/ We present results of a study in an intensively impacted and highly fragmented landscape in which we apply field-measured carbon (C) density values to land-use/land-cover (LU/LC) statistics to estimate the flux of C between terrestrial ecosystems and the atmosphere from the 1970s and 1990s. Carbon densities were assigned to common LU/LC classes on vegetation maps produced by Mexican governmental organizations and, by differencing areas and C pools, net C flux was calculated from the central highlands of Chiapas, Mexico, during a 16-year period. The total area of closed forests was reduced by half while degraded and fragmented forests expanded 56% and cultivated land and pasture areas increased by 8% and 30%, respectively. Total mean C densities ranged from a high of 504 tons C/ha in the oak and evergreen cloud forests class to a low of 147 tons C/ha in the pasture class. The differences in total C densities among the various LU/LC classes were due to changes in biomass while soil organic matter C remained similar. We estimate that a total of 19.99 thick similar 10(6) tons C were released to the atmosphere during the period of time covered by our study, equal to approximately 34% of the 1975 vegetation C pool. The Chiapas highlands, while comprising just 0.3% of Mexico's surface area, contributed 3% of the net national C emissions. KEY WORDS: Land use; Land cover; Carbon flux; Forests; Chiapas highlands; Mexico

We applied modeled biomass density estimates to changes in land use/land cover (LU/LC) statistics for the intensively impacted and highly fragmented landscape of tropical Mexico to estimate the flux of carbon (C) between terrestrial ecosystems and the atmosphere between 1977 and 1992. Biomass densities were assigned to hybrid LU/LC classes on vegetation maps produced by Mexican governmental organizations and, by differencing areas and biomass C pools, net C flux was calculated in the eight-state tropical region of southeast Mexico. These states, representing tropical Mexico, experienced a mean annual deforestation rate of nearly 559 000 ha/yr, or 1.9%, between 1977 and 1992. The total area of closed forests decreased by 26%, open/fragmented forests decreased by 31%, and agroecosystem areas increased by 64%. Total mean biomass densities ranged from a high of 265 Mg/ha in the Veracruz state tall/medium tropical evergreen forest class to a low of 12 Mg/ha in the cultivated land class (several states). We estimate that a total of 280 Tg C were released from the terrestrial biosphere during the 15-yr period covered by our study, equal to nearly 20% of the region's 1977 biomass C pool. The study region, while comprising just 24% of Mexico's surface area, contributed 36% of the net national C emissions from LU/LC change.

We applied modeled biomass density estimates to changes in land use/land cover (LU/LC) statistics for the intensively impacted and highly fragmented landscape of tropical Mexico to estimate the flux of carbon (C) between terrestrial ecosystems and the atmosphere between 1977 and 1992. Biomass densities were assigned to hybrid LU/LC classes on vegetation maps produced by Mexican governmental organizations and, by differencing areas and biomass C pools, net C flux was calculated in the eight‐state tropical region of southeast Mexico. These states, representing tropical Mexico, experienced a mean annual deforestation rate of nearly 559 000 ha/yr, or 1.9%, between 1977 and 1992. The total area of closed forests decreased by 26%, open/fragmented forests decreased by 31%, and agroecosystem areas increased by 64%. Total mean biomass densities ranged from a high of 265 Mg/ha in the Veracruz state tall/medium tropical evergreen forest class to a low of 12 Mg/ha in the cultivated land class (several states). We estimate that a total of 280 Tg C were released from the terrestrial biosphere during the 15‐yr period covered by our study, equal to nearly 20% of the region's 1977 biomass C pool. The study region, while comprising just 24% of Mexico's surface area, contributed 36% of the net national C emissions from LU/LC change.

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Soils support ecosystem functions such as plant growth and water quality because of certain physical, chemical, and biological properties. These properties have been studied at different spatial scales, including point scales to satisfy basic research needs, and regional scales to satisfy monitoring needs. Recently, soil property data for the entire USA have become available in the State Soil Geographic Data Base (STATSGO), which is appropriate for regional‐scale research. We analyzed and created models of STATSGO data in this study to serve as a research tool, for example, for linking the soil to regional water quality monitoring data in our companion paper. Map units in STATSGO define geographic land areas by soil characteristics (SCs) of similar soil series. We selected 27 SCs that influenced water properties (in varying degrees), aggregated the layer and component SCs to map unit SCs, and used SCs to calculate relationships among map units. The relationships were defined by equations of conditional mean for the qth SC (SCq), while using the remaining 26 SCs as predictors. The relative standard errors for 22 of the 27 SCs were less than 10%, and less than 22% for the remaining five. We conclude that spatial extrapolation of SCs is feasible and the procedures are a first step toward extrapolating information across a region using SC–water property relationships. Although our procedure is for regional scale monitoring, it is also applicable to finer spatial scales commensurate with available soil data.