Another unusual aspect of the Corcovado forests is the extremely low liana density ( only 54 individuals greater than 2.5 cm diameter including seven hemicryptophytes in a tenth hectare). Even more striking is the fact that almost none of the lianas are wind-dispersed, this forest having dramatically fewer wind-dispersed lianis (sic) than any other that I've sampled in the New World. The trees are also extremely skewed towards bird and mammal dispersal, just as one finds in the forests of Chocó.
Physiognomically, perhaps the most unusual aspect of the forest is the low density of small trees between 2.5 and 10 cm diameter. The Osa forest has fewer trees in this size category than any other lowland moist or wet forest site that I've sampled anywhere. It is the combination of the many large trees and very few small trees that give the forest its unusual physiognomic aspect. Thus, Sirena has quite a distinctive forest, and is especially unusual in being a preserved forest over good soil. The soil is even richer than at Manu, and this might well be the only preserved site over this type of substrate (I don't have soil data for LaSelva, however).
The Osa peninsula is also exciting floristically, since many American species, genera, and even families have disjunct populations there, but nowhere else in Central America. Good examples of genera with this distribution are Caryocar costaricensis, the only population of its genus and family north of the Panama-Colombia border (although it does get into Darien a bit), Caryodaphnopsis burgeri (Zamora and Poveda: Annals of the Missouri Botanical Garden 75:1160-1166. 1988) just described and the only Central American representative of its genus which turns out to be wide-spread in South America, but was previously known only from Asia.), Iryanthera (but my collection got lost, so I'm not sure what species), Tachigalia (that one is fairly common into Central Panama), Uribea (disjunct from northern Colombia), Tetrathylacium (I think; it also is at least in Darien), Cariniana pyriformis (the same species is common in the Magdalena Valley, and gets quite close to the Panama border in Colombia, but has never been collected in Panama), Couratari (this one also is fairly common into Central Panama), Chaunochiton (may be disjunct from the Guiana region!), Pterygota (again common in the Magdalena Valley, but to my knowledge never collected in Panama). To these might be added Anthodiscus, which was supposed to be an exclusively Amazonian genus, but which we discovered in the Chocó some years ago, and which is reported from the Osa by a sight record; as far as I know, there has never been a collection made on the Osa, however. (That's also Caryocaraceae, so it would mean that both genera of that family show exactly the same pattern of being only on the Osa outside of South America).
There are also similar patterns with South American species that only reach the Osa and not elsewhere in Central America, but in genera which have other species that go further north into Central America (eg. Clarisia racemosa and Vochysia allenii). Why all of these South American genera are disjunct on the Osa, but not on the Atlantic side of Costa Rica, nor apparently in intermediate Panama remains a mystery.
The Osa is, of course, also famous for high endemism, but I can't give you a list of the endemics off the top of my head.
I hope this helps.
Sincerely,
Al Gentry
Curator of Botany
AG/rys
P.S. I just thought of another characteristic you might want to mention. The Osa Peninsula forest is characterized by a greater prevalence of Moraceae than almost any other site that I've looked at. There were 15 or 16 species of that family, which is the ultimate rich soil family in South America. Since it is also a very important family for feeding monkeys and other vertebrates, you might want to emphasize something about the Moraceae-rich aspect of the Corcovado area. They even manage to beat out legumes for species richness in my sample - something that almost never happens (only on the other very rich wet forest sites like Rio Palenque).