NATURAL FORESTS OF QUERCUS ROBUR L. IN NORTHWEST ...

POLISH JOURNAL OF ECOLOGY
(Pol. J. Ecol.)
55 2 401–404 2007
Research note
Ignacio JAVIER DÍAZ-MAROTO, Pablo VILA-LAMEIRO
Department of Agroforestry Engineering, University of Santiago de Compostela, Campus Universitario
s/n, E-27002 Lugo, Spain. diazmaro@lugo.usc.es
NATURAL FORESTS OF QUERCUS ROBUR L. IN NORTHWEST
SPAIN – MAIN PROPERTIES OF SOILS
ABSTRACT: The main properties of the
soils under oak (Quercus robur L.) forests in
Galicia (NW Spain) were characterized. Eleven
edaphic parameters (pH, OM, N, C/N ratio, P,
K, Ca, Mg, sand, silt and clay) were estimated in
39 soil samples. Siliceous substrates were present
in all stands and the soils were found to be the
cambisols. The values of chemical parameters are
similar to those considered as the optimal ones by
other authors (i.e., average pH close to 5, average
C/N ratio close to 15) with the exception of the
organic matter which is slightly higher; the average
value is equal to 8.64 ± 5.19. The content of
nutrients can be considered as the low or medium
(for instance, the potassium is only 73 ppm) as
compared with other forests in the study area, except
the content of phosphorus, which is considerably
higher (21.8 ppm versus 1–6 ppm).
KEY WORDS: oak forests, soil properties,
Galicia, Spain
Soils in Galicia (NW Spain) are acidic.
The high precipitation, particularly the intensity
of rainfall during the rainy seasons
(autumn and spring) is decisive in this region
(Carballeira et al. 1983). The drainage
conditions favour water infiltration into soil
and increase the run-off and temperate thermic
regime allow the advance of acidification
(Camps et al. 2004). However, slightly acidic
soils are found to be more favourable than alkaline
ones to support the diverse forest vegetation
(Hardtle et al. 2005). The Galician
soils under oak forests that grow on acidic
substrates are oligotrophic or mesotrophic,
with a high C/N ratio, very low saturation
level and low contents of exchangeable cations;
however, phosphorus content is rather
high (Bara et al. 1985).
The studied stands are located in Galicia,
NW Spain (Fig. 1) at altitude average of 508
m; the slopes are steeper than 20% in half of
the study area. Siliceous substrates were present
in all stands, with dominance of granite
and schist and the soils are of cambisol type
(Díaz-Maroto and Vila-Lameiro 2006).
The climate presents generally an oceanic
pattern, with annual rainfall between 600
and 3000 mm (Carballeira et al. 1983).
According to the National Forest inventory,
stands of native hardwoods, mainly Quercus
spp., cover in Galicia 27% of the total woodland
area, i.e., 375922 ha. In 1986, these forests
covered only 20%. Pure stands of Quercus robur
cover 187789 ha, almost 14% of the woodland
area in the region (DGCONA, 2001).
A total of 39 sampling sites were chosen,
in relation to physiography and general soil
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Ignacio Javier Díaz-Maroto, Pablo Vila-Lameiro
SPAIN
Fig. 1. Distribution of the studied Quercus robur stands in Galicia (Spain).
characteristics (Fig. 1). To describe the soil
profiles, the pits of 2 m length and 80 cm
width were dug at each site. The horizons
were differentiated according to colour and/
or texture. One sample of each horizon (about
2 kg) was collected and following features
were analyzed (FAO, 1977): Munsell colour,
moisture (by oven drying to constant weight
at 105°C), pH (in H 2
O – 1:2.5 soil solution),
total nitrogen (semi-micro Kjeldahl method),
carbon (Walkley and Black method),
organic matter (% carbon × 1.724), C/N ratio,
exchangeable cations (by extraction with
1N ammonium acetate at pH=7 and quantified
by atomic absorption (Ca and Mg) and
by emission (K), available phosphorus (Bray
and Kurtz method), and physical analysis
(separation of the fine earth, elements < 2
mm of diameter: sand, silt and clay). In total
11 soil parameters were estimated (Table 1).
An univariable analysis was applied to
these parameters (Díaz-Maroto et al.
2006), with a normality test to verify the normal
distribution (Walpole et al. 1999). To
assess the variability of soils the bivariable
correlation and Principal Component Analysis
(PCA) (Sas Institute Inc., 2004) were applied.
The average values of following parameters:
pH, nitrogen and C/N ratio are similar
to these reported as the optimal ones by Bara
et al. (1985) and Rozados et al. (2000). The
values of OM are generally high and slightly
higher than given by above authors. Most of
soils are strongly acid (4.7 ≤ pH

Soil properties of oak forests
403
Table 1. Average, standard deviation (±SD), coefficient of variation (CV), and the range of values
(max-min) of soil parameters (n=39).
Parameter Average ± SD CV % max-min
pH 4.85 ± 0.46 9 6.15–3.92
OM (%) 8.64 ± 5.19 60 23.31–1.04
N (%) 0.307 ± 0.178 58 0.793–0.04
C/N 14.6 ± 4.5 31 29.6–6.9
P (ppm) 21.8 ± 28.9 133 117.2–0.4
K (ppm) 73 ± 40 55 231–9
Ca (ppm) 120 ± 216 180 1297–3
Mg (ppm) 29 ± 21 72 85–0
SAND (%) 66 ± 21 31 89–14
SILT (%) 23 ± 20 92 85–7
CLAY (%) 11 ± 5 45 27–1
Table 2. Importance of each parameter in the soil properties variability as revealed by PCA analysis.
Parameter Vector 1 Vector 2 Vector 3 Vector 4 Vector 5
pH -0.692 0.249 0.500 0.236 -0.177
OM (%) 0.870 0.185 0.339 -0.100 0.043
N (%) 0.848 0.139 0.301 -0.016 0.054
P (ppm) -0.392 0.436 0.185 0.502 0.247
K (ppm) 0.557 0.298 -0.456 0.428 0.222
SAND (%) -0.337 0.761 -0.119 -0.383 0.060
SILT (%) 0.196 -0.802 0.050 0.472 -0.061
CLAY (%) 0.568 0.246 0.276 -0.430 -0.035
the substrates poor in bases. For instance, the
phosphorus content is 21.8 ppm (Table 1);
it is much lower than the values given for a
loamy oak topsoil by Neirynck et al. (2000)
(294–387 ppm).
The contribution of sand, 66 %, is near
the data reported by Bara et al. (1985),
i.e. 72% but the silt fraction is higher, 23%
(Table 1). These values can result from the location
of studied stands on rather steep slope
with advanced soil erosion (Díaz-Maroto
et al. 2005).
Bivariable correlation analysis shows that
pH and OM contribute to the largest number
of significant relationships, including
the relation between these two parameters
(Pearson linear coefficient, r = –0.44), as
well as with other parameters, like OM-C/N
(r = 0.42). Despite the possible relationship
of some chemical parameters with the silt
fraction (Neir ynck et al. 2000), only percentage
of total clay is significantly correlated
with chemical parameters, particularly
with OM (r = 0.69) and N (r = 0.66).
The results were also analyzed with PCA
and five new variables or vectors combination
from 11 initial parameters (Table 2)
were obtained. The first vector is influenced,
above all, by OM and N content. It could be
interpreted as “absence of mineralization”
(Mansson and Falkengren-Grerup
2003; L andgraf et al. 2006). In the second,
the special importance of SAND and SILT
was manifested with opposite sign, positive
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Ignacio Javier Díaz-Maroto, Pablo Vila-Lameiro
for the first one. It could be called “the sandy
level” (Neir ynck et al. 2000). Third vector
represents the importance of pH, and it
could be called as “acidity”. In the fourth and
fifth vectors, P and K are mainly represented,
and they could be denominated as “trophic
properties” (Hagen-Thorn et al. 2006).
The results of study of the oak forest soils
from Galicia indicate that the variability of
the properties of acidic nutrient and poor
soils in this region is mainly explained by the
amount of organic matter and nitrogen as
well as by pH and contribution of sand and
clay.
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(Received after revising January 2007)
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