• To learn more about the tropical forests that are cleared when urban and residential development spreads, scientists assembled the most comprehensive time series of land cover maps to date for the island of Puerto Rico.  They learned that most of the forests cleared for land development from 1991 to 2000, 55%, were young (1–13 yr). Only 13% of the developed forest was older (41–55+ yr). However, older forest on rugged karst lands that long ago reforested is vulnerable to land development if it is close to an urban center and unprotected.
They then combined the time series of maps with forest inventory data on tree species diversity and carbon storage. They also characterized what drives both the spatial patterns of forest clearing for urbanization and the spatial patterns of forest recovery after large scale deforestation.
Based on their results and a synthesis of other work, the study concluded that accessibility, arability and spatial contagion emerge strongly as the overriding spatial controls on tropical forest age, determining 1) the pattern of agricultural abandonment that permits forest regrowth, and 2) where humans leave old-growth forest remnants.  In addition, similarities between the factors patterning forest age and land development explain why most forest cleared for land development is younger.  Forests are increasingly younger in more accessible and fertile areas where agriculture has lasted longer and land development is most common.  All else equal, more species-rich older forest on less arable lands are somewhat less likely to undergo development, but they are still vulnerable to clearing for land development if close to urban centers and unprotected. Accounting for forest age leads to a 19 percent lower estimate of forest biomass cleared for land development than if forest age is not accounted for.
Contributor: Eileen H. Helmer, USDA Forest Service
References:
Helmer, E.H., T.J. Brandeis, A.E. Lugo, and T. Kennaway (2008). Factors influencing spatial pattern in tropical forest clearance and stand age: Implications for carbon storage and species diversity. Journal of Geophysical Research, vol. 113, G02S04.
Kennaway, T. and E.H. Helmer (2007). The Forest Types and Ages Cleared from Land Development in Puerto Rico. GIScience & Remote Sensing, vol. 44, no. 4, pp: 356–382.