Water and Nutrient Limitations to Primary Production
187
fects on stem growth, we pooled plots, taking into account irrigation categories only: for all non-irrigated plots taken together (n = 12), mean diameter
increment was 0.33 ± 0.05 (SE) mm year-I, whilst for irrigated plots (n = 12)
it was 0.57 ± 0.05 (SE) mm year-I. Water addition during the warm season
thus increased the stem diameter increment on average by 66%.
13.4.2 Litterfall
The litterfall rate measured in our control plots (3.9 Mg ha- I year-I) was
similar to those recorded in many other holm oak forests, but, surprisingly, it
was much higher than in a nearby north-facing plot in the Avic catchment
(Bellot et al. 1992; Chap. 3). In our control plots, holm oak leaf fall averaged
2.1 Mg ha- I year-I and accounted for 54% of fine litterfall.
The enhancing effect of N fertilization on fine litterfall (Table 13.1) was
distributed among the different litterfall components. In particular, average
fruit fall increased by 54% and holm oak leaf fall by 11 %. Despite not having
an effect on total fine litterfall, irrigation did increase fruit fall on average by
48%, an effect similar to that of nitrogen. Surprisingly, irrigation slightly decreased holm oak leaf fall, by 10% when averaged over the 3 years and over
all plots receiving one of the irrigation treatments.
13.5 Mechanisms of Response
A detailed mechanistic analysis of the observed responses was not the objective of our experiment. Nonetheless, our studies conducted on the experimental plots shed light on some of the mechanisms underlying the effects of
increased resource availability on primary production.
13.5.1 Improved Water Status
Weekly irrigation during the warm season was generally instrumental in increasing the soil water content of the forest floor and upper mineral soil of
irrigated plots (V. Diego, unpubl. data). Mature sun-leaves from the topmost
branches produced during the second growing season after treatment application began had a significantly lower 13C content in irrigated than in control
plots (F. Roda et al. unpubl. data). This shows that, averaging over the lifespan of the leaf, irrigated trees had a lower water use efficiency than control
trees (Farquhar et al. 1989) as a result of their improved water status. In this
water-limited forest, where almost the entire annual precipitation is evapotranspired (Pifiol et al. 1995; Chap. 19), the supplementary water added via
irrigation was probably used to increase transpiration, either lengthening the
periods of active gas exchange at the leaf surface and/or increasing the mean
187
fects on stem growth, we pooled plots, taking into account irrigation categories only: for all non-irrigated plots taken together (n = 12), mean diameter
increment was 0.33 ± 0.05 (SE) mm year-I, whilst for irrigated plots (n = 12)
it was 0.57 ± 0.05 (SE) mm year-I. Water addition during the warm season
thus increased the stem diameter increment on average by 66%.
13.4.2 Litterfall
The litterfall rate measured in our control plots (3.9 Mg ha- I year-I) was
similar to those recorded in many other holm oak forests, but, surprisingly, it
was much higher than in a nearby north-facing plot in the Avic catchment
(Bellot et al. 1992; Chap. 3). In our control plots, holm oak leaf fall averaged
2.1 Mg ha- I year-I and accounted for 54% of fine litterfall.
The enhancing effect of N fertilization on fine litterfall (Table 13.1) was
distributed among the different litterfall components. In particular, average
fruit fall increased by 54% and holm oak leaf fall by 11 %. Despite not having
an effect on total fine litterfall, irrigation did increase fruit fall on average by
48%, an effect similar to that of nitrogen. Surprisingly, irrigation slightly decreased holm oak leaf fall, by 10% when averaged over the 3 years and over
all plots receiving one of the irrigation treatments.
13.5 Mechanisms of Response
A detailed mechanistic analysis of the observed responses was not the objective of our experiment. Nonetheless, our studies conducted on the experimental plots shed light on some of the mechanisms underlying the effects of
increased resource availability on primary production.
13.5.1 Improved Water Status
Weekly irrigation during the warm season was generally instrumental in increasing the soil water content of the forest floor and upper mineral soil of
irrigated plots (V. Diego, unpubl. data). Mature sun-leaves from the topmost
branches produced during the second growing season after treatment application began had a significantly lower 13C content in irrigated than in control
plots (F. Roda et al. unpubl. data). This shows that, averaging over the lifespan of the leaf, irrigated trees had a lower water use efficiency than control
trees (Farquhar et al. 1989) as a result of their improved water status. In this
water-limited forest, where almost the entire annual precipitation is evapotranspired (Pifiol et al. 1995; Chap. 19), the supplementary water added via
irrigation was probably used to increase transpiration, either lengthening the
periods of active gas exchange at the leaf surface and/or increasing the mean