190
Ferran Roda, Xavier Mayor, Santiago Sabate and Victoria Diego
high increase in this holm oak forest, where the canopy was already closed
before fertilization and where water availability strongly limits tree growth.
The effect of the experimental treatments on LAI and light interception
was, at least in part, due to the increased size of leaves and stems of new
shoots. We measured the size (weight, area, and length) of leaves and stems
of shoot cohorts borne on topmost branches sampled on several occasions
(Sabate 1993). For organs produced in the second spring after treatment application began, N fertilization increased the mean weight of individual
leaves and the mean length of individual stems of current-year shoots. For
this same cohort, irrigation increased the mean weight of leaves and standing fruits (sampled in August) and the mean weight and length of stems of
current-year shoots. These responses, added to possible (but unmeasured)
increases in the number of new leaves produced, accounted for the observed
increases in leaf area supported per branch unit (Fig. 13.2a) and in light interception in plots receiving either irrigation or N fertilization. A further
possible mechanism leading to increased LAl could be the extended retention of older leaves. IIi fact, the opposite was observed: when the leaf area
supported per unit of cross-sectional area of branch was split into leaf cohorts, all experimental factors showed accelerated shedding of the older cohort (2-year-old leaves; Fig. 13.2b). Thus, neither supplemental water, nitrogen, nor phosphorus increased the retention of older leaves. Total leaf area
borne on topmost branches in N-fertilized plots therefore increased despite
accelerated shedding of old leaves. For irrigated plots, total leaf area remained constant because the accelerated shedding of old leaves and the increased production of new leaves were of the same magnitude (Fig. 13.2c).
The above results show that, in irrigated or N-fertilized plots, the canopies
were not in steady state over the first 3 years of the experiment, but rather
they accumulated leaf biomass during this period. This was probably a major
factor behind the increased ANPP induced by both experimental factors.
However, a non-steady state leaf biomass implies that we underestimated the
net biomass increment because we used the same allometric regressions for
estimating the biomass at the start of the experiment and 3 years later. This
procedure was probably suitable for estimating the increment in woody biomass since, given the very low rates of diameter and height growth we found
even in treated plots, it was unlikely that allometric differences in woody
biomass between treated and untreated trees appeared in only 3 years. On
the other hand, for faster-aggrading tree components such as leaves, using
pre-treatment allometric regressions surely underestimated the net biomass
increment and the ANPP of, particularly, N-fertilized plots (Fig. 13.2c). Yet,
this simply reinforces our findings on the effect of N fertilization on canopy
dynamics and litterfall. The net effect of incorporating the increase in
standing leaf biomass in our estimates would be to strengthen the effect of N
fertilization. In particular, ANPP of this holm oak forest is somewhat more
responsive to N fertilization than Fig. 13.1 suggests.
Ferran Roda, Xavier Mayor, Santiago Sabate and Victoria Diego
high increase in this holm oak forest, where the canopy was already closed
before fertilization and where water availability strongly limits tree growth.
The effect of the experimental treatments on LAI and light interception
was, at least in part, due to the increased size of leaves and stems of new
shoots. We measured the size (weight, area, and length) of leaves and stems
of shoot cohorts borne on topmost branches sampled on several occasions
(Sabate 1993). For organs produced in the second spring after treatment application began, N fertilization increased the mean weight of individual
leaves and the mean length of individual stems of current-year shoots. For
this same cohort, irrigation increased the mean weight of leaves and standing fruits (sampled in August) and the mean weight and length of stems of
current-year shoots. These responses, added to possible (but unmeasured)
increases in the number of new leaves produced, accounted for the observed
increases in leaf area supported per branch unit (Fig. 13.2a) and in light interception in plots receiving either irrigation or N fertilization. A further
possible mechanism leading to increased LAl could be the extended retention of older leaves. IIi fact, the opposite was observed: when the leaf area
supported per unit of cross-sectional area of branch was split into leaf cohorts, all experimental factors showed accelerated shedding of the older cohort (2-year-old leaves; Fig. 13.2b). Thus, neither supplemental water, nitrogen, nor phosphorus increased the retention of older leaves. Total leaf area
borne on topmost branches in N-fertilized plots therefore increased despite
accelerated shedding of old leaves. For irrigated plots, total leaf area remained constant because the accelerated shedding of old leaves and the increased production of new leaves were of the same magnitude (Fig. 13.2c).
The above results show that, in irrigated or N-fertilized plots, the canopies
were not in steady state over the first 3 years of the experiment, but rather
they accumulated leaf biomass during this period. This was probably a major
factor behind the increased ANPP induced by both experimental factors.
However, a non-steady state leaf biomass implies that we underestimated the
net biomass increment because we used the same allometric regressions for
estimating the biomass at the start of the experiment and 3 years later. This
procedure was probably suitable for estimating the increment in woody biomass since, given the very low rates of diameter and height growth we found
even in treated plots, it was unlikely that allometric differences in woody
biomass between treated and untreated trees appeared in only 3 years. On
the other hand, for faster-aggrading tree components such as leaves, using
pre-treatment allometric regressions surely underestimated the net biomass
increment and the ANPP of, particularly, N-fertilized plots (Fig. 13.2c). Yet,
this simply reinforces our findings on the effect of N fertilization on canopy
dynamics and litterfall. The net effect of incorporating the increase in
standing leaf biomass in our estimates would be to strengthen the effect of N
fertilization. In particular, ANPP of this holm oak forest is somewhat more
responsive to N fertilization than Fig. 13.1 suggests.