Acorn Ecology
81
Nitrogen addition increased the production of both number and biomass
of fruits (Fig. 6.2) in most years, while phosphorus addition only produced a
non-significant increase of the crop biomass in two of the years. A positive
effect of irrigation treatment was only observed in years with high summer
drought (1989-1990 and 1990-1991; Fig. 6.2), in which it increased acorn
production both by decreasing summer abortion and by allowing larger fruit
sizes (Table 6.2). The effects of nitrogen and water availability, however, can
hardly be considered separately. Nitrogen fertilization increased acorn production especially in 1992-1993 (Fig. 6.2), which was a very rainy year. In the
very dry year 1989-1990, fruit production was extremely low, and although
the number of acorns was significantly higher in nitrogen-fertilized plots
(Table 6.2), there was no effect of nitrogen addition on acorn biomass production (Fig. 6.2). This result may have been also influenced by the short
time existing between fertilization (March) and flowering (May).
Therefore, the importance of the resources controlling the crop size
changes between years depending on the climatic conditions. Probably, the
initial flower production is mainly determined by nutrient availability and
also by climatic cues occurring in the winter-spring period. Acorn development seems to be very dependent on water availability. The hydric stress occurring during summer produces very high levels of acorn abortion (Stephenson 1981), while surviving acorns do not increase their size during this
period.
6.3 Acorn Predation
6.3.1 Pre-dispersal Predation
A large number of animals can consume acorns before they drop from the
tree (pre-dispersal predation). The most important are insects that can infect
a large proportion of the crop and may produce a premature fall of undeveloped acorns or may consume the embryos of the seed. This consumption
may sometimes be partial, allowing germination but decreasing the probabilities of seedling success. These different effects on seed fate often depend
on the infecting species. In the holm oak forests of northeastern Spain, including the Montseny and Prades massifs, the main insects feeding on acorns
are the larvae of the weevil Curculio glandium (0. Coleoptera, Fam. Curcilionidae), those of the wasp Callirhytis glandium (0. Hymenoptera, Fam.
Cynipidae) and of the moth Carpocapsa amplana (0. Lepidoptera, Fam.
Yponomeutidae).
Curculio glandium is the main pre-dispersal consumer in the studied holm
oak forests, with infection percentages ranging from 11 to 100% for individual trees and different years, with an overall mean of 49%. This weevil has a
life history strongly dependent on the reproductive cycle of several Quercus
81
Nitrogen addition increased the production of both number and biomass
of fruits (Fig. 6.2) in most years, while phosphorus addition only produced a
non-significant increase of the crop biomass in two of the years. A positive
effect of irrigation treatment was only observed in years with high summer
drought (1989-1990 and 1990-1991; Fig. 6.2), in which it increased acorn
production both by decreasing summer abortion and by allowing larger fruit
sizes (Table 6.2). The effects of nitrogen and water availability, however, can
hardly be considered separately. Nitrogen fertilization increased acorn production especially in 1992-1993 (Fig. 6.2), which was a very rainy year. In the
very dry year 1989-1990, fruit production was extremely low, and although
the number of acorns was significantly higher in nitrogen-fertilized plots
(Table 6.2), there was no effect of nitrogen addition on acorn biomass production (Fig. 6.2). This result may have been also influenced by the short
time existing between fertilization (March) and flowering (May).
Therefore, the importance of the resources controlling the crop size
changes between years depending on the climatic conditions. Probably, the
initial flower production is mainly determined by nutrient availability and
also by climatic cues occurring in the winter-spring period. Acorn development seems to be very dependent on water availability. The hydric stress occurring during summer produces very high levels of acorn abortion (Stephenson 1981), while surviving acorns do not increase their size during this
period.
6.3 Acorn Predation
6.3.1 Pre-dispersal Predation
A large number of animals can consume acorns before they drop from the
tree (pre-dispersal predation). The most important are insects that can infect
a large proportion of the crop and may produce a premature fall of undeveloped acorns or may consume the embryos of the seed. This consumption
may sometimes be partial, allowing germination but decreasing the probabilities of seedling success. These different effects on seed fate often depend
on the infecting species. In the holm oak forests of northeastern Spain, including the Montseny and Prades massifs, the main insects feeding on acorns
are the larvae of the weevil Curculio glandium (0. Coleoptera, Fam. Curcilionidae), those of the wasp Callirhytis glandium (0. Hymenoptera, Fam.
Cynipidae) and of the moth Carpocapsa amplana (0. Lepidoptera, Fam.
Yponomeutidae).
Curculio glandium is the main pre-dispersal consumer in the studied holm
oak forests, with infection percentages ranging from 11 to 100% for individual trees and different years, with an overall mean of 49%. This weevil has a
life history strongly dependent on the reproductive cycle of several Quercus