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Spatial and temporal patterns of fine root abundance in a mixed oak-beech forest
~ ~ ~ ~*:~i • ~ ~_~,~. E LS EV I ER
Forest Ecology and Management Forest Ecology and Management 70 (1994) 11-21
Spatial and temporal patterns of fine root abundance in a mixed oak-beech forest Volker B/ittner, Christoph Leuschner* LehrstuhlJ~r Geobotanik, Universityof Gfttingen, UntereKarspfile 2, D-37073 G6ttingen, Germany Accepted 7 July 1994
Abstract Spatial distribution and seasonal fluctuation of fine root density (mass per unit soil volume) and abundance (mass or surface area per unit ground surface area) were investigated by the sequential coring technique in a 100220 year old mixed Fagus sylvatica-Quercus petraea stand on acidic sandy soil in northwest Germany. The fine root systems of the two co-existing species overlapped completely with beech roots being twice as abundant as oak roots. Since Fagus and Quercus occupied equivalent parts of the canopy volume, oak appeared to be under-represented in the below-ground space. There was evidence for some degree of below-ground niche partitioning between the species in both the vertical and the horizontal direction. Oak fine roots were found to be more superficially distributed than beech roots in the organic layers, indicating a vertical stratification of the root systems of the two species. In the forest floor, fine roots were more abundant in the vicinity of oak stems where thicker organic layers occurred. However, this distribution pattern was not a consequence of a greater abundance of oak roots close to their parent stem, but was due to a higher frequency of beech roots here.
Keywords: Fine roots; Fagus sylvatica; Mixed stand; Quercuspetraea; Spatial distribution; Species differences
1. Introduction
Fine roots represent a functionally important part of the biomass of forest ecosystems. In many stands on acidic soils, the majority of active fine roots is concentrated in the uppermost soil horizons including the organic layers (Meyer, 1967; Persson, 1980). Here, fine roots are very densely packed and strong competition for water and nutrients may be expected among mycorrhizas and rootlets of neighbouring tree individuals. This was evidenced by various trenching and plant removal experiments (cf. Grubb, 1994). To re* Corresponding author.
duce exploitation competition, spatial and/or temporal separation of the root systems and their activities is likely to occur in multi-species communities. For example, Veresoglou and Fitter ( 1984 ) and Davis and Mooney ( 1986 ) reported differences in the location and timing of water and nutrient uptake by root systems of co-existing species in grassland and chaparral communities. Much less is known about the vertical and horizontal separation of root systems in mixed forest stands (e.g. Rachtejenko, 1952; Ehwald et al., 1961; Kern et al., 1961; McQueen, 1968; Kalisz et al., 1987). It is well known that, on similar substrates, tree species may differ markedly with
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V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
respect to both the extension of their coarse root system (e.g. Holstener-J/Srgensen, 1959; Ehwald et al., 1961; Abrams, 1990) and the abundance and the productivity of their fine roots (e.g. Kern et al., 1961; McQueen, 1968; Kochendorfer, 1973; Kalisz et al., 1987; Van Praag et al., 1988 ). However, most of these studies were conducted in monospecific tree stands where intra- but not interspecific competition can be expected. This paper reports on a study that examined the horizontal and vertical distribution, and the seasonal fluctuation of fine root abundance in a mixed old-growth oak-beech stand in northwest Germany. From canopy shape and forestry practice it is obvious that beech is the superior competitor for above-ground space in this stand (Leuschner, 1994) while the partitioning of below-ground space between the species is unknown. The aims of our studies were (1) to characterize possible species differences in the vertical and horizontal distribution of fine roots in a shared soil volume, and (2) to compare the seasonality of coexistent beech and oak fine roots. The study is part of an investigation into belowground co-existence in three successional communities on nutrient-poor, acidic soils in the region: a Calluna vulgaris heathland, an early successional birch-pine forest, and a late-successional oak-beech forest.
2. Materials and methods
2.1. Study site and environmental measurements A mixed stand ofFagus sylvatica L. ( 100-120 years of age ) and Quercuspetraea (Matt.) Liebl. (190-220 years of age) only minimally influenced by former forestry practice was selected in the neighbourhood of Unterlfiss (southern part of the Liineburger Heide, Lower Saxony, Germany, 52°45'N, 10°30'E). The ratio between the numbers of mature Fagus and Quercus trees is approximately 3:1 in the immediate vicinity of the sample plots (total stem density: 530 h a - l ) . Owing to the much larger canopy dimensions of the oaks, however, both species have comparable canopy projection areas in the stand (Fig. 1 ).
Soit pit
•
Beech
~Ook
Fig. 1. Location of the three sample plots (squares of 2 m × 2 m ) in the mixed oak-beech stand. Beech stems, black dots; beech canopy, white area; oak stems, open circles; oak canopy, dotted area.
There is no herbaceous layer. Situated in the diluvial lowlands of northwest Germany on Saalian melt water sands ( 115 m above sea level), the plot has a soil profile (spodo-dystric cambisol) with a thick organic layer (mean depth of the entire organic profile (OF + H layers ) 72 mm ) and a nutrient poor, highly acidic mineral soil (Leuschner et al., 1993; Rode et al., 1993). The organic layer is characterized by pH values of 3.0 and 2.6 (in 1 M KC1) and Ca2+/H + quotients of 0.2 in the equilibrium soil solution for the upper OF and the lower ON horizon, respectively. The climate is of a humid suboceanic type (an-
V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
nual means 8.0°C, 730 mm). Periods of low rainfall in summer may lead to water shortage in the soil as in August/September 1991 and in June/July 1992. The thickness of the organic layers was determined in 32 organic profiles each at 1 and 2 m distance from a beech or an oak stem in order to detect local differences in horizon depth. Care was taken not to compress the forest floor. These measurements were done in close vicinity to the root sampling plots. The temperature of the soil organic layer was monitored at intervals of 10 s with a thermistor placed at the upper edge of the OF horizon (i.e. roughly 2 cm below the litter surface ) and at 2.5 cm depth in the mineral soil profile. Sixteen cores, each 5 cm in diameter, were sampled at intervals of 7 days (during the summer period) to 14 days (during winter) in the organic layers for the gravimetric determination of water content. The depth of the OF and OH horizons in the samples was used to calculate volumetric water contents. With the pressure membrane technique the water potential-water content relationship was determined in samples of the OF+H layer.
2.2. Extraction of roots In the organic OF and OH layers, sequential coring was carried out roughly every 4 weeks between February and December 199 l, and at intervals of 12 weeks between January and December 1992. Four samples were collected in each of three plots of 2 m X 2 m with a sharp-edged corer of 5 cm diameter, producing 12 cores per sampling date. Coring locations were sited by random coordinates. The plots were situated halfway between the trunks of a beech and an oak tree roughly 4 m apart that were similar in demographic status. Consequently, the distance of a sample to an oak or a beech stem varied from approximately 1 m to 3 m. The cores were sliced into Ov and On horizons, transferred into plastic bags, sealed, transported to the laboratory, and kept at 4°C until processed within 5 days. The flesh litter (OL) was not investigated. Owing to a much lower fine root density, sampling in the
13
mineral soil profile was only conducted once in August 1992. Nine profiles each at a distance of 1 m to an oak or a beech stem were investigated with samples of 1000 cm 3 volume taken at six depths between 0-10 and 50-60 cm. The cores were soaked in water and passed through 2 and 1 mm sieves. Under a binocular, live roots were separated by hand from dead roots and decomposing organic material in the residue. Live and dead rootlets were distinguished by means of the degree of cohesion of cortex and periderm, root elasticity, and colour. More difficult was the separation of Fagus and Quercus fine roots: earlier investigations in pure stands had shown that the darker, reddish-brown colour of the root periderm of beech allows a reliable distinction from the lighter coloured, brown to yellow-brown roots of oak in intact root systems. Root fragments had to be sorted according to the ratio of intact beech and oak fine roots. Root tips of the two species showed a nearly complete infection by mycorrhiza and 'pseudomycorrhiza' forming fungi; data on the mycorrhizal status will be presented elsewhere. Two diameter (d) classes were separated: fine roots less than 1 mm in diameter (i.e. finest roots), and fine roots between 1 and 2 mm. Coarse roots (d> 2 mm) were not analysed. With a Delta-T Dias (Cambridge, UK) image processing unit the projected root surface area (root length (1) × diameter (d)) was determined, allowing the calculation of approximate values of the fine root surface area of a sample ( l × d × g , cm 2 g- ~dry mass). Dry mass ( 105 ° C, 24 h ) and surfaces of the fine roots in the organic layers were expressed in terms of root abundance (root mass or root surface per unit ground surface area, Am and As, respectively) and root density (root mass per humus or soil volume, Dm). In the following, these units are either used for the fine roots of beech or oak, or for the total (i.e. beech plus oak).
2.3. Statistical analysis A rank correlation test after Spearman with a 5% rejection level was used to detect significant correlations between a tree's fine root abun-
14
K Bidttner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
dance in an organic layer sample and its distance from the neighbouring beech or oak stem. Significant relationships could only be detected for less than one-third of the 15 sampling dates in 1991 / 1992, which was believed to be the result of an insufficient number (12) of cores per sampling date. Since the temporal variability in fine root abundance was found to be low in this stand, we treated the cores of four to five successive sampling dates as a collective in correlation analysis, leading to 50-60 samples per period. A non-parametric Mann-Whitney (Wilcoxon) two-sample test was used to determine ( l ) if there were differences in the depth of the organic profile for various stem distances, (2) whether root abundances were different in the OF and On layers, and (3) whether there were differences in the specific root surface area between beech and oak.
UNDER BEECH i ~BEECH OAK i i i"~ BEECH ';
~ I 02
J
I
I
O~
] sn~ 0
0
Live fine roots of both species showed highest densities in the upper, less decomposed Ov horizon of the organic profile with 300 mg per 100 ml (beech plus oak fine root biomass less than 1 mm diameter, i.e. finest roots) and declined sharply downwards in the profile to only 18 mg per 100 ml at 30-60 cm depth in the mineral soil (Fig. 2 ). Oak roots showed a faster decrease from the superficial OF horizon to the lower organic On layer than did beech roots (Table 1, columns 9 and 10, and Table 2). As a consequence, a higher proportion of oak roots than beech roots were concentrated in the uppermost, 5-cm-deep organic layer (35% and 51% of a species' fine root biomass in the Ov horizon for oak and beech, respectively). This coincided with a ratio of beech to oak in fine root biomass of approximately 2:1 (i.e. 68% beech) in the Ov layer but 4:1 or 6:1 ( 76-87% beech ) in the OH layer and the mineral soil (annual mean, see Table 1, columns 6 and 7 ). Over the course of the year, the proportion of beech roots (less than 1 mm) in the total fine root biomass (beech plus oak) varied between
I 120
I
I E:3 1 2jmm I 160
200
I
I
240
UNDER OAK ~
////~/////J
F
I
\',, 13o3o I
02
I
I
01
I \ ~ 0
Density (g-lOOm[4)
3.1. Vertical distribution
I
Abundance (g m-2)
Density {g .lOOmt-~)
I
3. Results
I 80
40
0
I
/~0
I
I
80
I
I
120
160
200
240
Abundance {gm -2)
Fig. 2. Vertical distribution of fine root biomass of beech and oak in the organic profile (Or and OH) and the mineral soil in the direct proximity of beech and oak stems in the mixed stand. Data from the organic profile are annual means of 78 and 96 samples taken at 1-2 m distance from either a beech or an oak stem, data from the mineral soil are the mean of each nine samples taken in August 1992 at 1 m distance from either a beech or an oak stem. Density values refer to fine roots less than 2 mm in diameter.
56% (September 1991 ) and 78% (March 1991 ) in the Ov horizon (Fig. 3), and between 70% (July 1992) and 94% (May 1991) in the On horizon. For both species, fine root biomass and surface area showed a good and, over the sampling period, constant relationship. However, beech roots had a significantly ( P < 0 . 0 1 ) lower specific fine root surface area than oak fine roots: 276 and 355 cm 2 g 1 (less than 1 mm fraction, i.e. finest roots), and 251 and 327 cm 2 g - i (less than 2 mm fraction, i.e. total fine roots) for Fagus and Quercus, respectively (for this analysis, only samples containing a minimum of 10 mg biomass were taken into account). As a result, the dominance of beech over oak was less pronounced for the root surface area than it was for
l~ Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
15
Table 1 Dry mass per unit ground surface area (Am, g m - 2 ) , root surface per unit ground surface area (As, m 2 m 2) and density (Dm, mg per 100 ml) of live fine roots less than 1 m m in diameter in the organic and the mineral soil profiles of a mixed oak ( O ) - b e e c h (B) stand Hor.
Depth (mm)
pH Nt (KCI) ( m o l m 3)
C/N (molmol-1)
H20 A m ( g m 2) Dm(mgperl00ml) (vo1.%) B O BOB O BO
A s ( m Z m 2)
2.89 1.73 4.62 (63) (37) 0.61 0.13 0.74 (82) (18) 3.50 1.86 5.36 (66) (34)
B
O
BO
Organicprofi# Ov
54
3.0
197
28.2
18.1
O,
18
2.6
782
29.8
29.6
OF+H
72
--
--
--
107 (68) 24 (87) 130 (70)
51 (32) 4 (13) 56 (30)
158 198
94
292
27 133
22
155
186
-
-
-
135 (81) 41 (76)
32 (19) 13 (24)
167
45
11
56
54
14
4
18
Mineral soil profile 0-30 cm
3.8
3960
29.6
17.5
30-60 cm
4.2
2083
22.5
6.9
3.73 1.14 4.87 (77) (23) 1.13 0.46 1.59 (71) (29)
For the organic horizons, means are shown of 15 sampling dates between February 1991 and October 1992. The data of the mineral soil are means of 18 samples per depth taken in August 1992. Values in parentheses indicate percentage of total ( b e e c h + o a k (BO)). Hor., horizon; Nt, total nitrogen per volume of a horizon; H20, annual mean of volumetric water content, chemical data are means of 20 samples.
Table 2 Dry mass per unit ground surface area (Am) and root surface per unit ground surface area (As) of live fine roots less than 2 m m in diameter in two organic horizons and the mineral soil profile of a mixed oak ( O ) - b e e c h (B) stand (data of Table 2 include those of Table 1 ). Values in parentheses indicate percentage of total (beech + oak ( B O ) ) . BO < 1, proportion of fine roots less than 1 m m in diameter (see Table 1 ) in the figures of fine roots less than 2 m m in diameter (%) Horizon
Am (g m -2)
BO< 1 (%)
B
O
BO
129 (68) 30 (86) 159 (70)
62 (32) 5 (14) 67 (30)
191
83
35
77
226
82
189 (77) 61 (75)
58 (23) 20 (25)
247
68
81
67
As (m 2 m -2)
BO< 1 (%)
B
O
BO
3.05 (63) 0.67 (82) 3.72 (66)
1.82 (37) 0.15 (18) 1.97 (34)
4.87
95
0.82
90
5.69
94
4.74 (71) 1.53 (70)
1.90 (29) 0.65 (30)
6.64
73
2.18
73
Organicprofile Ov OH Ov+n
Mineral soil profile 0-30 cm 30-60 cm
16
F. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
LOneburger Heide, OB5
1991 / 1992
25- I(Q)
- 80
20
0
o. "-1
° E
60
~
40
~
15 \ lO
0
T,
'^'
,(1, ]"i"i.
5l
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"/
~
,.,.
:"..,
;,,
c 0 o
20
¢ V JF
400
MAMJJA
S O ND
JFM
AMJ
JA
LOneburger Heide, OB5
S O N
1991 / 1992
• , 300Fagus + Quercus 200-
Faous I
"~./
0 100-
,°°°**% % ...°
°o..* .....
FMAMJJAS
-*
...°-
....
Quercus
..........
ONDJFMAMJJAS
° ......
ON
Fig. 3. (a) Seasonal course of temperature ( To, in the superficial organic layer (OL); Ts, at 2.5 cm depth in the mineral soil ) and water content of the organic layer (Or+n) 0 in the mixed oak-beech stand. (b) Fine root standing crop (less than 2 m m in diameter) of beech and oak in the entire organic layer (OF+n) from January 1991 to November 1992 (in dry mass per area ground surface). Each point is the mean of 12 samples. Notice the lower sampling frequency in 1992.
biomass (Table 1, columns 12 and 13, Table 2, Fig. 4).
3.2. Horizontal distribution According to the results of the correlation analysis, fine root biomass (less than 1 mm fraction) in the OF layer was not uniformly distributed in the horizontal direction in this mixed stand. For beech roots, a strong positive correlation was found between biomass and distance from a beech stem, i.e. a higher abundance of beech roots at a greater distance from a Fagus
stem (Table 3). As a result, beech roots were more abundant at 1 m than at 3 m distance from an oak stem. On the contrary, oak roots showed no change in abundance except for the winter interval with a significant positive correlation between biomass and oak stem distance. Total fine root biomass (beech plus oak) showed a highly significant negative correlation to the distance from an oak stem in all three seasons investigated, i.e. root abundance in the organic layer was higher in the close vicinity of the oaks than elsewhere (Table 3). This coincides well with the higher abundance of beech roots
K
Biittner, C. Leuschner/Forest Ecology and Management 70 (1994) 11-21
6 LOneburger Heide, OB5
17
1991 / 1992
4 c~ o3
A
Fagus
3 o3 o*'" "
2
;..
@
o" "" " ' "
Quercus
"" ""
g3
0 o
1
J F M A M J J A S
O N D J F M A M J J A S
ON
Fig. 4. Surface area of beech and oak fine roots (less than 2 mm in diameter) in the OF+. horizon of the mixed oak-beech stand from February 1991 to October 1992 (root surface area per unit ground area).
Table 3 Summary of results of a rank correlation analysis after Spearman to examine the relationship between the distance from a beech or an oak stem (dB and do, respectively, in cm), and the fine root (less than 1 mm diameter) dry mass of beech, oak or both species in the organic OF layer per unit ground surface area (AmB, Amo and AroOB,respectively, in g m-Z), or the relative contribution of beech roots to fine root dry mass totals in the Ov layer (PmB). For testing, all samples collected either between February and May, June and September, or October and January, 1991 / 1992, were treated as collectives with n elements Characteristic
Significance level
r
n
Total root biomass and oak stem distance do-A moB (Feb.-May) do-AmoB(June-Sept.) do-AmoB(Oct.-Jan.)
0.05 0.01 0.01
-0.32 -0.37 -0.42
54 60 60
Beech root biomass and beech stern distance dB-AmB (Feb.-May) dB-AmB (June-Sept.) dB-AmB (Oct.-Jan.)
0.001 0.001 0.001
0.48 0.44 0.52
54 60 60
Oak root biomass and oak stem distance do-Amo (Feb.-May) do-Amo (June-Sept.) do-Amo (Oct.-Jan.)
NS NS 0.05
0.01 0.21 0.28
54 60 60
0.40 0.42 0.44
54 60 60
Percental quota of beech roots and beech stem distance dB-Pr,B (Feb.-May) 0.01 dB-PmB (June-Sept.) 0.001 dB-Pm8 (Oct.-Jan.) 0.001 NS, not significant at P < 0.05.
18
V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
near oak stems, as mentioned above. Thus, total fine root biomass in the vicinity of oak stems was not higher because of a greater abundance of oak roots close to their (supposed) parent stem but was the result of a higher frequency of beech roots here. As a consequence, both the relative proportion of beech roots in the total and the total fine root biomass increased when approaching an oak stem.
3.3. Seasonal fluctuations The seasonal variability of fine root biomass was not very pronounced in the organic layers (OF+H) during the sampling period. In the intensively studied year 1991, total fine root biomass increased from its seasonal minimum in February to the maximum in August by 50%. For beech, a small peak occurred in April 1991, and the annual maximum was measured in August (210 g dry mass m-2, less than 2 m m fraction), while oak fine root biomass peaked in September 1991 ( 112 g m -2, Fig. 3(b) ). Seasonal minima were observed in February 1991 for beech (125 g m -2) and in April 1991 for oak (38 g m-2). A comparison of the figures for 1991 and 1992 showed no marked difference in the average fine root biomass of the OF+H layer.
4. Discussion
Our data show that two co-existing tree species can clearly differ with respect to root distribution in space and time in a shared soil volume although the root systems of both species overlap completely. This is consistent with the findings of Mikola et al. (1966) and McQueen (1968) who reported a vertical stratification of the fine roots of two co-existing tree species in the mineral soil under mixed Pinus-Picea and PinusFagus stands. In the acidic soil of the Lfineburger Heide stand, however, differences between the species were mainly restricted to the forest floor. Many studies on root distribution in forests have reported a concentration of fine root biomass and mycorrhizas in the superficial organic
layers and a concomitant exponential decline with increasing depth in the mineral soil of acidic profiles (e.g. G~ttsche, 1972; Kimmins and Hawkes, 1978; Deans, 1979; Vogt et al., !981 ). In the Liineburger Heide stand, fine root density declined by a factor of six from the organic layer to the mineral soil (organic Ov layer, 292 mg; 030 cm depth in the mineral soil, 56 mg total fine root biomass per 100 ml, Table 1 ), but, in addition, showed a significant decrease downwards within the organic profile itself. This sharp vertical decline was more pronounced for the oak fine root system than for beech and corresponded to an increasing mean root diameter with depth (see Table 2: proportion of fine roots less than 1 mm in biomass less than 2 ram). Thus, beech roots had a higher dominance in the lower OH horizon than in the Ov layer. The extremely superficial rooting patterns at the Lfineburger Heide site raises the question as to the impact of drought on the fine root systems of Quercus but also Fagus here. The moisture conditions of the organic layer apparently did not negatively influence the abundance of fine roots during the 24 months investigated. ( 1 ) In 1991, annual fine root biomass peaked in periods of lowest summer water content of the organic layer in August/September (Fig. 3 ( a ) ) when soil water matric potentials less than - 1.0 MPa were reached (G. G6rlitz, unpublished data, 1993). (2) Annual means and seasonal minima of the organic layer water content (Ch. Leuschner, unpublished data, 1994) were less favourable in the Ov layer with highest root densities than in the lower On horizon (Table 1, column 5), i.e. a negative correlation between moisture and root density existed. From the more superficial rooting patterns, it appears that the oak fine root system could be less susceptible to the influence of drought than that of beech. In the mineral soil however, we found no increase in the proportion of oak roots at a greater depth of the profile (Fig. 2). This is contrary to the observations of other authors (e.g. Abrams, 1990) who reported a deeper rooting of oak species than surrounding trees of other species. The rooting patterns at the Lfineburger Heide site could be a consequence of the highly
V. Biittno. C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
acidic soil profile which forces the trees to concentrate fine root growth in the nutrient-rich organic horizons. Furthermore, deep rooting as observed in other studies could be more pronounced in the coarse than in the fine root system of a species. Attempts to correlate tree root activity to soil water availability have yielded controversial results (Hoffmann, 1972; Santantonio and Hermann, 1985). Reports on a negative impact of water shortage on tree root biomass, root growth or root tip numbers (e.g. Kalela, 1955; Persson, 1980) contrast with studies that found minor or no reduction of root activity or biomass (e.g. Roberts, 1976; Vogt et al., 1981; Van Praag et al., 1988) or a stimulation of fine root and mycorrhiza growth during or following moderate water stress (Kausch, 1955; Teskey and Hinckley, 1981; Feil et al., 1988). Moreover, whether root growth is affected by soil moisture or not seems not to be dependent on the climatic regime as was suggested by Van Praag et al. (1988): Deans (1979) and G6ttsche (1972) observed negative drought effects even in stands under highly humid climates in Dumfries (southwest Scotland) and the Solling mountains (central Germany). In contrast, the Lfineburger Heide site with no clear negative drought influence on root abundance, has a much more continental climate with periods of low rainfall in summer which may show up in reversible depressions of the photosynthetic capacity of Fagus and other tree species (but not Quercus, see Greve and Terborg, 1993). We found significant gradients in root abundance not only in the vertical but also in the horizontal direction within the organic profile of this stand. It is well known that tree roots can extend considerably beyond the width of the crown, resulting in a certain degree of intermingling of the root systems of neighbouring trees (Grosskopf, 1950; Hermann, 1977; Persson, 1980; Caldwell, 1987). In only a small number of forest stands, however, was root biomass found to be distributed homogeneously between the trunks (e.g. Grosskopf, 1950; Moir and Bachelard, 1969; Kohman, 1972). The majority of studies reported a decrease of root abundance with rising
19
stem distance which points to stem-centred horizontal distribution patterns in many forests (e.g. Roberts, 1976; Ford and Deans, 1977; Persson, 1980). Our results from a more than 100-yearold mixed stand show that fine roots of the two species present, Fagus and Quercus, completely penetrate the organic layer between the trunks and also intermingle thoroughly, for all of 174 sampled cores in the organic profile had at least an equivalent of 35 g fine root biomass per square metre, and only seven cores contained no oak, and two cores no beech roots at all. Furthermore, a tree's fine root abundance apparently did not markedly decrease within a radius of roughly 34 m around the stem in this mixed stand as was reported from other stands. On the contrary, total fine root abundance differed in the horizontal between the three pairs of beech and oak trees with a significant increase towards oak stems that is attributable to an increase in beech (and not oak) roots. Therefore, it is likely that neighbouring fine root systems overlap to a high degree in this old-growth stand, irrespective of the species, and might even share soil space with stems several trees away. Although we did not measure the thickness of the organic layer in individual root sampling cores, the coincidence of a thicker organic layer (which probably is a result of local litter accumulation) and a higher root biomass in the vicinity of the three oak stems studied is obvious from the data in Tables 3 and 4. Since both the absolute and the relative abundance of beech fine roots were positively correlated with the total Table 4 Depth of the organic layer (OF+H) at 1 and 2 m distance from either a beech or an oak stem (averages of 32 measurements each). Significant differences were detected with a Wilcoxon test. The least significant difference between beech and oak profiles at 1 or 2 m is 9 mm in both distances Location
Depth of Or+ H layer (mm)
Beech (1 m) Beech (2 m ) Oak (2 m) Oak (1 m)
59 61 76 91
20
E Biittnep, C. Leuschner /Forest Ecology and Management 70 (1994) 11-21
(beech plus oak) fine root biomass in the samples (Table 3 ), it appears that the beech root system profited by a higher organic profile thickness while the oak root system did not. Seasonal fluctuations in fine root abundance in the organic layer were not very conspicuous for either Fagus or Quercusin 1991 with a single peak at the study site in August/September (in 1992, fluctuations could not be detected owing to a lower sampling frequency). This compares well with the results of Rapp (1991) and Van Praag et al. (1988) in monospecific beech forests of the Solling and the Ardennes (Belgium). In the two-species stand in the Ltineburger Heide, maxima in the fine root standing crop (or surface area) of the two species were not concurrent as is indicated in Fig. 4. One possible explanation is that oak root growth started later in spring than did that of beech, leading to a biomass peak later in summer similar to the above-ground phenological development of the two species. Whether interspecific competition between the root systems has a significant influence on these fluctuations, must remain open. Abundance and density of fine roots in the organic layers of the mixed oak-beech stand studied were not higher than in similar horizons of a monospecific beech stand on acidic soils in the Solling mountains (Rapp, 1991 ). A density of roughly 300 mg root dry mass ( d < 1 m m ) per 100 ml humus volume (Or layer) and an abundance of 150-190 g m -2 in the entire organic layer (Or+n) occurred in both the pure and the mixed stand under similar edaphic and demographic conditions. Whether these figures reflect a situation of maximum root occupancy in the crowded forest floor which restricts additional root growth (cf. Vogt et al., 1981 ) deserves further investigation. The percental ratio of beech and oak fine root biomass in the total figures of the profile (75:25) were roughly equivalent to the relative abundance of Fagus and Quercus stems (about 3:1 ) in the stand. Oak fine roots, however, seem to be under-represented in the forest floor as well as in the mineral soil under oak stems when taking into account the much larger canopy dimensions of the older oaks which result in similar canopy
projection areas of Fagus and Quercus in this stand. Remarkably, this finding coincides with the fact that oak will be outcompeted by beech in the stand in the long run (cf. Leuschner, 1994).
Acknowledgements We thank Dirk Gansert for helpful comments on the manuscript.
References Abrams, M.D., 1990. Adaptations and responses to drought in Quercus species of North America. Tree Physiol., 7: 227-238. Caldwell, M.M., 1987 Plant architecture and resource competition. In: E.-D. Schulze and H. Zw61fer (Editors), Potentials and Limitations of Ecosystem Analysis. Ecological Studies 61, Springer, Berlin. pp. 164-179. Davis, S.D. and Mooney, H.A., 1986: Water use patterns of four co-occurring chaparral shrubs. Oecologia, 70:172177. Deans, J.D., 1979. Fluctuations of the soil environment and fine root growth in a young Sitka spruce plantation. Plant Soil, 52: 195-208. Ehwald, E., Grunert, F., Schulz, W. and Vetterlein, E., 1961. Zur Okologie von Kiefern-Buchen-Mischbest~inden. Archiv Forstwes., 10: 397-416. Feil, W., Kottke, I. and Oberwinkler, F., 1988. The effect of drought on mycorrhizal growth and very fine root systems ofPicea abies (L.) Karst. under natural and experimental conditions. Plant Soil, 108:211-231. Ford, E.D. and Deans, J.D., 1977. Growth of a Sitka spruce plantation: spatial distribution and seasonal fluctuations of lengths, weights and carbohydrate concentrations of fine roots. Plant Soil, 47: 463-485. G6ttsche, D., 1972. Die Verteilung von Feinwurzeln und Mykorrhizen im Bodenprofil eines Buchen- und Ficbtenbestandes im Solling. Mitt. Bundesforsch.anst.f.Forst-u. Holzwirtsch. (Reinbek), 88: 1-120. Greve, A. and Terborg, O., 1993. Carbon assimilation of two early and two late-successional tree species in a heathland-forest succession in NW Germany. Scripta Geobot. (G/Sttingen), 21: 101-104. Grosskopf, W., 1950. Bestimmung der charakteristischen Feinwurzelintensit~iten in ungiinstigen Waldbodenprofilen und ihre ~kologische Auswertung. Mitt. d. Bundesanst, f. Forst- u. Holzwirtsch., No. 11. Grubb, P.J., 1994. Root competition in soils of different fer-
Id Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21 tility: a paradox resolved?. Phytocoenologia, 24: 495-505. Hermann, R.K., 1977. Growth and production of tree roots: A review. In: J.K. Marshall (Editor), The Belowground Ecosystem: A Synthesis of Plant-associated Processes. Range Sci. Dep. Sci. Ser. 26, Colorado State University, Fort Collins, CO, pp. 7-28. Hoffmann, G., 1972. Wachstumsrhythmik der Wurzeln und Sprossachsen von Forstgeh~51zen. Flora, 161:303-319. Holstener-J~Srgensen, H., 1959. Investigations of root systems of oak, beech and Norway spruce on groundwater affected moraine soils with a contribution to elucidation of evapotranspiration of stands. Forstl. Fors6gsv. Danmark, 25: 227-289. Kalela, E.K., 1955. Uber Ver~inderungen in den Wurzelverh~iltnissen der Kiefernbest~inde im Laufe der Vegetationsperiode. Acta For. Fenn., 65: 1-42. Kalisz, P.J., Zimmerman, R.W. and Muller, R.N., 1987. Root density, abundance, and distribution in the mixed mesophytic forest of Eastern Kentucky. Soil Sci. Soc. Am. J., 51: 220-225. Kausch, W., 1955. Saugkraft und Wassernachleitung im Boden als physiologische Faktoren. Planta, 45: 217-263. Kern, K.G., Moll, W. and Braun, H.J., 1961. Wurzeluntersuchungen in Rein- und Mischbest~inden des Hochschwarzwaldes (Vfl. Todtmoos 2/I-IV). Allg. Forst Jagdztg., 32: 241-259. Kimmins, J.P. and Hawkes, B.C., 1978. Distribution and chemistry of fine roots in a white spruce-subalpine fir stand in British Columbia: implications for management. Can. J. For. Res., 8: 265-279. Kochendorfer, J.N., 1973. Root distribution under some forest types native to West Virginia. Ecology, 54: 445-448. Kohman, K., 1972. Root-ecological investigations on pine (Pinus sylvestris). I. Problems of methodology and general relationships. Det Norske Skogsfors~Sksvesen (As), 30: 325-357. (In Norwegian, English summary. ) Leuschner, Ch., 1994. Walddynamik auf Sandb6den in der Lfineburger Heide, (NW-Deutsehland). Phytocoenologia, 22: 289-324. Leuschner, Ch., Rode, M.W., Danner, E., Ltibbe, K., Clauss, C., Margraf, S. and Runge, M., 1993. Soil profile alteration and humus accumulation during heathland-forest succession in NW Germany. Scripta Geobot. (G6ttingen), 21: 73-84. McQueen, D.R., 1968. The quantitative distribution of absorbing roots of Pinus sylvestris and Fagus sylvatica in a forest succession. Oecol. Plant., 3: 83-89.
21
Meyer, F.H., 1967. Feinwurzelverteilung bei Waldb~iumen in Abh~ingigkeit vom Substrat. Forstarchiv, 38: 286-290. Mikola, P., Hahl, S. and Torniainen, E., 1966. Vertical distribution of mycorrhizae in pine forests with spruce undergrowth. Ann. Bot. Fenn., 3: 406-409. Moir, W.H. and Bachelard, E.P., 1969. Distribution of fine roots in three Pinus radiata plantations near Canberra. Ecology, 50: 658-662. Persson, H., 1980. Spatial distribution of fine-root growth, mortality and decomposition in a young Scots pine stand in Central Sweden. Oikos, 34: 77-87. Rachtejenko, I.N., 1952. The root systems of tree and shrub species. Goslesbumizdat Moskve-Leningrad. (In Russian, German translation in KiSstler et aL, 1968). Rapp, Ch., 1991. Untersuchungen zum Einfluss von Kalkung und Ammoniumsulfat-Dtingung auf Feinwurzeln und Ektomykorrhizen eines Buchenaltbestandes im Solling. Ber. Forsch.zentr. Wald/Skosysteme, Univ. G6ttingen, Bd. A72, 293 pp. Roberts, J., 1976. A study of root distribution and growth in a Pinus sylvestris L. (Scots pine) plantation in East Angila. Plant Soil, 44: 607-621. Rode, M.W., Leuschner, Ch., Clauss, C., Danner, E., Gerdelmann, V., Margraf, S. and Runge, M., 1993. Changes in nutrient availability and nutrient turnover during heathland-forest succession in NW Germany. Scripta Geobot. (GiSttingen), 21: 85-96. Santantonio, D. and Hermann, R.K., 1985. Standing crop, production, and turnover of fine roots on dry, moderate, and wet sites of mature Douglas-fir in western Oregon. Ann. Sci. For., 42:113-142. Teskey, R.O. and Hinckley, T.M., 1981. Influence of temperature and water potential on root growth of white oak. Physiol. Plant., 52: 363-369. Van Praag, H.J., Sougnez-Remy, S., Weissen, F. and Carletti, G., 1988. Root turnover in a beech and a spruce stand of the Belgian Ardennes. Plant Soil, 105: 87-103. Veresoglou, D.S. and Fitter, A.H., 1984. Spatial and temporal patterns of growth and nutrient uptake of five coexisting grasses. J. Ecol., 72: 259-272. Vogt, K.A., Edmonds, R.L. and Grier, C.C., 1981. Seasonal changes in biomass and vertical distribution of mycorrhizal and fibrous-textured conifer fine roots in 23- and 180year-old subalpine Abies amabilis stands. Can. J. For. Res., 11: 223-229.
Forest Ecology and Management Forest Ecology and Management 70 (1994) 11-21
Spatial and temporal patterns of fine root abundance in a mixed oak-beech forest Volker B/ittner, Christoph Leuschner* LehrstuhlJ~r Geobotanik, Universityof Gfttingen, UntereKarspfile 2, D-37073 G6ttingen, Germany Accepted 7 July 1994
Abstract Spatial distribution and seasonal fluctuation of fine root density (mass per unit soil volume) and abundance (mass or surface area per unit ground surface area) were investigated by the sequential coring technique in a 100220 year old mixed Fagus sylvatica-Quercus petraea stand on acidic sandy soil in northwest Germany. The fine root systems of the two co-existing species overlapped completely with beech roots being twice as abundant as oak roots. Since Fagus and Quercus occupied equivalent parts of the canopy volume, oak appeared to be under-represented in the below-ground space. There was evidence for some degree of below-ground niche partitioning between the species in both the vertical and the horizontal direction. Oak fine roots were found to be more superficially distributed than beech roots in the organic layers, indicating a vertical stratification of the root systems of the two species. In the forest floor, fine roots were more abundant in the vicinity of oak stems where thicker organic layers occurred. However, this distribution pattern was not a consequence of a greater abundance of oak roots close to their parent stem, but was due to a higher frequency of beech roots here.
Keywords: Fine roots; Fagus sylvatica; Mixed stand; Quercuspetraea; Spatial distribution; Species differences
1. Introduction
Fine roots represent a functionally important part of the biomass of forest ecosystems. In many stands on acidic soils, the majority of active fine roots is concentrated in the uppermost soil horizons including the organic layers (Meyer, 1967; Persson, 1980). Here, fine roots are very densely packed and strong competition for water and nutrients may be expected among mycorrhizas and rootlets of neighbouring tree individuals. This was evidenced by various trenching and plant removal experiments (cf. Grubb, 1994). To re* Corresponding author.
duce exploitation competition, spatial and/or temporal separation of the root systems and their activities is likely to occur in multi-species communities. For example, Veresoglou and Fitter ( 1984 ) and Davis and Mooney ( 1986 ) reported differences in the location and timing of water and nutrient uptake by root systems of co-existing species in grassland and chaparral communities. Much less is known about the vertical and horizontal separation of root systems in mixed forest stands (e.g. Rachtejenko, 1952; Ehwald et al., 1961; Kern et al., 1961; McQueen, 1968; Kalisz et al., 1987). It is well known that, on similar substrates, tree species may differ markedly with
0378-1127/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
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12
V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
respect to both the extension of their coarse root system (e.g. Holstener-J/Srgensen, 1959; Ehwald et al., 1961; Abrams, 1990) and the abundance and the productivity of their fine roots (e.g. Kern et al., 1961; McQueen, 1968; Kochendorfer, 1973; Kalisz et al., 1987; Van Praag et al., 1988 ). However, most of these studies were conducted in monospecific tree stands where intra- but not interspecific competition can be expected. This paper reports on a study that examined the horizontal and vertical distribution, and the seasonal fluctuation of fine root abundance in a mixed old-growth oak-beech stand in northwest Germany. From canopy shape and forestry practice it is obvious that beech is the superior competitor for above-ground space in this stand (Leuschner, 1994) while the partitioning of below-ground space between the species is unknown. The aims of our studies were (1) to characterize possible species differences in the vertical and horizontal distribution of fine roots in a shared soil volume, and (2) to compare the seasonality of coexistent beech and oak fine roots. The study is part of an investigation into belowground co-existence in three successional communities on nutrient-poor, acidic soils in the region: a Calluna vulgaris heathland, an early successional birch-pine forest, and a late-successional oak-beech forest.
2. Materials and methods
2.1. Study site and environmental measurements A mixed stand ofFagus sylvatica L. ( 100-120 years of age ) and Quercuspetraea (Matt.) Liebl. (190-220 years of age) only minimally influenced by former forestry practice was selected in the neighbourhood of Unterlfiss (southern part of the Liineburger Heide, Lower Saxony, Germany, 52°45'N, 10°30'E). The ratio between the numbers of mature Fagus and Quercus trees is approximately 3:1 in the immediate vicinity of the sample plots (total stem density: 530 h a - l ) . Owing to the much larger canopy dimensions of the oaks, however, both species have comparable canopy projection areas in the stand (Fig. 1 ).
Soit pit
•
Beech
~Ook
Fig. 1. Location of the three sample plots (squares of 2 m × 2 m ) in the mixed oak-beech stand. Beech stems, black dots; beech canopy, white area; oak stems, open circles; oak canopy, dotted area.
There is no herbaceous layer. Situated in the diluvial lowlands of northwest Germany on Saalian melt water sands ( 115 m above sea level), the plot has a soil profile (spodo-dystric cambisol) with a thick organic layer (mean depth of the entire organic profile (OF + H layers ) 72 mm ) and a nutrient poor, highly acidic mineral soil (Leuschner et al., 1993; Rode et al., 1993). The organic layer is characterized by pH values of 3.0 and 2.6 (in 1 M KC1) and Ca2+/H + quotients of 0.2 in the equilibrium soil solution for the upper OF and the lower ON horizon, respectively. The climate is of a humid suboceanic type (an-
V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
nual means 8.0°C, 730 mm). Periods of low rainfall in summer may lead to water shortage in the soil as in August/September 1991 and in June/July 1992. The thickness of the organic layers was determined in 32 organic profiles each at 1 and 2 m distance from a beech or an oak stem in order to detect local differences in horizon depth. Care was taken not to compress the forest floor. These measurements were done in close vicinity to the root sampling plots. The temperature of the soil organic layer was monitored at intervals of 10 s with a thermistor placed at the upper edge of the OF horizon (i.e. roughly 2 cm below the litter surface ) and at 2.5 cm depth in the mineral soil profile. Sixteen cores, each 5 cm in diameter, were sampled at intervals of 7 days (during the summer period) to 14 days (during winter) in the organic layers for the gravimetric determination of water content. The depth of the OF and OH horizons in the samples was used to calculate volumetric water contents. With the pressure membrane technique the water potential-water content relationship was determined in samples of the OF+H layer.
2.2. Extraction of roots In the organic OF and OH layers, sequential coring was carried out roughly every 4 weeks between February and December 199 l, and at intervals of 12 weeks between January and December 1992. Four samples were collected in each of three plots of 2 m X 2 m with a sharp-edged corer of 5 cm diameter, producing 12 cores per sampling date. Coring locations were sited by random coordinates. The plots were situated halfway between the trunks of a beech and an oak tree roughly 4 m apart that were similar in demographic status. Consequently, the distance of a sample to an oak or a beech stem varied from approximately 1 m to 3 m. The cores were sliced into Ov and On horizons, transferred into plastic bags, sealed, transported to the laboratory, and kept at 4°C until processed within 5 days. The flesh litter (OL) was not investigated. Owing to a much lower fine root density, sampling in the
13
mineral soil profile was only conducted once in August 1992. Nine profiles each at a distance of 1 m to an oak or a beech stem were investigated with samples of 1000 cm 3 volume taken at six depths between 0-10 and 50-60 cm. The cores were soaked in water and passed through 2 and 1 mm sieves. Under a binocular, live roots were separated by hand from dead roots and decomposing organic material in the residue. Live and dead rootlets were distinguished by means of the degree of cohesion of cortex and periderm, root elasticity, and colour. More difficult was the separation of Fagus and Quercus fine roots: earlier investigations in pure stands had shown that the darker, reddish-brown colour of the root periderm of beech allows a reliable distinction from the lighter coloured, brown to yellow-brown roots of oak in intact root systems. Root fragments had to be sorted according to the ratio of intact beech and oak fine roots. Root tips of the two species showed a nearly complete infection by mycorrhiza and 'pseudomycorrhiza' forming fungi; data on the mycorrhizal status will be presented elsewhere. Two diameter (d) classes were separated: fine roots less than 1 mm in diameter (i.e. finest roots), and fine roots between 1 and 2 mm. Coarse roots (d> 2 mm) were not analysed. With a Delta-T Dias (Cambridge, UK) image processing unit the projected root surface area (root length (1) × diameter (d)) was determined, allowing the calculation of approximate values of the fine root surface area of a sample ( l × d × g , cm 2 g- ~dry mass). Dry mass ( 105 ° C, 24 h ) and surfaces of the fine roots in the organic layers were expressed in terms of root abundance (root mass or root surface per unit ground surface area, Am and As, respectively) and root density (root mass per humus or soil volume, Dm). In the following, these units are either used for the fine roots of beech or oak, or for the total (i.e. beech plus oak).
2.3. Statistical analysis A rank correlation test after Spearman with a 5% rejection level was used to detect significant correlations between a tree's fine root abun-
14
K Bidttner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
dance in an organic layer sample and its distance from the neighbouring beech or oak stem. Significant relationships could only be detected for less than one-third of the 15 sampling dates in 1991 / 1992, which was believed to be the result of an insufficient number (12) of cores per sampling date. Since the temporal variability in fine root abundance was found to be low in this stand, we treated the cores of four to five successive sampling dates as a collective in correlation analysis, leading to 50-60 samples per period. A non-parametric Mann-Whitney (Wilcoxon) two-sample test was used to determine ( l ) if there were differences in the depth of the organic profile for various stem distances, (2) whether root abundances were different in the OF and On layers, and (3) whether there were differences in the specific root surface area between beech and oak.
UNDER BEECH i ~BEECH OAK i i i"~ BEECH ';
~ I 02
J
I
I
O~
] sn~ 0
0
Live fine roots of both species showed highest densities in the upper, less decomposed Ov horizon of the organic profile with 300 mg per 100 ml (beech plus oak fine root biomass less than 1 mm diameter, i.e. finest roots) and declined sharply downwards in the profile to only 18 mg per 100 ml at 30-60 cm depth in the mineral soil (Fig. 2 ). Oak roots showed a faster decrease from the superficial OF horizon to the lower organic On layer than did beech roots (Table 1, columns 9 and 10, and Table 2). As a consequence, a higher proportion of oak roots than beech roots were concentrated in the uppermost, 5-cm-deep organic layer (35% and 51% of a species' fine root biomass in the Ov horizon for oak and beech, respectively). This coincided with a ratio of beech to oak in fine root biomass of approximately 2:1 (i.e. 68% beech) in the Ov layer but 4:1 or 6:1 ( 76-87% beech ) in the OH layer and the mineral soil (annual mean, see Table 1, columns 6 and 7 ). Over the course of the year, the proportion of beech roots (less than 1 mm) in the total fine root biomass (beech plus oak) varied between
I 120
I
I E:3 1 2jmm I 160
200
I
I
240
UNDER OAK ~
////~/////J
F
I
\',, 13o3o I
02
I
I
01
I \ ~ 0
Density (g-lOOm[4)
3.1. Vertical distribution
I
Abundance (g m-2)
Density {g .lOOmt-~)
I
3. Results
I 80
40
0
I
/~0
I
I
80
I
I
120
160
200
240
Abundance {gm -2)
Fig. 2. Vertical distribution of fine root biomass of beech and oak in the organic profile (Or and OH) and the mineral soil in the direct proximity of beech and oak stems in the mixed stand. Data from the organic profile are annual means of 78 and 96 samples taken at 1-2 m distance from either a beech or an oak stem, data from the mineral soil are the mean of each nine samples taken in August 1992 at 1 m distance from either a beech or an oak stem. Density values refer to fine roots less than 2 mm in diameter.
56% (September 1991 ) and 78% (March 1991 ) in the Ov horizon (Fig. 3), and between 70% (July 1992) and 94% (May 1991) in the On horizon. For both species, fine root biomass and surface area showed a good and, over the sampling period, constant relationship. However, beech roots had a significantly ( P < 0 . 0 1 ) lower specific fine root surface area than oak fine roots: 276 and 355 cm 2 g 1 (less than 1 mm fraction, i.e. finest roots), and 251 and 327 cm 2 g - i (less than 2 mm fraction, i.e. total fine roots) for Fagus and Quercus, respectively (for this analysis, only samples containing a minimum of 10 mg biomass were taken into account). As a result, the dominance of beech over oak was less pronounced for the root surface area than it was for
l~ Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
15
Table 1 Dry mass per unit ground surface area (Am, g m - 2 ) , root surface per unit ground surface area (As, m 2 m 2) and density (Dm, mg per 100 ml) of live fine roots less than 1 m m in diameter in the organic and the mineral soil profiles of a mixed oak ( O ) - b e e c h (B) stand Hor.
Depth (mm)
pH Nt (KCI) ( m o l m 3)
C/N (molmol-1)
H20 A m ( g m 2) Dm(mgperl00ml) (vo1.%) B O BOB O BO
A s ( m Z m 2)
2.89 1.73 4.62 (63) (37) 0.61 0.13 0.74 (82) (18) 3.50 1.86 5.36 (66) (34)
B
O
BO
Organicprofi# Ov
54
3.0
197
28.2
18.1
O,
18
2.6
782
29.8
29.6
OF+H
72
--
--
--
107 (68) 24 (87) 130 (70)
51 (32) 4 (13) 56 (30)
158 198
94
292
27 133
22
155
186
-
-
-
135 (81) 41 (76)
32 (19) 13 (24)
167
45
11
56
54
14
4
18
Mineral soil profile 0-30 cm
3.8
3960
29.6
17.5
30-60 cm
4.2
2083
22.5
6.9
3.73 1.14 4.87 (77) (23) 1.13 0.46 1.59 (71) (29)
For the organic horizons, means are shown of 15 sampling dates between February 1991 and October 1992. The data of the mineral soil are means of 18 samples per depth taken in August 1992. Values in parentheses indicate percentage of total ( b e e c h + o a k (BO)). Hor., horizon; Nt, total nitrogen per volume of a horizon; H20, annual mean of volumetric water content, chemical data are means of 20 samples.
Table 2 Dry mass per unit ground surface area (Am) and root surface per unit ground surface area (As) of live fine roots less than 2 m m in diameter in two organic horizons and the mineral soil profile of a mixed oak ( O ) - b e e c h (B) stand (data of Table 2 include those of Table 1 ). Values in parentheses indicate percentage of total (beech + oak ( B O ) ) . BO < 1, proportion of fine roots less than 1 m m in diameter (see Table 1 ) in the figures of fine roots less than 2 m m in diameter (%) Horizon
Am (g m -2)
BO< 1 (%)
B
O
BO
129 (68) 30 (86) 159 (70)
62 (32) 5 (14) 67 (30)
191
83
35
77
226
82
189 (77) 61 (75)
58 (23) 20 (25)
247
68
81
67
As (m 2 m -2)
BO< 1 (%)
B
O
BO
3.05 (63) 0.67 (82) 3.72 (66)
1.82 (37) 0.15 (18) 1.97 (34)
4.87
95
0.82
90
5.69
94
4.74 (71) 1.53 (70)
1.90 (29) 0.65 (30)
6.64
73
2.18
73
Organicprofile Ov OH Ov+n
Mineral soil profile 0-30 cm 30-60 cm
16
F. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
LOneburger Heide, OB5
1991 / 1992
25- I(Q)
- 80
20
0
o. "-1
° E
60
~
40
~
15 \ lO
0
T,
'^'
,(1, ]"i"i.
5l
"
"/
~
,.,.
:"..,
;,,
c 0 o
20
¢ V JF
400
MAMJJA
S O ND
JFM
AMJ
JA
LOneburger Heide, OB5
S O N
1991 / 1992
• , 300Fagus + Quercus 200-
Faous I
"~./
0 100-
,°°°**% % ...°
°o..* .....
FMAMJJAS
-*
...°-
....
Quercus
..........
ONDJFMAMJJAS
° ......
ON
Fig. 3. (a) Seasonal course of temperature ( To, in the superficial organic layer (OL); Ts, at 2.5 cm depth in the mineral soil ) and water content of the organic layer (Or+n) 0 in the mixed oak-beech stand. (b) Fine root standing crop (less than 2 m m in diameter) of beech and oak in the entire organic layer (OF+n) from January 1991 to November 1992 (in dry mass per area ground surface). Each point is the mean of 12 samples. Notice the lower sampling frequency in 1992.
biomass (Table 1, columns 12 and 13, Table 2, Fig. 4).
3.2. Horizontal distribution According to the results of the correlation analysis, fine root biomass (less than 1 mm fraction) in the OF layer was not uniformly distributed in the horizontal direction in this mixed stand. For beech roots, a strong positive correlation was found between biomass and distance from a beech stem, i.e. a higher abundance of beech roots at a greater distance from a Fagus
stem (Table 3). As a result, beech roots were more abundant at 1 m than at 3 m distance from an oak stem. On the contrary, oak roots showed no change in abundance except for the winter interval with a significant positive correlation between biomass and oak stem distance. Total fine root biomass (beech plus oak) showed a highly significant negative correlation to the distance from an oak stem in all three seasons investigated, i.e. root abundance in the organic layer was higher in the close vicinity of the oaks than elsewhere (Table 3). This coincides well with the higher abundance of beech roots
K
Biittner, C. Leuschner/Forest Ecology and Management 70 (1994) 11-21
6 LOneburger Heide, OB5
17
1991 / 1992
4 c~ o3
A
Fagus
3 o3 o*'" "
2
;..
@
o" "" " ' "
Quercus
"" ""
g3
0 o
1
J F M A M J J A S
O N D J F M A M J J A S
ON
Fig. 4. Surface area of beech and oak fine roots (less than 2 mm in diameter) in the OF+. horizon of the mixed oak-beech stand from February 1991 to October 1992 (root surface area per unit ground area).
Table 3 Summary of results of a rank correlation analysis after Spearman to examine the relationship between the distance from a beech or an oak stem (dB and do, respectively, in cm), and the fine root (less than 1 mm diameter) dry mass of beech, oak or both species in the organic OF layer per unit ground surface area (AmB, Amo and AroOB,respectively, in g m-Z), or the relative contribution of beech roots to fine root dry mass totals in the Ov layer (PmB). For testing, all samples collected either between February and May, June and September, or October and January, 1991 / 1992, were treated as collectives with n elements Characteristic
Significance level
r
n
Total root biomass and oak stem distance do-A moB (Feb.-May) do-AmoB(June-Sept.) do-AmoB(Oct.-Jan.)
0.05 0.01 0.01
-0.32 -0.37 -0.42
54 60 60
Beech root biomass and beech stern distance dB-AmB (Feb.-May) dB-AmB (June-Sept.) dB-AmB (Oct.-Jan.)
0.001 0.001 0.001
0.48 0.44 0.52
54 60 60
Oak root biomass and oak stem distance do-Amo (Feb.-May) do-Amo (June-Sept.) do-Amo (Oct.-Jan.)
NS NS 0.05
0.01 0.21 0.28
54 60 60
0.40 0.42 0.44
54 60 60
Percental quota of beech roots and beech stem distance dB-Pr,B (Feb.-May) 0.01 dB-PmB (June-Sept.) 0.001 dB-Pm8 (Oct.-Jan.) 0.001 NS, not significant at P < 0.05.
18
V. Biittner, C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
near oak stems, as mentioned above. Thus, total fine root biomass in the vicinity of oak stems was not higher because of a greater abundance of oak roots close to their (supposed) parent stem but was the result of a higher frequency of beech roots here. As a consequence, both the relative proportion of beech roots in the total and the total fine root biomass increased when approaching an oak stem.
3.3. Seasonal fluctuations The seasonal variability of fine root biomass was not very pronounced in the organic layers (OF+H) during the sampling period. In the intensively studied year 1991, total fine root biomass increased from its seasonal minimum in February to the maximum in August by 50%. For beech, a small peak occurred in April 1991, and the annual maximum was measured in August (210 g dry mass m-2, less than 2 m m fraction), while oak fine root biomass peaked in September 1991 ( 112 g m -2, Fig. 3(b) ). Seasonal minima were observed in February 1991 for beech (125 g m -2) and in April 1991 for oak (38 g m-2). A comparison of the figures for 1991 and 1992 showed no marked difference in the average fine root biomass of the OF+H layer.
4. Discussion
Our data show that two co-existing tree species can clearly differ with respect to root distribution in space and time in a shared soil volume although the root systems of both species overlap completely. This is consistent with the findings of Mikola et al. (1966) and McQueen (1968) who reported a vertical stratification of the fine roots of two co-existing tree species in the mineral soil under mixed Pinus-Picea and PinusFagus stands. In the acidic soil of the Lfineburger Heide stand, however, differences between the species were mainly restricted to the forest floor. Many studies on root distribution in forests have reported a concentration of fine root biomass and mycorrhizas in the superficial organic
layers and a concomitant exponential decline with increasing depth in the mineral soil of acidic profiles (e.g. G~ttsche, 1972; Kimmins and Hawkes, 1978; Deans, 1979; Vogt et al., !981 ). In the Liineburger Heide stand, fine root density declined by a factor of six from the organic layer to the mineral soil (organic Ov layer, 292 mg; 030 cm depth in the mineral soil, 56 mg total fine root biomass per 100 ml, Table 1 ), but, in addition, showed a significant decrease downwards within the organic profile itself. This sharp vertical decline was more pronounced for the oak fine root system than for beech and corresponded to an increasing mean root diameter with depth (see Table 2: proportion of fine roots less than 1 mm in biomass less than 2 ram). Thus, beech roots had a higher dominance in the lower OH horizon than in the Ov layer. The extremely superficial rooting patterns at the Lfineburger Heide site raises the question as to the impact of drought on the fine root systems of Quercus but also Fagus here. The moisture conditions of the organic layer apparently did not negatively influence the abundance of fine roots during the 24 months investigated. ( 1 ) In 1991, annual fine root biomass peaked in periods of lowest summer water content of the organic layer in August/September (Fig. 3 ( a ) ) when soil water matric potentials less than - 1.0 MPa were reached (G. G6rlitz, unpublished data, 1993). (2) Annual means and seasonal minima of the organic layer water content (Ch. Leuschner, unpublished data, 1994) were less favourable in the Ov layer with highest root densities than in the lower On horizon (Table 1, column 5), i.e. a negative correlation between moisture and root density existed. From the more superficial rooting patterns, it appears that the oak fine root system could be less susceptible to the influence of drought than that of beech. In the mineral soil however, we found no increase in the proportion of oak roots at a greater depth of the profile (Fig. 2). This is contrary to the observations of other authors (e.g. Abrams, 1990) who reported a deeper rooting of oak species than surrounding trees of other species. The rooting patterns at the Lfineburger Heide site could be a consequence of the highly
V. Biittno. C. Leuschner / Forest Ecology and Management 70 (1994) 11-21
acidic soil profile which forces the trees to concentrate fine root growth in the nutrient-rich organic horizons. Furthermore, deep rooting as observed in other studies could be more pronounced in the coarse than in the fine root system of a species. Attempts to correlate tree root activity to soil water availability have yielded controversial results (Hoffmann, 1972; Santantonio and Hermann, 1985). Reports on a negative impact of water shortage on tree root biomass, root growth or root tip numbers (e.g. Kalela, 1955; Persson, 1980) contrast with studies that found minor or no reduction of root activity or biomass (e.g. Roberts, 1976; Vogt et al., 1981; Van Praag et al., 1988) or a stimulation of fine root and mycorrhiza growth during or following moderate water stress (Kausch, 1955; Teskey and Hinckley, 1981; Feil et al., 1988). Moreover, whether root growth is affected by soil moisture or not seems not to be dependent on the climatic regime as was suggested by Van Praag et al. (1988): Deans (1979) and G6ttsche (1972) observed negative drought effects even in stands under highly humid climates in Dumfries (southwest Scotland) and the Solling mountains (central Germany). In contrast, the Lfineburger Heide site with no clear negative drought influence on root abundance, has a much more continental climate with periods of low rainfall in summer which may show up in reversible depressions of the photosynthetic capacity of Fagus and other tree species (but not Quercus, see Greve and Terborg, 1993). We found significant gradients in root abundance not only in the vertical but also in the horizontal direction within the organic profile of this stand. It is well known that tree roots can extend considerably beyond the width of the crown, resulting in a certain degree of intermingling of the root systems of neighbouring trees (Grosskopf, 1950; Hermann, 1977; Persson, 1980; Caldwell, 1987). In only a small number of forest stands, however, was root biomass found to be distributed homogeneously between the trunks (e.g. Grosskopf, 1950; Moir and Bachelard, 1969; Kohman, 1972). The majority of studies reported a decrease of root abundance with rising
19
stem distance which points to stem-centred horizontal distribution patterns in many forests (e.g. Roberts, 1976; Ford and Deans, 1977; Persson, 1980). Our results from a more than 100-yearold mixed stand show that fine roots of the two species present, Fagus and Quercus, completely penetrate the organic layer between the trunks and also intermingle thoroughly, for all of 174 sampled cores in the organic profile had at least an equivalent of 35 g fine root biomass per square metre, and only seven cores contained no oak, and two cores no beech roots at all. Furthermore, a tree's fine root abundance apparently did not markedly decrease within a radius of roughly 34 m around the stem in this mixed stand as was reported from other stands. On the contrary, total fine root abundance differed in the horizontal between the three pairs of beech and oak trees with a significant increase towards oak stems that is attributable to an increase in beech (and not oak) roots. Therefore, it is likely that neighbouring fine root systems overlap to a high degree in this old-growth stand, irrespective of the species, and might even share soil space with stems several trees away. Although we did not measure the thickness of the organic layer in individual root sampling cores, the coincidence of a thicker organic layer (which probably is a result of local litter accumulation) and a higher root biomass in the vicinity of the three oak stems studied is obvious from the data in Tables 3 and 4. Since both the absolute and the relative abundance of beech fine roots were positively correlated with the total Table 4 Depth of the organic layer (OF+H) at 1 and 2 m distance from either a beech or an oak stem (averages of 32 measurements each). Significant differences were detected with a Wilcoxon test. The least significant difference between beech and oak profiles at 1 or 2 m is 9 mm in both distances Location
Depth of Or+ H layer (mm)
Beech (1 m) Beech (2 m ) Oak (2 m) Oak (1 m)
59 61 76 91
20
E Biittnep, C. Leuschner /Forest Ecology and Management 70 (1994) 11-21
(beech plus oak) fine root biomass in the samples (Table 3 ), it appears that the beech root system profited by a higher organic profile thickness while the oak root system did not. Seasonal fluctuations in fine root abundance in the organic layer were not very conspicuous for either Fagus or Quercusin 1991 with a single peak at the study site in August/September (in 1992, fluctuations could not be detected owing to a lower sampling frequency). This compares well with the results of Rapp (1991) and Van Praag et al. (1988) in monospecific beech forests of the Solling and the Ardennes (Belgium). In the two-species stand in the Ltineburger Heide, maxima in the fine root standing crop (or surface area) of the two species were not concurrent as is indicated in Fig. 4. One possible explanation is that oak root growth started later in spring than did that of beech, leading to a biomass peak later in summer similar to the above-ground phenological development of the two species. Whether interspecific competition between the root systems has a significant influence on these fluctuations, must remain open. Abundance and density of fine roots in the organic layers of the mixed oak-beech stand studied were not higher than in similar horizons of a monospecific beech stand on acidic soils in the Solling mountains (Rapp, 1991 ). A density of roughly 300 mg root dry mass ( d < 1 m m ) per 100 ml humus volume (Or layer) and an abundance of 150-190 g m -2 in the entire organic layer (Or+n) occurred in both the pure and the mixed stand under similar edaphic and demographic conditions. Whether these figures reflect a situation of maximum root occupancy in the crowded forest floor which restricts additional root growth (cf. Vogt et al., 1981 ) deserves further investigation. The percental ratio of beech and oak fine root biomass in the total figures of the profile (75:25) were roughly equivalent to the relative abundance of Fagus and Quercus stems (about 3:1 ) in the stand. Oak fine roots, however, seem to be under-represented in the forest floor as well as in the mineral soil under oak stems when taking into account the much larger canopy dimensions of the older oaks which result in similar canopy
projection areas of Fagus and Quercus in this stand. Remarkably, this finding coincides with the fact that oak will be outcompeted by beech in the stand in the long run (cf. Leuschner, 1994).
Acknowledgements We thank Dirk Gansert for helpful comments on the manuscript.
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