The capability to switch gender in O. ten era sug-
gests a mechanism for negative spatial association of
the sexes, albeit one that is highly speculative. If a
plant improves its reproductive success when it is
near a plant of the opposite sex, and if its sex expres-
sion is flexible (as this study suggests), natural selec-
tion could favor sex switching in response to a plant's
social environment. Such a scenario would require
that plants be able to determine the sex of their neigh-
bors, perhaps by pheromonal communication, as pos-
tulated in plants attacked by herbivores (e.g., Rhoades
1983, Baldwin et al. 1990, but see Fowler and Lawton
1985). Another mechanism might be that plants are
able to monitor fruit set and adjust sex expression
accordingly. To test whether a plant's nearest neigh-
bors could influence its sex expression, I established
a third experimental population in 1991, planting O,
tenera seedlings near mature, isolated male or female
trees. The results of this experiment may bear fruit in
the future. Although not designed to determine the
mechanism of sex switching, the experiment may sug-
gest hypotheses and stimulate new studies to explain
the spatial distribution of tropical trees (Bawa and
Opler 1977).CLONAL REPRODUCTION THROUGH PLANT
FRAGMENTS IN POIKILACANTHUS MACRANTHUS
Stephen P. Bushn tropical forests, branches and twigs are routinely
broken from understory plants by falling limbs
and trees. These fragments may produce roots
and, ultimately, mature plants. A majority of shrub
species in Monteverde are capable of reproduction
through such plant fragments (Kinsman 1990). How-
ever, the importance of regeneration through fragmen-
tation in natural populations and its effects on genetic
diversity have not been studied. I determined the fre-
quency of regeneration via plant fragments in Poikila-
canthus macranthus (Acanthaceae), a shrub that per-
sisted throughout the five-year study period of Kinsman
(1990). Using DNA fingerprinting, I analyzed the
population genetic structure of P. macranthus.
To assess the significance of regeneration via plant
fragments, I determined the ratio of vegetatively to
sexually produced plants in natural populations of
P. macranthus. Plants derived from fragments and
those derived from seed can be distinguished, based
on the distinctive pattern of fragment development
(Kinsman 1990) and by the presence of a callous ring
at the root-stem junction of fragments (Sagers 1993).
Six plots (2 x 20 m) were sampled on leeward Pacific
slopes (1500-1800 m) below the Continental Divide.
Of all the small plants sampled in the transects, 158
were derived from plant fragments and only 2 were
derived from seed. Also, the density of these frag-
mentation-derived plants increased as elevation in-
creased, perhaps because mature populations of
F. macranthus increased in size and density at higher
elevations. Exposure to the intense northeast trade
winds is likely to favor regeneration through fragmen-
tation. The increased disturbance at higher elevations
likely produces a greater number of fragments, and
because fragment survivorship is correlated with in-
creased moisture levels (Kinsman 1990), establish-
ment rates are also likely to be higher in wet, exposed
locations.
Excavations to determine possible connections
between plants of P. macranthus in a mature isolated
patch indicated that most plants are physiologically
independent. In this plot, DNA fingerprints of uncon-
nected plants indicated that dense patches (more than
10 m across) may be composed of genetically identi-
cal individuals, that is, all units of the same clone.
Analysis of an entire clump, spanning 30 m across,
indicated that more than 50% of the plot (36 of 67
plants) was composed of only two clones; nine smaller
clones were intermingled. Thus, preliminary results
using DNA fingerprinting to identify clones in popu-
lations of P. macranthus confirm the significance of
regeneration via fragmentation. Repeated cycles of
fragment generation through disturbance, followed by
growth of fragments into tall shrubs, probably pro-
duces large clones. These data suggest that popula-
tion growth in P. macranthus is predominantly through
fragmentation.
Asexual reproduction through plant fragments in
P. macranthus may reduce genetic variability within
a population. Genetic differences may be maintained
primarily among clones of different populations. For
conservation of genetic diversity in P. macranthus, the
number of clumps conserved, rather than population
size, may be critical. Regeneration via fragmentation
may also alter community processes. Traditional
models of succession, in which disturbance is fol-
lowed by species replacement, may not hold true for
the Monteverde understory. Disturbance may be fol-
lowed by clonal growth, rather than species turnover
(Linhart et al. 1987, Kinsman 1990).88 Plants and VegetationI