Nutrient Distribution and Cycling
257
Holm oak leaves have similar or somewhat higher nutrient concentrations
than those of the major evergreen companion species at Prades (Table 18.3).
A coexisting deciduous oak (Quercus faginea) has higher foliar N concentrations than holm oak, reflecting the usual difference in photosynthetic capacity between evergreen and deciduous species.
18.3.2 Spatial Variations
Variations in tree nutrient concentrations among sites are usually related to
soil characteristics, which in turn vary with topographic position and other
factors. At Prades, Sabate et al. (1995) reported differences in mineral concentrations in holm oak leaves in two endpoints along a topographic gradient within the Avic catchment: they found higher foliar concentrations of N
and P, and lower concentrations of K, in a valley-bottom site than in a ridgetop site. Differences appeared both when nutrient concentrations were expressed on a dry weight basis and on a leaf area basis. Higher foliar Nand P
contents in the valley bottom can be, in part, explained by the higher nutrient
supply provided by a deeper soil: soil depth averaged 85 cm in the valley
bottom and only 45 cm in the ridge top. In contrast, the higher foliar K in the
ridge top site could derive from the stronger need to regulate the stomatal
aperture in this more xeric site, or from local differences in bedrock mineralogy.
Bedrock characteristics may contribute significantly to variations in soil
chemistry that can affect nutrient concentrations in tree tissues. In 15 different Catalan sites with contrasting bedrock (limestone and marl, volcanic
rock, and metamorphic schist) N, P and K concentrations in current-year
leaves of holm oak varied very little among sites, less so than in older leaves,
current-year twigs, or older twigs (Canadell and Vila 1992). This suggests a
tight regulation of the concentrations of these most important nutrients in
the youngest leaf cohort, which is the best illuminated. The higher variability
of nutrient concentrations in older leaves and in twigs found by Canadell
and Vila (1992) and Sabate (1993) can in part result from the higher heterogeneity of the light climate they experience, and from nutrient redistribution
within tree crowns. Mobile nutrients could be redistributed depending on
growth demands, with the different sites being sampled at different points of
the resultant depletion and replenishment cycles.
18.3.3 Foliar Retranslocation
When compared to mature leaves, senescent holm oak lea\,~s show lower
concentrations of N, P, and K, and sometimes also of Mg, and higher concentrations of Ca. These are usual changes in nutrient contents during leaf
senescence in most tree species. In our holm oak sites, from 30 to 40% of N, P,
K, and Mg in mature leaves is retranslocated during leaf senescence (Mayor
257
Holm oak leaves have similar or somewhat higher nutrient concentrations
than those of the major evergreen companion species at Prades (Table 18.3).
A coexisting deciduous oak (Quercus faginea) has higher foliar N concentrations than holm oak, reflecting the usual difference in photosynthetic capacity between evergreen and deciduous species.
18.3.2 Spatial Variations
Variations in tree nutrient concentrations among sites are usually related to
soil characteristics, which in turn vary with topographic position and other
factors. At Prades, Sabate et al. (1995) reported differences in mineral concentrations in holm oak leaves in two endpoints along a topographic gradient within the Avic catchment: they found higher foliar concentrations of N
and P, and lower concentrations of K, in a valley-bottom site than in a ridgetop site. Differences appeared both when nutrient concentrations were expressed on a dry weight basis and on a leaf area basis. Higher foliar Nand P
contents in the valley bottom can be, in part, explained by the higher nutrient
supply provided by a deeper soil: soil depth averaged 85 cm in the valley
bottom and only 45 cm in the ridge top. In contrast, the higher foliar K in the
ridge top site could derive from the stronger need to regulate the stomatal
aperture in this more xeric site, or from local differences in bedrock mineralogy.
Bedrock characteristics may contribute significantly to variations in soil
chemistry that can affect nutrient concentrations in tree tissues. In 15 different Catalan sites with contrasting bedrock (limestone and marl, volcanic
rock, and metamorphic schist) N, P and K concentrations in current-year
leaves of holm oak varied very little among sites, less so than in older leaves,
current-year twigs, or older twigs (Canadell and Vila 1992). This suggests a
tight regulation of the concentrations of these most important nutrients in
the youngest leaf cohort, which is the best illuminated. The higher variability
of nutrient concentrations in older leaves and in twigs found by Canadell
and Vila (1992) and Sabate (1993) can in part result from the higher heterogeneity of the light climate they experience, and from nutrient redistribution
within tree crowns. Mobile nutrients could be redistributed depending on
growth demands, with the different sites being sampled at different points of
the resultant depletion and replenishment cycles.
18.3.3 Foliar Retranslocation
When compared to mature leaves, senescent holm oak lea\,~s show lower
concentrations of N, P, and K, and sometimes also of Mg, and higher concentrations of Ca. These are usual changes in nutrient contents during leaf
senescence in most tree species. In our holm oak sites, from 30 to 40% of N, P,
K, and Mg in mature leaves is retranslocated during leaf senescence (Mayor