Stand Structure, Aboveground Biomass and Production
37
The shrub layer (stem diameters 2-5 cm) contributed little biomass in the
two sites where it was estimated (Table 3.3), and the field layer contributed
even less. Holm oak seedlings can be sometimes the dominant component in
the field layer.
3.5 Stand Dynamics
3.5.1 Demographic Changes
In Montseny, mean tree density increased by 0.21 % year- I during the decade
1985-1995. This increase was the balance between an ingrowth (stems incorporated into the tree layer) of 0.45% year- I and a mortality of 0.24% year-I.
This very limited recruitment may be explained because virtually all stems
with dbh < 5 cm are suppressed stems having no or negligible diameter
growth (Mayor and Roda 1993). In Avic (Prades), there was a slight reduction
in total stem density (050 ~ 2 cm) of 0.25% year- I from 1981 to 1986. On the
south-facing slope of Torners, stem mortality between 1988 and 1991 averaged 1.5% year- I (Mayor and Roda 1994). These figures indicate that selfthinning is slowly underway in the dense Prades forests. At both Montseny
and Prades mortality was concentrated in the smallest, suppressed stems.
Mortality of canopy trees was negligible during the above periods.
3.5.2 Tree Growth
Diameter growth rates of holm oak are very small, averaging 0.6-1.1 mm
year- I in Montseny and 0.3-1.2 mm year- I in Prades (Table 3.4). Holm oaks
grow much faster when they are in the overstory than when they remain in
the understory: in the Torrent de la Mina catchment, diameter growth of
overstory trees triple that of understory trees, while in Prades, growth of
dominant trees nearly doubles the average growth of all trees (Table 3.4).
Stand basal area increased by 0.2 m 2 ha- I year- I between 1981 and 1986 in the
Avic catchment at Prades (an increase of 0.8% year-I), and by 0.32 m 2 ha- I
year- I between 1985 and 1995 in the Torrent de la Mina catchment at Montseny (an increase of 2% year-I). Our results confirm that holm oak is a slowgrowing species (Cartan-Son et al. 1992; Lled6 et al. 1992; Mayor and Roda
1993,1994). When it occurs at high densities as a result of past coppicing the
outcome is a nearly stagnant forest where changes in stand structure are very
slow. Chapters 12 and 23 deal with the forest responses to experimental thinnings.
37
The shrub layer (stem diameters 2-5 cm) contributed little biomass in the
two sites where it was estimated (Table 3.3), and the field layer contributed
even less. Holm oak seedlings can be sometimes the dominant component in
the field layer.
3.5 Stand Dynamics
3.5.1 Demographic Changes
In Montseny, mean tree density increased by 0.21 % year- I during the decade
1985-1995. This increase was the balance between an ingrowth (stems incorporated into the tree layer) of 0.45% year- I and a mortality of 0.24% year-I.
This very limited recruitment may be explained because virtually all stems
with dbh < 5 cm are suppressed stems having no or negligible diameter
growth (Mayor and Roda 1993). In Avic (Prades), there was a slight reduction
in total stem density (050 ~ 2 cm) of 0.25% year- I from 1981 to 1986. On the
south-facing slope of Torners, stem mortality between 1988 and 1991 averaged 1.5% year- I (Mayor and Roda 1994). These figures indicate that selfthinning is slowly underway in the dense Prades forests. At both Montseny
and Prades mortality was concentrated in the smallest, suppressed stems.
Mortality of canopy trees was negligible during the above periods.
3.5.2 Tree Growth
Diameter growth rates of holm oak are very small, averaging 0.6-1.1 mm
year- I in Montseny and 0.3-1.2 mm year- I in Prades (Table 3.4). Holm oaks
grow much faster when they are in the overstory than when they remain in
the understory: in the Torrent de la Mina catchment, diameter growth of
overstory trees triple that of understory trees, while in Prades, growth of
dominant trees nearly doubles the average growth of all trees (Table 3.4).
Stand basal area increased by 0.2 m 2 ha- I year- I between 1981 and 1986 in the
Avic catchment at Prades (an increase of 0.8% year-I), and by 0.32 m 2 ha- I
year- I between 1985 and 1995 in the Torrent de la Mina catchment at Montseny (an increase of 2% year-I). Our results confirm that holm oak is a slowgrowing species (Cartan-Son et al. 1992; Lled6 et al. 1992; Mayor and Roda
1993,1994). When it occurs at high densities as a result of past coppicing the
outcome is a nearly stagnant forest where changes in stand structure are very
slow. Chapters 12 and 23 deal with the forest responses to experimental thinnings.