6. Constraints on Terrestrial Primary Productivity
99
subfreezing winter temperatures occur sufficiently often to prevent successful plantation of these species, even along the Pacific coast. One
might expect in the temperate-forest region of Chile that Northwest
conifers such as Douglas fir would not experience sufficient cold temperatures to meet their chilling requirements. This is not the case, however,
because young Douglas fir can be observed growing more than a meter in
height a year, side by side with eucalyptus and Monterey pine in the
Andes at 41°S latitude (Waring, personal observation with photographic
evidence available).
Effects of Climatic Change on the Temperate Forests of Chile
If global warming were to increase the growing-season temperature in
Chile, native tree species might be expected to respond far differently
from species in the Pacific Northwest. We speculate that an increase in
temperature would improve the ability of Chilean trees to take up water
and nutrients from soils and increase their tolerance to atmospheric
humidity deficits. Although a decrease in summer precipitation in Chile is
not predicted in climate models (Revkin, 1988), if drought were to .occur,
it should be less extreme than in the Pacific Northwest, because Chilean
forests receive at present from 10% to 25% of their annual precipitation
during summer months, whereas forests in the Pacific Northwest receive
only 3% to 15% of annual precipitation during the same interval
(Alaback, 1991). Other factors may contribute to changes in forest composition, but to assess these requires different kinds of models that
account for competition, reproduction, dispersal, introduction, and
extinction of species.
Conclusions
With an environmentally constrained light-absorption model, we quantified the influence of frost, drought, and humidity deficits upon
photosynthesis and growth across a steep climatic gradient in Oregon.
When account was made for reduced utilization of intercepted light due
to partial or complete stomatal closure, the solar conversion efficiency
was similar for all types of Pacific Northwest coniferous forests. Because
Northwest conifers are derived from boreal stock, they show little restriction on water uptake or growth once soil temperatures exceed 10°C. In
contrast, the native species of Chilean forests are derived mainly from
subtropical stock and thus are likely to be restricted in their performance
at soil temperatures below 20°C. If climatic warming occurs in Chile,
improved growth of native species is predicted from our model. The
opposite prediction applies to the Pacific Northwest forests, because
drought is expected to have more impact.
99
subfreezing winter temperatures occur sufficiently often to prevent successful plantation of these species, even along the Pacific coast. One
might expect in the temperate-forest region of Chile that Northwest
conifers such as Douglas fir would not experience sufficient cold temperatures to meet their chilling requirements. This is not the case, however,
because young Douglas fir can be observed growing more than a meter in
height a year, side by side with eucalyptus and Monterey pine in the
Andes at 41°S latitude (Waring, personal observation with photographic
evidence available).
Effects of Climatic Change on the Temperate Forests of Chile
If global warming were to increase the growing-season temperature in
Chile, native tree species might be expected to respond far differently
from species in the Pacific Northwest. We speculate that an increase in
temperature would improve the ability of Chilean trees to take up water
and nutrients from soils and increase their tolerance to atmospheric
humidity deficits. Although a decrease in summer precipitation in Chile is
not predicted in climate models (Revkin, 1988), if drought were to .occur,
it should be less extreme than in the Pacific Northwest, because Chilean
forests receive at present from 10% to 25% of their annual precipitation
during summer months, whereas forests in the Pacific Northwest receive
only 3% to 15% of annual precipitation during the same interval
(Alaback, 1991). Other factors may contribute to changes in forest composition, but to assess these requires different kinds of models that
account for competition, reproduction, dispersal, introduction, and
extinction of species.
Conclusions
With an environmentally constrained light-absorption model, we quantified the influence of frost, drought, and humidity deficits upon
photosynthesis and growth across a steep climatic gradient in Oregon.
When account was made for reduced utilization of intercepted light due
to partial or complete stomatal closure, the solar conversion efficiency
was similar for all types of Pacific Northwest coniferous forests. Because
Northwest conifers are derived from boreal stock, they show little restriction on water uptake or growth once soil temperatures exceed 10°C. In
contrast, the native species of Chilean forests are derived mainly from
subtropical stock and thus are likely to be restricted in their performance
at soil temperatures below 20°C. If climatic warming occurs in Chile,
improved growth of native species is predicted from our model. The
opposite prediction applies to the Pacific Northwest forests, because
drought is expected to have more impact.