128
Santiago Sabate, Anna Sala and Carlos A. Gracia
Leaf pubescence in holm oak (hairs are only present in the abaxial face)
increases from the bottom to the top of the canopy (Table 9.3). Increases in
leaf pubescence may result in increases in the thickness and humidity of the
leaf boundary layer, thus reducing the leaf to air water vapour pressure gradient and minimizing water loss by transpiration. Although increased water
use efficiency appears to be a direct consequence of increased pubescence,
the direct link between leaf pubescence and leaf gas exchange has never been
accurately tested.
9.4.2 Chlorophyll Content
Chlorophyll content on a leaf area basis changes considerably within holm
oak canopies. In the upper 3 m of the canopy at La Castanya, leaf chlorophyll
content is inversely related to the incident photosynthetically active radiation
(PAR) (Gracia 1983). Chlorophyll contents range from 600-700 mg m- 2 at the
top of the canopy to a maximum of approximately 1000 mg m- 2 at 3 m below
the top (Fig. 9.3). Below this point chlorophyll decreases in parallel with decreases of incident PAR to values between 400 and 200 mg m- 2 (Fig. 9.3).
Gracia (1983) reported very similar patterns of leaf-area-based chlorophyll
content in beech (Fagus sylvatica) while Ellsworth and Reich (1993) did not
find within-canopy variation in Acer saccharum.
At La Castanya, shaded leaves are relatively enriched with chlorophyll b:
the chlorophyll alb ratio decreases from 2.66 at the top of the canopy to 2.24
at 9 m below the top (Gracia 1983). The enrichment in chlorophyll b is due to
the extensive stacking of grana in chloroplasts of shaded leaves. These stacks
o
2
Incident PAR. (%J
o 20 40 60 80 100 0
•
I: (
11: 10 •
o
~ 12
U 14
16
18
Chlorophyll (a+bJ
(mg Chi I m 2 of leaf)
400
800 1200
--Ie<- 1-...- ~ ~ J
•
(
Cumulative
Chlorophyll (a+bJ
(g Chi I m 2 of ground)
o 1 2 3 4 5 o
2
4
6 §:
fj
8 go
10 11:
12 g til
14 u
16
18
Fig. 9.3. Percentage of PAR (le!t) , chlorophyll content on a leaf (middle) and ground (right) area
basis with increased canopy depth at La Castanya (Montseny) holm oak forest
Santiago Sabate, Anna Sala and Carlos A. Gracia
Leaf pubescence in holm oak (hairs are only present in the abaxial face)
increases from the bottom to the top of the canopy (Table 9.3). Increases in
leaf pubescence may result in increases in the thickness and humidity of the
leaf boundary layer, thus reducing the leaf to air water vapour pressure gradient and minimizing water loss by transpiration. Although increased water
use efficiency appears to be a direct consequence of increased pubescence,
the direct link between leaf pubescence and leaf gas exchange has never been
accurately tested.
9.4.2 Chlorophyll Content
Chlorophyll content on a leaf area basis changes considerably within holm
oak canopies. In the upper 3 m of the canopy at La Castanya, leaf chlorophyll
content is inversely related to the incident photosynthetically active radiation
(PAR) (Gracia 1983). Chlorophyll contents range from 600-700 mg m- 2 at the
top of the canopy to a maximum of approximately 1000 mg m- 2 at 3 m below
the top (Fig. 9.3). Below this point chlorophyll decreases in parallel with decreases of incident PAR to values between 400 and 200 mg m- 2 (Fig. 9.3).
Gracia (1983) reported very similar patterns of leaf-area-based chlorophyll
content in beech (Fagus sylvatica) while Ellsworth and Reich (1993) did not
find within-canopy variation in Acer saccharum.
At La Castanya, shaded leaves are relatively enriched with chlorophyll b:
the chlorophyll alb ratio decreases from 2.66 at the top of the canopy to 2.24
at 9 m below the top (Gracia 1983). The enrichment in chlorophyll b is due to
the extensive stacking of grana in chloroplasts of shaded leaves. These stacks
o
2
Incident PAR. (%J
o 20 40 60 80 100 0
•
I: (
11: 10 •
o
~ 12
U 14
16
18
Chlorophyll (a+bJ
(mg Chi I m 2 of leaf)
400
800 1200
--Ie<- 1-...- ~ ~ J
•
(
Cumulative
Chlorophyll (a+bJ
(g Chi I m 2 of ground)
o 1 2 3 4 5 o
2
4
6 §:
fj
8 go
10 11:
12 g til
14 u
16
18
Fig. 9.3. Percentage of PAR (le!t) , chlorophyll content on a leaf (middle) and ground (right) area
basis with increased canopy depth at La Castanya (Montseny) holm oak forest