ORIGINAL ARTICLE Does hydroelectric reservoirs affect the structure of surrounding tree communities? A test of hypotheses in subtropical South America Leonardo Marques Urruth 1 • Alexandre Fadigas Souza 2 • Juliano Morales Oliveira 1 Received: 20 April 2016 / Accepted: 22 February 2017 Ó Botanical Society of Sao Paulo 2017 Abstract Despite its advantages over other energy sour- ces, hydropower facilities are recognized as causes of ter- restrial and freshwater environmental impacts. A deeper understanding of hydropower effects on surrounding riparian forests is thus important to the development hydropower facilities of less impact on the environment as well as to define criteria to manage the vegetation in the vicinity of large hydroelectric dams. We compared tree community structure on forest slopes exposed to a hydro- power reservoir created in 2005 with control ones, in the biodiversity-rich subtropical forests of southern Brazil. Field data were collected between 2009 and 2011 on six forest slopes of which three were affected by the rise in water level caused by a hydropower dam, while the other three were in nearby sites unaffected by damming. Ninety 10 9 10 plots were set along six 50 m 9 10 m transects. Forest structure was significantly affected by vicinity of reservoir, but this effect differed significantly between different tributaries of the Pelotas River. Rarefaction curves showed homogeneity in species richness among the studied hills. Composition differed significantly between reservoir and control areas. Variance partition results showed that pure reservoir effects explained a fraction of the variance in floristic composition comparable to envi- ronmental factors. Vicinity to reservoir was the main factor able to distinguish between communities in the studied system in a multiple regression tree. Our results highlight that hydroelectric facility building produces more exten- sive damage to remnant native forests than anticipated and that even forests upslope from reservoir border may suffer rapid structural and compositional degradation due to changes in environmental conditions caused by altered hydrology. Keywords Atlantic forest Á Araucaria mixed forest Á Dam impacts Á Mixed forests Á Southern Brazil Introduction Hydroelectricity is regarded as one of the cleanest sources of energy because it avoids the burning of fossil fuels, minimizes CO 2 emissions, and does not produce long-term wastes as nuclear power plants, for example (Sperling 2012). Despite these advantages over other energy sources, hydropower impacts terrestrial and freshwater environ- ments (WCD 2000). Hydropower dams inundate neigh- boring ecosystems, change river flow, hinder aquatic species migration, can dry up natural springs, and affect microclimate through changes in water temperature and sediment load (Nilsson et al. 2005; Elerwein 2013). Such impacts occur at the catchment scale and act on biodiver- sity and ecosystem processes, both upstream and down- stream (Finer and Jenkins 2012; Liu et al. 2013; Sun et al. 2014). Dam reservoir impacts on neighboring riparian vegetation are, however, still poorly understood. Such impacts may be significant because reservoir formation Electronic supplementary material The online version of this article (doi:10.1007/s40415-017-0376-1) contains supplementary material, which is available to authorized users. & Leonardo Marques Urruth leourruth@gmail.com 1 Programa de Po ´s-Graduac ¸a ˜o em Biologia: Diversidade e Manejo da Vida Silvestre, Universidade do Vale do Rio dos Sinos, Av. Unisinos 950, Sa ˜o Leopoldo, RS 93022-750, Brazil 2 Departamento de Ecologia, CB, Universidade Federal do Rio Grande do Norte, Campus Universita ´rio, Lagoa Nova, Natal, RN 59072-970, Brazil 123 Braz. J. Bot DOI 10.1007/s40415-017-0376-1