A review of dipterocarps - Center for International Forestry Research

Seed Handling 74
viability of the seed but experimental proof is lacking.
In some years there are heavy infestations of the
developing seed by pests and insects. It is possible that
heavy infestations occur relatively more frequently in
years when there are light crops on the tree but
confirmation of this relationship is needed.
Maturity
Seed germination continues to improve up to near the
time of peak maturity, emphasising the need for optimal
harvest timing.
Physiological and other associated damage
During the period between collection and arrival at the
seed centre, material is at risk. This applies particularly
if seed is held under conditions that are either too humid
or too dry, and if temperatures are too high or too low.
Necrosis is liable to occur under such conditions,
associated with fungal growth and viability loss.
Seed Storage Categories
Researchers have divided seeds broadly into 3 major
groups on the basis of their storage behaviour. The
following descriptions give the general basis for each
type; more accurate definitions are presented in Chapter
3 (pages 60-61).
Orthodox seeds
This category includes seeds that can be dried to low
moisture contents (about 5%) without serious
deleterious effects. Under optimal conditions, the life
span of this group of seeds can be extended for decades
or longer.
Recalcitrant seeds
This group of seeds differs from orthodox seeds in two
ways; their seeds die if they are dried below relatively
high moisture contents (values are given for lowest-safe
moisture contents in Chapter 3, page 62) and if they are
subject to damage at low temperatures (< 16 o C). Even
under optimal conditions survival of seeds in this group
is limited. The difficulties in storing the seed led to their
being described as ‘recalcitrant’.
Intermediate (OLDA) seeds
A third category of seed storage physiology has been
recently defined. In practice, the seeds in this group have
desiccation characteristics that are intermediate between
those of the orthodox and recalcitrant seeds and they
have thus been termed ‘intermediate’. When harvested
in the usual way, seeds of this type can be dried to
moisture levels of about 8-12% whilst retaining a
substantial amount of (but not all) their original viability.
There is also a greater susceptibility to chilling and
freezing damage than is the case with orthodox seed, even
when the seeds are relatively dry.
When this type of seed was first studied in detail, its
physiological similarity to orthodox seeds led to the
description ‘orthodox with limited desiccation ability’
(OLDA). However, employing the term ‘intermediate’
to indicate a practical difference from orthodox seeds
is useful. This matter is further discussed in the Seed
Physiology chapter.
Tropical Forest Tree Seeds
Tompsett (1994) has estimated that 72% of tree species
found in the tropics may bear ‘recalcitrant’ seeds.
Recalcitrant seeds are shed from the mother plant with
very high moisture contents (about 40-60% on a wet
weight basis) and germinate soon after shedding. Whilst
recalcitrant dipterocarp species provide real problems,
those of the OLDA type are more amenable, as described
above. Tompsett (1994) found that, in the case of
dipterocarp species, 94% of those examined possessed
recalcitrant seed.
Seed Ontogeny
Ontogeny covers development from floral initiation
through growth and differentiation to maturity of the
seed. To date, very little work has been published on the
ontogeny of dipterocarp species; Owens et al. (1991)
presented a generalised, basic development diagram
which may relate to certain species of the dry forest in
Thailand.
Phenology
Phenology, in a broad sense, refers to the relationship
between changes in seasons and climate and to the
phenomena of leaf and bud formation, leaf fall, floral
anthesis, fruit set and ripening. In the aseasonal
dipterocarp forests from south Asia to Malesia
phenological observations are an essential part of the
strategy for seed procurement of dipterocarps, owing to
the irregularity of their flowering and fruiting patterns.