We present an overview of tropical montane forest (TMFs) that highlights the role of climate gradients and natural disturbance on vegetation zonation, with particular reference to discrete vegetation boundaries that have been reported on tropical mountains. We focus on the primary forests of the Cordillera Central, Dominican Republic where we hypothesize that a combination of discontinuities in environmental conditions, disturbance history and biogeographic affinities of the flora have resulted in a highly discrete ecotone between cloud forest below and seasonally dry monodominant forest of Pinus occidentalis above 2200 m elevation. Pinus occidentalis forest also covers extensive areas in the rainshadow to the south and west of the Cordillera Central, forming monospecific forest landscapes. As TMFs worldwide continue to face severe threats by direct human exploitation (e.g., logging, agricultural expansion) and the indirect influence of human-accelerated environmental change, an improved understanding of the controls on the species distributions in these ecological assemblages will contribute to the future management and protection of these unique forests.
We propose that a discontinuity in climatic factors (temperature, humidity) associated with the trade-wind inversion (TWI) is the primary cause of this and other ecotones in TMFs that occur at a generally consistent elevation. Low humidity, fires and occasional frost above the TWI favor pine over cloud forest species. Fires in the high-elevation pine forest have repeatedly burned down to the ecotone boundary and extinguished in the cloud forest owing to its low flammability, reinforced by high humidity, cloud immersion and epiphytic bryophyte cover. Analogous patterns are observed worldwide in other TMFs where the TWI is important, high-elevation fires are frequent, and the flora contains frost-tolerant species (often of temperate lineage). The importance of climate (TWI and El Niño) here suggests that rising global temperatures will have profound effects on fire regimes in tropical mountains. Models under doubled atmospheric CO2 concentrations forecast a general increase in wildfires in tropical forests as dry-season length, droughts, El Niño and lightning frequency are all predicted to increase. The influence of the TWI, in particular, has the potential to drive exceptionally strong spatial patterning in some tropical fire regimes.