96
R.H. Waring and W.E. Winner
utilized light and above-ground growth was observed across a broad range
of environments, as shown in Figure 6.6 . No specific threshold for soiltemperature responses was established eliminate because most species in
the Pacific Northwest have considerable tolerance for cool soils , perhaps
reflecting their boreal origins (Waring & Franklin, 1979). Old-growth
forests (1, Figure 6.6), because of reduced photosynthetic capacity and
increased maintenance respiration, grow much less than younger stands
with similar leaf area (Waring & Franklin, 1979).
Effects of Climatic Change on Forest Growth
in the Pacific Northwest
From a sequential analysis of the factors constraining photosynthesis
(Table 6.1), climatic warming combined with increased drought should
3 0 . . , . - - - - - - - - - - - - - - - - - - - ,
3
3F •
•
1
(Old-Growth)
1000
2000
Y = 0.0096x R~2= 0.99
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o
10
20
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t
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o II: a.
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UTILIZED
PHOTOSYNTHETICALLY ACTIVE RADIATION
MJ/m
2/yr
Figure 6.6. Estimated aboveground Net Primary Production correlates well with
estimates of photosynthetically active radiation that can be utilized by a range of
vegetation across a steep climatic gradient in western Oregon. Letters (A) refer to
a deciduous forest of alder, and (F) indicate stands that were fertilized with
nitrogen. The old-growth stand (1) was excluded from the regression. It utilized
30% less PAR than indicated . Deducting an additional 100MJ m-2 for increased
maintenance respiration compared to an adjacent 120 year-old stand brings the
calculated production in line with that observed . From Runyon et al. (1994).