most productive
forest types, especially along the coastline of northern California and the
Pacific Northwest. Fog is also a key climatic factor on the outer coast of
British Columbia and Alaska.
Dry interior valleys extend their climatic influence on coastal vegetation
patterns by allowing local domination by fire- or drought-resistant species.
This pattern is most significant in the warm temperate rainforest, in
particular the Klamath Mountain region , but also occurs intermittently in
the seasonal rainforest zone and in a few large valleys in the perhumid
zone. The only significant occurrence of Pseudotsuga in the northern
seasonal zone or perhumid rainforest, for example, is in these major
interior-influenced valleys (e.g. , Nimkish, Bella Coola in British
Columbia).
An equally significant but more consistent gradient in temperature also
characterizes the north-south transect. The temperature gradient is closely