Influência do florestamento de Acacia mangium Willd. no estabelecimento de espécies arbóreas nativas

Abstract

This study aimed to evaluate the effects of environmental changes (canopy openness and litter accumulation) induced by the Acacia mangium afforestation on the dynamics of native tree species natural regeneration on areas originally occupied by savannas. The effects of canopy openness, litter accumulation, and basal area of A. mangium on growth, recruitment, and mortality rates of native species established in the understory of the plantations were studied in a two-year interval. Individuals with base diameter greater or equal to 2 cm were measured in long, narrow plots (250m x 2m) of 0.05 hectare, with 8 plots located in savanna areas and 17 in forest plantations with an average age of 12 years. Canopy openness and litter (depth and dry mass) were obtained to evaluate the effects of these variables in the native tree species annual growth rates, mortality, and recruitment in the understory of the plantations. Canopy openness was obtained from digital photographs and litter depth was determined using the Marimon-Hay meter. The dry mass of litter and herbaceous biomass were determined by the direct method. In total, I found 1,436 individuals belonging to 29 families and 54 species. Savanna species represented only 16.67% of the individuals; the understory was dominated by forest (51.85%) and generalist (29.63%) species. The vegetation structure did not show significant changes over the time interval studied. Natural regeneration in the understory of acacia plantations studied was characterized by individuals of native species with diameter smaller than 4 cm and lower than 5 m in height. Regeneration of Acacia mangium in plantations was rare (only 3% of the sapling belonged to this species). However, after fire events, large number of acacia seedlings rapidly colonized the understory. The diameter growth, recruitment, and mortality of native species did not differ significantly between the study environments (savanna and plantation forest). There was no significant difference between the growth in diameter (F = 2.91, p <0.07) and height (F = 2.35, p <0.25) of native species established in plantations versus savanna, nor in the recruitment rate (t = 0.14; p = 0.89), and mortality (t = 0.86; p = 0.39) among acacia plantations and savanna. The effects of environment on relative growth rate of savanna species was dependent on the species. Curatella americana (t = -8.60, p <0, 0001) and Bowdichia virgilioides (t = -2.43, p <0.03) showed higher average growth rate in diameter in savanna areas, when compared to plantations. Mortality rate was also not related to canopy openness or basal area of A. mangium. However, the mortality rate was inversely related to the litter accumulated on the ground (F = 7.43, p = 0.01), indicating that lower mortality rates were expected in areas with higher amount of litter accumulation. The herbaceous biomass showed a negative relationship with the basal area of acacia (t = -3.82, p <0.001). The high dominance of A. mangium in the canopy, and the low density of individuals regenerating in the understory suggest that afforestation with A. mangium has a limited role in catalyzing secondary succession.