Stand Structure, Aboveground Biomass and Production
41
variation among topographic positions is less for litterfall than for woody
biomass increment, and spatial variations in aboveground primary production are better reflected by the latter.
Even with their low primary production, our holm oak forests accrete
biomass at appreciable rates: between 1985 and 1995 the mean aboveground
biomass of the tree layer in the Torrent de la Mina catchment increased from
78.8 to 96.5 Mg ha- l • This amounts to a mean biomass accumulation of 1.8
Mg ha- l year- l , or to a carbon sequestration of 90 g C m- 2 year- l into woody
tissues of living trees. Total sequestration of carbon by the ecosystem is
probably higher since these relatively young forests are also accumulating, at
least, dead wood. The mean annual increments in the woody biomass of living trees ranged from a low of 1.1 Mg ha- 1 in the control plots of the southfacing slope of Torners (Prades) to a maximum of 4.7 Mg ha- l obtained
around the year 1980 in the permanent plot of Torrent de la Mina catchment
(Montseny; Ferrt~s 1984). Intermediate values were found in Avic (Prades;
Fig. 3.2), in closed-canopy plots across the Torrent de la Mina catchment
(Fig. 3.2), and in later years in the permanent plot at Montseny (Mayor 1990).
The low rates of net aboveground primary production in our holm oak
forests probably result from a combination of soil water limitation, intrinsic
characteristics of the species (e.g. patterns of belowground allocation, see
Chap. 4, and respiration costs, see Chap. 12), and the effects of long-term use
and abuse by man on soil fertility and other forest properties. The role of
water and nutrient availabilities in limiting primary production in holm oak
forests is addressed in Chapter 13.
3.7 Structure, Biomass and Production
in Mediterranean Holm Oak Forests
A review of published studies on holm oak forests across the Mediterranean
Basin shows that their aboveground biomass varies greatly (Table 3.5). The
most striking outlier is the forest of Supramonte di Orgosolo (Sardinia; Susmel et al. 1976). This magnificent forest shows the highest biomass (339 Mg
ha- 1 ), and contains holm oaks with dbh > 60 cm. The holm oak forest of Le
Rouquet (southern France; Lossaint and Rapp 1971) also has a biomass
(269 Mg ha- l ) much higher than most holm oak stands (Table 3.5), a relevant
fact since Le Rouquet is the only Mediterranean forest ecosystem listed in
many worldwide compilations such as the IBP Woodland Data Set (Reichle
1981) or the review by Vogt et al. (1986). The high biomass and basal area of
the Supramonte di Orgosolo and Le Rouquet holm oak forests are related to
the long time these forests have remained without being felled (centuries in
the former and ca. 150 years in the latter), much longer than the other holm
oak forests studied (Table 3.5). This suggests a close relationship between
41
variation among topographic positions is less for litterfall than for woody
biomass increment, and spatial variations in aboveground primary production are better reflected by the latter.
Even with their low primary production, our holm oak forests accrete
biomass at appreciable rates: between 1985 and 1995 the mean aboveground
biomass of the tree layer in the Torrent de la Mina catchment increased from
78.8 to 96.5 Mg ha- l • This amounts to a mean biomass accumulation of 1.8
Mg ha- l year- l , or to a carbon sequestration of 90 g C m- 2 year- l into woody
tissues of living trees. Total sequestration of carbon by the ecosystem is
probably higher since these relatively young forests are also accumulating, at
least, dead wood. The mean annual increments in the woody biomass of living trees ranged from a low of 1.1 Mg ha- 1 in the control plots of the southfacing slope of Torners (Prades) to a maximum of 4.7 Mg ha- l obtained
around the year 1980 in the permanent plot of Torrent de la Mina catchment
(Montseny; Ferrt~s 1984). Intermediate values were found in Avic (Prades;
Fig. 3.2), in closed-canopy plots across the Torrent de la Mina catchment
(Fig. 3.2), and in later years in the permanent plot at Montseny (Mayor 1990).
The low rates of net aboveground primary production in our holm oak
forests probably result from a combination of soil water limitation, intrinsic
characteristics of the species (e.g. patterns of belowground allocation, see
Chap. 4, and respiration costs, see Chap. 12), and the effects of long-term use
and abuse by man on soil fertility and other forest properties. The role of
water and nutrient availabilities in limiting primary production in holm oak
forests is addressed in Chapter 13.
3.7 Structure, Biomass and Production
in Mediterranean Holm Oak Forests
A review of published studies on holm oak forests across the Mediterranean
Basin shows that their aboveground biomass varies greatly (Table 3.5). The
most striking outlier is the forest of Supramonte di Orgosolo (Sardinia; Susmel et al. 1976). This magnificent forest shows the highest biomass (339 Mg
ha- 1 ), and contains holm oaks with dbh > 60 cm. The holm oak forest of Le
Rouquet (southern France; Lossaint and Rapp 1971) also has a biomass
(269 Mg ha- l ) much higher than most holm oak stands (Table 3.5), a relevant
fact since Le Rouquet is the only Mediterranean forest ecosystem listed in
many worldwide compilations such as the IBP Woodland Data Set (Reichle
1981) or the review by Vogt et al. (1986). The high biomass and basal area of
the Supramonte di Orgosolo and Le Rouquet holm oak forests are related to
the long time these forests have remained without being felled (centuries in
the former and ca. 150 years in the latter), much longer than the other holm
oak forests studied (Table 3.5). This suggests a close relationship between