Structure and Dynamics of the Root System
55
of fine roots, which is in agreement with other species such as Acer saccharum (Hendrick and Pregitzer 1992; Burke and Raynal 1994) and Picea
sitchensis (Ford and Deans 1977), and is similar to the root diameter distribution of a northern hardwood forest, where two thirds of the total root
length was found in the 0.2- to 0.3-mm diameter class (Fahey and Hughes
1994).
To obtain biomass on a ground area basis from the minirhizotron observations, root growth was assumed to be isotropic, i.e. biomass density (g mm- 2 ;
all biomass and production data are on dry weight basis), observed from the
minirhizotron window, was supposed to be the same as the biomass density
one would see from above the soil. The 2-year average fine-root biomass obtained was 94.8 ± 6.8 g m- 2 (mean ± SE), which is similar to other reported
fine root biomasses (white oak: 115 g m- 2 , Aber et al. 1985; white pine: 97 g m- 2 ,
Aber et al. 1985; European beech: 150 g m- 2 , Ellenberg et al. 1986; American
beech: < 100 g m- 2 , Liu and Tyree 1997; lowlands of montane rainforest:
144 g m- 2 , Cavelier 1992), but substantially lower than estimates from some
northern hardwood forests (range: 510-990 g m- 2 , Harris et al. 1977;
McClaugherty et al. 1982; Joslin and Henderson 1987; Farrish 1991; Burke
and Raynal1994). A possible explanation for these differences is that a large
portion of the soil volume at Prades is composed of coarse gravel. Root production between time ti and time ti+1 was calculated by summing the biomass
of new roots and the positive increments of biomass of existing roots. To
obtain annual fine root production we summed the production of all the
field campaigns for the given year and then averaged the 2 years of observations. Root production (Fig. 4.3, top) was found to be 500 ± 4.4 g m -2 year-I,
thus giving a turnover rate of 5.27 year-lor a mean fine root lifespan of
68.3 days. However, longevity of individual roots ranged from 5 to more than
475 days.
The cost of fine root formation has been estimated assuming that 1 g of
carbohydrates produces 0.68 g of new root tissues (Chap. 12), which means a
cost of 7.35 kcal g-l of new root. Similarly, the average maintenance cost has
been estimated under the soil temperature at Prades as 50 cal g-l day-l
(Chap. 12). Using these simplifications, the formation cost of fine roots was
3675 kcal m- 2 , and the calculated cost of maintenance was 1730 kcal m- 2 year- 1
(Fig. 4.3, bottom). Fine root and leaf metabolic costs represent, in this forest,
more than 60% of total carbon fixed in gross primary production. Fine root
formation cost, on a ground area basis, is more than twice the leaf formation
cost, while the maintenance of fine roots is only 15.8% of leaf maintenance
cost. The high formation cost of fine roots is related to their high turnover.
Death of fine roots represents a very active mechanism of carbon transport
from the atmosphere to soil. This mechanism has been evaluated in Prades as
166 g C m- 2 year-I, 66% more than the carbon lost in leaf litterfall, making
fine roots the most important channel of carbon loss from these holm oak
trees.
55
of fine roots, which is in agreement with other species such as Acer saccharum (Hendrick and Pregitzer 1992; Burke and Raynal 1994) and Picea
sitchensis (Ford and Deans 1977), and is similar to the root diameter distribution of a northern hardwood forest, where two thirds of the total root
length was found in the 0.2- to 0.3-mm diameter class (Fahey and Hughes
1994).
To obtain biomass on a ground area basis from the minirhizotron observations, root growth was assumed to be isotropic, i.e. biomass density (g mm- 2 ;
all biomass and production data are on dry weight basis), observed from the
minirhizotron window, was supposed to be the same as the biomass density
one would see from above the soil. The 2-year average fine-root biomass obtained was 94.8 ± 6.8 g m- 2 (mean ± SE), which is similar to other reported
fine root biomasses (white oak: 115 g m- 2 , Aber et al. 1985; white pine: 97 g m- 2 ,
Aber et al. 1985; European beech: 150 g m- 2 , Ellenberg et al. 1986; American
beech: < 100 g m- 2 , Liu and Tyree 1997; lowlands of montane rainforest:
144 g m- 2 , Cavelier 1992), but substantially lower than estimates from some
northern hardwood forests (range: 510-990 g m- 2 , Harris et al. 1977;
McClaugherty et al. 1982; Joslin and Henderson 1987; Farrish 1991; Burke
and Raynal1994). A possible explanation for these differences is that a large
portion of the soil volume at Prades is composed of coarse gravel. Root production between time ti and time ti+1 was calculated by summing the biomass
of new roots and the positive increments of biomass of existing roots. To
obtain annual fine root production we summed the production of all the
field campaigns for the given year and then averaged the 2 years of observations. Root production (Fig. 4.3, top) was found to be 500 ± 4.4 g m -2 year-I,
thus giving a turnover rate of 5.27 year-lor a mean fine root lifespan of
68.3 days. However, longevity of individual roots ranged from 5 to more than
475 days.
The cost of fine root formation has been estimated assuming that 1 g of
carbohydrates produces 0.68 g of new root tissues (Chap. 12), which means a
cost of 7.35 kcal g-l of new root. Similarly, the average maintenance cost has
been estimated under the soil temperature at Prades as 50 cal g-l day-l
(Chap. 12). Using these simplifications, the formation cost of fine roots was
3675 kcal m- 2 , and the calculated cost of maintenance was 1730 kcal m- 2 year- 1
(Fig. 4.3, bottom). Fine root and leaf metabolic costs represent, in this forest,
more than 60% of total carbon fixed in gross primary production. Fine root
formation cost, on a ground area basis, is more than twice the leaf formation
cost, while the maintenance of fine roots is only 15.8% of leaf maintenance
cost. The high formation cost of fine roots is related to their high turnover.
Death of fine roots represents a very active mechanism of carbon transport
from the atmosphere to soil. This mechanism has been evaluated in Prades as
166 g C m- 2 year-I, 66% more than the carbon lost in leaf litterfall, making
fine roots the most important channel of carbon loss from these holm oak
trees.