Acorn Ecology
83
Long 1995). The infection rate does not appear clearly associated with acorn
production (Fig. 6.3), suggesting the existence of mechanisms, such as diapause and migration, that make the infection rate somewhat independent
from the high interannual variability in the acorn crop.
The infection rates by other insect species are lower, and data from holm
oak populations in the lowlands near Montseny indicate that they range from
o to 16% (mean 1.7%, SE 0.33) in Callirhytis glandium, and from 0 to 25%
(mean 4.9%, SE 0.42) in Carpocapsa sp. In both cases, infection typically
damages the embryos and germination is not possible.
6.3.2 Post-dispersal Predation
In the studied holm oak forests, many animals may consume the acorns after
they drop from trees. They include birds, such as jay (Garrulus glandarius),
magpie (Pica pica) and wood pigeon (Columba palumbus), and mammals,
such as wood mice (ApiJdemus sylvaticus), Mediterranean mice (Mus spretus), squirrel (Sciurus vulgaris) and wild boar (Sus scrofa) (Rossell 1988). Observations over several years in holm oak populations in the lowlands near
Montseny have shown that wood mice and Mediterranean mice are the only
effective post-dispersal consumers of acorns.
Both mice species consume a great proportion of the dropped crop. Predation may also occur on the remaining part of the acorn after seedling
emergence, and in these cases it does not necessarily imply the death of the
new plant. Acorns may be consumed in the same place where they are found,
or they may be carried to a safer place for the animal, where they are consumed immediately. Occasionally, they may be carried to the rodent belowground-refuge, where they are consumed or stored. When the size of the
acorn stock makes difficult their immediate consumption or their storage in
the belowground refuges, they can also be buried. Burying avoids desiccation
and acorns belowground remain appropriate for consumption longer than
those remaining at the soil surface. Some of these buried acorns remain in
the soil enough time to allow seedling emergence in March-April. In this way,
rodent activity increases the dispersal distance of acorns (Iida 1996) and,
when buried, favours their germination.
Acorn predation is especially intense in shrub lands close to holm oak forests because of the small acorn production of these areas and the relatively
high density of rodent populations. We have observed that the removal rate
of acorns by rodents is very high in secondary woodlands developing from
previous pine forests that burned 12-20 years ago. Most acorns are removed
on the first night after being labelled and experimentally placed on the
ground (Fig. 6.4), and almost all of them disappear in a few days. Some of
these acorns are consumed in the same spot where they were placed. We have
not found seedlings emerging from removed, labelled acorns, suggesting
high rates of consumption. Acorns placed in clearings more than 3 m in
diameter take longer to be removed and some of them remain untouched. In
83
Long 1995). The infection rate does not appear clearly associated with acorn
production (Fig. 6.3), suggesting the existence of mechanisms, such as diapause and migration, that make the infection rate somewhat independent
from the high interannual variability in the acorn crop.
The infection rates by other insect species are lower, and data from holm
oak populations in the lowlands near Montseny indicate that they range from
o to 16% (mean 1.7%, SE 0.33) in Callirhytis glandium, and from 0 to 25%
(mean 4.9%, SE 0.42) in Carpocapsa sp. In both cases, infection typically
damages the embryos and germination is not possible.
6.3.2 Post-dispersal Predation
In the studied holm oak forests, many animals may consume the acorns after
they drop from trees. They include birds, such as jay (Garrulus glandarius),
magpie (Pica pica) and wood pigeon (Columba palumbus), and mammals,
such as wood mice (ApiJdemus sylvaticus), Mediterranean mice (Mus spretus), squirrel (Sciurus vulgaris) and wild boar (Sus scrofa) (Rossell 1988). Observations over several years in holm oak populations in the lowlands near
Montseny have shown that wood mice and Mediterranean mice are the only
effective post-dispersal consumers of acorns.
Both mice species consume a great proportion of the dropped crop. Predation may also occur on the remaining part of the acorn after seedling
emergence, and in these cases it does not necessarily imply the death of the
new plant. Acorns may be consumed in the same place where they are found,
or they may be carried to a safer place for the animal, where they are consumed immediately. Occasionally, they may be carried to the rodent belowground-refuge, where they are consumed or stored. When the size of the
acorn stock makes difficult their immediate consumption or their storage in
the belowground refuges, they can also be buried. Burying avoids desiccation
and acorns belowground remain appropriate for consumption longer than
those remaining at the soil surface. Some of these buried acorns remain in
the soil enough time to allow seedling emergence in March-April. In this way,
rodent activity increases the dispersal distance of acorns (Iida 1996) and,
when buried, favours their germination.
Acorn predation is especially intense in shrub lands close to holm oak forests because of the small acorn production of these areas and the relatively
high density of rodent populations. We have observed that the removal rate
of acorns by rodents is very high in secondary woodlands developing from
previous pine forests that burned 12-20 years ago. Most acorns are removed
on the first night after being labelled and experimentally placed on the
ground (Fig. 6.4), and almost all of them disappear in a few days. Some of
these acorns are consumed in the same spot where they were placed. We have
not found seedlings emerging from removed, labelled acorns, suggesting
high rates of consumption. Acorns placed in clearings more than 3 m in
diameter take longer to be removed and some of them remain untouched. In