Automated measurements of the net forestfloor
CO2 exchange (NFFE) were made in a mature (130yearold)
boreal black spruce forest over an 8year
period (2002–2009) with the objectives of (1) quantifying
the spatial and temporal (seasonal and interannual) patterns in NFFE, soil respiration (SR) and gross forestfloor
photosynthesis (GFFP), and (2) better understanding the key climatic controls on each component
at both time scales. Scalingup
of the component fluxes to the stand level showed that the feather moss
community accounted for more than 85% of NFFE and SR, and more than 70% of GFFP. The remainder was
partitioned almost equally between the sphagnum and lichen communities for all components fluxes,
while the exposed mineral soil in hollows accounted for less than 1% of NFFE and SR. Soil temperature (Ts)
was the dominant climate variable determining seasonal trends in NFFE and SR. The shape of the exponential
response was, however, strongly modulated by soil water content (SWC) in the surface organic
horizon, with reduced apparent temperature sensitivity at low SWC. A lowering of the water table depth
also had an effect on NFFE and SR, although very weak, with increased CO2 loss from the hollows likely due
to improved soil aeration. Air temperature (Ta) was the dominant climate variable determining seasonal
trends in GFFP, while plant water status seemed to have played a minor role. Although not statistically
significant (p = 0.9907), annual totals of scaledup
NFFE varied from 505 ± 121 to 601 ± 144 g C m−2 y−1
over the 8year
period. The lowest NFFE was observed in 2004, the coldest and wettest year on record,
while the highest was observed in 2005, a warmer year with slightly aboveaverage
precipitation. SR, by
far the dominant component of the forestfloor
CO2 exchange, closely followed the interannual
trends
in NFFE, while GFFP was lowest in 2004 and highest in 2003, also a cold year but with very low precipitation.
Over the 8year
period, winter NFFE contributed 7% to annual NFFE while GFFP during the growing
season reduced losses due to SR by 18%.
While strong correlations were observed between the component fluxes and temperature (Ts or Ta) and
SWC at the seasonal time scale, the mean annual values of these climate variables were poor predictors of
the interannual
trends when considered individually. Combining multiplicatively Ts and SWC for NFFE
and SR, and Ta and SWC for GFFP, significantly increased the predictive ability of the models. The difference
in predictability of the two time scales poses some interesting challenges for interpreting and modeling
the longterm
temporal trends in NFEE and its components. The results obtained in this relatively longterm
study suggest that the interannual
variability in the component fluxes was not driven by the mean
annual climate conditions, but rather the shorter time scale changes in climate conditions, i.e. changes
that occurred within days, weeks and/or seasons. Moreover, it appeared that the timing of the climatic
changes within each year was also critical, spring and summer conditions having a far greater impact
than fall and winter conditions in this stand.
