Biomass, Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Abstract

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pas- ture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of Mexico, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha -1 , whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha -1 . The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha -1 . Dead wood (standing and downed combined) comprised 27%-29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass av- eraged 32.0 Mg ha -1 in floodplain forest, 17.1 Mg ha -1 in dry forest, and 5.8 Mg ha -1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha -1 in pasture and reached 180 and 12.9 Mg ha -1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha -1 in pasture and reached 2623 and 264 kg ha -1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha -1 in floodplain forest, 141 Mg ha -1 in dry forest, and 124 Mg ha -1 in pasture. Soil C comprised 37%-90% of the total ecosystem C, whereas soil N comprised 85%-98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in Mex- ico, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmo- sphere from the burning of biomass in the dry tropical landscapes of Mexico may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.