Tropical Ecology

Forest Ecotones, Zonation and Pattern in Tropical Montane Cloud Forests

For the last 10 years, we have conducted research on the community organization of tropical montane cloud forests in the Dominican Republic. Tropical montane forests remain poorly studied compared with lowland tropical forests, and many basic questions about their organization remain debated. A major gap has been the lack of research linking disturbance regimes with environmental drivers in these systems; our research has shown these drivers can work in unison to spatially pattern forest structure, composition and species richness. Landscape-scale disturbances can interact with biogeographic factors (the overlap of in flora of temperate and tropical lineages commonly found in subtropical montane systems) to form discrete vegetation zones in tropical montane forests. This research is published in the Journal of Biogeography (PDF), and the Journal of the Torrey Botanical Society (PDF).

The forest vegetation mosaic in Parque Bermudez, Cordillera Central, Dominican Republic. Note the patches of brown forest in the high elevation areas where the 2005 fires burned through the windward, high elevation forest of Pinus occidentalis.

Photo: Patrick Martin, January 2006.

Fire Ecology of Tropical Montane Forests

Tree ring reconstructions of fire history have played a pivotal role in our understanding of fire's critical function in many temperate ecosystems. A similar appreciation of fire's historical importance in tropical ecosystems has lagged despite the common occurrence of natural fires, particularly in montane tropical systems. Fire history studies in the tropics have been hampered by a lack of long-term fire records and a dearth of tropical species suitable for dendrochronology. The paucity of tropical fire history studies hinders our ability to elucidate fire regimes across broader spatial scales and the links between fires and interregional climate synchronicities (e.g., El Niño). We reconstructed the fire history in the Cordillera Central mountain range of the Dominican Republic where two adjacent national parks comprise a protected area over 1500 km2. We performed a tree ring reconstruction of fire history using an endemic pine species shown to have (mostly) annual ring growth. This study documented a number of firsts: (1) the long-standing role of frequent fires in a tropical ecosystem, (2) the dramatic increase in fire frequency after permanent human settlement in the region, and (3) an historic link between El Niño events and drought-synchronized fires in the subtropics. The fire history component of this research is published in the Journal of Tropical Ecology (PDF).

In March–April 2005, a severe fire burned through the Cordillera Central, Dominican Republic, resulting in patchy, yet landscape-scale mortality. The fire escaped from a farmer’s field adjacent to the southern, leeward side of the park. The fire burned for 28 days, eventually spreading through most of the high-elevation pine forests on both the leeward and windward slopes of the Cordillera Central. This was the first extensive fire in the study area since 1965. The fire occurred after a pronounced drought that was coincident with an El Niño–Southern Oscillation event. Using satellite imagery and field plots, our study found that fire burned through 96% of the pine forest but quickly extinguished at the pine–cloud forest boundary along most of the ecotone. Topographic factors and fire severity had no influence on fire behavior at the ecotone. These observations support our original hypothesis that fire interacts with vegetation to maintain the discrete ecotone between the pine and cloud forests in these mountains. Vegetation structure and composition played a direct role in regulating fire spread and behavior in this landscape. This study is published in Ambio (PDF).

Fire damage to monospecific Pinus occidentalis forest on the leeward slopes of the Cordillera Central, Dominican Republic . Fires swept up from Valle Tetero in late March of 2005 and resulted in 100% mortality in some areas.

Photo : Patrick Martin, February 2007.

Recovery of Tropical Forests from Agriculture

Secondary forests in the tropics have received increased attention in recent years as their value in conserving biological diversity has been recognized. Their importance in tropical landscapes is not new, however; in most tropical countries, secondary forests have comprised larger areas than primary forest for decades. Yet, fundamental aspects of tropical forest succession following agricultural remain poorly understood, in part because most studies of tropical secondary forests have been confined to the first decade of vegetation recovery. As part of my doctoral studies, I undertook research on the resiliency of tropical forests to clearing for agriculture in secondary forests which had undergone natural recovery for 40 years. My research was organized around questions critical for the future conservation of tropical forests: do protected areas of primary tropical forests act as an effective source of native plant species for the recolonization of abandoned lands? Does the high diversity of life-forms which typify mature tropical forests recover along the same trajectory as species richness? Major findings of this research were: (1) forest structure and species richness of woody species is able to recover within 40 years when soils are fertile, and (2) the species richness and abundance of non-woody plants, especially epiphytes, recovers much more slowly. This research is published in Biotropica (PDF).

40 year-old secondary riparian forests in the Dominican Republic

Photo : Patrick Martin, January 2006.