6. Constraints on Terrestrial Primary Productivity
97
lead to a reduced fraction of IPAP being utilized by forests in the Pacific
Northwest. Slower-growing species better adapted to drought should
invade and replace more mesic types of forests as the total area occupied
by forests contracts in comparison to the current distribution (Franklin &
Dyrness, 1973). The transition could be abrupt, spurred by outbreaks of
insects, disease, and fire across much of the region (Waring, Savage,
Cromack, Jr. , & Rose, 1992; Waring & Schlesinger, 1985).
Application of the Light Utilization Model in Chile
In the last 2 decades, much of the native forests of Chile have been
logged and the land replanted with introduced species such as Pinus
radiata (Lara, 1985). The pine forests are planted very densely and
absorb >95% of all visible light, as do coniferous species composing
forests on the west side of the Cascade Range in the states of Oregon and
Washington (Runyon et aI., 1994; Waring & Franklin, 1979). Read and
Hill (1985) reported that in 21 sampled native Chilean forests, >30% of
the light penetrated to the forest floor. The temperate forest region in
Chile has a milder climate, with more uniform distribution of rainfall,
than the Pacific Northwest (Alaback, 1991; Lawford, 1995). The milder
climates result in an extra month of growing season (Goward, Dye ,
Kerber, & Kalb, 1987). During the growing season, however, the climate
is much cooler than at a comparable latitude in the Pacific Northwest.
The mild and moist climate in the mid-coastal zone of Chile permits
plantations of Pinus radiata to produce wood volumes from 22-24
rrr' ha -1 yr"". On the other hand, second-growth stands of the fastestgrowing native species (Nothofagus obliqua and N . alpina) produce only
12-14m
3ha- 1yr- 1 (Schmidt & Lara, 1985).
Why might native species in Chile form more open canopies than
introduced species and generally grow more slowly? One possibility is
that most of the native flora is less than perfectly adapted to the current
climate as a result of being derived from subtropical stock (Axelrod,
Kalin-Arroyo, & Raven , 1991; Raven & Axelrod, 1974). Related genera
in New Zealand derived from subtropical origin exhibit a temperature
optimal for photosynthesis of 27°C, as compared to 20°C for Pacific
Northwest conifers (refer to Table 6.2). Even presumably well-adapted,
relatively fast-growing species from Chile such as Nothofagus obliqua and
N. alpina have temperature optima of 23°C, which is well above mean
monthly temperatures during the growing season recorded within most of
the current range of these species (Alaback , 1991; Read & Bu sby, 1990;
Read & Hill, 1985).
We infer that cool soil temperatures provide a much greater constraint
on the growth of most Chilean tree species than equivalent temperatures
do on Northwest conifers (Figure 6.3). Unfavorably cool soil temperatures can restrict stomatal opening, photosynthesis, the growth of roots,
97
lead to a reduced fraction of IPAP being utilized by forests in the Pacific
Northwest. Slower-growing species better adapted to drought should
invade and replace more mesic types of forests as the total area occupied
by forests contracts in comparison to the current distribution (Franklin &
Dyrness, 1973). The transition could be abrupt, spurred by outbreaks of
insects, disease, and fire across much of the region (Waring, Savage,
Cromack, Jr. , & Rose, 1992; Waring & Schlesinger, 1985).
Application of the Light Utilization Model in Chile
In the last 2 decades, much of the native forests of Chile have been
logged and the land replanted with introduced species such as Pinus
radiata (Lara, 1985). The pine forests are planted very densely and
absorb >95% of all visible light, as do coniferous species composing
forests on the west side of the Cascade Range in the states of Oregon and
Washington (Runyon et aI., 1994; Waring & Franklin, 1979). Read and
Hill (1985) reported that in 21 sampled native Chilean forests, >30% of
the light penetrated to the forest floor. The temperate forest region in
Chile has a milder climate, with more uniform distribution of rainfall,
than the Pacific Northwest (Alaback, 1991; Lawford, 1995). The milder
climates result in an extra month of growing season (Goward, Dye ,
Kerber, & Kalb, 1987). During the growing season, however, the climate
is much cooler than at a comparable latitude in the Pacific Northwest.
The mild and moist climate in the mid-coastal zone of Chile permits
plantations of Pinus radiata to produce wood volumes from 22-24
rrr' ha -1 yr"". On the other hand, second-growth stands of the fastestgrowing native species (Nothofagus obliqua and N . alpina) produce only
12-14m
3ha- 1yr- 1 (Schmidt & Lara, 1985).
Why might native species in Chile form more open canopies than
introduced species and generally grow more slowly? One possibility is
that most of the native flora is less than perfectly adapted to the current
climate as a result of being derived from subtropical stock (Axelrod,
Kalin-Arroyo, & Raven , 1991; Raven & Axelrod, 1974). Related genera
in New Zealand derived from subtropical origin exhibit a temperature
optimal for photosynthesis of 27°C, as compared to 20°C for Pacific
Northwest conifers (refer to Table 6.2). Even presumably well-adapted,
relatively fast-growing species from Chile such as Nothofagus obliqua and
N. alpina have temperature optima of 23°C, which is well above mean
monthly temperatures during the growing season recorded within most of
the current range of these species (Alaback , 1991; Read & Bu sby, 1990;
Read & Hill, 1985).
We infer that cool soil temperatures provide a much greater constraint
on the growth of most Chilean tree species than equivalent temperatures
do on Northwest conifers (Figure 6.3). Unfavorably cool soil temperatures can restrict stomatal opening, photosynthesis, the growth of roots,