A review of dipterocarps - Center for International Forestry Research

Biogeography and Evolutionary Systematics of Dipterocarpaceae
dispersal and seedling survival. It has been shown that
the species diversity of ovary and style sizes and shapes
in the Shorea sensu lato subdivisions correspond to
their pollinator insect groups (Appanah 1990). The
successive ontogenic phases of flower aestivation and
seed germination, and seedling construction, also reveal
the existence of particular directions such as the change
of sepal position from imbricate in the young flower to
subvalvate or valvate in ripe fruit (not the reverse), the
multiplication of vascular bundles and resin canals (never
the reverse) and the presence/absence of a postgerminative
growth of cotyledonary limbs (Maury 1978,
Maury-Lechon 1979a, b).
The biological plasticity of ripe seeds and seedlings
depends on their ability to maintain their biological
functions (which are dependent on their structures). This
plasticity determines the possibilities of survival in a
changing environment or in new and different places. The
exact knowledge of the present geographical distribution
and the main ecological features of these places already
allow useful speculations for the choice of species
potentially able to adapt in more drastic conditions
(Maury-Lechon 1993, 1996, Xu and Yu 1982). This is,
for example, the case of species from the seasonal
tropics, or from the aseasonal regions subjected to great
changes (diurnal, seasonal or unpredictable climatic
events) on dry sands of sea coastal areas, or temporary
and alternatively flooded or dried areas. The evolutionary
trends established by Hopea or Shorea allow a much
wider range of possible future adaptations than do those
established by Vateria or Stemonoporus (Maury-Lechon
1979b). Thus a second level of prediction for adaptability
is possible.
Overall, the striking feature of the family is the high
variability of characters within and between species,
within and between individual trees in many cases, and
even within a single seed in certain species. Furthermore,
the present classification demonstrates a heterogeneity
of levels between the two Asian subgroups Dipterocarpi
and Shoreae sensu Ashton. A notable case is the 11
sections of the Shorea genus in the Shoreae subgroup.
They have unequal hierarchic levels when comparisons
are established between the two Asian subgroups.
Sections such as Anthoshorea, Shoreae or Richetioides
of the Shoreae tribe have much higher rank than sections
Rubellae or Ovalis for example, and a similar level to
Doona, Pentacme, and Parashorea in the Imbricate
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group (sensu Maury-Lechon). A similar situation appears
for the Vatica genus in the Dipterocarpi subgroup (sensu
Ashton). For this reason Sunaptea (Vatica pro-parte in
certain cases) has again been raised to generic rank
(Kostermans 1987). These difficulties underline the
complexity of the family. However certain well defined
genera exist, such as Dryobalanops, Dipterocarpus,
Anisoptera and Upuna. It could thus be hoped that a more
equal weighting of characters is still possible in building
a more homogeneous classification in the complex parts
of the family, and that criteria can be defined for Asian
dipterocarps to determine generic rank.
Present supraspecific taxa are mainly defined by
groups of characters concerning the morphological
aspects of leaves, the sequence fruit-seed-embryoseedling,
flowers, bark and wood, and colour and
consistency of resins. Anatomical structures (wood, bark,
petioles, epidermis, germinating seeds and seedlings),
stomatal types, and chemistry of resins, have historically
clarified the definition of supraspecific taxa (but very
few wood characters are specific) in Dipterocarpaceae.
However anatomical or chemo-taxonomical groups are
not yet totally integrated into the present taxonomic
divisions. This is the case for genus Dipterocarpus for
example, in which phytochemistry has recognised main
groups without evident correspondence with the previous
morphological divisions based on fruit characters
(Meijer 1979). The main reason for this is that too few
species have been studied in this way to permit a rigorous
understanding of within group variability and thereby
establish which characters provide useful criteria for
defining groups and hierarchic levels.
The morphological variability in the seasonal tropics
may be the result of the frequent great changes with time
in distribution of habitats and geographical boundaries.
These changes favoured variability but rarely provided
prolonged isolation mechanisms for fertility barriers to
evolve. The frequency of hybrids suggests the same
conclusion. It is not really known to what extent
competition eliminates these hybrids. In the aseasonal
tropics dipterocarps appear to be outbreeding species.
Many other species in aseasonal tropics present
allopatric differentiation and clear discontinuities in
variation. Facultative apomixis may produce successful
genotypes and accelerate ecotypic differentiation and
short term evolution. Apomixis could serve to maintain
fecundity where sexual reproduction is inadequate, and