Lh. Gu et al., Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests, J GEO RES-A, 104(D24), 1999, pp. 31421-31434
We analyzed half-hourly tower-based flux measurements of carbon dioxide (CO
2) from a boreal aspen forest and a temperate mixed deciduous forest in Can
ada to examine the influences of clouds on forest carbon uptake. We showed
that the presence of clouds consistently and significantly increased the ne
t ecosystem exchanges (NEE) of CO2 of both forests from the level under cle
ar skies. The enhancement varied with cloudiness, solar elevation angles, a
nd differed between the two forests. For the aspen forest the enhancement a
t the peak ranged from about 30% for the 20 degrees-25 degrees interval of
solar elevation angles to about 55% for the 55 degrees-60 degrees interval.
For the mixed forest the enhancement at the peak ranged from more than 60%
for the 30 degrees-35 degrees interval of solar elevation angles to about
30% for the 65 degrees-70 degrees interval. Averaged over solar elevation a
ngles >20 degrees, the aspen and mixed forests had the maximal NEE at the i
rradiance equivalent to 78 and 71% of the clear-sky radiation, respectively
. The general patterns of current shy conditions at both sites permit furth
er increases in cloudiness to enhance their carbon uptake. We found that bo
th forests can tolerate exceedingly large reductions of solar radiation (53
% for the aspen forest and 46% for the mixed forest) caused by increases in
cloudiness without lowering their capacities of carbon uptake. We suggest
that the enhancement of carbon uptake under cloudy conditions results from
the interactions of multiple environmental factors associated with the pres
ence of clouds.