Influence of secondary forest and grass on deflagration of shallow landslides in Nova Friburgo, Rio De Janeiro

Abstract

The mountain region of Rio de Janeiro has one of the largest continuous remnants of Atlantic Forest being characterized at the same time by the recurrence of mass movements associated with intense rains. In the disaster of this mountain region, more than 3, 600 landslides were triggered, of which 24.6% occurred in grass cover, while 54.8% occurred in forest cover. The objective of this work was to analyze the influence of these vegetations on soil moisture control for the understanding of landslides deflagration in heavy rains. Two slopes were selected in Nova Friburgo (RJ) to monitor precipitation, interception and suction at depths of 5, 20, 50, 100, 150 and 220 cm, which were also analyzed for biomass and length of thick (> 2 mm) and fine roots (<2 mm) and in relation to the physical properties of the soil (grain size, aggregate stability and porosity). The results showed significant biomass and long root length differences between forest (1785.0 g.m-3 and 40.1 m.m-3) and grass (121.2 g.m-3 and 6.4 m.m-3). The fine roots biomass was similar among the vegetations, but in the grass were found up to 100 cm, while in the forest up to 200 cm. Regarding the soil physical properties, the main difference was found in the higher stability of aggregates and porosity in the forest, reflecting the better soil structure by the roots. Forest interception was 19% of precipitation, indicating the recovery of this hydrological function. The forest roots contributed to the entry and removal of moisture along the entire profile, while in the grass the depths of 100, 150 and 220 cm showed moisture conservation (-20 kPa) throughout the monitoring. In the heavy rains, however, both vegetations favored soil saturation (-3 kPa), which in the grass occurred up to 50 cm, while in the forest occurred from 100 cm.