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Plant and Soil 264: 1–11, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Relationships between tree dimension and coarse root biomass in mixed stands of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.) A. Bolte1,5 , T. Rahmann1 , M. Kuhr2 , P. Pogoda3 , D. Murach4 & K. v. Gadow3 1 Georg-August-University,

Göttingen, Institute of Silviculture, Dept. I, Büsgenweg 1, 37077 Göttingen, Germany. FIV, Prof.- Oelkers-Str. 6, 34346 Hann. Münden. 3 Georg-August-University Göttingen, Institute of Forest Management, Büsgenweg 5, 37077 Göttingen. 4 University of Applied Science, Faculty of Forestry, A.Moeller-Str. 1, 16225 Eberswalde, Germany. 5 Corresponding author∗

2 Hessen-Forst,

Received 26 March 2003. Accepted in revised form 23 March 2004

Key words: allometry, biomass, Fagus sylvatica, forest, Picea abies, root/shoot ratio, root system

Abstract Relationships between tree parameters above ground and the biomass of the coarse root system were examined in six mixed spruce-beech stands in the Solling Mountain region in northwest Germany. The selected stands were located on comparable sites and covered an age range of 44 to 114 years. Coarse roots (d ≥ 2 mm) of 42 spruce and 27 beech trees were sampled by excavating the entire root system. A linear model with logarithmic transformation of the variables was developed to describe the relationship between the coarse root biomass (CRB, dry weight) and the corresponding tree diameter at breast height (DBH). The coefficients of determination (R 2 ) attained values between 0.92 for spruce and 0.94 for beech; the logarithmic standard deviation values were between 0.29 and 0.43. A significantly different effect of tree species on the model estimates could not be detected by an analysis of covariance (ANCOVA). For spruce, the derived relationships were similar to those reported in previous studies, but not for beech. Biomass partitioning in the tree compartments above and below ground differs significantly between spruce (coarse root/shoot ratio 0.16 ± 0.06) and beech (coarse root/shoot ratio 0.10 ± 0.03) in the mixed stands. These results are similar to those given in other studies involving pure spruce and beech stands on comparable sites in the region, although the ratios of pure stands in other regions growing under different site conditions are somewhat higher. Comparing trees of the same DBH classes, root/shoot ratios of spruce are 1.2 to 3 times higher than those of beech. Dominant spruce trees (DBH > 60 cm) attained the highest ratios, suppressed beech trees (DBH < 10 cm) the lowest. Site conditions of varying climate and soils and interspecific tree competition are likely to affect root/shoot ratio and DBH-coarse root biomass relationships. The greater variability in beech compared with spruce indicates a high ‘plasticity’ and adaptability of beech carbon allocation. Thus, the derived equations are useful for biomass estimates of coarse roots involving trees of different ages in mixed stands of spruce and beech in the Solling Mountains. However, application of these relationships to stands in other regions would need further testing.

Introduction Tree roots contribute significantly to the total biomass and carbon storage of forests (Grier et al., 1981; Kurz et al., 1996; Li et al., 2003). Direct assessment of root biomass in the field is, however, difficult. The diffi∗ FAX No: +49-551-393270. E-mail: [email protected]

culties include large variations in the root samples, the need to use destructive methods, high labour cost and limited capacity to describe the spatial distribution of roots (Bengough et al., 2000). Therefore, modelling tree root biomass has gained wide acceptance (Drexhage and Collin, 2001). During the past 20 years, scientists have made numerous attempts to establish a

2 relationship between the tree attributes above and below ground on stand level, mostly using models of the relative allocation of dry weights between roots and above ground parts (Cairns et al., 1997; Gower et al., 1996; Kurz et al., 1996; Li et al., 2003; Nadelhoffer et al., 1985). These have produced varying root biomass and root production estimates, mainly because of the lack of information on root size and number, tree species, sampling methods and the interaction of these variables with different site and climatic conditions. For single trees within stands, several authors used estimated relationships between root biomass and stem diameter at breast height (DBH), tree height (h) or both parameters (Drexhage and Colin, 2001; Drexhage and Gruber, 1999b; Hofmann and Usoltsev, 2001; Kuiper and Coutts, 1992; Laiho and Finer, 1996; Le Goff and Ottorini, 2001; Lee, 1998; Pellinen, 1986; Santantonio et al., 1977; Thies and Cunningham, 1996; Usoltsev and Vanclay, 1993). Some of these studies suggest a consistent relationship between root biomass and DBH for different tree species within one stand or even at different locations (Kira and Shidei, 1967; Le Goff and Ottorini, 2001; Santantonio et al., 1977) and would support the hypothesis that a balance exists between the above and below ground tree dimensions and that their biomass partitioning is very conservative (see Bartelink, 1998; Lacointe, 2000; Spinnler et al., 2003). Other studies, however, showed a small correlation in those relationships when data of different tree species and locations were compared (Drexhage and Colin, 2001). European beech (Fagus sylvatica) and Norway spruce (Picea abies) are two of the most common tree species in German forests (BMELF, 1992). Currently silviculture practices in Western Europe favour near natural forest management by using the site adapted tree species and selective harvesting (Gadow and Puumalainen, 1998; Olsthoorn et al., 1999). Such practices will increase the area of mixed beech–spruce stands in the low mountain ranges of north Germany (Otto, 1992). However, there are few root biomass studies of large trees (DBH > 25 cm) in mixed beech and spruce stands (Nihlgård, 1972; Pellinen, 1986). The objectives of this study are (1) to establish relationships between certain dimensional variables and the coarse root biomass of large trees in mixed spruce and beech stands and (2) to evaluate the application of the derived relationships for mixed and pure spruce and beech stands.

Materials and methods Locations and sites Studies were conducted in six mixed beech–spruce stands in the Solling Mountain region in Lower Saxony (northwest Germany). The altitude ranges between 300–500 m above sea level, the climate is characterised by a mean annual air temperature of 6.5– 7.5 ◦ C and an annual precipitation of 900–1050 mm (AK Standortskartierung, 1985). All stands grow on loess and sandstone sediments of the Lower Triassic. Soils are characterised as Dystric Cambisols (Podzolic brown earth) with a moderate nutrient status and a good water supply (Otto, 1991). Sampling and measurement In 1999, 42 spruce and 27 beech trees were selected for coarse root sampling in six different stands (Table 1). All roots and root parts greater than or equal to 2 mm in diameter were defined as coarse roots (see Linder and Troeng, 1980; Vogt et al., 1989). Before excavating the root system, the ground level and the north side of every selected tree were marked. The tree height [h] was measured with the electronic hypsometer Forester Vertex (Haglöf, Sweden) and the DBH with a beam calliper (the mean of two diameters taken perpendicular to each other, Hush et al., 2003). In addition, the crown status of every tree was assessed using the dominance criteria developed by Kraft (1884). The excavation of the large root systems proved to be demanding. Under water-saturated soil conditions, a skidder was used to pull down the trees carefully with a cable winch (16 t traction power). To prevent root breakage in the large trees (DBH > 50 cm), a small excavator was used which fixed the root system and supported the excavation of the samples (Dahmer, 1998). Broken roots were excavated manually and tagged (see Kuiper and Coutts, 1992). At the marked ground level, the stem and root system were separated with a chain saw. The roots were then turned over and placed upside down on the ground. Subsequently the roots were cleaned by using pressurised air (7 bars), and manually by using (semi-circular) spades, picks and chisels. The assessment method resembled the one reported by Kuhr (2000), Nielsen (1995) and Redde (2002). For the subsequent measurements, the root system was partitioned into three-dimensional sections as

3 Table 1. Selected stands and trees for the coarse root sampling (d ≥ 2 mm). Stand location according to the German forest administration system (district, compartment), age class and tree characteristics (age, dimension) is displayed; DBH values are rounded to full centimetres, height to full metres No.

1 2 3 4 5 6

Forest district and compartment no.

n

Norway spruce age DBH height (a) (cm) (m)

n

European beech age DBH height (a) (cm) (m)

Steinhoff 129 Relliehausen 144 Abbecke 112 Relliehausen 35 Abbecke 79 Eschershausen 1084

8 1 7 1 13 12

44 45 77 98 112 114

9 1 3 2 8 4

44 53 80 110 118 127

suggested by Nielsen (1995) and Kuhr (2000). The sections consist of concentric rings taking into consideration (1) the distance from the stem basis in 50 cm sections up to a maximum distance of 400 cm of the horizontal coarse roots, (2) the soil depth using 25 cm sections (top soil) and 50 cm sections to a maximum depth of 200 cm and (3) the orientation, using four discrete directions: NE, SE, SW and NW (Figure 1). The dry weight of the coarse root biomass (CRB) was derived indirectly from the coarse root volume (CRV) and the dry density of the root wood including bark (DD, Pellinen 1986). CRV was calculated using Equation (1) assuming that the root sections may be approximated by a frustum of a cone (Fehrmann et al., 2003): π (1) CRV = l(r12 + r1 r2 + r22 ), 3 where r = root radius entering a section (r1 ) and leaving a section (r2 ), and l = root length. We recorded the length of every coarse root within each section using a measuring tape, and each root radius entering and leaving a section with a calliper. Volumes (CRV) were summarized for each root section to derive the total coarse root volume for every tree. A separate estimate of the stump volume below ground was made by recording the height and diameter of the core wood cylinder (the ‘stump cylinder’). The stump cylinder represented the solid part of the root system without branches. Oven dry mass values (CRB) were derived from the dry weight of a random sample of different root parts (beech: n = 313, spruce: n = 230); coarse root parts were oven-dried for at least 48 h at 105 ◦ C to a constant weight. Average root dry densities (DD) were calculated using a linear regression between coarse

16–36 29 43–55 38 30–58 29–74

16–36 29 24–30 38 23–33 19–37

4–26 21 14–32 37–38 15–46 18–53

7–21 21 18–24 24–28 17–28 20–28

root volume (CRV) and coarse root biomass (CRB) without intercept (CRB = DD · CRV). DD values were 451 kg m−3 for spruce (R 2 = 0.91) and 536 kg m−3 for beech (R 2 = 0.94). Analysis Biomass partitioning between tree components above and below ground were calculated using root biomass results from this study and above ground data of trees from the same stands provided by Rademacher (pers. comm.). We used this data to derive root/shoot ratios excluding fine roots for all trees and for identical trees of the above ground and coarse root studies. For identical trees, a Mann–Whitney U-test was undertaken to test differences in root/shoot ratios for spruce and beech. The software STATISTICA (StatSoft Inc., Tulsa, Oklahoma) was used for the statistical analysis. The non-linear allometric relationships between tree dimensions (diameter at breast height, height) and root biomass are often described with linear functions using transformed variables (Drexhage and Collin, 2001; Hofmann and Usoltsev, 2001; Le Goff and Ottorini, 2001). This approach alleviates problems with heteroscedasticity and allows the application of simple linear least square regressions. The following equation was used: ln y = β0 + β1 ln x,

(2)

where y = coarse root biomass [CRB, dry weight]; x = breast height diameter [DBH], tree height [h]; β0 , β1 = empirical parameters. As proposed by Hofmann and Usoltsev (2001), the following bias correction factor K (Finney, 1941) for the reverse transformation of ln-transformed values was calculated:

4

Figure 1. Schematic illustration showing the framework for recording coarse root dimensions, based on the horizontal and vertical sectioning of the root systems. (Kuhr, 2000; number of sections modified).  2 /2 K = es ln

 2  4    sln sln 2 4 2 3sln + 44sln + 84 , 1− s +2 + 4n ln 96n2

(3) where sln = standard deviation of ln-transformed values (ln x, ln y) n = number of samples. We calculated the amount of unexplained variation using the coefficient of determination (R 2 ) and performed a residual analysis including a Shapiro–Wilk normality test of the residuals. An analysis of covariance (ANCOVA) was used to evaluate the effect of the predictor variable tree species (covariate) on the relationship between DBH and coarse root biomass. This refers to the parallelism of the regression lines in the two species classes, beech and spruce. If those lines are parallel, then the relationship in both classes is the same, and the relationship between DBH and coarse root biomass is not moderated by tree species (StatSoft Inc., 2003).

Results Allometric DBH–CRB relationships Close relationships were found between diameter at breast height (DBH) and coarse root biomass (CRB, dry weight) of individual trees, with or without the stump cylinder (Table 2; Figures 2a and b). After data transformation, linear functions [Equation (2)] with coefficients of determination of at least 0.92 were obtained (Table 2). The relationships are consistent for all investigated stands of varying age as shown in Table 1. The residuals are normally distributed according to a Shapiro–Wilk normality test (P < 0.05), except for one outlier for the relationship between DBH and CRB with stump cylinder for beech. No significant improvement of the relationships was obtained by including tree height [h] as an additional variable. When comparing the relationships separately for spruce and beech, it was found that the differences between the two were small (Figure 2). The standard error of estimation and the bias correction factor (K,

5

Figure 2. Relationships between diameter at breast height (ln DBH) and coarse root biomass (ln CRB); (a) without stump cylinder, (b) with stump cylinder. The equations of the regression lines are presented in Table 2.

Table 2) are a little higher for beech than for spruce and the root volume and root biomass of the spruce trees increase only slightly with a greater DBH when compared to the beech trees. The differences are even less evident for data including stump cylinders (Figure 2b), because the beech stump cylinder biomass contributes more to the root biomass (Table 3 and last section). An analysis of covariance (ANCOVA), did not show any significant effect (P < 0.05) of tree species as a predicator variable on the coarse root biomass estimates (without stump cylinder P = 0.33, with stump cylinder P = 0.56).

class < 10 cm). Root/shoot ratios for spruce trees are somewhat constant; higher values are found in dominant trees (DBH classes > 60 cm). For the suppressed to co-dominant trees (DBH classes > 10 cm to 50 cm), the differences in the root/shoot ratios between spruce and beech for identical trees (method 2) are significant according to the Mann–Whitney U-test (U = 34, Z = −3.09869, P < 0.01) for independent samples (n = 15 spruce trees, n = 14 beech trees).

Biomass partitioning

In the mixed stands under investigation, the diameter at breast height (DBH) is a reliable predictor of the volume and biomass of coarse roots for European beech and Norway spruce. This result supports findings in earlier studies of pure stands of spruce reported by Drexhage and Gruber (1999b) and Lee (1998), and of beech by Le Goff and Ottorini (2001) and Pellinen (1986). The derived relationships between DBH and total root biomass are comparable with those in this study because of the relatively small contribution of fine roots (d < 2 mm) to the total root system biomass of individual adult trees (< 10% for beech: Bartelink, 1998; Hertel, 1999; Le Goff and Ottorini, 2001; < 10% for spruce: DeAngelis et al. 1981, Nihlgård 1972). Figure 3 shows the re-transformed equations of the present study and compare them with the equations taken from previous studies. For spruce without stump cylinder (Figure 3a), similar root biomass relationships were reported for 34–40-year-old pure stands on

Tree biomass data of spruce and beech trees in the mixed stands indicated that the allocation pattern differed for the above and below ground fractions (Table 3). The above ground biomass for different DBH classes in spruce was 0.49 to 0.75 of the equivalent beech biomass in a corresponding DBH class. However, the total CRB values were mostly similar (spruce/beech ratio: 0.96 to 1.15; 1.58 for DBH class > 40 to 50). Stump cylinder biomass contributed significantly to the CRB (spruce: 5% to 14%; beech: 7% to 20%). Coarse root/shoot ratios for (1) all sampled trees and (2) identical trees were mostly similar. Greater differences can be noticed, particularly in those DBH classes where the sample size differs between the assessments above and below ground. Root/shoot ratios for spruce exceed the ratios for beech (spruce/beech ratio: 1.2 to 3.0). For beech, the values vary without a clear tendency; a low ratio is found in trees with mostly suppressed crowns (DBH

Discussion

6 Table 2. Regression equations for predicting coarse root biomass from breast height diameter (DBH). All equations are of the form ln y = β0 + β1 ln x, see Eq. (2), y = coarse root biomass of beech or spruce, with or without stump cylinder (CRB, kg), x = breast height diameter (DBH, cm), β0 , β1 = regression coefficients (empirical parameters, rounded to two decimals) with standard errors in parentheses (± SE). R 2 = coefficient of determination (R 2 ), s = standard deviation of residuals (‘standard error’ of estimation) and K = bias correction factor, see equation (3) are displayed. All model parameters were statistically significant (α = 0.05) in accounting for variation in CRB Breast height diameter (DBH)

Coarse root biomass (dry weight) (CRB) Norway spruce European beech without stump with stump without stump with stump cylinder (kg) cylinder (kg) cylinder (kg) cylinder (kg)

β0 (± SE) β1 (ln DBH) (± SE) n R2 s K

−5.90 (± 0.49) 2.85 (± 0.13) 42 0.92 0.30 1.04

different sites in the Harz Mountains, Göttinger Wald and Fichtelgebirge [Bavaria] (Figure 3a; Drexhage and Gruber, 1999a, b; Lee, 1998). This, and the correspondence of beech and spruce relationships (Figure 2, Table 2), support the hypothesis that ‘the nature of the relation of root-system biomass to stem diameter at breast height is remarkably consistent’ (Santantonio et al., 1977, p. 23) and support the application of a DBH-root biomass relationship for different tree species on different sites and in different geographical regions (see results of Kira and Shidei, 1967; Kuiper and Coutts, 1992; Laiho and Finer, 1996). However, (Figure 3b), the root biomass relationships for beech trees in this study differ clearly from those of Le Goff and Ottorini (2001) and Pellinen (1986). For example, their equations predict for beech trees with a DBH of 20 cm approximately twice the coarse root biomass than was observed in our study. This, and the findings of Drexhage and Colin (2001), obviously contradict the idea of a consistent relationship. Drexhage and Colin (2001) compared eleven studies of diameter–root biomass relationships of different tree species including spruce and beech and found significant differences of seven regressions. A likely cause for such different results reported in previous studies is the method of assessment. In contrast with the present study, Le Goff and Ottorini (2001) estimated that for 30-year-old beeches

−5.59 (± 0.49) 2.79 (± 0.13) 42 0.92 0.29 1.04

−4.04 (± 0.40) 2.27 (± 0.13) 27 0.93 0.43 1.09

−4.00 (± 0.37) 2.32 (± 0.12) 27 0.94 0.40 1.08

(DBH < 25 cm) about 13% of the total biomass could be assigned to roots missing after excavating the root systems with a mechanical shovel. They then adjusted their observations accordingly. For the estimates, they used biomass equations derived from unbroken root ends. Studying large, wind thrown Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) with a DBH ranging between 94 and 135 cm, Santantonio et al. (1977) added 10.8% to 15.1% of the fresh root weight to correct for losses due to missing root ends. In that study, however, the reported biomass losses refer mainly to breakage of strong roots (root end d ≥ 50 mm, according to Sutton and Tinus, 1983), losses which in most cases could be avoided by a careful and controlled root system extraction as in the present study. Careful extraction in water saturated soil conditions and the manual excavation and tagging of broken root ends, probably reduced coarse root biomass losses to an insignificant quantity in the present study. Different definitions of biomass components are other factors affecting a valid comparison between biomass and DBH. The term ‘Stockholz’ used by Pellinen (1986) included above ground parts of the stump which remained after the tree was felled (Erlbeck et al., 1998). Due to lack of further information about the dimensions of the stump parts above ground

7 Table 3. Biomass fractions and root/shoot ratios for beech and spruce in different DBH classes. Crown classes are assessed according to the criteria proposed by Kraft (1884). All displayed biomass values are arithmetic averages of tree biomass ± standard deviation for every DBH class (kg, dry weight); n = sample size of the above and below ground biomass studies. The above ground biomass studies were conducted in the same stands of the present below ground studies; 21 identical spruce, 15 identical beech trees and 36 additional trees were studied (data by P. Rademacher, pers. communication). Root/shoot ratios (kg kg−1 ) are calculated without fine root biomass (roots d < 2 mm). Root/shoot ratios 1 are calculated using the displayed average biomass values in the table. Root/shoot ratios 2 are only derived from data for identical trees.

0–10 Norway spruce Crown class Above ground parts (n) Total biomass (stem, branches, leafs) Belowground parts (n) Coarse roots Stump Coarse root system incl. stump Root∗ /shoot ratio 1 Root∗ /shoot ratio 2 (21 identical trees) European beech Crown class Above ground parts (n) Total biomass (stem, branches. leafs) Below ground parts (n) Coarse roots Stump Coarse root system incl. stump Root∗ /shoot ratio 1 Root∗ /shoot ratio 2 (15 identical trees)

>10–20

DBH classes (cm) >20–30 >30–40 >40–50

>50–60

>60–70

>70–80

intermediate to suppressed 2 9 9 62.1 249.2 531.1 ± 3.6 ± 83.3 ±157.4

co-dominant 5 7 800.2 1438.4 ± 86.9 ± 120.4

dominant 3 1 2030.1 2978.6 ± 158.5

2 10.9 ± 5.5 1.8 ± 0.5 12.7 ± 6.0

6 70.1 ± 20.9 10.7 ± 6.3 80.8 ± 26.0

11 128.6 ± 50.4 12.1 ± 13.9 140.7 ± 57.9

12 262.6 ± 104.8 20.1 ± 7.4 282.7 ± 109.4

3 553.0 ± 265.1 30.0 ± 17.0 583.0 ± 281.7

0.15 0.15 ± 0.01

0.18 0.13 ± 0.02

0.20 0.14 ± 0.04

co-dominant 3 1455.2 ± 89.1 2

3 2459.3 ± 243.2 1

0.21 – –

7 41.8 ± 20.0 3.2 ± 1.1 45.0 ± 19.7

±

0.18 0.16 0.06

intermediate to suppressed 2 12 7 38.0 126.9 332.2 ± 5.7 ± 48.0 ± 108.1 4 9 4

9 870.0 ± 136.1 7

1.3 ± 1.0 0.1 ± 0.03 1.4 ± 1.0 0.04 0.03 –

56.9 ± 19.3 14.2 ± 5.9 71.1 ± 19.1 0.08 0.07 ± 0.01

11.3 ± 4.7 2.0 ± 1.5 13.3 ± 6.0 0.10 0.11 ± 0.02

36.8 ± 17.1 6.8 ± 2.1 43.6 ± 15.7 0.13 0.13 ± 0.03

71.6 ± 19.3 17.3 ± 3.1 88.9 ± 22.4 0.06 0.11 –

±

0.29 0.25 0.07

1 840.8 – 47.6 – 888.4 – 0.30 0.30 –

202.1 – 43.3 – 245.4 – 0.10 – –

∗ Without fine root biomass

(Pellinen, 1986), it was not possible to make further adjustments for comparison. However, differences in the method cannot explain all the discrepancies in the relationships for beech. The relationship between the DBH and various shoot biomass fractions is relatively stable (Causton, 1985). This is not so for the coarse root biomass. Comparing the low overall root/shoot ratios for Solling beech of 0.10 ± 0.03 kg kg−1 (method 2; see Table 3) and of

0.13 kg kg−1 for the IBP Solling beech stand B1 (Ellenberg et al., 1986) with the higher root/shoot ratios of 0.16 kg kg−1 in the Göttinger Wald (Pellinen, 1986) and 0.19 kg kg−1 at the French site in Hesse (Lebaube et al., 2000; Le Goff and Ottorini, 2001), major differences in the environment at different beech sites can be expected (Cannel, 1985; Larcher, 1995). This describes differences between the long-term equilibrium

8

Figure 3. Comparison of root biomass relationships [regression lines, root biomass vs. diameter at breast height (DBH), (a) Norway spruce, (b) European beech] of this study in comparison with previous studies (Drexhage and Gruber, 1999b; Le Goff and Ottorini, 2001; Lee, 1998; Pellinen, 1986).

of root/shoot biomass ratios as against the short-term fluctuations (Cannel and Willett, 1976). Root/shoot ratio (type 2) for spruce of 0.16 ± 0.06 kg kg−1 was found to be similar to 0.14 to 0.16 kg kg−1 calculated from the data provided by Drexhage and Gruber in the Harz Mountains (1999a). However, spruce root/shoot ratios taken from the literature varied between 0.20 and 0.33 kg kg−1 (DeAngelis et al., 1981). It is believed that low soil fertility and water deficiency will increase biomass partitioning to roots (Axelson and Axelson, 1986; Keyes and Grier, 1981; Linder and Axelson, 1982; Murphy and Lugo, 1986). The sites in Hesse (Le Goff and Ottorini, 2001) and the Göttinger Wald (Pellinen, 1986) had relatively dryer and warmer climates with 100 mm to 340 mm less annual precipitation and a 0.5 ◦ C to 2.7 ◦ K lower annual temperature than the Solling sites. In addition, the beeches in the Göttinger Wald grow on soil derived from shell limestone sediments with a lower water retaining capacity than the Solling sites (Horn, 2002). However, the Solling site probably has a poorer nutrient status than the sites on clay-sandstone (Hesse: Le Goff and Ottorini, 2001) and limestone (Göttinger Wald: Horn, 2002) with mull humus forms; this may counteract a lower biomass partitioning to roots. High root/shoot ratios in spruce under stress have been reported (Clemensson-Lindell and Persson, 1993; Ericson et al. 1996) that may contradict a consistent DBH-root biomass ratio for different sites. Due to paucity of studies describing the effects of site and climate on the DBH-root biomass, it is difficult to eval-

uate the application of the allometric relationships in different regions. Some authors observed that the interspecific competition could reduce root/shoot biomass ratios (Hertel, 1999; Newton and Cole, 1991). This observation corresponds with our finding that the dominant spruce trees had the highest root/shoot ratios and the suppressed beeches the lowest (Table 3). Fine root studies by Bolte et al. (2003) in stands adjacent to our site no. 6, and by Rothe (1997), and coarse root studies reported by Schmid and Kazda (2001) gave a different pattern for vertical root distribution in mixed spruce-beech stands when compared with pure beech stands. In mixed stands the root systems of beech and spruce separated into depth zones with most of the beech roots confined to the deeper soil depths (high root densities) and a much shallower rooting pattern in the surface soil layers than that observed in pure beech stands. Similar results for beech were reported in admixtures with other species such as ash (Fraxinus excelsior L.) and Douglas fir (Hendriks and Bianchi, 1995; Rysavy and Roloff, 1994). These variations in root system architecture due to interspecific competition may influence the balance of root/shoot biomass partitioning of beech trees (Hertel, 1999). Spruces in a mixed stand in the Solling Mountains, however, did not show such variations in the rooting pattern as did beech and were found to retain a more consistent architecture with a high root density in the surface soil layers (Bolte et al., 2003). Therefore, when comparing studies in pure and mixed stands, a wider range of root/shoot biomass ratios and a greater

9 variability of DBH-root biomass relationships may be expected in beech than in spruce.

P.K. Khanna and two anonymous reviewers for comments that improved previous drafts of the manuscript.

Conclusions

References

The study is based on a comparatively large data set, involving root systems of mature spruce and beech trees. Detailed analysis of the root systems produced four equations for estimating root biomass from DBH. They can be used to estimate the coarse root biomass (CRB) of beech and spruce in mixed stands in the Solling region. In our study, the DBH-CRB relationships are not significantly affected by the different tree species. However, the common idea of a consistent relationship for different tree species, sites and geographic regions cannot be upheld. The varying relationships for beech shown in our study and cited in the literature could not be explained simply by different assessment methods. Biomass partitioning between tree compartments above and below ground is variable, particularly in beech growing in different regions. Root/shoot ratios seem also to be affected by tree competition for both tree species, too. For beech, the variability of vertical root biomass distribution is high compared to spruce. This finding has been reported by several authors comparing pure and mixed stands. The greater variability in beech indicates a high ‘plasticity’ and adaptability of beech carbon allocation. Further studies dealing with biomass allometry and biomass distribution in pure and mixed spruce-beech stands in different regions and with different site conditions may support or reject previous findings, including our own. In future studies more attention should be given to describing the biomass compartments and their fractions more adequately. Consistency in the classification and description of the main components of the root system, including solid stump cylinder, is needed.

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Acknowledgements This study was funded by the German Federal Ministry of Education, Science, Research and Technology within the cooperative project ‘Indicators and strategies of a sustainable, multi-functional forestry – case study Solling’ (BMBF FKZ 033947C/3) of the Forest Ecosystems Research Centre at the GeorgAugust-University, Göttingen. The authors are grateful to P. Rademacher for kindly providing data and to

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