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.

The interfacial effects arising in electrical measurements of rocks, when a two‐electrode system is used, are analyzed via the Maxwell‐Wagner effect. The similarity in electrical behavior between rock samples and heterogeneous dielectrics with nonnegligible ohmic conductivities leads to a general analysis of the Maxwell‐Wagner effect. We conclude that, in general, it is necessary to have volume charge accumulations at interfaces within the sample and at the electrode‐sample interface in order to have electrical steady‐state conditions in rock samples. The expression for the charge accumulation at the interface as a function of frequency is obtained as well as expressions for the effective dielectric permittivity in a composite material; the relaxation time associated with the charge accumulation and that of the effective dielectric permittivity are the same. Given a sample consisting of two different materials, it is shown that whenever both components have nonnegligible ohmic conductivities, the imaginary component of the effective dielectric permittivity tends to infinity when the frequency tends to zero; such behavior is not observed when one of the two conductivities is negligible. This fact constitutes the basis of the method of measurement for heterogeneous dielectrics with nonnegligible ohmic conductivities proposed herein. The method consists essentially of utilizing a dielectric with negligible ohmic and dielectric conductivities as one of the two components of the sample, which isolates the other one from the electrodes. Theoretical examples of the behavior of such a composite material for various cases are analyzed and three experimental examples are presented. The method presents the advantage of permitting application of theory developed for dielectrics in general.

No abstract

No abstract

No abstract

No abstract

No abstract