54
J. CanadelJ, A. Djema, B. L6pez, F. L., S. Sabate, D. Siscart and C.A. Gracia
and it falls at the high end of the range of 15-25% given by Harris et al.
(1980). Thus, the Montseny holm oak forest is not strikingly different from
temperate forest ecosystems in its pattern of root to shoot biomass allocation, at least in an undisturbed mesic site without major water stress during
the dry season and dominated by single-stemmed trees. At the tree level, and
not taking into account roots of 0 < 1 cm, the mean root:shoot ratio of single-stemmed holm oaks was 0.41 (SE 0.02, n = 30) for all excavated trees in
Montseny.
A very different picture emerges for stands dominated by multi-stemmed
holm oak (as in Prades and the Montseny xeric site). Through repeated resprouting after fire and coppicing, current stems are here much younger than
the stools bearing them. Under these conditions, holm oak, a long-lived species, develops massive lignotubers in which belowground biomass keeps accumulating while aboveground biomass is burned or harvested at different
frequencies. Belowground biomass in these multi-stemmed stands can exceed the aboveground biomass, as is the case at the Montseny xeric and
Prades sites where 58 and 55% of total biomass is belowground, respectively
(the latter figure representing an underestimation of the actual belowground
biomass because roots of 0 < 1 cm were not accounted for; Canadell and
Roda 1991; Djema 1995). Belowground biomass accounted for 45 and 46% of
the total biomass for two small- and medium-sized multi-stemmed trees in
La Castanya, and 66% for the largest excavated tree.
4.7 Fine Roots
Fine roots of trees (0 < 2.5 mm) are the most dynamic fraction of the root
system, having turnover times of between a few weeks and more than 8 years
(Hendrick and Pregitzer 1992). Fine roots are responsible for nutrient and
water uptake. However, fine root distribution, phenology and turnover have
only recently been studied for holm oak. The following account is based on
repeated observations with inflatable minirhizotrons placed down to 60-cm
depth at Prades (Lopez et al. 1996, 1998).
Fine root density, averaged over 2 years of study (excluding the first
3 months after minirhizotron installation), was greatest in the upper soil layers, with 32% of the roots counted down to 60-cm depth being in the top
10 cm of soil, 60% in the top 20 cm and 80% in the top 30 cm. Root biomass,
however, was more evenly distributed along the soil profile, with only 51 % of
the biomass in the top 30 cm. Only 7.4% of the fine root biomass was found
between 50 and 60 cm deep. Deep fine roots, although representing small
quantities of the total amount of biomass, may playa key role in extracting
deep soil moisture during dry periods (Canadell et al. 1996; Hendrick and
Pregitzer 1996), which are an important feature of the Mediterranean climate. Roots of 0 < 0.5 mm accounted at Prades for 95% of the total number
J. CanadelJ, A. Djema, B. L6pez, F. L., S. Sabate, D. Siscart and C.A. Gracia
and it falls at the high end of the range of 15-25% given by Harris et al.
(1980). Thus, the Montseny holm oak forest is not strikingly different from
temperate forest ecosystems in its pattern of root to shoot biomass allocation, at least in an undisturbed mesic site without major water stress during
the dry season and dominated by single-stemmed trees. At the tree level, and
not taking into account roots of 0 < 1 cm, the mean root:shoot ratio of single-stemmed holm oaks was 0.41 (SE 0.02, n = 30) for all excavated trees in
Montseny.
A very different picture emerges for stands dominated by multi-stemmed
holm oak (as in Prades and the Montseny xeric site). Through repeated resprouting after fire and coppicing, current stems are here much younger than
the stools bearing them. Under these conditions, holm oak, a long-lived species, develops massive lignotubers in which belowground biomass keeps accumulating while aboveground biomass is burned or harvested at different
frequencies. Belowground biomass in these multi-stemmed stands can exceed the aboveground biomass, as is the case at the Montseny xeric and
Prades sites where 58 and 55% of total biomass is belowground, respectively
(the latter figure representing an underestimation of the actual belowground
biomass because roots of 0 < 1 cm were not accounted for; Canadell and
Roda 1991; Djema 1995). Belowground biomass accounted for 45 and 46% of
the total biomass for two small- and medium-sized multi-stemmed trees in
La Castanya, and 66% for the largest excavated tree.
4.7 Fine Roots
Fine roots of trees (0 < 2.5 mm) are the most dynamic fraction of the root
system, having turnover times of between a few weeks and more than 8 years
(Hendrick and Pregitzer 1992). Fine roots are responsible for nutrient and
water uptake. However, fine root distribution, phenology and turnover have
only recently been studied for holm oak. The following account is based on
repeated observations with inflatable minirhizotrons placed down to 60-cm
depth at Prades (Lopez et al. 1996, 1998).
Fine root density, averaged over 2 years of study (excluding the first
3 months after minirhizotron installation), was greatest in the upper soil layers, with 32% of the roots counted down to 60-cm depth being in the top
10 cm of soil, 60% in the top 20 cm and 80% in the top 30 cm. Root biomass,
however, was more evenly distributed along the soil profile, with only 51 % of
the biomass in the top 30 cm. Only 7.4% of the fine root biomass was found
between 50 and 60 cm deep. Deep fine roots, although representing small
quantities of the total amount of biomass, may playa key role in extracting
deep soil moisture during dry periods (Canadell et al. 1996; Hendrick and
Pregitzer 1996), which are an important feature of the Mediterranean climate. Roots of 0 < 0.5 mm accounted at Prades for 95% of the total number