188
Ferran Roda, Xavier Mayor, Santiago Sabate and Victoria Diego
stomatal conductance. Both responses would allow a higher rate of net carbon fIxation, eventually increasing stem diameter growth in irrigated trees,
as we observed.
N fertilization did not signifIcantly change the I3C content of holm oak
leaves. Thus, and although improved N nutrition often leads to higher water
use effIciency, the increased N supply did not have such an effect in our holm
oak plots.
13.5.2 Increased Photosynthetic Capacity
N fertilization signifIcantly increased the N concentration in topmost sun
leaves produced during the second growing season after fertilization, from a
mean of ca. l.3% in non N-fertilized plots to ca. l.5% in N-fertilized plots
(Sabate and Gracia 1994). Irrigation and P fertilization did not affect the foliar N concentration. Given the general relationship between foliar N content
and photosynthetic capacity (Field and Mooney 1986), it is likely that our Nfertilized holm oaks had an increased rate of net carbon fIxation, leading to a
higher ANPP. Such an effect was not involved in the ANPP response to irrigation. At the canopy level, increased carbon gain resulted probably also
from the increased proportion of current"year leaves making up the leaf
population in N-fertilized plots and, to a lesser extent, in irrigated plots
(Fig. 13.2b,c).
13.5.3 Increased Light Interception
Light interception capacity is closely linked to primary production
(Landsberg 1986; Cannell 1989). By the second summer after treatment application began, the total leaf area displayed per unit of cross-sectional area
of topmost branches had increased as a result of N fertilization (Fig. 13.2a).
This implies that N-fertilized canopies had a greater capacity to intercept
light. At this time, irrigation and P fertilization had no effect on total leaf
area supported per branch.
In April 1992,3 years after the start of the experiment, light penetration
was measured below the canopy of each plot. Irrigation and N fertilization
each lowered light penetration, in both cases from average values of 91 IlE
PAR m- 2 S-1 in plots not receiving one of the above factors to 65 IlE m- 2 S-1 in
those receiving one of them. The effects of water and N were additive, and P
fertilization had no effect on light penetration.
Thus, both N fertilization and irrigation seemed to increase the leaf area
index (LAl) and the light interception capacity of the stand, though the effects of N were detected 1 year earlier than those of irrigation. An increase in
LAI after N fertilization of forests is commonly reported in the literature (e.g.
Binkley and Reid 1984; Vose et al. 1994), but we were surprised to fInd such a
Ferran Roda, Xavier Mayor, Santiago Sabate and Victoria Diego
stomatal conductance. Both responses would allow a higher rate of net carbon fIxation, eventually increasing stem diameter growth in irrigated trees,
as we observed.
N fertilization did not signifIcantly change the I3C content of holm oak
leaves. Thus, and although improved N nutrition often leads to higher water
use effIciency, the increased N supply did not have such an effect in our holm
oak plots.
13.5.2 Increased Photosynthetic Capacity
N fertilization signifIcantly increased the N concentration in topmost sun
leaves produced during the second growing season after fertilization, from a
mean of ca. l.3% in non N-fertilized plots to ca. l.5% in N-fertilized plots
(Sabate and Gracia 1994). Irrigation and P fertilization did not affect the foliar N concentration. Given the general relationship between foliar N content
and photosynthetic capacity (Field and Mooney 1986), it is likely that our Nfertilized holm oaks had an increased rate of net carbon fIxation, leading to a
higher ANPP. Such an effect was not involved in the ANPP response to irrigation. At the canopy level, increased carbon gain resulted probably also
from the increased proportion of current"year leaves making up the leaf
population in N-fertilized plots and, to a lesser extent, in irrigated plots
(Fig. 13.2b,c).
13.5.3 Increased Light Interception
Light interception capacity is closely linked to primary production
(Landsberg 1986; Cannell 1989). By the second summer after treatment application began, the total leaf area displayed per unit of cross-sectional area
of topmost branches had increased as a result of N fertilization (Fig. 13.2a).
This implies that N-fertilized canopies had a greater capacity to intercept
light. At this time, irrigation and P fertilization had no effect on total leaf
area supported per branch.
In April 1992,3 years after the start of the experiment, light penetration
was measured below the canopy of each plot. Irrigation and N fertilization
each lowered light penetration, in both cases from average values of 91 IlE
PAR m- 2 S-1 in plots not receiving one of the above factors to 65 IlE m- 2 S-1 in
those receiving one of them. The effects of water and N were additive, and P
fertilization had no effect on light penetration.
Thus, both N fertilization and irrigation seemed to increase the leaf area
index (LAl) and the light interception capacity of the stand, though the effects of N were detected 1 year earlier than those of irrigation. An increase in
LAI after N fertilization of forests is commonly reported in the literature (e.g.
Binkley and Reid 1984; Vose et al. 1994), but we were surprised to fInd such a