262
Antoni Escarrt!, Ferran Rod!, Jaume Terradas and Xavier Mayor
Montseny and, even more spectacularly, about four times as much Ca goes
annually into new coarse roots at Prades than at Montseny. This difference is
a result of the higher base saturation of the soil, the higher Ca concentration
in coarse roots, and the higher estimated root production at Prades.
The above-mentioned nutrient fluxes can be integrated into three major
derived fluxes: return, retention, and uptake. We have computed them for the
aboveground components, for which data are reasonably complete. Aboveground nutrient return is defined as the sum of nutrient fluxes in litterfall
and canopy leaching. Nutrient return measures the intensity of nutrient cycling between the trees and the soil. Nutrient return fluxes are moderate at
both sites. Aboveground retention is defined as the amount of nutrient annually incorporated into the production of new woody tissues in boles and
branches. Nutrient retention is rather low at both sites, but except for P it is
notably higher at Montseny than at Prades; reflecting the higher current production of boles plus branches at the first site (2.4 vs. 1.6 Mg ha- I year-I).
Annual nutrient uptake needed to sustain the functioning of aboveground
biomass is defined as ,the sum of aboveground retention and return. The involved nutrient fluxes are again broadly similar at both sites, and they reach
(kg ha- I year-I) 46-51 for Ca, 30-39 for K, and 30-42 for N. If the nutrients
needed to construct coarse roots are added to these figures, total net nutrient
uptake (except for fine roots) reaches (kg ha- I year-I) 56-66 kg for Ca, 34-42
for K, and 34-45 for N.
The overall picture that emerges from these data is that these holm oak
forests exhibit a parsimonious nutrient cycle, i.e. that they exert a low to
moderate demand on the site nutrient capital. However, given the long-lived
nature of holm oak and particularly of its stools, the amounts of nutrients
accumulated in the biomass can be quite high, as we have seen in the previous section. Figure 18.1 shows for four major nutrients how uptake is split
between retention and return. Return makes up a higher proportion of annual uptake of Nand K than of P and Ca, reflecting the higher relative allocation of Nand K to leaves and reproductive structures, of Ca to bark, and of
P (surprisingly) to wood. As a result, Nand K cycle faster between trees and
the soil than P and Ca.
The intrasystemic nutrient fluxes discussed above can be compared to
ecosystem input and output nutrient fluxes (Chap. 20). In common with
other undisturbed forest ecosystems (e.g. Monk and Day 1985; Johnson and
Henderson 1989; Likens et al. 1995), our holm oak forest sites show much
larger within-ecosystem nutrient fluxes than between-ecosystem fluxes. Nutrient demand by trees is determinant in providing this degree of closure of
the forest nutrient cycle. The trenching experiment mentioned in Chapters
16 and 17 revealed indeed that holm oak forests have a large potential to lose
nitrogen through nitrate leaching after disturbance (Bonilla and Roda 1989,
1990).
Antoni Escarrt!, Ferran Rod!, Jaume Terradas and Xavier Mayor
Montseny and, even more spectacularly, about four times as much Ca goes
annually into new coarse roots at Prades than at Montseny. This difference is
a result of the higher base saturation of the soil, the higher Ca concentration
in coarse roots, and the higher estimated root production at Prades.
The above-mentioned nutrient fluxes can be integrated into three major
derived fluxes: return, retention, and uptake. We have computed them for the
aboveground components, for which data are reasonably complete. Aboveground nutrient return is defined as the sum of nutrient fluxes in litterfall
and canopy leaching. Nutrient return measures the intensity of nutrient cycling between the trees and the soil. Nutrient return fluxes are moderate at
both sites. Aboveground retention is defined as the amount of nutrient annually incorporated into the production of new woody tissues in boles and
branches. Nutrient retention is rather low at both sites, but except for P it is
notably higher at Montseny than at Prades; reflecting the higher current production of boles plus branches at the first site (2.4 vs. 1.6 Mg ha- I year-I).
Annual nutrient uptake needed to sustain the functioning of aboveground
biomass is defined as ,the sum of aboveground retention and return. The involved nutrient fluxes are again broadly similar at both sites, and they reach
(kg ha- I year-I) 46-51 for Ca, 30-39 for K, and 30-42 for N. If the nutrients
needed to construct coarse roots are added to these figures, total net nutrient
uptake (except for fine roots) reaches (kg ha- I year-I) 56-66 kg for Ca, 34-42
for K, and 34-45 for N.
The overall picture that emerges from these data is that these holm oak
forests exhibit a parsimonious nutrient cycle, i.e. that they exert a low to
moderate demand on the site nutrient capital. However, given the long-lived
nature of holm oak and particularly of its stools, the amounts of nutrients
accumulated in the biomass can be quite high, as we have seen in the previous section. Figure 18.1 shows for four major nutrients how uptake is split
between retention and return. Return makes up a higher proportion of annual uptake of Nand K than of P and Ca, reflecting the higher relative allocation of Nand K to leaves and reproductive structures, of Ca to bark, and of
P (surprisingly) to wood. As a result, Nand K cycle faster between trees and
the soil than P and Ca.
The intrasystemic nutrient fluxes discussed above can be compared to
ecosystem input and output nutrient fluxes (Chap. 20). In common with
other undisturbed forest ecosystems (e.g. Monk and Day 1985; Johnson and
Henderson 1989; Likens et al. 1995), our holm oak forest sites show much
larger within-ecosystem nutrient fluxes than between-ecosystem fluxes. Nutrient demand by trees is determinant in providing this degree of closure of
the forest nutrient cycle. The trenching experiment mentioned in Chapters
16 and 17 revealed indeed that holm oak forests have a large potential to lose
nitrogen through nitrate leaching after disturbance (Bonilla and Roda 1989,
1990).