Signature of Ecosystem Respiration

Recently, an increasing number of studies have evaluated the

13CO, exchange of terrestrial ecosystems, applying the experimental methods outlined above. Results for 51 different forest and

TABLE 2 Plant-Specific Physiological Parameters*

Trees

Tropical evergreen Tropical raingreeir' Temperate broadleaved evergreen Temperate sumrnergreeir' Subtropical/temperate conifer Boreal evergreen Boreal deciduous1' Nontrees

Temperate grass'""1''' Tropical grass" Desert woody shrub'"'1 Tundra woody shrulr Cold herbaceous® Cushion forb/lichen/moss'

10 10

70 67 65

52 83 83

83 57

53 93 93 93

18 9 18

30 24 6

8 10

optratio

0.95

0.65

0.65

0.75

10 10 5

Tcurve

Rfact

Alloc

25 20 40 50 40 33 33

40 40 33 33 33 33

*P, phenological type (e, evergreen, r, raingreen, s, summergreen); Gmill, minimum canopy conductance (mm/s); Em„, maximum daily transpiration rate (mm/day); K !0, percent of roots in the top 30 cm of soil; Im, leaf longevity (months); optratio, the maximum allowed C/C, ratio; kk, the Beer's law extinction coefficient; Tp,-j, minimum monthly temperature for C, photosynthesis; '/curve, modifier to the curve response of photosynthesis to temperature; Rfact, modifier to the curve response of maintenance respiration to temperature; Alloc, modifier to the minimum allocation; Fire, the soil moisture percent threshold at which a fire day is counted.

'Soil moisture threshold of 60 % for leaf flushing and 50 % for leaf fall.

''Requirement of 200 growing degree-days on a 5°C basis to grow a full canopy.

"Soil moisture threshold of 30% for leaf flushing and 20% for leaf fall.

dRequirement of 100 growing degree-days on a 0°C basis to grow a full canopy.

'Presence of C, photosynthesis, minimum monthly temperature for C4 photosynthesis is 10°C.

'Presence of sapwood respiration.

sRequirement of five growing degree-days on a 0°C basis to grow a full canopy.

agricultural sites were available to the authors (published and unpublished datasets) and were used in the following analysis (original data are given in the Appendix). When more than one estimate of the l3C signature of ecosystem respiration was published, a growing season mean was calculated (arithmetic mean). Thus, seasonal variability of ¿>I3CER was not considered (but see Buchmann etal, 1998).

Estimates of ¿>I3CER varied globally between — 29.4%o (Yakir and Wang, 1996; Ilarwood, 1997) and -20%o (Yakir and Wang, 1996; Buchmann and Ehleringer, 1998), averaging —25.3 ±2.2 (SD) %o (Fig. 2A). Tropical forests and agricultural stands exhibited very low <5I3CER values, while agricultural C4 stands in Mediterranean and temperate regions showed the highest <5I3CEK values. Physiological constraints of C, photosynthesis as well as the expression of the Q photosynthetic pathway resulted in this large global spread of <5I3CER values. The smallest variability of £>I3Cer ratios was observed in the humid tropics (< 10° N or S), reflecting relatively stable microclimatic conditions as well as the restriction of study sites to forest stands. Greatest variability was found for agricultural stands, illustrating the pronounced effects of land-use change (Buchmann and Ehleringer, 1998). Under these circumstances, changes in the photosynthetic pathway of vegetation cover from C3 to C., (and vice versa) result in a mixture

tropical subtropica

temperate

boreal

-20

-

i 1 i o

A ¿O

A

0

<1 *

0

-24

-25.3

O

-28

crops i i

coniferous

-20

-

i i i •

i I •

B

-24

*

-28

-

i i i

i i

i i

FIGURE 2 Observed carbon isotope ratios of ecosystem respiration. S'^Cf,, was determined as the intercept of "Keeling plots," i.e., the regression of inverse canopy [CO,] against the corresponding 513Ccjnu[,y. A: 6l3Crl! based on measurements of canopy air for broad-leaved and coniferous forests and crop stands. Original data are given in the Appendix. B: S'-'Cpu based on tropospheric measurements within the NOAA network (after Bakwin et al., 1998).

FIGURE 2 Observed carbon isotope ratios of ecosystem respiration. S'^Cf,, was determined as the intercept of "Keeling plots," i.e., the regression of inverse canopy [CO,] against the corresponding 513Ccjnu[,y. A: 6l3Crl! based on measurements of canopy air for broad-leaved and coniferous forests and crop stands. Original data are given in the Appendix. B: S'-'Cpu based on tropospheric measurements within the NOAA network (after Bakwin et al., 1998).

of organic matter in the soil that is being respired carrying a mixture of both 13C signatures. No clear latitudinal trend was found, supporting results from tropospheric air measurements (Fig. 2B). Calculating S13CER ratios from the NOAA/CMDL flask sampling network and from very tall towers (up to 600 m), Bakwin et al (1998) found an average S13CER of -24.7 ± 0.8%o (mean ±SD) for the biogenic carbon exchange with the atmosphere, slightly higher than the average from measured <513CER estimates.

0 0

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