Leaf Traits and Canopy Organization
127
1997). These different responses may reflect a growth response to the most
limiting nutrient in each soil type.
Concurrent with decreases of LSM with increased canopy depth, there are
substantial increases in leaf size from top to bottom of the canopy. While leaf
size at the top of the canopy is similar at the ridge and valley sites of Avic,
leaves below 1 m from the top of the canopy are significantly larger at the
valley site than at the ridge site (Fig. 9.2). These differences may be due, in
part, to increased leaf growth at the valley site due to greater water availability compared to the ridge site. Stronger radiation extinction within canopies
at the valley site (due to greater LAI and the presence of understory) may
also contribute to increased leaf size at lower canopy levels at this site. Small
leaf size in the upper canopy layers where LAI is greater allows for efficient
overall direct light capture while it improves light penetration deeper in the
canopy. Radiation capture at the bottom of the canopy, where only 10% of
the incident radiation is available (Chap. 11), is facilitated by exposing large
surface areas.
In Mediterranean climates, where periods of greatest evaporative demand
coincide with periods of reduced soil water availability, changes in LSM and
leaf size within the canopy have adaptive value. For instance, the decrease in
leaf size at the top of the canopy confers a mechanism to improve thermoregulation and heat dissipation during summer drought (Gates 1980).
Improved thermoregulation may significantly contribute to leaf cooling,
which, in turn, may prevent tissue mortality due to excessive leaf temperatures. Similarly, higher LSM at the top of the canopy may be advantageous to
reduce water loss by transpiration during the summer, because increases in
LSM are associated with increased resistance to water loss (Specht 1988;
Specht and Specht 1989). However, increased LSM is also associated with increased resistance to diffusion of CO2, thus posing a potential limitation to
carbon uptake. Relatively high leaf stomatal density at the top of the canopy
(Table 9.3) may compensate for the negative effects of increased LSM and
improve CO2 uptake during favourable periods. Stomata of holm oak are
highly sensitive to atmospheric and soil drought and strong stomatal closure
prevents excessive water loss under high evaporative demand and/or low soil
moisture, when the risk of desiccation is higher (Sala and Tenhunen 1994;
Chaps. 10 and 11).
Table 9.3. Stomatal and hair density in holm oak leaves from
top to bottom of the canopy in the permanent plot at La Castanya, Montseny (Save 1986)
Canopy depth
Stomatal density
Hair density
from top (m)
(no. mm- 2 )
(no. mm- 2 )
o
2
4
6
135.5
108.1
98.5
76.8
108.4
101.8
85.4
12.5
127
1997). These different responses may reflect a growth response to the most
limiting nutrient in each soil type.
Concurrent with decreases of LSM with increased canopy depth, there are
substantial increases in leaf size from top to bottom of the canopy. While leaf
size at the top of the canopy is similar at the ridge and valley sites of Avic,
leaves below 1 m from the top of the canopy are significantly larger at the
valley site than at the ridge site (Fig. 9.2). These differences may be due, in
part, to increased leaf growth at the valley site due to greater water availability compared to the ridge site. Stronger radiation extinction within canopies
at the valley site (due to greater LAI and the presence of understory) may
also contribute to increased leaf size at lower canopy levels at this site. Small
leaf size in the upper canopy layers where LAI is greater allows for efficient
overall direct light capture while it improves light penetration deeper in the
canopy. Radiation capture at the bottom of the canopy, where only 10% of
the incident radiation is available (Chap. 11), is facilitated by exposing large
surface areas.
In Mediterranean climates, where periods of greatest evaporative demand
coincide with periods of reduced soil water availability, changes in LSM and
leaf size within the canopy have adaptive value. For instance, the decrease in
leaf size at the top of the canopy confers a mechanism to improve thermoregulation and heat dissipation during summer drought (Gates 1980).
Improved thermoregulation may significantly contribute to leaf cooling,
which, in turn, may prevent tissue mortality due to excessive leaf temperatures. Similarly, higher LSM at the top of the canopy may be advantageous to
reduce water loss by transpiration during the summer, because increases in
LSM are associated with increased resistance to water loss (Specht 1988;
Specht and Specht 1989). However, increased LSM is also associated with increased resistance to diffusion of CO2, thus posing a potential limitation to
carbon uptake. Relatively high leaf stomatal density at the top of the canopy
(Table 9.3) may compensate for the negative effects of increased LSM and
improve CO2 uptake during favourable periods. Stomata of holm oak are
highly sensitive to atmospheric and soil drought and strong stomatal closure
prevents excessive water loss under high evaporative demand and/or low soil
moisture, when the risk of desiccation is higher (Sala and Tenhunen 1994;
Chaps. 10 and 11).
Table 9.3. Stomatal and hair density in holm oak leaves from
top to bottom of the canopy in the permanent plot at La Castanya, Montseny (Save 1986)
Canopy depth
Stomatal density
Hair density
from top (m)
(no. mm- 2 )
(no. mm- 2 )
o
2
4
6
135.5
108.1
98.5
76.8
108.4
101.8
85.4
12.5