European Journal of Forest Research

, Volume 124, Issue 1, pp 1–8 | Cite as

The carbon budget of an adult Pinus cembra tree at the alpine timberline in the Central Austrian Alps

Original Paper

Abstract

We investigated carbon (C) uptake and respiratory losses of an adult Pinus cembra tree at the alpine timberline throughout an entire year by means of an automated, multiplexing gas exchange system. These chamber measurements were then combined with biomass data for scaling up the C budget to the tree level. Integrated over an entire year, the cumulative C gain of the tree under study was 23.5 kg of C in 2002. The daily C balance was negative for 5 months and the estimated total wintertime respiratory losses were 9% of the amount of C fixed during the growing season. The total annual C loss of the tree consumed 55% of the annual net C gain and the remaining surplus was stored in new tissues (36%) and used for fine root growth (9%). Thus, the overall C budget of P. cembra at the upper timberline is balanced fairly well, although the C sink strength in fine roots is strongly limited owing to low root zone temperatures when compared to conifers at lower elevation sites.

Keywords

Pinus cembra Net carbon gain Respiration Carbon balance Carbon allocation Alpine timberline 

Introduction

The dry matter production of a tree is the balance between the photosynthetic carbon (C) yield of the foliage and the amount of C lost via autotrophic respiration of the foliage and woody tissues. The decisive factor in C fixation is the time span over which a positive daily CO2 balance is maintained. At the tree level, however, the net C yield is dependent on the ratio of assimilating organs to that of respiring tissues. The few data published for Pinus cembra trees older than 20 years indicate a leaf mass ratio (LMR=leaf mass per total tree biomass) between 13% and 20%, with the remaining biomass lying in the stem, branches, and coarse roots (Oswald 1963; Bernoulli and Körner 1999). Costs for maintaining these non-productive tissues are high in trees, when compared to dwarf shrubs and herbaceous plants with their more favourable LMR (Boysen-Jensen 1932; Stevens and Fox 1991; Körner 2003; Cairns 1998; Cairns and Malanson 1998). In forest trees, the fraction of net photosynthetic production consumed by autotrophic respiration is approximately 30–70% (Sprugel et al. 1995). Recently, it has been argued that besides low temperatures and a short vegetation period, an imbalance in C-accumulating foliage versus respiring tissues might burden the C balance of the tree life-form at high elevation (Hättenschwiler et al. 2002).

Despite the many studies on foliar gas exchange (Cartellieri 1935; Pisek and Winkler 1958; Tranquillini 1959a, b, 1979; Stecher et al. 1999; Wieser 1997, 2004a) and woody tissue respiration (Tranquillini 1959; Tranquillini and Schütt 1970; Havranek 1981; Wieser and Bahn 2004) in P. cembra, we have only estimates of the annual C balance for young trees at the alpine timberline (Tranquillini 1959b). It was the aim of this study to examine the seasonal dynamics of C fluxes throughout an entire year for an adult P. cembra in situ at the alpine timberline. We used chamber techniques to measure the C gain of the foliage, and woody tissue C efflux. We also sampled vegetation to facilitate scaling up chamber measurements to both space (tree) and time (an entire year). Our objectives were to examine seasonal variations in the uptake and loss of C and to quantify the annual balance of C within an entire tree.

Material and methods

Study site and plant material

The study was carried out in an open (approximately 95-year-old in 2002) P. cembra forest near Klimahaus Research Station on Mt. Patscherkofel near Innsbruck, Austria (47°N, 11°E; 1,950 m a.s.l.) from 7 October 2001 to 21 January 2003. The field site is characterised by a cool subalpine climate with low temperatures and the possibility of frost in all the months. Annual precipitation averaged over the last 40 years was 950 mm, with the major portion falling during the growing season between May and October. The 40-year mean annual temperature was 2.4°C, with summer maxima up to 32°C and winter minima down to −28°C. The soil is a haplic podzol with a saturation water content of 72±3% volume in the raw humus layer and 54±6% volume in the mineral horizons, respectively (for further details see Wieser 2004b).

Environmental and gas exchange measurements

Scaffolding provided access to the crown of an adult 8-m-high P. cembra tree which was not shaded by neighbouring trees. Its stem diameter at breast height was 30 cm, and the projected crown area was 28 m2. A LI-190 PAR quantum sensor (Li-Cor, Inc., Lincoln, Nebraska) was installed on a horizontal aluminium rod 2 m above of the top of the tree. Air temperatures and soil temperature were monitored with type-T thermocouples at 8, 5.5, 3.5, and 1.5 m above ground, and at 5 cm soil depth, respectively. Soil water potential in the top 10-cm soil layer was measured with an equitensiometer (Model EQ3, Ecomatic, Dachau-Munic, Germany).

Carbon-dioxide exchange of the last three flushes was measured continuously throughout the snow-free period using two climate-controlled cuvettes (Walz, Effeltrich, Germany). One cuvette was installed in the upper sun crown and another in the lower shade crown on the south and the north side of the tree, respectively (Fig. 1). Air temperature and humidity tracked ambient conditions and were controlled by means of 100 Ω platinum resistance thermometers and dew-point mirrors (Walz, Effeltrich), respectively. Irradiance was monitored with LI-109 PAR quantum sensors (Li-Cor, Inc., Lincoln). Shoots in the chambers were changed at weekly to biweekly intervals. Cuvettes were removed during the winter owing to freezing of dew-point mirrors and cold traps.
Fig. 1

Position of the cuvettes and chambers installed for estimating of foliar gas exchange (F) and CO2 efflux from branches (B), stem sections (S), and coarse roots (R). in the experimental P. cembra tree at the alpine timberline near the Klimahaus Research Station, Mt. Patscherkofel (1,950 m), Tyrol, Austria

Woody tissue respiration was measured by means of a multiplexing open gas exchange system as described previously (Wieser and Bahn 2004). A total of ten unclimatised, clear ‘Perspex’ chambers were fitted on the stem, branches, and coarse roots for continuous monitoring of CO2 efflux from the main stem, coarse roots, and branches (Fig. 1). Stem respiration was measured at 5.9 m and 3.5 m above ground on the south side of the tree and 1.5 m above ground on both the south and the north side of the stem. These stem heights corresponded to diameters of 80, 160, and 320 mm. Coarse root respiration was measured on one south-facing and one north-facing root corresponding to diameters of 123 mm and 96 mm, respectively. For the estimation of CO2 efflux from branches, one cuvette each was installed in the inner and outer sections of the upper and the lower crown (Fig. 1). The corresponding diameters were 18.6 mm and 17.4 mm in the upper crown, and 47 mm and 7.4 mm in the lower crown, respectively. The restriction to one single tree was necessary owing to the high technical requirements of the gas exchange system and the fact that at the study site the distance from one tree to another is more than 20 m on average.

Additionally, cambium temperature was measured in and outside the chambers with 1-mm-thin type-T thermocouples. Relative to the tissue position outside the chambers, mean maximum overheating of the tissue in the chambers reached 2.1 K during short periods of high irradiance, but otherwise was negligible.

All pneumatic tubing was heated and insulated. Gas streams from all the cuvettes, chambers, and from the reference air, sampled 2 m above the top of the tree, were measured alternately by means of a solenoid-based, gas-switching system. CO2 concentration of the air streams through the two foliage-cuvettes was measured with a Li-Cor 6262 gas analyser (Li-Cor, Inc., Lincoln) operating in the differential mode and the corresponding flow rates were monitored with electronic mass flow-meters (Tylan, Eching, Germany), each cuvette being sampled for 3-min intervals. CO2 efflux from each of the ten woody tissues chambers was measured every 15 min with a second Li-Cor 6262 gas analyser operating in the absolute mode.

All data were transmitted to an AM416 multiplexer (Campbell Scientific, Ltd, Shepshed, U.K.) and recorded with a Campbell CR10 data logger, programmed to record 30-min means. The data logger also controlled the switching of the solenoids. Foliar CO2 gas exchange data were based on total needle surface area estimated by the glass bead technique (Thompson and Leyton 1971). The specific leaf area was 61.3±3.8 and 66.7±2.7 cm2 (total needle area) g−1 in the upper sun crown and the lower shade crown, respectively. Woody tissue CO2 efflux rates were related to both surface area and sapwood volume.

Scaling to the annual budget

Components of C allocation with respect to space (entire tree) and time (entire year) were estimated from tissue-specific gas exchange measurements and measured or estimated biomass (foliage, stem, branches, and coarse roots).

Following the seasonal gas exchange measurements, the tree was felled in early spring 2003 for estimating above-ground biomass distribution and growth increment. The stem and all the branches were harvested, cut into segments of 100 cm and 50 cm length, respectively. A sample disc was cut from the base of each stem and branch segment for estimating annual ring increment, sapwood, and heartwood radii. From these measurements, the total surface area and the sapwood volume of each stem and branch segment was then computed. Finally, total above-ground woody and needle biomass was estimated after oven drying (85°C) to constant dry weight.

Preliminary gas exchange measurements on 12 shoots distributed randomly within the sun crown and shade crown of an entire P. cembra tree throughout one growing season (Wieser 2000, unpublished) failed to find significant differences in the daily carbon balance of the foliage with respect to crown position. Therefore, foliar gas exchanges rates measured in each of the two cuvettes were weighted equally (c.f. also Rayment and Jarvis 1999) and combined with data measured previously during the cold season (Wieser 1997), in order to estimate the annual time-course of daily total carbon gain, daily total night-time respiration, and the annual carbon balance of the foliage.

Total daily woody tissue respiration throughout the year was estimated using data obtained from ten different locations and surface areas or sapwood volumes of twigs, branches, stem, and coarse roots. For the crown, we used seasonal courses derived from each of four branches of different diameter. Respiration data from the smallest branch diameter (d=7.4 mm) were applied to twigs (d<13 mm). For branches, the results from the 17.2 mm to 47 mm branches were applied to the diameter classes 13<d<25 mm and d>25 mm, respectively. No branch had a diameter ≥ 50 mm. Total stem respiration was calculated by averaging respiration data obtained for the four different stem segments, because using individual respiration data with respect to height above ground and exposure gave the same result.

In order to avoid any destruction of the soil system, root biomass was not determined and coarse root biomass was assumed to be 25% (=RMR) of the total tree biomass. This RMR was obtained from biomass allocation patterns derived from a wide range of P. cembra trees sampled within the timberline ecotone of the central Tyrolean (Oswald 1963) and Swiss Alps (Bernoulli and Körner 1999) and was found to be conservative in trees that were older than 23 years. Total coarse root surface area, sapwood volume, and radial growth were estimated by using the allometric relationship with stem diameter (H. Krofuß, personal communication). Biomass increment of woody tissue components was estimated from volume increment and the specific weight of the sapwood (0.6 g cm−3). A wood dry weight to C ratio of 0.5 (Larcher 2001) was used in the estimation of C allocation.

Results

After harvest in 2003, total above-ground biomass of the tree under study was 131.1 kg. Most of the above-ground biomass was concentrated in branches (71%). The needle and stem fractions were 20% and 9%, respectively. Phytomass stratification, cumulative leaf area (on a total needle surface area basis), and the vertical distribution of photon flux density and mean air temperature (≈mean tissue temperature) are shown in Fig. 2.
Fig. 2

Canopy structure and microclimate profiles within the P. cembra tree under study. Open bars needles, grey bars woody biomass, thick solid line cumulative total needle surface area, hatched line mean annual air temperature, dotted line vertical distribution of growing season mean photon flux density (PFD); the solid points indicate mean growing season PFD at the two locations of foliar gas exchage measurents

Although photon flux density was, at an average, 58% lower in the lower crown than in the upper crown, sun and shade needles did not differ significantly with respect to in situ photosynthetic capacity (i.e. maximum photosynthetic rate under optimum ambient conditions; Larcher 2001), night-time dark respiration rate at 10°C, and daily mean C gain (Table 1).
Table 1

Photosynthetic capacity, night-time dark respiration at 10°C, and daily mean carbon gain of fully expanded foliage in the upper sun and the lower shade crown

Parameter

Upper sun crown

Lower shade crown

Photosynthetic capacity (μmol m−2 s−1)

4.47±0.2

4.61±0.11

Night-time dark respiration (μmol m−2 s−1)

0.31±0.07

0.22±0.07

Daily mean carbon gain (g C m−2 d−1)

0.70±0.45

0.65±0.41

Total woody tissue CO2 efflux corrected to 10°C (R10), calculated on a volume-base, did not vary consistently at diameters >80 mm and averaged 12.7±2.5 μmol m−3 s−1 within the entire stem and coarse roots (Fig. 3). Within branches (diameter <50 mm) estimated R10 was negatively correlated to branch diameter (Fig. 3). Area-based R10, in contrast, did not show any clear trend with respect to woody tissue diameter (r2=0.02) and averaged 0.95±0.48 μmol m−2 s−1.
Fig. 3

Total woody tissue respiration rate per sapwood volume of stems and coarse roots (>80 mm in diameter) as well as branches (<80 mm in diameter) at a tissue temperature of 10°C (R10) as a function of tissue diameter. Branch data were fit by linear regression: R10= −10.7x+557; r2=0.96

The seasonal patterns of environmental parameters were representative of the climatic conditions in the timberline ecotone of the central Tyrolean Alps (Fig. 4). Daily mean air temperature varied between −14.4°C on 14 December 2001 and +19.4°C on 19 June 2002 (Fig. 4). Snow persisted from mid-November until the end of April. The lowest daily mean soil temperature recorded during this study was −3.1°C on 15 January 2002, and the highest daily mean soil temperature was 12.8°C on 23 June 2002 (Fig. 4). Owing to frequent precipitation during the snow-free period, soil water potential always remained above −0.06 MPa (Fig. 4), indicating that the tree was not subjected to water stress.
Fig. 4

Time-course of meteorological data; daily sum of photon flux density (PFD), daily mean air (Tair, solid line) and soil temperatures at 5 cm soil depth, (Tsoil, dotted line), winter snow cover (solid area), soil water potential (Ψsoil), and daily sum of precipitation (P), as well as daily totals for carbon uptake and loss of the foliage (thick solid lines), total above-ground woody tissue (dotted line), and coarse root respiration (thin solid line) of the P. cembra tree under study between 5 November 2001 and 31 December 2002

In late fall and early winter net photosynthesis permanently declined, in parallel with shorter days, lower irradiance, and near-freezing temperatures (Fig. 4), and was almost completely suppressed during the period of winter dormancy. Recovery of photosynthesis began in spring, in response to the diminishing occurrence of frost, higher air and soil temperatures, as well as an adequate water utilisation (Fig 4). Although photon flux density decreased significantly with increasing depth of the canopy (Fig. 2), foliage in the lower shade crown was remarkably productive throughout the growing season and reached maximum rates of 1.18 g C m−2 (total surface area) day−1 as compared to 1.20 g C m−2 day−1 in the sun crown, thus giving an estimated maximum daily total C gain of the tree of 219 g C (Fig. 4).

The seasonal patterns of foliar night-time respiration and daily total woody tissue CO2 efflux generally followed seasonal trends in temperature (Fig. 4). Rates were highest during the growing season and reduced to the level of maintenance respiration during the dormant period. The fall decline and spring increase in respiration was coincident with seasonal temperature trends (Fig. 4), and tissue temperature explained 85–93% of the variation in total foliage night-time respiration and total woody tissue CO2 efflux (Fig. 5).
Fig. 5

Daily sums of fully expanded foliar night-time carbon loss (open circles), total above-ground woody tissue (closed circles), and total coarse root carbon loss (R) (filled squares) in relation to the corresponding mean night-time (Tn) and daily mean tissue temperatures (Td), respectively. Values were fitted by exponential regression. Foliage: growing season only, R=1.37exp(0.146 Tn), r2=0.93; above-ground woody tissue: R=14.36exp(0.083 Td), r2=0.87; coarse roots: R=9.53exp(0.057 Td), r2=0.85

Daily C balance of the tree was negative from November upto mid-April (≈5 months), and it took a further month to compensate the total of 2.1 kg C lost by respiration during the dormant season (Fig. 6).
Fig. 6

Seasonal course of the daily carbon balance and the cumulative net carbon exchange of the experimental P. cembra tree between 5 November 2001 and 31 December 2002

The annual carbon balance for the entire tree estimated for 2002 is shown in Table 2. Because of uncertainties in root biomass (see Sect. 2) we considered the sum of dry matter production and respiration of above-ground components and total below-ground allocation to be the best estimation of the annual carbon budget. Total C loss of the foliage owing to growing season night-time respiration and total (24 h) respiration during the cold season was 17.1% of the annual net C gain. Area-based C effluxes of above-ground woody tissues did not differ significantly from sapwood volume-based values (Table 2) and total above-ground woody tissue respiration was 2.5% higher when calculated on an area basis compared to volume basis. The difference between total below-ground C allocation and the total of coarse root production plus respiration is an unknown sink to fine-root turnover, C allocation to root exudation, and mycorrhizae or may also represent a cumulative error in sampling and up-scaling.
Table 2

Estimated annual carbon balance of a 95-year-old P. cembra tree at the alpine timberline

Parameter

Area-based

Sapwood-volume-based

Net photosynthetic carbon gain

23,542

100

Foliage increment

2,001

Foliage growing season night-time respiration

3,783

Foliage total winter respiration

224

Twig increment

752

Twig respiration

332

729

Branch increment

2,746

Branch respiration

6,623

7,510

Stem increment

1,800

Stem respiration

1,421

706

Total below-ground carbon allocation

3,854

3,291

Coarse root increment

942

Coarse root respiration

1,050

188

Unknown sink

1,862

2,161

Estimates were calculated on area or sapwood volume based woody-tissue respiration. Total below-ground carbon allocation was estimated as the difference between net photosynthetic carbon gain and above-ground respiratory losses plus biomass increment. All values are given in g carbon year−1

Discussion

In the P. cembra tree under study, the total above-ground biomass accumulated in foliage, branches, and stem was 20, 71, and 9%, respectively. Such above-ground C accumulation ratios are typical for P. cembra trees in open stands within the timberline ecotone of the central European Alps, as shown previously for the study site by Tranquillini and Schütt (1970) and also along elevational transects across the timberline ecotone of the Austrian (Oswald 1963) and Swiss Alps (Bernoulli and Körner 1999).

Seasonal variations in photosynthesis and respiration at the alpine timberline have been attributed to the prevailing temperature. Previous studies in the timberline ecotone showed a dormancy in C gain of P. cembra for about 4–5 months of the year (Cartellieri 1935; Pisek and Winkler 1958; Havranek 1981; Tranquillini 1962; Wieser 1997), with the bulk of C fixed during a 5–6-month growth period (Tranquillini 1959a, 1959b; Tranquillini and Schütt 1970).

During the growing season, the daily C gain of the entire crown of P. cembra at the timberline was up to 1.20 g C m−2 (total needle surface area) day−1. This amount is within the range of 0.5 g C m−2 day−1 and 1.6 g C m−2 day−1 reported for other forest trees at lower elevation sites (Schulze 1970; Schulze et al 1977; Benecke and Nordmeyer 1982; Troeng and Linder 1982a, b; Matyssek and Schulze 1988; Häsler et al. 1991; Rayment and Jarvis 1999). Owing to physiological (Pisek and Winkler 1958) and morphological adaptations (Wieser 2004a) foliage in the shade crown was remarkably productive and contributed more than 45% to the overall total daily and annual C gain of the tree. In an open high elevation Pinus canariensis stand in Tenerife, Canary Islands, Peters (2001) also failed to find significant differences daily uptake and loss of C between the upper and the lower canopies. In contrast, at low elevation sites in northern Germany, the contribution of the shade crown of Fagus sylvatica (Schulze 1970), Picea abies (Schulze et al. 1977) and Larix spp. (Matyssek and Schulze 1988) contributed an average to less than 30% to the total annual C gain of the tree. The major difference between species, however, is crown architecture and leaf area distribution. In consequence, a better illumination of needles in lower canopy layers in the open crown of P. cembra at timberline is a suggested explanation of the high daily C gain, similar to trees at lower elevations with more dense canopies. Thus, the length of the growing season (i.e. the time span over which a positive daily CO2 balance is possible) seems to be the decisive factor in the annual C gain at the alpine timberline.

With the transition to full winter dormancy, foliage (Pisek and Winkler 1958; Wieser 1997) and woody tissue respiration (Havranek 1981; Wieser and Bahn 2004) in P. cembra was reduced to the level of maintenance respiration. Although CO2 uptake is, in general, terminated during the winter at the alpine timberline (Tranquillini 1979), respiratory C losses during the dormant season did not burden the annual C balance. The daily C balance of the tree under study was negative for 160 days. The total estimated C loss of the tree during this period was 2.1 kg C, which is 9% of the annual net photosynthetic C gain, and it took a further month to compensate this loss. During the 5-month dormancy, the total respiratory C loss of snow-covered young P. cembra trees amounted to 7% of the annual net C gain (Tranquillini 1959b). For a mature Larix decidua respiration of the whole tree during the leafless period from October upto April was less than 3% of the annual net C gain (Havranek, unpublished) in Havranek and Tranquillini 1995). Hence, rather than being a burden the dormant season seems to improve the tree’s C balance because of a reduced respiration activity in cold winters owing to generally low air and soil temperatures.

The below-ground portion on the carbon budget, however, was based on several assumptions. Coarse root biomass was obtained from biomass allocation patterns derived from P. cembra trees and was found to be conservative within the timberline ecotone of the central Tyrolean (Oswald 1963) and Swiss Alps (Bernoulli and Körner 1999). Coarse root increment as well as surface area and sapwood volume were estimated with an allometric relationship derived from a wide range of P. cembra trees (H. Krofuß, personal communication). Fine root biomass measurements were similar in spring and fall. Therefore, based on these assumptions, we consider that roughly 55% of the annual net C gain was respired by foliage and woody-tissues. Kimura et al. (1968) estimated autotrophic respiration to comprise 40% of the annual net C gain in young Abies veitchii trees at timberline on Mt Shimagare, Japan. Benecke and Nordmeyer (1982) estimated autotrophic respiration to be 57% and 43% of the annual net C gain for Pinus contorta and Nothofagus solandri trees, at a subalpine site in the Craigieburn Range, New Zealand. For the boreal zone, Linder and Troeng (1981) and Linder and Axelsson (1982) reported autotrophic respiration of Pinus sylvestris to consume 28–34% of the annual C gain. Thus, the overall annual C budget of P. cembra at the alpine timberline is balanced fairly well and is in accordance with the overall C budget estimated for P. contorta and N. solandri trees growing at high elevation sites in New Zealand (Benecke and Nordmeyer 1982).

From the remaining surplus in the carbon budget (=100% ≈ net primary production; NPP) the P. cembra tree under study allocated 62% into woody tissues (stem + twig + branch + coarse roots), 20% into new foliage, and 18% into fine-root production (including respiratory losses and exudates to mycorrhizae). Similar proportions in annual C allocation into these three components (wood, foliage, and fine production) have also been reported for P. contorta (64, 20, and 15) and N. solandri (56, 28, and 16) at upper montane and subalpine sites in New Zealand (Benecke and Nordmeyer 1982). Thus, the annual total fine-root production of trees at timberline was about 50–56% lower than the mean rates reported for conifers in boreal (32%) and cold temperate (37%) climates, respectively (Gower et al. 1995). On a global scale, fine-root biomass production has been shown to be positively correlated to mean annual temperature (Gower et al. 1995). Fine-root growth of P. cembra (Turner and Streüle 1983) and L. decidua (Häsler et al. 1999) reached only 5% of its maximum at soil temperatures lower than 6°C, strongly suggesting that low root zone temperatures limit fine root production at timberline. In addition, low soil temperatures limit the metabolic activity (Schinner et al. 1989), and respiration costs of mycorrhizae at timberline are unknown. Annual soil CO2 efflux, measured continuously throughout the entire investigation period below the crown of the tree under study, averaged 42.8±26.4 g C m−2 year−1 (Wieser 2004b) and matched the unknown sink in the C balance of the tree under study (Table 2) when calculated on a ground surface area (≈70 g C m−2 ground surface area year−1). However, further investigations on the components of soil C efflux (respiration of roots, microbes, and mycorrhizae) is required to improve our understanding in below-ground C allocation patterns in conifers within the timberline ecotone. Such experiments may also contribute to a conclusive explanation of the upper end of tree life at high altitudes.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.Bundesamt und Forschungszentrum für WaldAbt. ForstpflanzenphysiologieInnsbruckAustria

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