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AOBPreview originally published online on December 5, 2007
Annals of Botany 2008 101(3):469-477; doi:10.1093/aob/mcm304
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Seasonal Variation in CO2 Efflux of Stems and Branches of Norway Spruce Trees

Manuel Acosta1,*, Marian Pavelka1, Radek Pokorny1, Dalibor Janous1 and Michal V. Marek1,2

1 Laboratory of Plants Ecological Physiology, Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic, Porici 3b, 603 00 Brno, The Czech Republic
2 Institute of Forest Ecology, Faculty of Forestry and Woody Technology, Mendel University of Agriculture and Forestry, Zemedelská 3, 613 00 Brno, The Czech Republic

* For correspondence. E-mail manuel{at}usbe.cas.cz

Received: 10 July 2007    Returned for revision: 27 September 2007    Accepted: 2 November 2007    Published electronically: 5 December 2007


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 LITERATURE CITED
 

Background and Aims: Stem and branch respiration, important components of total forest ecosystem respiration, were measured on Norway spruce (Picea abies) trees from May to October in four consecutive years in order (1) to evaluate the influence of temperature on woody tissue CO2 efflux with special focus on variation in Q10 (change in respiration rate resulting from a 10 °C increase in temperature) within and between seasons, and (2) to quantify the contribution of above-ground woody tissue (stem and branch) respiration to the carbon balance of the forest ecosystem.

Methods: Stem and branch CO2 efflux were measured, using an IRGA and a closed gas exchange system, 3–4 times per month on 22-year-old trees under natural conditions. Measurements of ecosystem CO2 fluxes were also determined during the whole experiment by using the eddy covariance system. Stem and branch temperatures were monitored at 10-min intervals during the whole experiment.

Key Results: The temperature of the woody tissue of stems and branches explained up to 68 % of their CO2 efflux. The mean annual Q10 values ranged from 2·20 to 2·32 for stems and from 2·03 to 2·25 for branches. The mean annual normalized respiration rate, R10, for stems and branches ranged from 1·71 to 2·12 µmol CO2 m–2s –1 and from 0·24 to 0·31 µmol CO2 m–2 s–1, respectively. The annual contribution of stem and branch CO2 efflux to total ecosystem respiration were, respectively, 8·9 and 8·1 % in 1999, 9·2 and 9·2 % in 2000, 7·6 and 8·6 % in 2001, and 8·6 and 7·9 % in 2002. Standard deviation for both components ranged from 3 to 8 % of the mean.

Conclusions: Stem and branch CO2 efflux varied diurnally and seasonally, and were related to the temperature of the woody tissue and to growth. The proportion of CO2 efflux from stems and branches is a significant component of the total forest ecosystem respiration, approx. 8 % over the 4 years, and predictive models must take their contribution into account.

Key words: Stem respiration, branch respiration, Picea abies, seasonal variation, temperature, Q10, R10


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 LITERATURE CITED
 
Respiration is a principal component of a plant's carbon budget, estimated to consume between 30 and 70 % of assimilatory fixed carbon (e.g. Sprugel and Benecke, 1991; Hagihara and Hozumi, 1991; Ryan, 1991; Ryan et al., 1994). On the forest-stand or ecosystem scales, the importance of respiration is indisputable and is estimated to be 40–60 % of gross photosynthesis in cool temperate forest, and up to 90 % in tropical forests (Whittaker, 1975; Linder, 1985). Respiration is a physiological process that is sensitive to environmental conditions and can depend on the current physiological state of the cell or whole organ. It takes place continuously in all the living tissue of a tree, but differs quantitatively between organs, changing in relation to the availability of substrates, with the state of development or organ ontogeny, and with temperature. It is widely accepted that temperature is a key environmental factor, in that the rate of respiration generally increases exponentially with an increase in temperature (Amthor, 1984; Stockfors, 2000). This response is usually expressed in terms of Q10 (Sprugel et al., 1995), i.e. the proportional change in respiration rate resulting from a 10 °C increase in temperature. For a wide variety of plants, Q10 ranges from 1·3 to 3·0, with an average of about 2 (Amthor, 1994). However, the relationship between respiration and temperature of organs, measured in the air near them, but not at the site of respiration, is complex.

An important feature is the time-lag in the response of respiration to temperature (Ryan et al., 1995; Lavigne et al., 1996). For stems this reflects the large resistance to CO2 diffusion from the stem interior to ambient air (Eklund and Lavigne, 1995), because of the inner wood structure and outer bark resistance. Eklund (1990) reported that CO2 concentration inside the stem, which can be 10–30 times larger that in the ambient air, drives CO2 diffusion. Ryan et al. (1995) reported that CO2 efflux (CO2E) lagged behind air temperature by up to 5 h, and was slower for species with thicker bark (ponderosa pine and slash pine) than for species with thin bark (red pine and western hemlock). Another important factor influencing CO2E from woody tissue is the concentration of CO2 transported in the transpiration stream, representing an aqueous transport of CO2 (Martin et al., 1994; Levy et al., 1999; Saveyn et al., 2007).

The analysis suggests a paucity of research on CO2E from tree stems and branches. This paper presents the findings from the following: (1) determination of the daily course of stem and branch CO2E from Norway spruce over four growing seasons in relation to cambium temperature with particular focus on the seasonal and intra-seasonal variation in Q10; (2) estimation of the relationship between stem and branch CO2E, to provide a physiological understanding of tree growth and as a measure of incremental changes in biomass during the seasons; (3) estimation of the influence of sap flow on stem and branch CO2E; and (4) quantification of the contribution of above-ground woody tissue (stem and branch) CO2E to the carbon balance of a whole Norway spruce forest.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 LITERATURE CITED
 
Site description
The experiment was carried out in a Norway spruce (Picea abies [L.] Karst) forest stand at the Experimental Ecological Study Site (EESS) Bíly Kríz (49°30'17''N, 18°32'28''E, at an altitude of 908 m a.s.l.) located in the Moravian–Silesian Beskydy Mts (north-east Moravia, Czech Republic). The 6-ha forest stand was planted in 1981, using 4-year-old seedling stock, and is located on a gentle slope (13·5°) with a south-southwest exposure. The basic climatological parameters at EESS Bíly Kríz for the period of investigation, i.e. 1999–2002, were: mean annual temperature 5·5 °C, mean annual total precipitation 1400 mm and mean annual relative air humidity 80 %. The soil is a predominantly Haplic Podzol (FAO soil classification system). The under-canopy is dominated by Vaccinium myrtillus. Main characteristics of the forest stand over the experiment period are presented in Table 1.


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  		TABLE 1. Main characteristics of the forest stand over four investigated growing seasons

 
Stem and branch CO2E measurements
CO2E was measured on stems and branches of four sample trees, characteristic of the forest canopy, randomly selected from across the experimental area. Two branches located in the lower parts of the crown body (i.e. at 1·3 m height above ground, with west and east orientation) were investigated on each tree. CO2E was determined with a portable closed gas exchange system (Li-6250; Li-Cor, Lincoln, NE, USA). Stem CO2E was measured using a self-made respiration chamber consisting of a half-cylinder PVC tube, 12 cm high and 6·5 cm in diameter, with edges covered by neoprene. The stem chamber was attached near the measurement of diameter at breast height (dbh, i.e. 1·33 m) with belts, and orientated to the north to minimize the effect of direct sunshine. The stem surface area enclosed in the chamber was calculated on the assumption that the stem was a circle with a radius of half the diameter (measured). The branch chamber was also made from PVC tubing with edges covered by neoprene, and was cylindrical, divided in two halves 12 cm long and 6·5 cm in diameter. It was attached as near as possible to the branch base and hermetically sealed by pressing the chamber halves against each other and securing with belts. An airtight seal was obtained using sticky non-hardening mastic (UHU patafix, Bühl-Baden, Germany) on corners of the chamber. The surface area of the branch enclosed in a chamber was estimated, assuming that it was cylindrical. Although stem and branch chambers were always fixed at the same place during the period of CO2E measurement, they were removed between measurements. This ensured the device had minimal impact on growth. Stem and branch CO2E measurements were made 3–4 times per month under different temperatures, under dry conditions. Because respiration was measured by recycling the gas within the chamber and analysis system, a leak test was performed before each series of measurements, using the standard procedure offered by the Li-Cor system (Li-6250).

Stem and branch CO2E were measured during the growing seasons of 1999–2002. In 1999 measurements were made from July onward. In subsequent years they were made from early May until late October. Stem and branch CO2E were expressed per unit surface area. The reason for choosing surface area as the reference lies in an assumption that respiration takes place mainly in the cambium region (Zagirova and Kuzin, 1998). Moreover, stem wood volume changes with water supply as a result of water loss and absorption. Surface area is thus the most stable reference unit.

In addition, measurements of ecosystem CO2 fluxes were performed at EESS Bíly Kríz using the eddy covariation system (Aubinet et al., 2005) (Edisol, University of Edinburgh, UK) during the whole period of the experiment (1999–2002).

Temperature measurements
Over the growing seasons in the four years, stem and branch temperatures were measured with thermistors (PT1000; HIT, Uherske hradiste, Czech Republic) permanently installed at each measured position at dbh in the cambium layer in stem and in the sapwood close to the base of branches. Thermistors were inserted with a northern orientation (to avoid direct solar radiation) and connected to a data logger (Delta-T Ltd, Cambridge, UK). In addition, air temperature was measured with a thermistor installed at a height of 2 m within the crown of each tree. The temperatures were measured at 10-min intervals.

Woody tissue diameter increment
Increase in stem diameter at dbh was measured twice per month during the whole growing season (May–October), using dendrometer bands (accuracy 0·01 cm) on 50 sample trees, including those on which CO2E was measured. Change in height was observed on a subset of ten trees (including sample trees). The increase in diameter of sample branches was measured monthly using a digital calliper (Digimatic CD-20D; Mitutoyo Ltd, Andover, UK), as the mean of two measurements perpendicular to each other at the beginning and at the end of the area covered by the respiration chamber.

Sap flow measurement
The heat pulse method (Hatton et al., 1990) was used to measure sap flow on ten trees (including those on which CO2E was measured) during the growing season of 2002. Five sap flow meters (SF 300; Greenspan Technology, Coffs Harbour, Australia) were used. The sap flux values (L h–1) obtained for the sampled trees and sapwood area at each measurement point were used to calculate the mean specific sap flux (SSF; L h–1 cm–2 sapwood). Sapwood area was evaluated on the basis of a site-specific allometric equation, which was also used to scale up the sampled trees SSF values to the whole stand transpiration (for more details see Pokorny, 2000). Monthly estimated relationships between SSF values and incident global radiation were used to fill gaps in the seasonal transpiration data. In addition, the following environmental factors were measured: incident global radiation (tube solarimeter; Delta-T) and relative air humidity (RHA1; Delta-T) at 2 m height. From the air temperature and relative air humidity (RH), vapour pressure deficit (VPD) was derived. Soil moisture was estimated at three different soil depths (1–17, 16–32 and 27–34 cm) using the time domain reflectometry (TDR) technique (IMKO, Ettlingen, Germany). Incident precipitation was measured automatically (ENVITECH, Brno, Czech Republic) on an open area close to the stand.

Data analysis
The natural logarithm of the CO2E rate, i.e. respiration rate, and the woody-tissue temperature were regressed using a linear model:


Formula 304M1

(1)
where β0 and β1 are the regression coefficients. Q10 was calculated (Linder and Troeng, 1981) from:


Formula 304M2

(2)
where β1 is the regression coefficient obtained from eqn (1). Then, a reference value of R10 (CO2E normalized to a temperature of 10 °C) was calculated as:


Formula 304M3

(3)
where Ri is the measured respiration rate at T woody tissue temperature. Using the calculated values of Q10 and R10, a woody tissue respiration rate was modelled on the basis of the measured temperature. Modelled respiration rate Rt (µmol CO2 m–2 s–1) was calculated as:


Formula 304M4

(4)
Modelled stem and branch respiration were averaged per month. The time-lag in the response of CO2E to tissue temperature for each month was taken into account and then expressed on a stand-level basis. This recalculation was based on the stem area index and branch area index values obtained for each year (Pokorny and Marek, 2000; R. Pokorny, unpubl. res.). To determine the time-lag between CO2E and woody tissue temperature (Ryan et al., 1995), the measured CO2E rates were shifted in time against stem and branch temperatures recorded at different times relative to the moment of CO2E measurement. The highest coefficient of determination (r2) for this relationship was accepted. Monthly carbon losses of woody tissue (stem and branch) were calculated from the modelled respiration rate and expressed per unit surface area and scaled up to the forest stand. Moreover, modelled stem and branch respiration rate based on chamber measurements were used to estimate the contribution of stem and branch respiration to total ecosystem respiration measured by eddy covariance.

Data distribution (i.e. normality) was tested by a Shapiro–Wilk's test. Levene's and Brown Forsythe's tests were performed to determine the homogeneity of the variance. In the case of breaching the assumptions, the non-parametric Mann–Whitney U-test was used. Tukey's HSD test was used to detect statistically significant differences, and differences were tested on the level {alpha} = 0·05. Statistica software (StatSoft Inc., Tulsa, OK, USA) was used for statistical analysis.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ACKNOWLEDGEMENTS
 LITERATURE CITED
 
Daily CO2 efflux
The CO2E from stems, for all growing seasons (1999–2002), showed a distinctive dynamic, with a maximum between 1300 and 1600 h (Fig. 1, which shows data for one day) and a minimum usually between 0600 and 0800 h. Branch CO2E rate (Fig. 2, data for a typical day) showed similar daily dynamics as for the stem, but there were small differences related to branch orientation (west or east) linked to the different temperature in the different positions. The CO2E rates in the four growing seasons ranged from 0·34 to 6·52 µmol CO2 m–2 s–1 for stems and from 0·12 to 1·53 µmol CO2 m–2 s–1 for branches (Table 2).


Figure 1
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  		FIG. 1. Measured CO2 efflux and temperature of the stem (at breast height, 1·3 m) in a single P. abies tree, during the day, 1 August, 2000. Each symbol represents the mean of 12 consecutive measurements on the stem. Means are shown ± s.d.

 


Figure 2
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  		FIG. 2. Measured CO2 efflux and temperature of the branch in a single P. abies tree, during the day, 3 August, 2000. Measurements were made on two branches in the same whorl at breats height (1·3 m) close to the branch base. Resp E, measured branch CO2 efflux rate in easterly orientation; Resp W, measured branch CO2 efflux rate in westerly orientation (and similarly for temperature). Each symbol represents the mean of 12 consecutive measurements per branch.

 


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  		TABLE 2. Range of stem and branch respiration rate in Norway spruce trees for each year of the investigated growing seasons (1999–2002)

 
Seasonal variation of Q10 and R10
Stem and branch CO2E, normalized to 10 °C, was greater (up to 34 %) in spring than in autumn. CO2E rate increased during spring in all years. The highest rates were during the summer months, especially in June and July, as a result of active woody tissue growth, high temperature and transpiration. In some summer days CO2E rates were low due to short periods of drought. The lowest CO2E rates for stem and branches were in the autumn, possibly caused by a cessation of growth processes and a decrease in temperature. Averaged monthly values of Q10 for stem and branch (Fig. 3) were derived from diurnal measurements, in each growing season (1999–2002); when averaged for each season and over years, these values were 2·25 and 2·17, respectively (Table 3). Stem and branch Q10 for respiration showed similar seasonal patterns, i.e. the highest values were obtained in early summer. Small differences were found in Q10 between stems and branches during the summer period, and increased during the autumn. Normalized stem and branch CO2E rate (R10) showed similar seasonal patterns between years, with the exception of 2000 (Fig. 4), and stem and branch R10 averaged over all growing seasons were 1·92 and 0·25 µmol CO2 m–2 s–1, respectively (Table 3). CO2E rate reached a maximum in late June and then decreased from July to October. This was also observed for branches (Fig. 4). Furthermore, a pronounced increase of stem and branch respiration was observed in spring for all years as a consequence of increased temperature and physiological activity after the dormant period. Stem CO2E rate values showed a small coefficient of variation between measured sample trees (CV 12 %). The same was observed for branch CO2E rate (CV 5 %).


Figure 3
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  		FIG. 3. Seasonal variation in Q10 of stems (A) and branches (B) of P. abies trees. Values were derived from measurements made on four trees and eight branches during four growing seasons (1999–2002). Mean are shown ± s.d.

 


Figure 4
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  		FIG. 4. Seasonal variation in stem (closed circles) and branch (open circles) respiration rate at 10 °C (R10, respiration normalized to 10°C) in Norway spruce trees. Results are based on measurements of four trees and eight branches during four growing seasons: 1999 (A), 2000 (B), 2001 (C) and 2002 (D). Means are shown ± s.d.

 


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  		TABLE 3. Range of stem and branch mean Q10 value and mean normalized respiration rate (R10, µmol CO2 m–2 s–1) for each year of the investigated growing seasons (1999–2002)

 
Temperature effects
Temperature differences between air and stem (cambium layer) were up to 3 °C during the day. Temperatures of stems of individual trees did not differ significantly (P = 0·05). Air and branch temperatures changed similarly during daylight hours whereas stem temperature showed less change during the day. The highest r2 values for this relationship were chosen to be able to evaluate the appropriate time-lag between respiration and temperature in the woody tissue. For seasons, r2 values for stem and branch were 0·69 and 0·72, respectively, in 1999, 0·84 and 0·81 in 2000, 0·79 and 0·85 in 2001, and 0·82 and 0·80 in 2002. The woody tissue temperature explained most of woody tissue CO2E (up to 68 %). Analysis of the time-lag showed that the stem temperature recorded between 60 and 120 min before the respiration measurement fitted CO2E from surface of the stem better than the temperature at the time of CO2 measurement. The time-lag for branches ranged from 60 to 90 min. The time-lag also varied seasonally, with an increasing trend in the summer and shorter in the autumn compared with the middle of the growing season.

Woody tissue diameter increment
The increasing slope of the relationship between cumulative diameter and date indicated that growth occurred between days 150 and 200 in the year, i.e. in May–July, in each growing season (Fig. 5A–D). The fastest rates of growth in diameter occurred in the middle of spring and at the beginning of summer in 1999 (days 139–170), i.e. 0·37–0·43 mm d–1. The lowest rate (0·01–0·04 mm d–1; Fig. 5E–H) was from the end of summer to late autumn (days 223–328) in 2000. The small variation in diameter in 2000 and 2002 was related to the lack of precipitation during the growing season. Although branch growth was not as high as stem growth, differences in growth of branches between growing seasons were found. Branches grew fastest during 2000 and slowest during 1999.


Figure 5
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  		FIG. 5. Averaged cumulative increase in stem diameter increment of four sample trees on which CO2 was measured during four growing seasons: (A) 1999, (B) 2000, (C) 2001 and (D) 2002. Means are shown ± s.d. Also shown are the rate of increase in stem diameter of the four sample trees on which CO2 efflux was measured during four growing seasons: (E) 1999, (F) 2000, (G) 2001 and (H) 2002. Each symbol represents the mean.

 
Sapflow
Daily courses of SSF differed significantly between trees, were highly correlated with air temperature, RH and VPD, and were similar in stems and branches to the changes in CO2 efflux. The SSF rates in 2002 ranged from 0·005 during the night to 0·075 L h–1 cm–2 during sunny days without water stress; the lowest rates were during periods of drought. Detailed correlation analysis between SSF and different microclimatic parameters on three, separate, different types of day (bright, cloudy and overcast days) revealed that low soil water content was a considerable limiting factor to stand transpiration, particularly under high irradiance. Time lags between transpiration stream and incident solar radiation were found: (1) 1 h in the case of bright days, (2) from 1 to 1·5 h in the case of cloudy days and (3) from 1 to 2 h in the case of overcast days (R. Pokorny, unpubl. res.). Soil moisture ranged from 14 to 30 % in all three depth layers during the growing season of 2002. The upper layer showed the greatest variation, with a decreasing trend from early spring to mid-summer and increasing to the end of the growing season, with some peaks after the wet season (days 195–202 and 224–231). During analysis of the transpiration stream, a decrease (up to 5 %) in stem respiration was found. The seasonal dynamic of transpiration corresponded to a decline in respiration from spring to autumn (Fig. 6).


Figure 6
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  		FIG. 6. Seasonal course of Norway spruce stand transpiration during the growing season 2002 (determined by sap-flux velocity monitoring).

 
Modelling and scaling up of woody tissue respiration
The implicit assumption for scaling woody tissue respiration from small sample measurements to the whole above-ground woody tissue is that respiration depends on temperature. Monthly total above-ground carbon losses of woody tissue (stem and branch) were calculated (Fig. 7). The annual above-ground carbon losses of stems and branches were 1·41 ± 0·6 t C ha–1 in 1999, 2·31 ± 0·3 t C ha–1 in 2000, 2·02 ± 0·5 t C ha–1 in 2001 and 2·35 ± 0·8 t C ha–1 in 2002. There were small differences between the proportion of stem and branch carbon losses in the different growing seasons. Carbon fluxes were much higher in summer than in spring and autumn, with maxima in July and August owing to higher temperatures. From the eddy covariation measurements and modelled respiration (based on chamber measurements) the annual contribution of stem and branch CO2E to total ecosystem CO2E were, respectively, 8·9 ± 6 and 8·1 ± 8 % in 1999, 9·2 ± 4 and 9·2 ± 7 % in 2000, 7·6 ± 3 and 8·6 ± 8 % in 2001, and 8·6 ± 5 and 7·9 ± 7 % in 2002.


Figure 7
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  		FIG. 7. Monthly carbon losses during the growing season from stem and branch woody tissue of Norway spruce trees at the Experimental Ecological Study Site Bíly Kríz in each year of the study: (A) 1999, (B) 2000, (C) 2001 and (D) 2002. Error bars indicate s.d.

 


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ACKNOWLEDGEMENTS
 LITERATURE CITED
 
Respiration (determined as CO2E) of stem and branches during the day changed in response to the temperature of the tissue, with the highest rates in the middle of the growing season when temperatures were highest and growth fastest (Fig. 5). Zha et al. (2004) showed that diurnal and seasonal changes in stem respiration were in response to a combination of temperature and assimilation. Zabuga and Zabuga (1985) pointed out that seasonal change in stem CO2E is related to cambial activity, and the present data are in accordance with this. Stem and branch respiration rates in all years reached maxima in June and July, decreasing from July to October. Indeed, seasonal and intra-seasonal variations in stem and branch respiration are related to changes in temperature and growth activity (Fig. 5). Rapid growth in diameter correlated with faster respiration. Branches on the same whorl had statistically significant differences in temperature (P < 0·05); those with a westerly orientation had higher temperatures than those with an easterly orientation. This may be related to canopy closure and location of the branch within the canopy so that those on the west are exposed to sunlight radiation for longer periods than those on the east. Respiration correlated with woody tissue temperature, so branch respiration rate is also influenced by exposure to solar radiation.

Differences between stem and branch Q10 are associated with the range of temperature used for its determination. Maier et al. (1998) found differences in Q10 between branches and stems and explained it on the basis of the faster response of respiration to changes in temperature for smaller-diameter organs, which show a more rapid equilibration between tissue and air temperature. We suggest that obtaining a representative Q10 coefficient for each ecosystem component depends on temperature sensitivity of the individual component and also on a large quantity of measurements being made over a range of temperatures. Q10 of woody tissue was highest at the beginning of the growing season and decreased as temperature increased. This trend was confirmed in all four investigated seasons (Fig. 3). Moreover, the variations in the estimated Q10 values in this study were directly linked to woody tissue temperature sensitivity during continuous and long-term monitoring.

The time-lag between CO2E and temperature has already been reported for several species (Ryan et al., 1995; Stockfors and Linder, 1998; Acosta and Brossaud, 2001), yet many studies of respiration of woody tissue have not taken this into account when modelling carbon balances and when scaling-up measurements. This has often led to an under- or over-estimation of woody tissue CO2E as a component of total ecosystem respiration. Ceschia (2001) pointed out that this time-lag could be caused by a delay in warming the internal parts of the stem, as compared with the surface. Alternative explanations for the time-lag include the possibility that the measured temperatures do not represent those experienced by most of the respiring biomass (Derby and Gates, 1966), and that fluctuations in seasonal time-lag could be explained by changing rates of respiration in combination with seasonal variation of cambial function (Stockfors and Linder, 1998). Another aspect that may be connected with the time-lag is the effect of CO2 transport in the transpiration stream (Saveyn et al., 2007). CO2 produced by respiration of roots and parenchyma cells within the woody tissues is transported by the transpiration stream and is partly released through the bark and partly consumed by photosynthesis in the leaves (Levy et al., 1999). This additional CO2E could cause errors in measurement (over- or under-estimation) of the total respiration of stem and branches. Moreover, transpiration can cool the stem, thereby reducing the metabolic activity of the growing tissues. Teskey and McGuire (2002) noted that the CO2 of xylem sap affects the rate of CO2E through the bark. During the sap flow measurements and analysis here, a decrease in stem respiration was found (up to 5 %) caused by the transpiration stream, and this would influence branch respiration as well. Low woody tissue respiration rates were observed on some days with high SSF rates, supporting this model. As solar radiation affects stomatal conductance and transpiration rate, which in turn affects the rate of sap flow, it is possible that radiation will affect respiration independent of temperature. In addition, the diffusivity of gases is closely correlated to the water content in woody tissue (Sorz and Hiezt, 2006) and may thus influence respiration during periods of water stress. All the above factors influence estimation of stem and branch CO2E and are often confounded or difficult to define. Indeed, there is a need to learn more about the influence of the transpiration stream on stem and branch respiration.

The following main conclusions can be drawn from the present study. Changes in stem and branch respiration rates during daylight hours and over the growing season correlated with the diurnal variation in woody tissue temperature and growth activity, with a maximum in spring and summer. Temperature of the tissue explained most of the CO2E. Therefore, the temperature of components of the canopy must be considered, rather than air temperature. Stem and branch respiration lagged behind temperature and must be taken into account when collecting data and interpreting the results. The proportions of carbon losses from woody tissue of stems and branches within the Norway spruce forest are of considerable importance. Although eddy covariance is a useful tool for measuring CO2E at the ecosystem level, the determination of CO2E from individual components is still needed for a better understanding of carbon budgets in forest ecosystems. Such data are important for evaluating the effect of global climate or other possible influences on carbon cycling and sequestration in forest ecosystems.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
LITERATURE CITED
 
This work was supported by research intention AV0Z60870520 (Academy of Sciences of the Czech Republic) and grants from the Ministry of the Environment of the Czech Republic VaV/640/18/03 and the Ministry of Education, Youth and Sport 2B06068. We would like to thank Dr Quentin Dawson for linguistic assistance.


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 LITERATURE CITED
 

    Acosta M, Brossaud J. Stem and branch respiration in a Norway spruce forest stand. Journal of Forest Science (2001) 47:136–140.

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