Leaf Traits and Canopy Organization
125
the top of the canopy suggests maximum leaf functional activity in higher
canopy levels where irradiance does not limit photosynthesis (see below).
Total projected stem area on a ground area basis (SAl; excluding main
trunks) was very similar at the two Avic sites (Table 9.0. SAl contributed ca.
19 and 23% of the total phytomass projected area (leaves and stems) at the
ridge and valley sites, respectively. Thus, stems contribute significantly to
light extinction within the canopy and should be taken into account in modelling efforts to estimate canopy gas exchange.
9.4 Morphological and Functional Leaf Traits
Leaves of holm oak are typically sclerophyllous, with thick cuticles and
abundant sclerification. Short spines on the leaf margins are not uncommon.
In general, sclerophyllous leaves are considered to be adaptive in dry, nutrient-poor (especially phosphorus-poor) environments due to their increased
resistance to water loss, nutrient conservation mechanisms and herbivory
protection (Dunn et al. 1976; Rundel 1988; Specht and Rundel 1990; Herms
and Mattson 1992; Turner 1994; Feller 1995, 1996). The typical long dry
summer period characteristic of Mediterranean areas imposes severe water
limitations on growth. Growth is also limited by phosphorus, particularly on
calcareous soils (Sardans 1997), and by nitrogen and potassium (Sabate
1993). Herbivory is another important biotic factor that may strongly influence holm oak growth and production (Picolo and Terradas 1989). Thus,
sclerophyllous leaves in holm oak appear to be a response to the combined
pressures imposed by limited water and nutrients and by herbivory.
9.4.1 Leaf Size and Leaf Specific Mass
Mean area and dry weight of holm oak leaves are highly variable, ranging
from 2 to 10 cm 2 , and from 40 to 110 mg, respectively, or even less during extremely dry years (Mayor 1990; Sabate et al. 1992; Sabate 1993; Sala et al. 1994;
and unpubl. data from Prades and Montseny). Leaf mass per unit of leaf area,
or leaf specific mass (LSM), is an important trait strongly dependent on the
incident radiation and water availability (Specht and Rundel 1990; Rambal
et al. 1996). Because of the high sensitivity of LSM to incident radiation and
water availability, LSM varies between sites and, particularly, within closed
canopies where strong gradients of radiation occur (Hollinger 1989;
Ellsworth and Reich 1993; Rambal et al.1996). For instance, LSM of holm oak
leaves at the top of the canopy is 22 mg cm- 2 at Avic (Prades, xeric site) compared with 16-17 mg cm- 2 in the permanent plot at La Castanya (Montseny,
mesic site). However, within-canopy variations in LSM in response to incident
radiation are much stronger than site-to-site variations resulting from changes
125
the top of the canopy suggests maximum leaf functional activity in higher
canopy levels where irradiance does not limit photosynthesis (see below).
Total projected stem area on a ground area basis (SAl; excluding main
trunks) was very similar at the two Avic sites (Table 9.0. SAl contributed ca.
19 and 23% of the total phytomass projected area (leaves and stems) at the
ridge and valley sites, respectively. Thus, stems contribute significantly to
light extinction within the canopy and should be taken into account in modelling efforts to estimate canopy gas exchange.
9.4 Morphological and Functional Leaf Traits
Leaves of holm oak are typically sclerophyllous, with thick cuticles and
abundant sclerification. Short spines on the leaf margins are not uncommon.
In general, sclerophyllous leaves are considered to be adaptive in dry, nutrient-poor (especially phosphorus-poor) environments due to their increased
resistance to water loss, nutrient conservation mechanisms and herbivory
protection (Dunn et al. 1976; Rundel 1988; Specht and Rundel 1990; Herms
and Mattson 1992; Turner 1994; Feller 1995, 1996). The typical long dry
summer period characteristic of Mediterranean areas imposes severe water
limitations on growth. Growth is also limited by phosphorus, particularly on
calcareous soils (Sardans 1997), and by nitrogen and potassium (Sabate
1993). Herbivory is another important biotic factor that may strongly influence holm oak growth and production (Picolo and Terradas 1989). Thus,
sclerophyllous leaves in holm oak appear to be a response to the combined
pressures imposed by limited water and nutrients and by herbivory.
9.4.1 Leaf Size and Leaf Specific Mass
Mean area and dry weight of holm oak leaves are highly variable, ranging
from 2 to 10 cm 2 , and from 40 to 110 mg, respectively, or even less during extremely dry years (Mayor 1990; Sabate et al. 1992; Sabate 1993; Sala et al. 1994;
and unpubl. data from Prades and Montseny). Leaf mass per unit of leaf area,
or leaf specific mass (LSM), is an important trait strongly dependent on the
incident radiation and water availability (Specht and Rundel 1990; Rambal
et al. 1996). Because of the high sensitivity of LSM to incident radiation and
water availability, LSM varies between sites and, particularly, within closed
canopies where strong gradients of radiation occur (Hollinger 1989;
Ellsworth and Reich 1993; Rambal et al.1996). For instance, LSM of holm oak
leaves at the top of the canopy is 22 mg cm- 2 at Avic (Prades, xeric site) compared with 16-17 mg cm- 2 in the permanent plot at La Castanya (Montseny,
mesic site). However, within-canopy variations in LSM in response to incident
radiation are much stronger than site-to-site variations resulting from changes