Manual de Plantas de Costa Rica 191 
below), Nicoya, Osa, and Burica are also being uplifted, while zones of subsidence in- 
clude the Llanuras de San Carlos and Tortuguero (Limon Basin), the basins of the Rios 
Tempisque, Grande de Térraba, and Coto Colorado, and the Gulf of Nicoya and Golfo 
Dulce. Continuity of Isthmian Central America and South America was not established 
until the late Pliocene, some 3.5—2.5 million years ago. The active stratovolcanoes of the 
Cordilleras de Guanacaste (including Volcan Arenal) and Central, related to the subduc- 
tion of the Cocos Plate, have been erected since the upper Pleistocene. 
The creation of upland habitats resulting from the uplift of cordilleras in southern 
Central America, concomitant with a major drop in temperature beginning in the mid- 
dle Miocene, fostered the southward migration of temperate North American (Laur- 
asian) plants, to Isthmian Central America and (in many cases) beyond. Examples in- 
clude members of the genera Alnus, Berberis, Cornus, Lupinus, Morella, Morus, Nyssa, 
Oenothera, Prunus, Quercus, Rhamnus, Ribes, Rubus, Salvia, Scutellaria, Ulmus, Vac- 
cinium, Valeriana, and Viburnum. Meanwhile, the establishment of a dry-land connec- 
tion with South America permitted migration of tropical Gondwanan plants northward 
into Central America and Mexico. These, thriving in both wet and seasonally dry 
forests at lower elevations, include members of the genera Amaioua, Clytostoma, 
Coussapoa, Cremastosperma, Dicella, Herrania, Mansoa, Maripa, Peritassa, Segui- 
eria, Selysia, Siparuna, Socratea, Thinouia, and Trigonia. 
The composition and origin of much of Costa Rica’s flora can probably be ex- 
plained by the above simplistic scenario: temperate elements from North America 
(Laurasian) colonizing the uplands, tropical elements from South America (Gond- 
wanan) at lower elevations. But complications arise, especially involving the tropical 
elements. A large contingent of Gondwanan plants of African origin appears to have 
reached southern Central America from the north, including members of many im- 
portant genera: Acacia, Beilschmiedia, Bursera, Caesalpinia, Cedrela, Chrysophyl- 
lum, Dalbergia, Diospyros, Ficus, Lonchocarpus, Nectandra, Ocotea, Oreopanax, 
Persea, Sabal, Sapium, Sterculia, and Terminalia. There is also evidence that mem- 
bers of some tropical genera may have arrived from South America prior to the land- 
bridge connection, e.g., Alchornea, Allophylus, Apeiba, Astronium, Bauhinia, Ber- 
noullia, Casearia, Dendropanax, Erythrina, Eugenia, Homalium, Luehea, Mabea, 
Ochroma, Pouteria, Tapirira, Thouinia, and Trichilia. Moreover, the Santa Elena Pen- 
insula, which originated some 85 million years ago as an oceanic island, presumably 
developed its own endemic flora, which would have mingled with the mainland flora 
when peninsular status was attained (Janzen, 1998). Janzen (1998) suggested the fol- 
lowing species as possible holdovers from an endemic Santa Elena flora: Acacia ri- 
paria (Fabaceae), Agave seemanniana (Agavaceae), Amphipterygium adstringens 
(Anacardiaceae), Bursera permollis and B. schlechtendalii (Burseraceae), and Melo- 
cactus curvispinus (Cactaceae). Costa Rican lowland forests are thus believed to have 
a heterogeneous origin, with elements arriving from various sources via different 
routes and at different times. 
