Leaf Traits and Canopy Organization
129
contain a high concentration of the light-harvesting complex II, where most
of the chlorophyll b is found (Anderson 1986).
In holm oak, the patterns of leaf-area-based chlorophyll content within
the canopy appear to be a balance between the optimal use of resources and
the maximization of canopy carbon input. Leaf photosynthetic activity in the
top 3 m of the canopy is high due to adequate light and relatively high N
content (see below). Within this active portion of the canopy, where more
than 90% of the total canopy LAI is found (Fig. 9.1), PAR levels decrease
rapidly and leaf chlorophyll concentration increases (Fig. 9.3). This is a typical plastic response to increase light absorption in the shade, where photosystems are no longer light saturated. It appears, then, that at PAR levels
above 30% of maximum incident PAR, investments in chlorophyll in leaves
located up to 3-m deep into the canopy result in a positive photosynthetic
return. However, below 3 m, where PAR levels decrease below 30% of the
maximum, investment in chlorophyll is no longer advantageous, probably as
a result of excessively low light levels. Most leaves already absorb 80-85% of
the available light and a further doubling of chlorophyll concentration only
increases absorption by 3 to 6% (Evans 1988).
9.4.3 Nutrient Content
Holm oak leaves are important nutrient sinks and have a relatively high nutrient concentration compared to other tree components (Chap. 18). Seasonal
trends of leaf-area-based Nand P concentrations of a single age cohort of
holm oak leaves at different canopy depths are shown in Fig. 9.4 for the ridge
and valley sites of the Avic catchment.
Leaf N content is higher at the valley site than at the ridge site. Higher N
content at the valley site may be related to higher total soil nutrient content at
the former site due to increased soil depth (Sabate et al. 1995). Nand P content differences between both sites of Avic are larger when concentrations are
expressed on a dry weight basis (Sabate et al. 1995). Presumably leaf tissue at
the ridge site (more xeric) has a higher content of structural compounds
with little or no Nand P than at the valley. Leaf K concentration is higher at
the ridge site than at the valley site on both a leaf area and a dry weight basis
(Sabate 1993; Sabate et al. 1995). As K plays a key role in osmotic processes
and stomatal control of water loss (Marschner 1995) it is not surprising to
find higher K contents at the more xeric ridge site. This is indeed consistent
with the fact that trees at this site exhibit a higher resistance to water stress
compared with trees at the valley bottom (Sala et al. 1994).
Leaf-area-based Nand P contents decrease significantly from top to bottom of the canopy (Fig. 9.4). However, on a dry weight basis, differences are
very small (Sabate et al. 1995), indicating that changes in area-based nutrient
content result from the strong decrease of LSM from the top to the bottom of
the canopy. As increases in N content are related to increases in photosynthetic capacity (Field and Mooney 1986; Evans 1989; Reich et al. 1992; Rambal
129
contain a high concentration of the light-harvesting complex II, where most
of the chlorophyll b is found (Anderson 1986).
In holm oak, the patterns of leaf-area-based chlorophyll content within
the canopy appear to be a balance between the optimal use of resources and
the maximization of canopy carbon input. Leaf photosynthetic activity in the
top 3 m of the canopy is high due to adequate light and relatively high N
content (see below). Within this active portion of the canopy, where more
than 90% of the total canopy LAI is found (Fig. 9.1), PAR levels decrease
rapidly and leaf chlorophyll concentration increases (Fig. 9.3). This is a typical plastic response to increase light absorption in the shade, where photosystems are no longer light saturated. It appears, then, that at PAR levels
above 30% of maximum incident PAR, investments in chlorophyll in leaves
located up to 3-m deep into the canopy result in a positive photosynthetic
return. However, below 3 m, where PAR levels decrease below 30% of the
maximum, investment in chlorophyll is no longer advantageous, probably as
a result of excessively low light levels. Most leaves already absorb 80-85% of
the available light and a further doubling of chlorophyll concentration only
increases absorption by 3 to 6% (Evans 1988).
9.4.3 Nutrient Content
Holm oak leaves are important nutrient sinks and have a relatively high nutrient concentration compared to other tree components (Chap. 18). Seasonal
trends of leaf-area-based Nand P concentrations of a single age cohort of
holm oak leaves at different canopy depths are shown in Fig. 9.4 for the ridge
and valley sites of the Avic catchment.
Leaf N content is higher at the valley site than at the ridge site. Higher N
content at the valley site may be related to higher total soil nutrient content at
the former site due to increased soil depth (Sabate et al. 1995). Nand P content differences between both sites of Avic are larger when concentrations are
expressed on a dry weight basis (Sabate et al. 1995). Presumably leaf tissue at
the ridge site (more xeric) has a higher content of structural compounds
with little or no Nand P than at the valley. Leaf K concentration is higher at
the ridge site than at the valley site on both a leaf area and a dry weight basis
(Sabate 1993; Sabate et al. 1995). As K plays a key role in osmotic processes
and stomatal control of water loss (Marschner 1995) it is not surprising to
find higher K contents at the more xeric ridge site. This is indeed consistent
with the fact that trees at this site exhibit a higher resistance to water stress
compared with trees at the valley bottom (Sala et al. 1994).
Leaf-area-based Nand P contents decrease significantly from top to bottom of the canopy (Fig. 9.4). However, on a dry weight basis, differences are
very small (Sabate et al. 1995), indicating that changes in area-based nutrient
content result from the strong decrease of LSM from the top to the bottom of
the canopy. As increases in N content are related to increases in photosynthetic capacity (Field and Mooney 1986; Evans 1989; Reich et al. 1992; Rambal