Dinâmica na produção de biomassa radicular em anos secos, em Floresta Tropical Seca, Caatinga

Abstract

The Caatinga, one of the largest remaining areas of Dry Tropical Forest (DTF), has been inadequately investigated and is considerably threatened, as are the other DTF around the world. This adds to the knowledge gap in biomass dynamics in these forests, especially of root biomass, which plays a key role in nutrient cycling, particularly in the carbon cycle. This work investigates root biomass dynamics in the Caatinga DTF in the long term, and its relationship to climate seasonality. In July 2015, 142 ingrowth cores were installed to monitor root biomass in the 0-30 cm soil layer in an area of Caatinga under regeneration for 31 years. An initial survey of the root biomass (10 cores collected) was carried out in July 2015. From November 2015, six cores were collected every four months until November 2018, the roots were separated from the soil by washing and their dry weight was determined. Precipitation and temperature data were collected from the agrometeorological station close to the experiment. In July 2015, the biomass was 16.46 Mg ha-1; this value represents the root dynamics for the 31 years without anthropogenic intervention. A significant correlation (Spearman = 0.329**) was found between the rainfall accumulated during the three months prior to collection and the collected biomass. During the first two years (2016 and 2017), the increases in biomass were greater than the reductions. Over the three years, total biomass production during the rainy season was 16.22 Mg ha-1, while reductions during the dry season resulted in the contribution of 10.51 Mg ha-1 of OM to the soil. Fine roots accounted for more than 80% of the biomass in each collection following installation, and were more sensitive to variations in water availability than the coarse roots. The vegetation of the Caatinga DTF has a better response to rainfall distribution than to the annual total rainfall, with root biomass proving to be significantly sensitive to rainfall seasonality, increasing during the rainy season and decreasing during the dry season. Despite being a dry tropical forest, root production in the Caatinga took place even in years of below average rainfall. Initial results show that the vegetation requires a minimum period of accumulated rainfall (3 months) to respond significantly to the increase in root biomass. Considering the results obtained until now, it can be seen that in order to clearly understand root dynamics in the DTF, studies of longer than the period analysed here are necessary.