Water and Nutrient Limitations to Primary Production
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Primary production responses
to Irrigation and fertilization
Irrigation
N fertilization P fertilization
185
Fig. 13.1. Aboveground net primary production (ANPP) in holm oak forest at Prades during the
first 3 years of the irrigation/f¢rtilization experiment. Each column is the mean (+ 1 SE) of the
12 plots receiving/not receiving application of the specified factors
In control (Le. untreated) plots, ANPP averaged only 5.1 Mg ha- I year-I, a
typical figure for holm oak forests under dry conditions (Chap. 3). Irrigation
and N fertilization each significantly increased ANPP (Fig. 13.1), while P fertilization did not. This demonstrates that ANPP in this forest is simultaneously limited by water and N availability. The effects of irrigation and of N
fertilization were of similar (and moderate) magnitude: each factor increased ANPP on average by 0.8-1.1 Mg ha- I year-I. There were no significant interactions between the experimental factors, perhaps because the
power of the experiment to detect interactions was rather low. Even so, the
highest mean ANPP (8.1 Mg ha- I year-I) was observed in plots receiving irrigation plus NP-fertilization (INP treatment).
13.4 Components of Primary Production
We measured or estimated the two major components of ANPP: net increment of aboveground biomass and litterfall. Mean biomass increment ranged
from 1.1 Mg ha- I year- I in control plots to 3.0 Mg ha- I year- I in INP plots.
Mean fine litterfall ranged from 3.9 Mg ha- I year- I in control plots to 5.2 Mg
ha- I year- I in INP plots.
Biomass increment and litterfall were affected by the experimental treatments in contrasting ways. Adding water to this holm oak forest significantly
increased biomass increment but not litterfall; adding nitrogen had the opposite effect (Table 13.1). Therefore, increasing the availabilities of these two
resources (water and nitrogen) had a similar overall effect on ANPP, but by
-
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";'
ro
.J::
....
........
CL
CL
z
«
8
7
6
5
4
3
2
1
0
Primary production responses
to Irrigation and fertilization
Irrigation
N fertilization P fertilization
185
Fig. 13.1. Aboveground net primary production (ANPP) in holm oak forest at Prades during the
first 3 years of the irrigation/f¢rtilization experiment. Each column is the mean (+ 1 SE) of the
12 plots receiving/not receiving application of the specified factors
In control (Le. untreated) plots, ANPP averaged only 5.1 Mg ha- I year-I, a
typical figure for holm oak forests under dry conditions (Chap. 3). Irrigation
and N fertilization each significantly increased ANPP (Fig. 13.1), while P fertilization did not. This demonstrates that ANPP in this forest is simultaneously limited by water and N availability. The effects of irrigation and of N
fertilization were of similar (and moderate) magnitude: each factor increased ANPP on average by 0.8-1.1 Mg ha- I year-I. There were no significant interactions between the experimental factors, perhaps because the
power of the experiment to detect interactions was rather low. Even so, the
highest mean ANPP (8.1 Mg ha- I year-I) was observed in plots receiving irrigation plus NP-fertilization (INP treatment).
13.4 Components of Primary Production
We measured or estimated the two major components of ANPP: net increment of aboveground biomass and litterfall. Mean biomass increment ranged
from 1.1 Mg ha- I year- I in control plots to 3.0 Mg ha- I year- I in INP plots.
Mean fine litterfall ranged from 3.9 Mg ha- I year- I in control plots to 5.2 Mg
ha- I year- I in INP plots.
Biomass increment and litterfall were affected by the experimental treatments in contrasting ways. Adding water to this holm oak forest significantly
increased biomass increment but not litterfall; adding nitrogen had the opposite effect (Table 13.1). Therefore, increasing the availabilities of these two
resources (water and nitrogen) had a similar overall effect on ANPP, but by