<em>TIMONIUS</em> <em>PASOHENSIS</em> (RUBIACEAE), A RARE NEW SPECIES FROM PASOH FOREST, MALAYSIA, AND ITS POPULATION CHARACTERISTICS IN THE 50-HA FOREST DYNAMICS PLOT

Abstract

The intensely rich flora of lowland tropical rain forests includes rare species that represent a significant portion of the overall diversity. In the Pasoh Forest Reserve, Malaysia, Timonius is represented by two sympatric species: T. wallichianus, and a new species described and illustrated here. The new species T. pasohensis is rare and so far known only from this reserve. The population characteristics of the new species and the commoner and more widespread T. wallichianus were compared in the 50-ha plot. The distribution of both species overlapped in several habitat types. The torus translation test indicated that T. pasohensis is positively associated with swamps and slopes, whereas T. wallichianus prefers alluvium. However, other factors such as competition, fecundity, susceptibility to droughts, and herbivory, may also be important in influencing the distribution of the two species. They have a similar population structure but are distinctly different in other aspects. From 1986 to 2015, the abundance of T. pasohensis has greatly declined to less than half its original population, and saplings and trees < 10 cm diameter at breast height appear to be the most affected. In contrast, the population size of T. wallichianus has remained relatively stable during the same period, possibly owing to the abundance of suitable habitat type and higher seed set.