Above- and below-ground biomass, surface and volume, and stored water in a mature Scots pine stand
This study describes the amount and the spatial distribution of the above- and below-ground tree skeleton—defined as the woody structure of stem, branches and roots—in a mature Scots pine (Pinus sylvestris L.) stand in Belgium. Tree skeleton data were linked to the respective needle area, and as such, this work provides the background framework for modeling the tree hydraulic architecture and the carbon balance of the forest stand. Using validated allometric equations, we were able to calculate the amount of the volume, of the biomass and of the corresponding surface areas of individual trees in the stand. Total woody biomass of the 66-year-old forest stand was 155 Mg ha−1, i.e., 126 Mg ha−1 above ground and 29 Mg ha−1 below ground. The total bio-volume of the woody mass of the stand was 314 m3 ha−1. The highest fraction of this value was the stem bio-volume, i.e., 236 m3 ha−1 or 75 % of the total. The total volume of all roots was 57 m3 ha−1 (18 % of the total volume), and the volume of branches was 20 m3 ha−1 (7 % of the total volume). The surface area of the roots ranged from 38,000 m2 ha−1 in the winter to 68,000 m2 ha−1 in the spring. The surface area of the stems was 2,700 m2 ha−1, and the surface area of all branches reached 4,400 m2 ha−1. The total above-ground water storage in the xylem was 94 m3 ha−1 (or 9.4 mm), while the accessible stored water was 2 mm of that quantity. A comparative analysis of the biometric parameters showed the balance between the different functionally connected, operational surface areas of the trees. The needle surface area was similar to the root surface area and in the same order of magnitude as the surface area of woody cambium. The results allow to link water uptake with transpiration and assimilation with respiration.
KeywordsAllometry Biomass Carbon budget Huber value Respiration Sap flow Water balance Pinus sylvestris
Forests contain about 90 % of the carbon stored in the terrestrial vegetation and account for 40 % of the carbon exchange between the atmosphere and the terrestrial biome (Schlesinger 1997). Forest stands contribute to the terrestrial carbon balance in two different ways. First, forest stands are the principal pools of the stored carbon. It is therefore of interest to quantify tree biomass and its increment to enable the quantification of the carbon pool size. Secondly, forest trees exchange carbon with the atmosphere, not only through the uptake of carbon in photosynthesis, but also through the release of carbon dioxide via the respiration of living cells. The amount of carbon released by the woody skeleton (stem and branches) is usually quantified on a surface area basis (Edwards and Hanson 1996; Damesin et al. 2002; Kim et al. 2007; Acosta et al. 2008). For a proper extrapolation to the tree and the stand levels, the tree surface area should be known. Scots pine (Pinus sylvestris L.) is the most widely spread pine species across Eurasia, covering 24 % of Europe’s forested area (Stanners and Bourdeau 1995) and growing in a wide range of ecotypes (Richardson 1998). Scots pine stands are thus an important factor affecting the carbon and water balances in Europe and Asia (Richardson 1998; Poyatos et al. 2007). Although there is a good knowledge of the stems of Scots pine (e.g., Claesson et al. 2001; Landsberg et al. 2005), much less is known about the surface area and the biomass of branches and roots.
The vertical distribution of roots and branches plays an important role in the competitiveness of an individual tree (Stoll and Schmid 1998). The branches hold the needles, exposing them to incoming radiation, and thus affect photosynthesis and transpiration of the individual tree (Sala and Tenhunen 1996; Peters et al. 2008). With their high surface area, branches significantly contribute to the overall tree respiration (Damesin et al. 2002). Therefore, an optimal needle to branch (and stem) surface area improves the carbon economy of the tree. Similarly, the rate and the structure of colonization of the soil by roots affect both water and nutrient uptake. Usually, most of the roots grow in the topsoil layers, exploiting the most fertile soil horizons and acquiring rain water (Jackson et al. 1996; Monserud et al. 1996; Janssens et al. 1999; Xiao et al. 2003; Konopka et al. 2005, 2006). However, in sites with easily accessible groundwater and low precipitation, a significant fraction of the roots grow in deeper soil layers, as, e.g., observed in Scots pine (Nadezhdina et al. 2007; Čermák et al. 2008a) and in oak trees (Vyskot 1976; Tatarinov et al. 2008), among others. The distribution of biomass and biosurface area (i.e., the surface area of the plant/tree parts) is similar in roots and branches: The highest amount of biomass is located in the coarse roots and in the coarse branches, whereas the highest surface area is generally found in the fine roots and in the thin branches (Helmisaari et al. 2002).
Tree allometry reflects the environmental conditions and stand properties as well as individual tree status—especially age, social position and tree health (Cannell 1982; Bartelink 1997; Konôpka et al. 2010; Wang et al. 2011; Poorter et al. 2012). Theoretically, allometric equations describing tree dimensions are affected by the physiological requirements of the tree; i.e., form and function are related. The most important of these requirements are water transport, light interception, and mechanical support against gravity or wind (Niklas 1994). The present allometric study provides the necessary background information and knowledge for more detailed studies on water relations of Scots pines of different social positions. From the hydraulic point of view, a tree may be viewed as a network of interconnected pipes (Zimmerman 1983). According to the “pipe model theory” (Shinozaki et al. 1964), the quantity of roots is proportional to the conductive stem cross-sectional area of a tree (i.e., sapwood), while the sapwood area is linked to the amount of foliage. Any disproportion in this balance will affect tree function. For example, larger crowns relative to the sapwood area result—if other tree parameters of the overall hydraulic conductivity remain the same—in lower water potentials and in a higher risk of cavitation (Maherali and de Lucia 2000; Cochard 2006; Ogasa et al. 2013). The Huber value (HV, Huber 1928; Tyree and Ewers 1991)—defined as the sapwood area supporting a unit amount of foliage—is therefore a good measure of tree adaptation to the environment and of the susceptibility of a tree to stress (e.g., Berninger et al. 1995; Poyatos et al. 2007, Martínez-Vilalta et al. 2009). Sapwood water storage is a key feature that helps the tree to cope with diurnal peaks in water demand as well as with a longer drought stress. A considerable amount of water for the daily transpiration is supplied from the stored water (Čermák et al. 1982; Goldstein et al. 1998; Zweifel et al. 2000; Čermák et al. 2007; Urban et al. 2013). Therefore, this paper also describes the amount of available water storage in the skeleton organs.
The aims of the current study were as follows: (1) to describe the amount and the spatial distribution—vertical, radial as well as within individual trees—of the skeleton (i.e., the tree structure consisting of stem, branches and roots) of a mature Scots pine stand in terms of their biomass, volume, surface area and water storage; (2) to describe the overall crown and canopy architecture, as well as the root geometry; (3) to provide and evaluate the necessary scaling-up tools and allometric relations so that these can be applied to various parameters and processes of main interest to canopy carbon and water fluxes; and (4) to describe the ecological consequences of the skeleton parameters linked to the other functional tree organs. As such, this study extends a previous paper (Čermák et al. 1998) covering the needle and coarse root distribution of the same set of Scots pine trees. In this study, ‘skeleton’ is defined as the structural part of the tree, i.e., the branches, the stem and the coarse roots.
Materials and methods
Experimental site—location, climate and soil
The study was performed at the experimental plot of a Scots pine (Pinus sylvestris L.) forest in Brasschaat, Campine region of the province of Antwerp, Belgium (51°18′33′′N and 4°31′14′′E, altitude 16 m, orientation N.N.E.). This forest is part of the regional forest ‘De Inslag’ (parcel no. 6, Flemish Region) located nearly 15 km northeast from Antwerp. The site is almost flat (slope 1.5 %) and belongs to the plateau of the northern lower plain basin of the Scheldt River. Soil characteristics are as follows: (1) moderately wet sandy soil with a distinct humus and/or iron B-horizon (psammenti haplumbrept in the USDA classification, umbric regosol or haplic podzol in the FAO classification); (2) very deep (1.75–2.25 m) eolian sand (Dryas III), somewhat poorly drained (neither receiving nor shedding water); and (3) rarely saturated but moist with rapid hydraulic conductivity for all horizons (Baeyens et al. 1993; Van den Berge et al. 1992). The groundwater depth normally ranges between 1.2 and 1.5 m and might be lower due to non-edaphic circumstances. Human impacts mainly include deep (up to 45 cm) forest tillage in the past. The occurrence of a Rhododendron ponticum (L.) shrub in the understorey layer causes (probably also because of allelopathic effects) an unfavorable O-litter characterized by very low biological activity. A mycelium and many ants are present in the litter layer. The climate of the Campine region is moist subhumid (C1), rainy and mesothermal (B’1) (Köppen 1936). The Campine region has a temperate maritime climate with a mean (over 28 years) annual temperature of 9.8 °C and a mean annual precipitation of 767 mm. The mean annual potential evapotranspiration is 670 mm (Carrara et al. 2003).
The original climax vegetation (natural forest) in the area was a Querceto-Betuletum (Tack et al. 1993). The studied pine stand was planted in 1929. The stocking density was 1,390 trees ha−1 in 1980, decreasing to 899 trees ha−1 in 1987, 743 trees ha−1 in 1990 and 716 trees ha−1 in 1993. Due to windfall in 1994, a remaining 542 trees ha−1 were still present in 1995. The most recent forest inventory dated from spring 1995, including the frequency distribution of stem diameter at breast height (DBH at 1.30 m above the ground), tree height to the top and to the basis of the crown (i.e., the lowest green whorl). All the forest inventory data were collected for the entire area of the experimental plot (1.996 ha, Čermák et al. 1998)). The sparse pine canopy allowed a rather dense vegetation of a few understorey species as Prunus serotina (Urban et al. 2009) and Rhododendron ponticum (Nadezhdina et al. 2004) which were partially removed in 1993 until a ground cover of about 20 % of the area. The herbaceous layer was composed of a dominant grass (Molinia caerulea, covering about 50 % of the area), and some mosses (Hypnum cupressiforme and Polytrichum commune) that created a compact layer in about 30 % of the surface area.
The stand was measured in diameter at breast height (DBH) classes of 2-cm resolution. The number of trees in each DBH class was recorded, and these numbers were used in up-scaling calculations. Six sample pine trees were selected for the harvest and for the destructive measurements as representatives for the entire stand. The selection of these trees was based on their basal area using the technique of the quantils of the total (Čermák and Kučera 1990; Čermák and Michalek 1991). Biometric data were measured on these six sample trees, i.e., stem diameter at breast height including the bark (DBH), stem diameter below the green crown including the bark (DGC), the corresponding bark thickness, total tree height, height of the base of the crown and crown projected area. In the summer of 1995, each sample tree was cut and slowly put on the ground, using ropes, to prevent significant breakage of branches. For three sample trees—with a DBH of 21.4, 28.5 and 40.6—a suppressed, a co-dominant and a dominant tree, respectively—the spatial distribution of all branches and needles within the crown was analyzed in detail using the ‘cloud’ technique (Čermák 1990; Čermák et al. 1998). Only the total amounts of needles and of branches were assessed in the other three sample trees. Needle and (partially) root distribution of the same set of trees were described previously (Čermák et al. 1998).
Additional measurements on a set of seven randomly chosen trees were taken in the same forest stand in the same year (August 1995) to describe properties of coarse roots. Stem diameters of these seven trees ranged from 19 to 31 cm with a mean of 26 cm. After a rough excavation from the sandy soil, the mean diameter of the root system, the total rooting depth, the mean length and diameter of the main roots were measured. The below-ground part of the stump was not included into the calculations of the coarse root biomass. To get a crude estimate of the total volume and the dry mass of the root systems, the volume to dry mass ratio of the base of the stem was also used for the roots. The up-scaling of the biometric root parameters from the individual trees to the entire stand was approached in the same way as for the above-ground biomass. Data related to the fine and small root biomass of the investigated forest site were obtained and reported by Janssens et al. (1999) for the spring and by Xiao et al. (2003) for the winter, thus, respectively, representing the maximum and the minimum amounts of root biomass. These data were obtained by the method of soil coring with subsequent washing and scanning. The surface area and the dry root biomass were estimated. Three diameter classes of fine and small roots were distinguished, i.e., less than 1, 1–2 and 2–5 mm. The vertical profile of the root distribution was described in 20 cm steps to the depth of 80 cm. In analogy with the leaf area index (LAI) of the needles, the total root surface area was also presented as the root area index (RAI). RAI (m2 m−2) was calculated as the surface area of the roots of all size classes in a particular forest area divided by the ground surface area.
Biomass, volume, surface area and amount of water
In general, the same approach was applied as was used for the description of the needle distribution reported previously (Čermák et al. 1998). For each branch, the following parameters were measured with a taper, a caliper and a protractor, respectively: The branch diameter at 10 cm from the main stem as well as just below the green part with needles; the bark thickness; the branch orientation (azimuth); the vertical angles of the branch to the main stem and to the center of the leaf cluster—‘cloud’; the total branch length; and the length up to the green part of the branch. Furthermore, all branches were cut into pieces and separated into three categories, i.e., large branches (diameter of the branch at the point of cutting larger than 2.5 cm), small branches (diameter ≤2.5 cm) and shoots (maximum 3-year-old branches, actually holding the needles). Unless further specified otherwise, all shoots were assessed together with the small branches. The main stem was divided into sections with a length of 20 cm. The mean diameter and the bark thickness were measured for each section. The volume was calculated from the length and the mean diameter of the stem sections. From each section, samples of the wood (side length of the block taken from the stem was 5 cm) were collected for the quantification of the amount of water and of air.
Depth of the sapwood was estimated from the drop in the radial profile of the water content, typical for some tree species including Scots pine (Kravka et al. 1999; Gartner and Meinzer 2005). The amount of utilizable water at the time of sampling was measured as the difference between the RWC in the sapwood and in the heartwood following the procedure of Kravka et al. (1999). Two mean values of the wood density, one for the sapwood and one for the heartwood, were used in further calculations. The volume of the skeleton was calculated for each branch and for each stem separately from the measured wood density. The amount of water was calculated from the volume of the sapwood, of the heartwood and of the branches, and from the corresponding volumetric water content.
Surface area of the stem was calculated for the different sections from the mean diameter and the length of each section (Eq. 3) and then cumulated toward the entire stem. The surface area of the stem and of the branches was calculated both with and without bark. The surface area without bark referred to as the cambium surface area. To allow comparisons with other indices (as LAI), the cambium surface area was expressed as cambium area index (CAI). CAI (m2 m−2) was calculated as the total cambium surface area in the forest site divided by the respective ground area (Honzová and Čermák 2011).
Vertical and radial distribution of skeleton parameters
Scaling-up total surface area, dry mass, volume and water content of the skeleton
Allometric relationships for volumes, surface areas and water content—all assessed at breast height diameter (DBH, cm)—of the above-ground skeleton
Generalized trees (DBH, cm)
Sample trees (DBH, cm)
Amount per ha
Surface area of cambium (m2)
Volume without bark (m3)
Surface outside bark (m2)
Volume including bark (m3)
Volume of bark (m3)
Biomass of bark (kg)
Volume of heartwood (m3)
Volume of sapwood (m3)
Total dry wood biomass (kg)
Sapwood depth (cm)
Heartwood biomass (kg)
Sapwood biomass (kg)
Water content in sapwood (kg)
Water content in heartwood (kg)
Total water content in stem (kg)
Branch biomass (kg)
Water content (kg)
Surface area (m2)
Total above-ground woody biomass
Biomass of dry wood (kg)
Surface area of cambium (m2)
Biomass including bark (kg)
Outer surface area (m2)
Water content (kg)
Wood volume (m3)
Volume of wood and bark (m3)
Volume of all roots (m3)
Volume of small and fine roots (m3)
Volume of coarse roots (m3)
Biomass of all roots (kg)
Surface area of all roots (m2)
Surface of small and fine roots (m2)
Surface area of coarse roots (m2)
Total woody biomass
Dry biomass (kg)
Surface area (m2)
Scaling-up of distribution of biomass, volume, surface area and water storage
The vertical distribution of the skeleton (volume, surface area, dry mass, water content) was scaled-up to the stand level using a two-steps approach (Čermák et al. 2008b). The distribution of the skeleton biomass and the surface area in different layers above the ground (hi = height in m) was approximated by a single equation for each sample tree, separately for the branches (total amount of branches, large branches and small branches) and for the stem. Canopy layers with a depth of 0.2 m (along the axis of the stem) were considered, so that the skeleton distribution (yi) could be expressed in (kg per 0.2 m layer) and/or in (m2 per 0.2 m layer).
Regression parameters (a–g) of Eq. 10 for three sample trees and coefficients of determination (R2) of the models
Regression parameters (a–c) of Eq. 11 for the estimation of tapering of the stem radius in the three sample trees and coefficients of determination (R2) of the models
The amount and the spatial distribution of the parameters of the skeleton were calculated from the above-mentioned equations for the three generalized trees of a different social position (i.e., suppressed, co-dominant and dominant). The size of the generalized trees was calculated from the cumulated value of the tree basal areas in the diameter classes using the quantils of the total (Čermák and Michálek 1991). Each of the three generalized trees represented on average 33.3 % of the tree asal area of the stand the experimental plot. The DBHs of the generalized trees were 22, 28 and 34 (for the suppressed, co-dominant and dominant tree, respectively).
Huber values (HV; Huber 1928; Tyree and Ewers 1991) were calculated as the ratio of the cross-sectional area of the sapwood (cm2) to the unit of needle dry biomass (kg) supported by it. While sapwood area measurements are part of this contribution, the needle biomass (estimated for the same set of the trees) was taken up from the previous contribution (Čermák et al. 1998). The HV values were calculated at different levels in the tree: i.e., at breast height; below the living crown; as the sum of the sapwood areas at the base of the branches; at the onset of the green parts of the second-order axis; and at the base of the shoots. From the approach used in this contribution, the units of HV therefore were cm2 kg−1. Some authors (e.g., Mencuccini and Grace 1994; Mencuccini and Bonosi 2001; Sellin and Kupper 2006) prefer to express the Huber value as the needle area to the sapwood area ratio (m2 cm−2). We therefore also calculated HV in this way.
All statistical analyses were performed using software STATISTICA 8.0 (StatSoft, Inc., Tulsa, USA). Allometric models were calculated using the methods of the linear and nonlinear least square regression analysis (Niklas 1994). We mainly used linear and power type functions and determined their corresponding r2. Significance of the r2 and of the model were tested by one-way ANOVA (analysis of variance, α = 0.05). In case the calculated F value was higher than the threshold value, we considered the proposed model as significant. In linear models, we also tested the significance of the absolute coefficient by a t test for t0.025, n−2. If the calculated value was lower than the threshold value, the coefficient was excluded from the equation. The model with the highest r2 was selected for the up-scaling function. Significance of differences between two different groups was tested by t test. If there were more than two groups, we used a one-way ANOVA.
Individual tree level
Huber values (cm2 kg−1) of Scots pine trees of three different social categories
Branches at stem
Branches at green
The total above-ground (woody) biomass reached 126 Mg ha−1 (Table 1). The stems contributed to 93 % and the branches to 7 % of this total woody biomass. The large branches (diameter ≥2.5 cm) represented 60 % of the biomass of all branches (5,200 kg ha−1). The total volume of xylem in the stems was 212 m3 ha−1; the volume of the bark was 24 m3 ha−1 (10 % of the above-ground biomass and volume). Volume of the sapwood was 187 m3 ha−1, good for 88 % of the total stem volume. The total volume of branches was 20 m3 ha−1 (Table 1). The total surface area of the above-ground skeleton was 7,152 m2 ha−1, while the surface area of woody cambium was 7,035 m2 ha−1 (Table 1). Above-ground cambium area index (CAI) was 0.7 when LAI was 3.0.
Water in the xylem
Individual tree level
Our results on the distribution of the branches in the crown confirmed earlier studies (Kellomäki et al. 1980; Baldwin et al. 2000; Mäkëla and Vanninen 2001) indicating that the vertical maximum of the needle and branch biomass of the dominant trees was lower than in the suppressed trees. The ratios of the needle to the branch biomass and the needle to the branch surface areas provide an insight in the assimilation and respiration patterns of the tree. In the case of our forest site, branches contribute for the largest part to the above-ground tree cambium surface area. The above-ground woody skeleton cambium area to needle surface area ratio, which is more favorable in larger trees (Fig. 1), therefore serve as a relative measure of the economy in the investment of assimilates. The ratio of the needle biomass to the total crown biomass (40 %) was comparable to the fraction of 34 % reported by Monserud et al. (1996) for Scots pine in Russia.
The HV is an indicator of the long-time hydraulic adjustment to local climatic conditions, also in Scots pine (e.g., Berninger et al. 1995; Poyatos et al. 2007; Martínez-Vilalta et al. 2009). Our observations on HV at breast height were in line with the values reported for Scots pine from a range of various sites (Mencuccini and Bonosi 2001; Poyatos et al. 2007) and were closer to drier sites (Martínez-Vilalta et al. 2009). The HV values for the base of the crown and for branches were slightly higher than HV reported by Mencuccini and Grace (1994) and Mencuccini and Bonosi (2001). In trees of a different social status, the HV is a measure of their investment into the conductive pathways per unit of the leaf area fed by these pathways (Tyree and Ewers 1991). The largest sapwood area per leaf area was found in the smallest trees (Table 4). This feature protects suppressed trees against xylem cavitation because the cavitation risk in their xylem is increased by maintaining lower water potentials (Sellin 2001) and by lower xylem hydraulic conductivity (Reid et al. 2003).
The total stem volume and the fraction of the branches were similar to the values reported for Scots pine stands in the temperate zone (Vogt 1991). The vertical distribution of the stem biomass reflected stem taper (Fig. 4, Garber and Maguire 2003; Younger et al. 2008). The proportion of branch biomass from total above-ground woody mass (7 %) was similar to the values at old stands on fertile sites (Vanninen et al. 1996) but lower than 11 % in a stand of similar age in Eastern Finland (Helmisaari et al. 2002). However, the mean DBH, height and stocking volume of the above-mentioned stand in Finland were lower, as a result of less favorable growing conditions, which inevitably resulted in a higher allocation of biomass into the branches (Poorter et al. 2012).
The above-ground woody cambium surface area (7,035 m2 ha−1) was very close to the 6,500 m2 ha−1 (Korf 1974) for Scots pine. The surface areas of the cambium of these two Scots pine stands (in Belgium and in the Czech Republic) were very similar, despite the significantly smaller tree size in the study of Korf (1974). The comparable CAI in these two forests supports the hypothesis that the cambium surface area of a forest stand increases until the age of 40–50 years and then remains approximately constant (Anučin 1959). As in our study, Korf (1974) estimated cambium area on harvested trees from mean diameter and length of a section (Eq. 3). Above-ground CAI is usually very close to the woody area index (WAI), (defined, i.e., in Bréda 2003) which can be estimated by the optical-based methods. However, different methods of measurements as well as variation in tree age, site quality and stand density may impose large source of variation within a specific species (Smolander and Stenberg 1996; Weiskittel and Maguire 2006). WAI of our forest site comprised 19 % from the above-ground plant area index (PAI), which fells into the range 3–33 % listed by Bréda (2003) for various pine species. But it is almost twice as much as 10 % estimated by hemispherical photography (Walter and Grégoire-Himmler 1996) and still more than 14 % from direct measurements in a 25-year-old (i.e., three times younger) Scots pine stand (Smolander and Stenberg 1996).
The fraction of root biomass from the total (18 %) was identical to the value reported for a 36-year-old boreal forest (Ilvesniemi and Liu 2001) and similar to the 13 % in a 100-year-old Scots pine stand in Eastern Finland (Helmisaari et al. 2002). It was also within the range of 5–40 % reported in the literature for various coniferous forests (i.e., Jackson et al. 1996; Thies and Cunningham 1996; Drexhage and Gruber 1999; Do-Hyung 2001). Most of the below-ground biomass (76 %) was located in the top 20 cm of the soil and in the coarse roots with a diameter larger than 5 mm (Fig. 5a). The amount of the roots decreased with depth. Various authors described a similar root distribution (e.g., Roberts 1975; Persson 1983; Nadezhdina et al. 2007; Čermák et al. 2008a; Børja et al. 2008). This type of vertical distribution of coarse roots emphasizes the high importance of precipitation compared to the groundwater (Tatarinov et al. 2008). The seasonal variation of the root surface area reflected the different demand for the absorption of water and nutrients. A twofold change in the surface area between the spring and the winter (Janssens et al. 1999; Xiao et al. 2003) was linked with the changes in the surface area of small and fine roots and was in line with the observed variability in a Scots pine forest stand in Sweden (Persson 1978).
The RAI observed in this study was close to the RAI values of 6.8 for a European beech and 5.4 for a Norway spruce stand (van Praag et al. 1988). Higher values of RAI than LAI were often reported for different forest ecosystems (Jackson et al. 1996), for Scots pine forests (Ilvesniemi and Liu 2001; Addington et al. 2006) and for a Norway spruce stand (Do-Hyung 2001). But in some cases, i.e., in a short-rotation poplar coppice, values of RAI (1.7–3.7 m2 m−2) could be much lower than those of LAI (2.1–6) (Al Afas et al. 2008).
Water in the xylem
The water content in the xylem is a dynamic value that changes on a diurnal as well as on a seasonal basis. Upon rapidly changing weather conditions and low availability of the soil water, as much as half of the transpired water can be extracted from the water stored in the sapwood over short periods (Waring et al. 1979; Phillips et al. 2003; Čermák et al. 2007; Hernández-Santana et al. 2008). Therefore, our results provide only a one-time estimate of the water storage in the Scots pine stand. Our data of water storage in the sapwood corresponded well with the measured water storage of 6.4 to 7.5 mm in the stem sapwood of 50- and 100-year-old Scots pine stands on a sandy soil in central Sweden (Kravka et al. 1999). Similarly, the mean water content in the sapwood of 400 kg m−3 (Kravka et al. 1999) was very close to the 408 kg m−3 in the present study. On the contrary, the xylem water content of 120 kg m−3 in the heartwood measured by Kravka et al. (1999) in central Sweden was two times lower than here. The water content in the heartwood does not change much over the season; this reflects the hydraulic separation between the living portions of the stem and of the heartwood (Holbrook, 1995). Therefore, high moisture in the heartwood may have been induced by constantly high water availability at our research site (Ehleringer and Dawson 1992; Kumagai et al. 2009).
Ecophysiological significance of relations among various biometric parameters
On average, 7 m2 of needles colonized each unit of surface area at branches. The photosynthetically active organs—needles—feed the “heterotrophic” parts of the tree, primarily the living cells of the cambium layers. On average, one unit of surface area of needles fed 0.25 m2 of the above-ground cambium and 1–2 m2 of the total cambium surface area. The amount of needles supporting the unit of the tree cambium area (Table 4, Fig. 1) was smaller in smaller trees. This ratio may be an indicator of the lower growth increment potential of the suppressed trees, but also, on the one hand, of their better hydraulic safety margins (Choat et al. 2012). The LAI (reaching a value of 3) was comparable to the RAI (3–7), emphasizing the mutual physiological roles of those two active surfaces (i.e., the transpiratory surface area vs. the water absorptive surface area).
In this contribution, we provided allometric equations on biomass and surface areas of above- and below-ground tree organs in a mature Scots pine stand. These equations are necessary background information for up-scaling of physiological data measured on the spatially limited level (i.e., respiration of various tree organs). Similarly, process-based models operate on a unit area level and the operator needs to know the stand biometry to either downscale empirically measured input data or to upscale the model outputs. Total woody biomass of the 66-year-old forest stand was 155 Mg ha−1, i.e., 126 Mg ha−1 above ground and 29 Mg ha−1 below ground. These values can be linked to the carbon storage. The amount of stored water in this biomass available for transpiration (2 dm3 m−2) plays a role in mitigating the effect of drought stress on a tree. Speaking about stress, the surface area of needles supported by unit of cross-sectional area of sapwood provides information about individual tree resistance. Total surface area of the woody skeleton was 75,000 m2 ha−1. Most of the surface area of the skeleton (90 %) was in the roots. This could be linked to the number of living cells and sheds light on the partitioning of the above-ground and below-ground tree respiration. The relation between the above-ground woody surface area and the needle surface area defines the fraction of intercepted radiation available for photosynthesis. A comparative analysis of the biometric parameters showed the balance between the different functionally connected, operational surface areas of the trees in the stand (i.e., root to needle surface areas).
We thank I.A. Janssens, D. de Pury, V. Gond and N. Calluy for their valuable help with field and laboratory measurements. We are also grateful to J. Van Slycken(+) and S. Overloop (INBO, Geraardsbergen) for their logistic support at the forest site. This research was financially supported by the Sixth Framework Programme of the European Commission as Carbo-Europe IP (contract no. GOCE-CT-2003-505572), by the Flemish-Czech Bilateral Scientific Cooperation project (UA-BOF-1-2006-19), by the COST MŠMT LD13017 and by the Investments in Education Development (contract no. OPVK CZ.1.07/2.3.00/30.0017) co-financed by the European Social Fund and the state budget of the Czech Republic. This work was part of the Global Change and Terrestrial Ecosystems Core project of the International Geosphere-Biosphere Programme (IGBP).
Conflict of interest
The authors declare that they have no conflict of interest.
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