The threat imposed by increased sediment loading rates ranks among the most important stressors affecting coral reef ecosystems worldwide. This study represents an effort to quantify the effects of unpaved roads on erosion rates in a dry sub-tropical area of Puerto Rico and is intended to aid in developing scientifically-based erosion mitigation strategies. Hence, the specific objectives of this study were to: (1) measure sediment production rates from unpaved roads; (2) evaluate the effect of precipitation, rainfall erosivity, slope, plot length, and vegetation cover on sediment production rates; and (3) compare measured sediment production rates to published surface erosion data from roaded and natural sites in the Eastern Caribbean. Sediment production from nine abandoned road segments with varying slopes and plot lengths were measured with sediment traps in southwestern Puerto Rico from August 2003 to September 2005. The overall average sediment production rate was 0.84 Mg ha−1 yr−1, and the range of observed values was 15–50 times higher than locally-measured natural erosion rates. Only four of the nine study sites had a statistically significant correlation between sediment production and total rainfall and this is attributed to progressive changes in some of the conditions controlling erosion rates. Sediment production rates were dependent on slope raised to the 1.6th power, as well as to the product of plot length times slope1.6. Average erosion rates were inversely but poorly related to vegetation cover. An observed decline in sediment production rates was observed for all nine study segments, and this amounted to a statistically significant difference between observations made during the early stages of monitoring (Period 1: August 2003–April 2004) relative to those during the latter parts of the study (Period 2: May 2004– September 2005). Annual erosion rates during Period 1 amounted to 0.18 to 4.0 Mg ha−1 yr−1 for road segments with 1% and 22% slopes, respectively; rates during Period 2 were between 0.024 and 0.52Mg ha−1 yr−1, or only 13% of those during Period 1. Differences in sediment production rates between the two periods are attributed to more intense rainfall during Period 1 and to a notably higher vegetation cover during Period 2. Rainfall appears to play a paradoxical role in controlling surface erosion rates on abandoned road surfaces in a sub-tropical dry region.While ample rainfall is needed to generate erosion by rainsplash and overland flow, once rainfall satisfies soil moisture requirements for sustaining vegetation colonization it may also contribute to declining sediment production rates. Therefore, any model that attempts to properly address the temporal variation in erosion rates occurring on abandoned roads in a climatic setting where moisture availability is a limiting factor must not only follow the more traditional surface armoring-based approach but must also integrate the effects of re-vegetation. Such types of modelswill eventually become useful tools to properly assess the effects of past, current, and future land use practices on erosion rates, and to improve mitigation and land development strategies to lessen the impact on vital marine habitats

Puerto Rico’s forest cover continues to increase and is now 57 percent for mainland Puerto Rico, 85 percent for Vieques, and 88 percent for Culebra. Subtropical dry forest occupies 50 346 ha, 6832 ha, 2591 ha, and 6217 ha on the islands of Puerto Rico, Vieques, Culebra, and Mona, respectively. Subtropical moist forest, the most prevalent forested life zone on mainland Puerto Rico, had 49 percent forest cover or 258 861 ha of forest. Subtropical wet and rain forest occupies 161 503 ha, lower montane wet and rain forest occupies 11 723 ha at the highest elevations, and mangrove forest occupies 7920 ha in coastal areas. Puerto Rico’s forests were found to have over 1,602,378,689 trees over 2.5 cm in diameter and 10 607 847 m2 of basal area, and to hold 36.6 million Mg of sequestered carbon. There were 3,112 trees, 19.2 m2 of basal area, 68.25 m3 of merchantable stem volume, and 80 Mg of aboveground biomass in an average hectare of forest. The subtropical moist and wet and rain secondary forests inventoried in 1990 are still young and increasing in average basal area, which rose from 13.2 mm2/ha in 1980, to 15.2 m2/ha in 1990, to the current level of 20.9 mm2/ha. The most important tree species were the African tuliptree [Spathodea campanulata] Beauv., American muskwood [<="" i="">] (L.) Sleumer, cabbagebark tree [Andira inermis] (W. Wright) Kunth ex DC., and pumpwood [Cecropia schreberiana] Miq. Few unhealthy, stressed trees werenoted and widespread pest and disease problems were not observed. Only 12.9 percent of live trees had some type of damage or disease. Average per-hectare amounts of down woody material, forest floor duff, and forest floor litter generally increased as the forest environment became more humid. Small-to-medium forest fire fuels were most common in subtropical dry forests, while medium-to-large fuels were most common in more humid forest life zones.

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to 32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4 þ NO
3 ) suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.

In the Caribbean island of Puerto Rico, rapid land-use changes over the past century have included recent land-cover conversion to urban/built-up lands. Observations of this land development adjacent to reserves or replacing dense forest call into question how the changes relate to forests or reserved lands. Using existing maps, this study first summarizes island-wide land-cover change between 1977-78 and 1991-92. Then, using binomial logit modeling, it seeks evidence that simple forest cover attributes, reserve locations, or existing land cover influence land development locations. Finally, this study quantifies land development, reserve protection and forest cover by ecological zone. Results indicate that 1) pasture is more likely to undergo land development than shrubland plus forest with low canopy density, 2) forest condition and conservation status appear unimportant in that development locations neither distinguish between classes of forest canopy development nor relate to forest patch size or reserve proximity, and 3) most land development occurs in the least-protected ecological zones. Outside the boundaries of strictly protected forest and other reserves, accessibility, proximity to existing urban areas, and perhaps desirable natural settings, serve to increase land development. Over the coming century, opportunities to address ecological zone gaps in the island’s forest reserve system could be lost more rapidly in lowland ecological zones, which are relatively unprotected.