258
Antoni Escarre, Ferran Rodil, Jaume Terradas and Xavier Mayor
and Roda 1992; Sabate 1993). These are low retranslocation rates (retranslocation of up to 60-80% of Nand P in mature leaves is not uncommon, particularly in deciduous trees). Nutrient retranslocation (or resorption) is an
active process that removes nutrients from senescing leaves before abscission. Resorbed nutrients are transported to growing organs (sinks) or to
storage in perennial tissues (Chapin 1980). Retranslocation means for the
plant a higher control of nutrient resources, and allows it to reutilize them.
Though retranslocation from senescing leaves has received most attention,
retranslocation is not restricted to senescing organs, but it can occur at other
times of the year, particularly during growth pulses in evergreens. At Prades,
Nand P concentrations in holm oak leaves were lower during summer and
larger in autumn and spring, while the contrary was found for stems (Sabate
1993; Chap. 9) suggesting that intra-annual nutrient concentration changes
are linked to nutrient retranslocation process occurring in spring and
autumn in association with growth periods. If retranslocation is computed
by substracting nutrient concentrations (on a leaf area basis) in leaf litterfall
from the maximum seasonal concentrations attained by mature leaves, holm
oak at Prades retranslocates 34-52% of the maximum foliar N contents, and
47-65% of those of P (Sabate 1993), i.e. a substantially higher resorption than
when only retranslocation during leaf senescence is considered.
Independently of when it occurs, retranslocation could be ecologically
significant for Mediterranean evergreen species. A plant is expected to increase its leaf longevity when nutrient status is low, and the proportion of
supporting tissue increases with leaf longevity (Chabot and Hicks 1982). Retranslocation is linked to nutrient mobility, which depends on tissue composition. Therefore, in situations where growth is limited by resource availability the retranslocation capacity is expected to decrease (Del Arco et al. 1991;
Escudero et al. 1992). As noted above, evergreen species like holm oak tend to
have smaller retranslocation rates than deciduous trees. This limits the
adaptive capacity of evergreen species for increasing their nutrient-use efficiency through retranslocation in nutrient-poor soils. In fact, increasing leaf
longevity seems to be much more effective for increasing nutrient-use efficiency in these sites (Escudero et al. 1992). In this context, Pugnaire and
Chapin (1993) concluded that retranslocation is not an adaptation to low
nutrient availability but a phenotypic response to variations in nutrient
status. Killingbeck (1996) distinguished two components of such response:
potential resorption (the maximum amount of nutrients than can be withdrawn from senescing leaves) and resorption proficiency (the levels to which
nutrients have been reduced in senesced leaves). According to Killingbeck
(1996), plants reducing Nand P concentrations in their senesced leaves to
levels below 0.7% for Nand 0.05% for P can be considered highly proficient.
Mean nutrient concentrations in holm oak leaf litterfall were 0.87-0.97% for
Nand 0.055-0.010% for P, depending on site and period (Verdu 1984; Mayor
1990; Bellot et al. 1992; V. Diego, unpubl. data). This means that holm oak is
not very proficient at resorbing Nand P.
Antoni Escarre, Ferran Rodil, Jaume Terradas and Xavier Mayor
and Roda 1992; Sabate 1993). These are low retranslocation rates (retranslocation of up to 60-80% of Nand P in mature leaves is not uncommon, particularly in deciduous trees). Nutrient retranslocation (or resorption) is an
active process that removes nutrients from senescing leaves before abscission. Resorbed nutrients are transported to growing organs (sinks) or to
storage in perennial tissues (Chapin 1980). Retranslocation means for the
plant a higher control of nutrient resources, and allows it to reutilize them.
Though retranslocation from senescing leaves has received most attention,
retranslocation is not restricted to senescing organs, but it can occur at other
times of the year, particularly during growth pulses in evergreens. At Prades,
Nand P concentrations in holm oak leaves were lower during summer and
larger in autumn and spring, while the contrary was found for stems (Sabate
1993; Chap. 9) suggesting that intra-annual nutrient concentration changes
are linked to nutrient retranslocation process occurring in spring and
autumn in association with growth periods. If retranslocation is computed
by substracting nutrient concentrations (on a leaf area basis) in leaf litterfall
from the maximum seasonal concentrations attained by mature leaves, holm
oak at Prades retranslocates 34-52% of the maximum foliar N contents, and
47-65% of those of P (Sabate 1993), i.e. a substantially higher resorption than
when only retranslocation during leaf senescence is considered.
Independently of when it occurs, retranslocation could be ecologically
significant for Mediterranean evergreen species. A plant is expected to increase its leaf longevity when nutrient status is low, and the proportion of
supporting tissue increases with leaf longevity (Chabot and Hicks 1982). Retranslocation is linked to nutrient mobility, which depends on tissue composition. Therefore, in situations where growth is limited by resource availability the retranslocation capacity is expected to decrease (Del Arco et al. 1991;
Escudero et al. 1992). As noted above, evergreen species like holm oak tend to
have smaller retranslocation rates than deciduous trees. This limits the
adaptive capacity of evergreen species for increasing their nutrient-use efficiency through retranslocation in nutrient-poor soils. In fact, increasing leaf
longevity seems to be much more effective for increasing nutrient-use efficiency in these sites (Escudero et al. 1992). In this context, Pugnaire and
Chapin (1993) concluded that retranslocation is not an adaptation to low
nutrient availability but a phenotypic response to variations in nutrient
status. Killingbeck (1996) distinguished two components of such response:
potential resorption (the maximum amount of nutrients than can be withdrawn from senescing leaves) and resorption proficiency (the levels to which
nutrients have been reduced in senesced leaves). According to Killingbeck
(1996), plants reducing Nand P concentrations in their senesced leaves to
levels below 0.7% for Nand 0.05% for P can be considered highly proficient.
Mean nutrient concentrations in holm oak leaf litterfall were 0.87-0.97% for
Nand 0.055-0.010% for P, depending on site and period (Verdu 1984; Mayor
1990; Bellot et al. 1992; V. Diego, unpubl. data). This means that holm oak is
not very proficient at resorbing Nand P.