Graduate Thesis Or Dissertation
 

Biomass and nutrient dynamics associated with deforestation, biomass burning and conversion to pasture in a tropical dry forest in Mexico

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  • The effects of deforestation and biomass burning in tropical dry forests (TDF) remain a little studied phenomenon. We quantified total aboveground biomass (TAGB), carbon and nutrient (N,S,Ca,P,K) loss under two separate fire severity scenarios; one early when the fuels were higher in moisture content, one later when the slash fuels were drier and then compared the loss and the regrowth of the sites. The TAGB and nutrients were measured (1993-1995) after the forest was cut, after a forest slash fire, one year after pasture establishment and, two years after the slash fire, biomass was quantified before and after a pasture fire. The treatments were based upon time from slash to burn. The low severity fires (Baja) were burned 65 days and the higher severity fires (Alta) were burned 95 days after the initial slash of TDF on ≈ 3.5 ha near the Chamela Biological Research Station on the Ejido San Mateo, Jalisco, Mexico. As a result of the 1993 slash fire, TAGB declined from 118.2 to 43.6 Mg ha⁻¹ (62%) in the Baja treatment and from 134.9 to 26.8 Mg ha⁻¹ (80%) in the Alta treatment. Nutrients pools declined 57-88% with ≈ 10% higher combustion of the Alta pools. In 1995, after the pasture fires, TAGB declined from 40.3 to 14.8 Mg ha⁻¹ (63%) and from 29.0 to 7.6 Mg ha⁻¹ (75%) in the Alta treatment and nutrient pools declined 57-88%. Total aboveground biomass loss from 1993-1995 was 103.4 Mg ha⁻¹ (87%) in the Baja treatment and 127.3 Mg ha⁻¹ (94%) in the Alta treatment. Carbon and nutrient losses ranged from 87-96% over the three-year study. We found little ash retention after fire, no increase in nutrient soil concentrations and, highly volatized nutrients (i.e. Ca and P) were essentially lost due to wind and water erosion on 40 to 60% slopes. Wood decomposition between fires reduced TAGB and nutrient pools by 15% in the Baja treatment and 3% in the Alta treatment.
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