A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14 7
Fig. 5. Seasonal variation of forest floor CO
2
exchange. a Daily averages of soil temperature, T
s
, at 5 cm depth thick line, air temperature, T
a
, at 8.5 m dotted line, and soil water content, θ , thin line, b average night-time, c average day-time, and d daily totals of forest floor CO
2
efflux, which during day corresponds to respiration reduced by photosynthetic uptake, F
G
Eq. 2, and during night only respiration, R. Filled circles are measurements made at locations C1P1, C1P3 and C1P4, filled triangles measurements at location C1P2,
and open circles measurements made at locations C2P1–C2P4.
F
G
was calculated by rearranging Eq. 1 cf. Goulden et al., 1997. Because data were noisy and
the nocturnal temperature range was often small, even for periods of several weeks, temperature response
functions were difficult to establish for single mea- surement periods Eq. 2. Therefore all available
data were used, and two groups established. Data from spot C1P2, were treated separately because of
higher fluxes Table 1, and because those measure- ments were representative of only a very small part
of the stand. The other group contained data from the other seven spots.
3. Results
The system showed a distinct light response Fig. 3. At low PPFD, i.e. night, dusk and dawn, respira-
tion was the dominating component. Already at PPFD in the range 3–30
m
mol m
− 2
s
− 1
, moss and dwarf
8 A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14
Fig. 6. Mean night-time respiration rates, R. a Respiration plotted against soil temperature at 5 cm, T
s
. The best fit for open circles was R =
0.0401 exp0.1559T
s
, Q
10 e
= 4.8, R
2
= 0.49, n = 195, where Q
10 e
is an effective Q
10
, see text; b Respiration plotted against moss temperature, T
m
. The best fit for open circles was R = 0.0599 exp0.1067T
m
, Q
10 e
= 2.9, R
2
= 0.29, n = 195; c Respiration plotted against
chamber temperature, T
ch
. The best fit for open circles was R = 0.1092 exp0.0638T
ch
, Q
10 e
= 1.9, R
2
= 0.17, n = 195; and d Respiration
plotted against soil water content at 10 cm. The fit for open circles was R = −0.9024θ + 0.3341, R
2
= 0.16, n = 195. Thick solid lines are
regression lines and thin solid lines are extrapolations of the regression lines. Open circles show respiration rates at locations C1P1, C1P3, C1P4 and C2P1–C2P4. Filled circles show respiration rates at location C1P2.
shrubs began to photosynthesise Fig. 3, and the system apparently was light-saturated at PPFD above
50
m
mol m
− 2
s
− 1
. The effect of photosynthesis was clearly seen when F
N
was plotted against temperature for different PPFD classes. At night, F
N
showed an exponential increase Fig. 4a. At PPFD in the range
3−30
m
mol m
− 2
s
− 1
Fig. 4b, photosynthesis clearly affected F
N
, and at PPFD above 30
m
mol m
− 2
s
− 1
Fig. 4c, a temperature response only was seen at temperatures below 10
◦
C. Average nocturnal respiration rates and the daytime
net CO
2
exchange of the forest floor were at their lowest in May, increased through June and July, max-
imum in August and decreased in September and Oc- tober Fig. 5a–c. The increase was slow throughout
May to the middle of July, and was followed by a more rapid increase from July before reaching the maximum
in August. The corresponding temperature increase, on the other hand, was faster in May and first half
of June and was slower from mid-June until August. From August until September, both nocturnal respira-
tion and daytime net CO
2
efflux were more strongly correlated with temperature. Net CO
2
efflux, accumu- lated to diurnal 24-h values, showed a trend similar
to that of the average nocturnal and daytime values Fig. 5d. Locations with denser vegetation cover had
generally higher respiration and net efflux rates, than sparsely vegetated locations. Excluding location C1P2
Table 1, Fig. 5b–d, the highest average nocturnal respiration rate was 0.65 mg m
− 2
s
− 1
and the highest daytime efflux rate was 0.51 mg m
− 2
s
− 1
. Similarly, the largest diurnal efflux was 52 g m
− 2
. Nocturnal respiration was fairly well described
by exponential relationships with temperature Fig. 6. Soil temperature at 5 cm explained 49 of the
variation in forest floor respiration, while moss and air temperature explained 29 and 17 of the
variation, respectively. For location C1P2, air temper-
A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14 9
Fig. 7. Measured and modelled instantaneous net forest floor CO
2
exchange. a Half-hourly average values of net forest floor CO
2
exchange, F
N
, taken as an average of measurements at locations C1P4 and C2P1 thick line, and modelled respiration thin line for 11 May–16 June, Measured and modelled values during the period 1–13 June, see detail in b, b measured F
N
circles for 7–13 June together with modelled respiration, R, thin line, and gross forest floor exchange, F
G
, thick line, and c accumulated R, F
N
, and F
G
. Horizontal lines in b indicate when sun was above horizon.
ature explained 61 of the variation in respiration, while soil and moss temperatures both explained
50 of the variation. The relationship with soil temperature at 5 cm gave a base respiration rate of
0.04 mg m
− 2
s
− 1
, Q
10 e
was 4.75, and the respiration rate at 10
◦
C was 0.19 mg m
− 2
s
− 1
Fig. 6a. Base respiration rate increased, while Q
10 e
decreased, with increasing temperature range; hence with chamber
temperature as predictor, the base respiration rate was 0.11 mg m
− 2
s
− 1
and Q
10 e
1.89 Fig. 6c. The relationship with soil-water content was weak, and
explained only 20 of the variation Fig. 6d. To estimate the photosynthetic uptake respiration
was modelled as a function of soil temperature at 5 cm Eq. 2 with parameters according to Fig. 5a. Com-
parison with an average of the measurements at loca- tions C1P4 and C2P1 from 11–14 May and from 23
May–16 June, indicated that the base respiration rate, and possibly also Q
10 e
, varied throughout the season Fig. 7a. From 11–31 May, nocturnal respiration was
underestimated by the model as compared to measure- ments when data was available, from 1–13 June mod-
elled and measured data agreed, and from 13–16 June the model overestimated respiration Fig. 7a and b.
Because of these discrepancies, photosynthetic uptake was estimated only for the period for which modelled
and measured nocturnal fluxes showed good agree- ment Fig. 7b. During this period, chamber tempera-
ture ranged from 6–26
◦
C, and soil temperature from 7–11
◦
C. F
G
was calculated as the difference between modelled respiration and measured net CO
2
efflux Eq. 1, and was at most 0.19 mg m
− 2
s
− 1
Fig. 7b. R, F
N
and F
G
, accumulated for the period 7–13 June, showed that photosynthetic uptake reduced the amount
10 A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14
Fig. 8. Estimated instantaneous gross forest floor CO
2
exchange, F
G
, from Fig. 7b, plotted against a incident light, PPFD, and b chamber temperature, T
ch
for 7–13 June 1996.
of CO
2
released through respiration by about 28 Fig. 7c. Of the 96.6 g m
− 2
respired, 27.3 g m
− 2
was used in photosynthesis, which resulted in a net CO
2
efflux over 7 days of 69.3 g m
− 2
. F
G
showed a large scatter, and only a weak light-response Fig. 8a. Fur-
Fig. 9. Average net forest floor exchange rate, F
N
, based on measurements from the two chambers, accumulated per
day, and plotted against soil temperature, T
s
, at 5 cm. Regres- sion was made for three periods: 11 May–16 June circles,
F
N
= 5.236 + 0.483T
s
, R
2
= 0.42, n = 20; 17 June–8 July squares,
F
N
= − 19.77 + 3.227T
s
, R
2
= 0.24, n = 33; 9 July–7 October tri-
angles, F
N
= − 1.046 + 2.150T
s
, R
2
= 0.69, n = 43. Arrows indi-
cate the seasonal course. Fig. 10. Net forest floor exchange rate, F
N
, accumulated from 1 May–31 October 1996. Daily net CO
2
efflux, F
N
, was based on measurements from the two chambers, accumulated per day and
missing data filled in by linear regression with soil temperature, T
s
, at 5 cm cf. Fig. 9. Bars show F
N
accumulated per month.
thermore, F
G
increased as temperature increased. The temperature optimum was difficult to identify, but the
response in relation to chamber temperature appeared to level out at about 18
◦
C Fig. 8b. From 11 May–7 October, chamber measurements
covered 96 complete days. Soil temperatures were available for 179 days, from 6 May–31 October.
A linear relationship was established between daily accumulated net CO
2
efflux of the forest floor and mean soil temperature. The model was improved by
dividing data into three groups, 11 May–16 June, 17 June–8 July and 9 July–7 October Fig. 9. The third
period showed the strongest correlation with tem- perature, describing 69 of the variation in F
N
. To cover 6 months, 1 May–31 October, F
N
was modelled from soil temperature where no data were available
from soil chambers. Over the 6-month period, about 3.1 kg m
− 2
of CO
2
was released to the atmosphere. On a monthly basis, F
N
was lowest in May, with 6 of the total F
N
, and highest in August, with 30 of the 6-month total F
N
Fig. 10.
4. Discussion
