Monitoring the impacts of a mega-dam on Amazonian understorey herbs

Abstract

Mega-dams have drastically altered the pulse of natural flooding in rivers of the Amazon, negatively affecting the biodiversity of different forest ecosystems. This study aimed to investigate the impacts of the Jirau dam on herbaceous assemblages, a relatively little studied biodiversity component. To accomplish this, we surveyed understorey herbs in 22 permanent plots (375 m2 each) between 2012 and 2019, before and after filling of the Jirau dam, and also measured a set of environmental predictors. Sampling comprised 178 morphospecies and 43,335 individuals. We found that species compositions in the pre-dam period differed in each of the sampled habitats (terra firme, transitional forest, várzea, and campinarana), and that such differences were mostly correlated with sum of bases, topography, C/N ratio, silt and pH. Environmental associations differed among three major groups of herbs (Ferns, Poales and Zingiberales). During the post-dam period, flooded plots showed high rates of mortality and recruitment close to 100 % and a sharp decline in species richness while non-flooded plots showed relative stability for these parameters. The number of flooded days and habitat type were strong predictors of mortality and recruitment rates. A combination of canopy opening driven by tree mortality and seasonal flooding after filling of the Jirau reservoir altered herb species composition in flooded plots, favouring the recruitment of Poales. Overall, herbaceous plants showed higher sensitivity to flooding caused by the Jirau dam when compared to trees sampled in the same site, reinforcing their potential as biodiversity targets for environmental assessments.