Plant recruitment six years after the Samarco's tailings-dam disaster: Impacts on species richness and plant growth

Abstract

One of the greatest tragedies in Brazilian mining history occurred in November 2015 in Mariana, Minas Gerais state, when a dam from the mining company Samarco was breached. Millions of mine tailings from this upstream embankment were dumped over the Doce River basin, impacting an area of approximately 1469 ha of riparian vegetation. Our objective was to experimentally investigate whether plant recruitment and establishment are impaired in areas affected by tailings six years after the deposition. To achieve this goal, in 2021 we compared soil chemical properties between affected and unaffected areas, performed a soil seed bank experiment in controlled conditions, and conducted a greenhouse growth experiment using the two most abundant plant species. Affected soils presented lower fertility and organic matter content. At the same time, the mean abundance and richness of emerging plants did not differ between soils. Still, affected areas exhibited approximately 35% lower accumulated species richness (gamma diversity) than unaffected ones. The three most abundant species in both areas represented 34% of the individuals, being Marsypianthes chamaedrys (Vahl) Kuntze, Ludwigia octovalvis (Jacq.) P.H. Raven and Ageratum conyzoides L. In the growth experiment, plants growing in affected soils presented reduced height and stem diameter increment (L. octovalvis) or allocated fewer resources to root production than aerial parts (M. chamaedrys), potentially in response to soil infertility and density. Even after six years, our results showed that tailings-affected areas continue to experience negative impacts on plant recruitment, highlighting its adverse effects on ecosystem functions and services.

Fig 1. Location of the areas sampled in the three regions of the study.

Above, from left to right: South America, Brazil, and the state of Minas Gerais. Bellow: Sampling carried out in the regions: 1) downstream of the Fundão dam, 2) downstream of Paracatu de Baixo and 3) upstream of the Risoleta Neves Hydroelectric Power Plant. The tailings deposition zone is highlighted along the sampled region (shape file “PG23_Area_Afetada_Lama” provided by Fundação RENOVA). Map created using QGIS 3.34.14. Map data from openstreetmap.org/copyright.

Fig 2. Riverscapes of areas in the upper Doce River basin during the dry season.

Areas (A) affected by the Fundão dam disaster and (B) unaffected tributaries.

Fig 3. Substrate samples from riverine banks spread in plastic trays for seedling emergence counting.

(A) Substrate from affected areas and (B) soil from unaffected tributaries.

Fig 4. Species richness and abundance of emerging plants in the seed bank experiment.

Sample-sized rarefaction curves (solid lines) and extrapolation (dashed lines) of the entire study. Curves from areas affected and unaffected by the passage of the tailings from the Fundão Dam, collected six years after the disaster. Species diversity based on the Hill numbers (q = 0) and shaded areas represent 95% confidence intervals, which do not overlap (p < 0.05).

Fig 5. Variation in height and diameter of individuals from the study’s two most abundant species.

Vertical bars indicate the standard deviation of the mean. Different lower-case letters indicate significant differences in the parameters of the same species in time. *Significant overall difference for individuals of L. octovalvis plants.

Fig 6. Growth habit and flowers from individuals of the growth experiment.

(A) Ludwigia octovalvis and (B) Marsypianthes chamaedrys.