Experimental Areas of Prades and Montseny
21
2.2.4 Soils
At La Castanya, soils are rather shallow with maximum depths of 1.5-2 m,
and a high level of spatial heterogeneity in the landscape because of the rugged topography. Ten soil profIles excavated in the north-facing slope of the
Torrent de la Mina catchment yielded a mean depth to the bedrock of 60 cm.
The organic layers (0 horizon) are 3 cm deep on average. The Land F layers
are usually well developed, but the H horizon is sometimes lacking. Most of
the soils are colluvial with discontinuities in the distribution of the very
abundant stones and little vertical distinction in morphological features.
They are classified (Soil Survey Staff 1992) as Entisols (Lithic Xerorthents) or
Inceptisols (Typic, Lithic or Dystric Xerochrepts). The main pedogenetic
process is the formation of a cambic horizon, with moderate illuviation
(Hereter 1990).
Major soil characteristics are shown in Table 2.2. They are stony soils with
loamy textural classes, afld the bulk density tends to increase with depth.
Below the first centimeters of mineral soil, the concentration of organic carbon averages around 2% (Table 2.2). The soils are acidic, acidity being buffered mainly by silicate weathering and cation exchange. Calcium is the
dominant exchangeable base cation, and it is especially abundant in the upper mineral soil. Base saturation is low - for a Mediterranean soil - under the
first centimeters of mineral soil, and reaches the highest values in both Aand C-horizons. There is a significant positive relationship between the
cation exchange capacity and the content of soil organic matter. Organic
phosphorus is the dominant phosphorus form in all soil horizons.
2.2.5 Vegetation
At La Castanya, holm oak forest covers the lower 53% of the Torrent de la
Mina catchment (TMO). On the north-facing slope, holm oak extends up to
an altitude of 1000 m, while on the east-facing slope it reaches 1200 m. The
headwaters of the catchment are occupied by heathlands and grasslands,
with Calluna vulgaris, Juniperus communis, Erica arborea, Erica scoparia, and
Pteridium aquilinum as major species. A beech forest on the north-facing
slope covers 10% of the catchment. The TM9 catchment is wholly covered by
a dense holm oak forest, undisturbed in the last decades. Throughout the
holm oak forests within the Torrent de la Mina catchment, holm oak is virtually the only tree species. The tree canopy is usually 5-9 m high. Current
holm oak stems are derived both from seed and from stump sprouts. On the
north-facing slope of the Torrent de la Mina catchment 52% of stems having
a diameter at breast height (dbh) of 2:5 cm originated from seed, but only
17% on the east-facing slope. Seed-derived stems dominate in the permanent
plot, with 84% of tree stems. The understory is very sparse where the holm
oak canopy has remained closed for some time, but a thick understory of
the shrub Erica arborea is present where the tree canopy is open or was open
21
2.2.4 Soils
At La Castanya, soils are rather shallow with maximum depths of 1.5-2 m,
and a high level of spatial heterogeneity in the landscape because of the rugged topography. Ten soil profIles excavated in the north-facing slope of the
Torrent de la Mina catchment yielded a mean depth to the bedrock of 60 cm.
The organic layers (0 horizon) are 3 cm deep on average. The Land F layers
are usually well developed, but the H horizon is sometimes lacking. Most of
the soils are colluvial with discontinuities in the distribution of the very
abundant stones and little vertical distinction in morphological features.
They are classified (Soil Survey Staff 1992) as Entisols (Lithic Xerorthents) or
Inceptisols (Typic, Lithic or Dystric Xerochrepts). The main pedogenetic
process is the formation of a cambic horizon, with moderate illuviation
(Hereter 1990).
Major soil characteristics are shown in Table 2.2. They are stony soils with
loamy textural classes, afld the bulk density tends to increase with depth.
Below the first centimeters of mineral soil, the concentration of organic carbon averages around 2% (Table 2.2). The soils are acidic, acidity being buffered mainly by silicate weathering and cation exchange. Calcium is the
dominant exchangeable base cation, and it is especially abundant in the upper mineral soil. Base saturation is low - for a Mediterranean soil - under the
first centimeters of mineral soil, and reaches the highest values in both Aand C-horizons. There is a significant positive relationship between the
cation exchange capacity and the content of soil organic matter. Organic
phosphorus is the dominant phosphorus form in all soil horizons.
2.2.5 Vegetation
At La Castanya, holm oak forest covers the lower 53% of the Torrent de la
Mina catchment (TMO). On the north-facing slope, holm oak extends up to
an altitude of 1000 m, while on the east-facing slope it reaches 1200 m. The
headwaters of the catchment are occupied by heathlands and grasslands,
with Calluna vulgaris, Juniperus communis, Erica arborea, Erica scoparia, and
Pteridium aquilinum as major species. A beech forest on the north-facing
slope covers 10% of the catchment. The TM9 catchment is wholly covered by
a dense holm oak forest, undisturbed in the last decades. Throughout the
holm oak forests within the Torrent de la Mina catchment, holm oak is virtually the only tree species. The tree canopy is usually 5-9 m high. Current
holm oak stems are derived both from seed and from stump sprouts. On the
north-facing slope of the Torrent de la Mina catchment 52% of stems having
a diameter at breast height (dbh) of 2:5 cm originated from seed, but only
17% on the east-facing slope. Seed-derived stems dominate in the permanent
plot, with 84% of tree stems. The understory is very sparse where the holm
oak canopy has remained closed for some time, but a thick understory of
the shrub Erica arborea is present where the tree canopy is open or was open