Reproductive
ecology of Shorea roxburghii G. Don
(Dipterocarpaceae), an Endangered semievergreen species tree of peninsular
India
A.J. Solomon Raju 1, K. Venkata Ramana 2& P. Hareesh Chandra 3
1,2,3 Department
of Environmental Sciences, Andhra University, Visakhapatnam, Andhra Pradesh
530003, India
Email:1 ajsraju@yahoo.com (corresponding author), 2 vrkes.btny@gmail.com,3 hareeshchandu@gmail.com
Date of publication
(online): 26 September 2011
Date of publication
(print): 26 September 2011
ISSN 0974-7907
(online) | 0974-7893 (print)
Editor: K.R. Sasidharan
Manuscript details:
Ms # o2763
Received 13 April
2011
Final received 19
July 2011
Finally accepted 29
August 2011
Citation: Raju, A.J.S., K.V. Ramana & P.H. Chandra (2011). Reproductive ecology of Shorea roxburghii G. Don
(Dipterocarpaceae), an Endangered semievergreen tree species of peninsular
India. Journal of Threatened
Taxa 3(9): 2061–2070.
Copyright: © A.J. Solomon Raju, K. Venkata Ramana & P.
Hareesh Chandra 2011. Creative Commons Attribution 3.0
Unported License. JoTT allows unrestricted use of this article in any medium
for non-profit purposes, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
Author Details: Dr. A.J. Solomon Raju, Professor
and Head in the Department of Environmental Sciences, is on the editorial board
of several international journals. He is
presently working on endemic and endangered plant species in southern Eastern
Ghats forests with financial support from University Grants Commission and the
Ministry of Environment and Forests, Government of India. K.
Venkata Ramana and P. Hareesh
Chandra are Junior Research Fellows working in All India Coordinated
Research Project on Reproductive Biology of Four Rare Endangered and Threatened
(RET) Tree species namely, Hildegardia
populifolia (Roxb.) Schott. & Endl., Eriolaena lushingtonii Dunn (Sterculiaceae), Syzygium alternifolium (Wt.) Walp. (Myrtaceae) and Shorea roxburghii(Dipterocarpaceae) of Andhra Pradesh, funded by the Ministry of Environment and
Forests, Government of India, under the supervision of Dr. A.J. Solomon Raju.
Author Contribution: AJSR has done part of
the field work and write-up of the manuscript while VR and HC were involved in
field work and provided assistance in writing.
Acknowledgements: This study is a part
of the research work carried out under an All India Coordinated Research
Project on Reproductive Biology of RET Tree species funded by the Ministry of
Environment & Forests, New Delhi sanctioned to AJSR. We thank Dr. V.V. Ramamurthy, Division of
Entomology, Indian Agricultural Research Institute, New Delhi, for
identification of some insects reported in the present study.
Abstract: Shorea roxburghii is an Endangered semievergreen tree species restricted to
peninsular India in the Eastern Ghats. Leaf shedding and leaf flushing are annual events while flowering is not
annual, but when it does flower, in March, it shows massive blooming. Massive blooming, drooping inflorescence with
pendulous flowers, ample pollen production, gradual pollen release as a
function of anther appendage and aerodynamic pollen grains - all suggest
anemophily. The characteristics of
nectar secretion, hexose-rich sugars and amino acids in nectar are additional
adaptations for entomophily. The plant
is functionally self-incompatible, obligately outcrossing and ambophilous. The natural fruit set does not exceed 15%
despite the plant being ambophilous. Scarabaeid beetle by causing flower damage and bruchid beetle by using
buds, flowers and fruits for breeding greatly affect fruit set rate and thus
the success of sexual reproduction in this plant species is also affected. Seeds are non-dormant, the embryo is
chlorophyllous while the fruits are on the plant. Healthy seeds germinate as soon as they reach
the forest floor but their establishment is seemingly affected by resource constraints
due to the rocky habitat. The study
suggests that non-annual flowering, massive flowering for a short period, high
bud/flower and fruit infestation rate, absence of seed dormancy and rocky
habitat could attribute to the endangered status of S. roxburghii.
Keywords: Ambophily, anemochory, bud, flower
and fruit predation, self-incompatibility, Shorea
roxburghii.
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Introduction
Shorea is an important timber genus with most of its species classified
as Critically Endangered in the IUCN Red List (IUCN 2011). James & Chan (1991) stated that Shorea species are
insect pollinated; a variety of insects have been implicated in its
pollination. Shorea species occurring within one habitat and sharing the same insect
pollinators, flower sequentially to prevent competition for pollinators (James
& Chan 1991). S. megistophylla, an endemic canopy tree species in Sri Lanka has been reported to
be pollinated by Apis bees (Dayanandan et al. 1990). Shorea flowers
with large yellow elongate anthers have been reported to be pollinated by bees
while those with small, white anthers by thrips. Thrips are implicated as pollen vectors for
several Malaysian species of Shorea (Appanah & Chan 1981). In India, the genus Shorea is represented by S. assamica, S.
robusta, S. tumbuggaia and S. roxburghii. S. robusta is an anemophile with explosive pollen release pollination
mechanism (Atluri et al. 2004). S. tumbuggaia is a Data Deficient (Ashton 1998a) semievergreen tree species
restricted to the southern Eastern Ghats in Andhra Pradesh and Tamil Nadu. It is anemophilous as well as anemochorous
(Solomon Raju et al. 2009). S. roxburghii is a semievergreen Endangered (Ashton 1998b)tree species of
peninsular India, which is included in the list of medicinal plants of
conservation areas of Eastern Ghats of Andhra Pradesh (Rani & Pullaiah
2002; Jadhav & Reddy 2006). It is a
constituent species of southern tropical dry deciduous forests in the Eastern
Ghats (Chauhan 1998) and extends its distribution to dry evergreen or deciduous
forest and bamboo forest, often on sandy soils in Burma, Thailand, Indochina
and peninsular Malaysia in tropical Asia. It is an important timber and resin source; the latter is used as a
stimulant and for fumigation (Ashton 1963, 1982; Anonymous 1985). There is absolutely no information on the
reproductive ecology of this species, hence the present study was contemplated
to provide a comprehensive account on its reproductive ecology and discuss the
same in the light of relevant published information.
Materials and
Methods
Shorea roxburghii populations growing on rocky areas at Akasaganga, Papavinasanam
and Talakona sites of Tirupati Hills of the Eastern Ghats (Talakona—13040’N
& 79019’E, elevation 744m; Akasaganga and Papavinasanam are 3km
apart from each other but both the sites are about 80km to the west of
Talakona) in Andhra Pradesh State were selected for study during
2008–2010. The study aspects included
flowering, fruiting, seed dispersal and seedling ecology. Ten inflorescences, two each from five trees were
tagged and followed for their flowering duration. Thirty flowers collected from six trees were
used to record floral details. Mature
flower buds on ten inflorescences were tagged and followed for recording the
time of flower opening. The same flowers were followed for recording the time
of anther dehiscence. The pollen grain
characteristics were recorded by consulting the book of Bhattacharya et al.
(2006). Pollen production per flower was
calculated following the method described by Cruden (1977). Pollen fertility was assessed by staining
them in 1% acetocarmine. Stigma receptivity
and nectar volume, sugar concentration and sugar types were recorded by
following the protocols given in Dafni et al. (2005). Nectar was also analyzed
for amino acid types by following the paper chromatography method of Baker
& Baker (1973). Fifty mature buds,
five each from 10 inflorescences on five trees were bagged a day before
anthesis, without manual self pollination, to know whether fruit set occurs
through autogamy. Another set of 50
mature buds was selected in the same way, then emasculated and bagged a day
prior to anthesis. The next day, the
bags were removed and the stigmas were brushed with freshly dehisced anthers
from the flowers of the same tree and rebagged to know whether fruit set occurs
through geitonogamy. Five trees each at
Akasaganga, Papavinasanam and Talakona were selected for manual
cross-pollination and open-pollination. Fifty flowers were used per tree for manual cross-pollination. For this, mature buds were emasculated and
bagged a day prior to anthesis. The next
day, the bags were removed; freshly dehisced anthers from the flowers of
another tree were brushed on the stigma and rebagged. Ten inflorescences on
each tree were tagged for fruit set in open pollination. The bagged flowers and tagged inflorescences
were followed for four weeks to record the results. Observations on flower visitors and their
foraging activity period with reference to pollination were made by using
binoculars. The insect species visiting
the flowers and the forage sought by them were recorded. Five-hundred flowers collected at random from
20 trees were examined to record the percentage of flower damage by the
scarabaeid beetle. Another set of 500
flowers collected from 20 trees were examined for flower infestation rate by
the bruchid beetle. Further, 385 fallen
fruit were collected to record fruit infestation rate by the same bruchid
beetle. Fruit set rate, maturation and
fruit fall timing and dispersal aspects were observed in the field. Field observations were made to record natural
seed germination and establishment rate. One-hundred mature fruits collected from the trees were sown in an
experimental plot to record seed germination rate.
Results
Shorea roxburghii is a semievergreen tree species. Leaf shedding, flowering and leaf flushing are annual events in this
species. Leaf shedding occurs during the
winter season from mid-November to mid-February (Image 1a). About a week later, leaf flushing begins and
ends in July (Image 1b). Flowering
begins in the first week of March and ceases by the end of March at population
level. A tree flowers for about three
weeks only. Trees grow up to a height of
12m and flowering at canopy level is quite visible from a long distance due to
the presence of newly emerging bright green leaves. Inflorescence is a drooping terminal or
axillary racemose panicle with an average of 37±6 flowers which anthese over an
average period of 5±2 days (Image 1c). Flowers are pedicellate, hang downwards, milky white with light reddish
tinge, fragrant, 2cm long and 3cm across, bisexual, zygomorphic, cup-like at
base and star-like terminally (Image 1d). Sepals are five, blunt-lobed, 0.7cm long, light green, imbricate,
basally united into a cup, free terminally and persistent. Petals are five, milky white, fragrant, 2.2cm
long, connate at base forming a cup-like structure, free terminally. Stamens are 15, free, arranged closely in two
whorls to the base of the corolla; inner row consists of five stamens while
outer row with ten stamens. They are situated below the level of stigma. Each stamen consists of a 0.2cm long
filament with a 0.2cm long anther. Anthers are light yellow, dorsifixed but appear to be basifixed;
tetrasporangiate and dehisce by longitudinal slits ca. 30min after
anthesis. The connectival part of the
filament of each anther extends into a 0.3cm long sterile tip constituting
“anther appendage” (Image 1e). The
pollen production per anther is 3,379.8±196.62 grains, and per flower it is
50,697. The pollen grains are yellow,
powdery, radially symmetric, tricolporate, 24.9µm long and have reticulate
exine with muri separated by lumina (Image 1f). In a flower, fertile pollen is 92% while the remaining 8% is sterile.
Pollen to ovule ratio is 8,449.5:1. Ovary is semi-inferior, syncarpous with three
united locules having a total of six light yellow ovules on axile
placentation. Style is 0.6cm long,
semi-wet and extended into a trilobed stigma (Image 1g,h). The petals, stamens, style and stigma fall
off on the third day while the sepals remain until the fruits fall off.
The flower opening occurs at
0500–0600 hr while anther dehiscence occurs after three hours. The petals being twisted in bud gradually
unfold and spread upwards gradually giving a star-like appearance. The cup-like flower base with stamens is
exposed to the outside environment. The
flowers are nectariferous and each flower produces 2.15±0.28 µl of nectar. The nectar sugar concentration is 11.7±1.9 % consisting
of glucose, fructose and sucrose but the first two sugar types are dominant. The nectar also contains both essential and
non-essential amino acids; the essential ones are histidine, arginine,
iso-leucine and threonine while the non-essentials are proline, aspartic acid,
alanine, glutamic acid, glysine, tyrosine and cystine. The stigma lobes are erect and united in bud
but unfold at anthesis indicating receptivity which lasts for two days by being
in semi-wet state; it is dry and shows signs of withering by the end of the
second day. The same duration of stigma receptivity was recorded when tested
with hydrogen peroxide. The flowers in
the hanging position do not allow nectar flow along the length of the corolla
since it is in a minute quantity and held intact by the ovary base and staminal
filaments. The pollen release from
dehisced anthers was gradual when the flowers were shaken manually. The dehisced anthers became empty after 3–5
manual shakes. This gradual pollen
release was considered to be an adaptation for anemophily. The insects probing the flowers for forage
collection also caused the anthers to release pollen gradually. The flowers were foraged during day time by
eight insect species belonging to Hymenoptera [Apis
dorsata (Image 1k), A. cerana (Image 1l), A. florea (Image 1m), Trigona
iridipennis (Image 1n) and Vespa cincta (Image 1o)], Diptera [Helophilus sp. (Image 2a)] and Lepidoptera [Euploea
core (Image 2b) and Tirumala limniace (Table 1)]. Bees were
found to collect both nectar and pollen; they were regular foragers throughout
the flowering season. Their foraging
activity pattern showed two schedules, one during 0700–1200 hr with hectic
activity and the other during 1600–1800 hr with low activity. Wasps and fly foragers were also regular but
only a few individuals visited the flowers. They foraged for nectar only during
the forenoon period from 1000 to 1200 hr (Fig. 1). Nymphalid butterflies were also exclusive
nectar foragers but their visits were not consistent during the day. The data collected on the foraging visits of
these foragers on a given day indicated that bees accounted for 77%, wasps and
flies each 5% and butterflies 13% of the total foraging visits (Fig. 2). All insects after landing probed the flowers
for nectar and/or pollen. Both nectar
and pollen collecting insects were found to be contacting the anthers and
stigma invariably while collecting the forage and such contact with the sex
organs was considered to be resulting in pollination. Trigona bees tended to stay mostly on the same tree for forage collection
effecting mostly selfpollinations while Apis bees and wasps made frequent inter-tree flights in search of more
pollen/nectar causing cross-pollination simultaneously. The flies tended to forage mostly on the same
tree; it could effect mostly selfpollinations. The nymphalid butterflies being
inconsistent foragers also made frequent inter-tree flights in search of nectar
and in doing so effecting crosspollinations.
Further, swarms of Coleopteran Popillia
impressipyga (Scarabaeidae) (Image 2c-e)
were found to be consistent flower-feeders. Its newly emerging offspring especially juveniles fed on the sap of
floral petals while the adults on all parts of the flowers effecting the
success of sexual reproduction to a great extent. The breeding site of this beetle was not
known. Flower damage rate by this beetle
was 48% and these flowers subsequently fell off. An unidentified bruchid beetle was found to
be breeding in flowers and the flowers hosting this beetle were found
subsequently to be falling off without fruit set. In each flower, there was only one green
coloured larva of this beetle (Image 1i,j) and the larva falls off along with
the petals and stamens. The larvae upon
reaching the ground pupate within the soil to produce adults. Flower infestation rate by this beetle was 36.6%.
The manual pollinations for
autogamy and geitonogamy did not set fruit while those of xenogamous
pollinations set fruit ranging from 15.7 to 28.4 %. The fruit set was 8.4–15.4 % in
open-pollinations (Table 2). The number of fruits set in open-pollinations at
inflorescence level was 5.03±0.52. Each
fruit produces only one seed against the actual number of six ovules. The fruits take about five weeks to mature
and fall to the ground by the end of May (Image 2f-g). They are winged and wings represent sepals
which are accrescent in that they are thickened and three of them expand into
wings and are larger than the other two sepals (Image 2h). They are 1.41±0.29 gm in weight while the
fruits without winged sepals are 1.18 ± 0.26 (Image 2i). The fruits show colour change from green to
brown to dark brown gradually and finally become dry. The fruit wall is free from calyx, woody,
with a thin inner membranous lining invaginated into the folds of cotyledons
and split into two parts at the apex. The seed is non-dormant and the embryo is chlorophyllous.
The fruits also contained the
same bruchid beetle which was found in flowers. Each fruit contained a single larva which was creamy white in
colour. The larva feeds on the internal
soft parts of the developing fruit and emerges from the exit hole drilled by it
(Image 2j-l). When the fruit falls to
the ground, the larva leaves the fruit through the hole for pupation in the
soil. The pupal stage was observed for
six weeks but there was no emergence of adult in the lab set up; this long
period was considered as dormant stage of pupa for the emergence of the adult
when conditions are favourable in the forest soil. Further, in 2% of fallen fruits, the larva
remains inside to pupate and produce the adult beetle. Fruit infestation rate was 87%. The dry winged fruits fall to the ground and
disperse within a 10–20 m area of the tree due to wind action. Healthy seeds germinate in field conditions
following monsoon showers. A small
number of seedlings withered initially (Image 2m) while most of them perished
after some growth and development. Finally, a few seedlings grew continually
(Image 2n-p). Seed germination rate was
8% in the experimental plot.
Discussion
Shorea roxburghii is an important constituent of deciduous forests in the Eastern
Ghats. It is a semievergreen tree
species due to its very brief leafless state during the dry season. Leaf shedding, leaf flushing and flowering
occur almost sequentially one after the other. Leaf flushing however extends beyond fruit dispersal. In S. robusta and S. tumbuggaia also, these three phenological events occur in sequence (Singh
& Kushwaha 2005; Raju et al. 2009). In S. roxburghii, the flowering is not an annual event as only a few trees flowered
at each study site but leaf shedding and flushing occurred annually. Flowering occurred on all branches of the
tree. In S.
tumbuggaia also, flowering is not
annual and in the flowering individuals, the flowering is restricted to branches
which are exposed to sunlight (Raju et al. 2009). The flowering period is very brief in S. roxburghii while its duration is further reduced in S. tumbuggaia (Raju et al. 2009). In
both the species, the flowering pattern represents the massive flowering
pattern in which more flowers are produced per day during the flowering period
(Gentry 1974; Opler et al. 1980). Mass
flowering is considered as a property of the individuals of a plant species
(Bawa 1983) and this pattern of flowering may have evolved among individuals ofS. roxburghii for effective pollen movement between trees. The new leaves are known for their
photosynthetic efficiency and hence have the ability to provide the required
photosynthate to the growing fruits.
In S. roxburghii, the flowers are morphologically and functionally bisexual. The absence of fruit set in autogamy and
geitonogamy suggests that the plant is self-incompatible. The sterile pollen present in the flowers
appears to be a derived trait to promote self-incompatibility. The protogyny is an important functional
mechanism to promote out-crossing but it is weak in this species. Bertin & Newman (1993) stated that
protogyny is a characteristic associated with self-compatible anemophilous
flowers to reduce selfing rate. S. roxburghii being self-incompatible exhibits weak protogyny and hence it is a
residual character and does not serve to achieve cross-pollination. On the contrary, S.
tumbuggaia and S. robusta are self-compatible anemophiles; but protogyny is weak in the former
while it is strong in the latter species (Atluri et al. 2004; Raju et al.
2009). In S.
roxburghii, the drooping inflorescence,
hanging flowers with compactly arranged anthers at the base and held above by
anther appendages and the exposed cup-like flower base collectively aid in the
gradual dispersal of pollen by wind. Gradual pollen release occurs when the flowers are manually shaken; it
suggests that wind force does not make the anthers release the pollen at once,
hence there is an in-built device for the gradual and economical release of
pollen from oscillating flowers due to wind force. As the flowers are at the canopy level, the
wind force can easily make flowers release pollen into the air and then carry the
same to the receptive stigmas of flowers on different trees. The pollen grain
size is a characteristic typical aerodynamic particle, which permits effective
wind transport and deposition on the stigma through impaction (Gregory 1973;
Reddi 1976) and the characters such as reticulate exine and muri separated by
lumina may reduce terminal velocity and contribute to the increased dispersal
range of the pollen (Niklas 1985). Synchronous anthesis and high pollen production enable anemophily to be
more effective. In S. robusta and S. tumbuggaia also, a similar pollen release mechanism and anemophily exist
(Atluri et al. 2004; Raju et al. 2009). The study sites experienced moderate
turbulent atmospheric conditions especially during the forenoon period and this
favoured efficient transport of the entrained pollen (Mason 1979).
The self-compatible S. robusta and S. tumbuggaia are strictly anemophilous. The flowers of both the species do not secrete nectar and hence pollen
is the only floral reward for the insects which visit them. Atluri et al. (2004) reported that honey bees
may visit S. robusta for pollen collection and their foraging activity is of no use to
this plant. Raju et al. (2009) reported
that the stingless bee, Trigona iridipennis visits S. tumbuggaia for pollen collection and its foraging activity is important for
self-pollination due to its slow mobility. The fruits formed from selfed-flowers have been considered to be
abortive. In S.
roxburghii, the flowers are
nectariferous and produce hexose-rich nectar with low sugar concentration. Since the flowers offer both nectar and
pollen, they attract nectar and pollen foraging bees, nectar foraging wasps,
flies and butterflies; flies are important for self-pollination and all the
other insects for both self-and cross-pollination. Their foraging activity on S. roxburghii flowers is not in line with the generalization that they visit
flowers rewarded with sucrose-rich nectar with high sugar concentration (Baker
& Baker 1982, 1983; Cruden et al. 1983). The nectar of S. roxburghii is also a source of four of the ten essential amino acids and
seven non-essential amino acids (DeGroot 1953). They add taste to the nectar and serve as an important cue for insects
to pay visits to the flowers; and the insects while collecting the forage
effect pollination. The amino acids are
especially important for the growth and development of flower-visiting insects
(DeGroot 1953). Therefore, S. roxburghii being a self-incompatible species has evolved to produce nectar
with sugars and amino acids as rewards to attract insects for increasing the
cross-pollinate rate. The ability to
have both anemophily and entomophily is adaptive for S. roxburghii to set fruit to permitted level through cross-pollination. The function of a pollination system
involving both wind and insects as vectors of pollen transfer is referred to as
‘ambophily’ (Culley et al. 2002) and hence S.
roxburghii is functionally ambophilous.
S. roxburghii flowers attract two beetle species. The scarabaeid beetle causes flower damage by
sucking sap from petals and the damaged flowers whether pollinated or
un-pollinated fall off. The bruchid
beetle uses the floral buds for its breeding. A single larva emerges when the buds mature and bloom; such buds and
flowers fall off together with the larvae without fruit set. The bud and flower infestation rate by these
two beetle species is very high and hence have a great bearing on the success
of sexual reproduction in S. roxburghii.
In S. roxburghii, the fruits mature quickly during the dry season. They are winged, light-weight and
characteristically produce a single seed. The embryo is chlorophyllous while on the parent plant, suggesting that
the seed is non-dormant and such a characteristic may aid in better survival in
unpredictable habitats with irregular supply of light, nutrients and water
during the germination period (Maury 1978; Maury-Lechon & Ponge 1979). A similar situation exists in S. tumbuggaia (Raju et al. 2009). The
winged character of fruits is seen in most dipterocarps and it is an important
adaptation for dissemination by wind (Ashton 1982). The winged structure of the sepals allows
1-seeded fruits to gyrate toward the ground and hence the seed dispersal is
anemochorous. Seeds disperse by wind to
a short distance only, due to the semi-closed nature of the canopy cover of the
forest. The dispersal of winged fruits
takes place much more efficiently by wind if the forest is of the open,
seasonal, dry deciduous type (Maury-Lechon & Curtet 1998). The seeds fallen on the ground have no possibility
for further dispersal by the sweeping action of the wind due to litter
accumulation and grass growth in the study sites during rainy season. In S. tumbuggaia, seed dissemination by wind takes place up to a distance of 10m
(Raju et al. 2009) and up to 2km in S. albida (Ashton 1982).
Different insect species
attack seeds of Shorea species during their development (Singh 1976). Insect pests attack at the pre- or
post-dispersal stage of the seed; in the pre-dispersal stage, the pest attacks
the fruit on the tree before dispersal while in the post-dispersal stage, the
pest attacks fruits on the ground (Toy 1988). In S. roxburghii, the same bruchid beetle which uses buds and flowers for breeding
also attacks at an early stage of the development of the fruit. Its larva leaves through the exit hole
created by it when the mature fruits fall to the ground. It pupates in the soil and perhaps, produces
an adult only when favourable conditions return to the forest floor to repeat
its life cycle. Khatua & Chakrabarti
(1990) reported that many bruchid species spend a dormant stage as pupae in the
soil and it holds true in the case of the bruchid beetle which is a pest of the
seeds of S. roxburghii. Further, in a small
percentage of fallen fruits, the larva remains within, pupates and produces an
adult beetle suggesting that the beetle has the ability to use the fruit for
its entire life cycle. The same bruchid
beetle attacks the co-occurring S. tumbuggaia seeds in the study sites but the per cent of infested seeds is
comparatively less due to its late flowering which occurs for two weeks in the
last week of April and the first week of May (Raju et al. 2009). They also reported that in India, the seed
weevil Sitophilus (Calandra) rugicollis attacks seeds of Shorea robusta, survives as a dormant adult in the forest floor and emerges with
the first monsoon rain, which coincides with the commencement of seed fall
(Khatua & Chakrabarti 1990).
Mass fruiting appears to
favour seed predators, but it can also be a strategy to escape complete seed
destruction (Janzen 1974). Seed
predation can be very high, and the crop can be completely wiped out. Natawiria et al. (1986) observed that weevils
(Curculionidae) damage 40–90% of the seeds of Shorea
pauciflora, S. ovalis, S. laevis and S. smithiana. In S. tumbuggaia, seed damage is 70% and only about half of the remaining healthy
seed crop established seedlings in the forest (Raju et al. 2009). In S. roxburghii, seed predation is 87% suggesting that this tree is threatened by
bruchid beetle in terms of reproductive success.
In S. roxburghii, fruit set in open pollination is up to 15% while it has almost
doubled in manual cross-pollination. This suggests that fruit set does not exceed beyond 30% even if the cross-pollination
rate increases by wind and insects. The
tree appears to have resource constraints to increase fruit set; the rocky
nature of the forest floor with dry conditions during the fruit set period is
perhaps the main constraint. Such a low
fruit set in open-pollination has also been reported in S. tumbuggaia in the same study sites by Raju et al. (2009). The study suggests that non-annual flowering,
massive flowering for a short period, high bud/flower and fruit infestation
rate, and the absence of seed dormancy could be attributed to the endangered
status of S. roxburghii. Field situation relating
to the mortality rate of seedlings and the seed germinate rate evidenced in the
experimental plot also substantiate this conclusion.
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