Massive tree mortality from flood pulse disturbances in Amazonian floodplain forests: The collateral effects of hydropower production

Highlights

• Considerable floodplain forest loss occurred downstream of the Balbina dam from flood pulse changes;

• Several tree species are still suffering mortality, which may increase dead stand areas;

• Remote sensing methods used here may be applied to other floodplains with massive tree mortality by damming;

• Downstream igapó forests are highly sensitive to the loss of low water periods resulting from flow alteration by dam operations;

Abstract

Large dams built for hydroelectric power generation alter the hydrology of rivers, attenuating the flood pulse downstream of the dam and impacting riparian and floodplain ecosystems. The present work mapped black-water floodplain forests (igapó) downstream of the Balbina Reservoir, which was created between 1983 and 1987 by damming the Uatumã River in the Central Amazon basin. We apply remote sensing methods to detect tree mortality resulting from hydrological changes, based on analysis of 56 ALOS/PALSAR synthetic aperture radar images acquired at different flood levels between 2006 and 2011. Our application of object-based image analysis (OBIA) methods and the random forests supervised classification algorithm yielded an overall accuracy of 87.2%. A total of 9800 km² of igapó forests were mapped along the entire river downstream of the dam, but forest mortality was only observed below the first 49 km downstream, after the Morena rapids, along an 80-km river stretch. In total, 12% of the floodplain forest died within this stretch. We also detected that 29% of the remaining living igapó forest may be presently undergoing mortality. Furthermore, this large loss does not include the entirety of lost igapó forests downstream of the dam; areas which are now above current maximum flooding heights are no longer floodable and do not show on our mapping but will likely transition over time to upland forest species composition and dynamics, also characteristic of igapó loss. Our results show that floodplain forests are extremely sensitive to long-term downstream hydrological changes and disturbances resulting from the disruption of the natural flood pulse. Brazilian hydropower regulations should require that Amazon dam operations ensure the simulation of the natural flood-pulse, despite losses in energy production, to preserve the integrity of floodplain forest ecosystems and to mitigate impacts for the riverine populations.

Introduction

Hydroelectric dams are a growing threat to the extensive Amazon wetlands, having several negative environmental effects and disturbing the hydrological balance that controls the structure and function of these environments (Castello and Macedo, 2016; Foote et al., 1996; Kahn et al., 2014; Latrubesse et al., 2017; Timpe and Kaplan, 2017). Although dams are important for electricity production, they also increase greenhouse gas emissions, disrupt indigenous and riverine communities, and lead to forest degradation and losses of biodiversity as a consequence of the large areas artificially inundated by the reservoirs behind storage dams (Benchimol and Peres, 2015; Cochrane et al., 2017; Fearnside, 1995; Junk and de Mello, 1990; Kemenes et al., 2007). Most ecological studies of hydropower dam impacts assess only the inundated and surrounding areas of reservoirs, while downstream environmental alterations caused by the hydrological changes created by dams remain poorly understood (Assahira et al., 2017; Manyari and de Carvalho, 2007; Timpe and Kaplan, 2017; Williams and Wolman, 1984).

Dams typically alter the natural “flood pulse” (Junk et al., 1989), negatively affecting the ecological dynamics of downstream floodplain ecosystems (Benchimol and Peres, 2015; Fearnside, 1995; Junk and Mello, 1990; Kemenes et al., 2007). Under natural conditions, the flood pulse of large rivers is predictable, characterized by a monomodal flood pulse with an annual cycle of rising, high, receding and low water phases, which can span more than 10 m of amplitude in the case of Central Amazonia (Junk et al., 2011; Schöngart and Junk, 2007). This natural water level oscillation floods over 750,000 km² annually in the Amazon Basin (Wittmann and Junk, 2017), varying in amplitude and duration depending on the amount and seasonality of rainfall within catchments, and on river discharge and topography (Junk et al., 2011). The flood pulse is the main driver of ecological and biogeochemical processes in floodplain forests (Junk, 1989; Lewis et al., 2000), where trees display anatomical, morphological, physiological, and biochemical adaptations to survive in this seasonal environment (De Simone et al., 2003; Ferreira and Parolin, 2011; Junk, 1989; Parolin et al., 2004). For this reason, a large number of tree species are endemic to floodplain environments (Wittmann et al., 2006, Wittmann et al., 2013, Wittmann et al., 2017).

Within the Amazonian floodplains, igapós are forests influenced by black or clear water (oligotrophic) rivers with catchments in the Precambrian Guiana shield and Central Brazilian archaic shield, receiving much lower sediment loads than the eutrophic várzea forests, which are inundated by sediment-rich white-water rivers with catchments in the Andean region (Junk et al., 2011; Prance, 1979; Sioli, 1984). Due to their low sediment load, black water and clear water rivers have been the main targets of hydroelectric projects across the Brazilian Amazon. Furthermore, several igapó areas along clear water rivers are close to the so-called “Arc of Deforestation”, the expanding southwestern agricultural frontier in the Amazon, further compounding the threats to these systems (Lees et al., 2016).

Floodplain forest trees are recognized as good bioindicators of hydrological disturbances, due to their longevity (Schöngart et al., 2005). Many species have a seasonal growth pattern closely linked to the hydrological cycle (Schöngart et al., 2002), and changes in the regularity and duration of the natural flood cycle can lead to growth reduction or interruption and even to mortality. Assahira et al. (2017) found that a substantial increase in annual minimum water levels observed for several consecutive years in the Uatumã River, caused by water released from the Balbina Reservoir for power generation, led to the mortality of large populations of the highly flood-adapted tree species Macrolobium acaciifolium (Benth.) Benth. (Fabaceae) (Schlüter and Furch, 1992).

The number of hydroelectric power plants in the Amazon has grown steadily in the last 30 years, and this growth is expected to continue because these ventures are considered to be important for economic development, with efforts to understand and manage their socioecological implications often rushed and inadequate (Tilt, 2012; Tilt et al., 2009). Recent studies show conflicting data on the number of hydroelectric plants in operation, under construction and planned, even for papers published within the same year (Castello et al., 2013; Latrubesse et al., 2017; Lees et al., 2016; Timpe and Kaplan, 2017). Latrubesse et al. (2017) identified 140 dams in operation or under construction and 288 planned dams in the entire Amazon region. However, Anderson et al. (2018) reported 142 dams in operation or under construction and 160 planned dams for the Andean Amazon alone, implying a considerable underestimation from the remaining studies (Anderson et al., 2018; Castello and Macedo, 2016; Finer and Jenkins, 2012; Forsberg et al., 2017).

Disruptions in connectivity are also known to compromise the physical habitat and the ecological integrity of river systems around the world, threatening endemic species and allowing the colonization of invasive ones (Ligon et al., 1995; Mumba and Thompson, 2005; Nilsson, 2000; Ward and Stanford, 1995). In the case of large-scale Amazonian hydroelectric dams, existing studies have a very limited spatial distribution, and thus the true impacts and the extent of environmental disturbances caused by them remain only partially understood (Fearnside, 2014; Junk and Mello, 1990; Wittmann and Junk, 2017). There is a pressing need to quantify and understand the processes that occur downstream of Amazonian dams, where the natural flood pulse is completely altered by the opening and closing of floodgates, thereby altering the natural seasonality of the rivers (Assahira et al., 2017; Junk and Mello, 1990).

The construction of the Balbina Dam (Amazonas, state, Brazil) between 1983 and 1987 led to an abrupt change in the Uatumã River hydrology, which has been maintained for the past 30 years, providing an ideal system to assess ecohydrological responses to damming in igapó environments. Therefore, in the present study we address the following questions: (1) What is the spatial distribution and extent of tree mortality in igapó floodplain forests in the Uatumã River, downstream of the Balbina Dam? (2) Which recommendations can be made for future evaluations of environmental impacts of future hydroelectric dams in the Amazon region with focus on downstream areas? (3) Is igapó vegetation loss still occurring, and if so, in which areas?