River logjams cause frequent large-scale forest die-off events in southwestern Amazonia

Umberto Lombardo

doi:10.5194/esd-8-565-2017

Abstract

Abstract. This paper investigates the dynamics of logjam-induced floods and alluvial deposition in the Bolivian Amazon and the effects these have on forest disturbance and recovery cycles. It expands on previous work by Gullison et al. (1996) who reported a case of catastrophic floods triggered by logjams in the Chimane Forest in the Bolivian Amazon. No further studies have followed up on this observation and no research has been published on the effects of large wood in tropical lowland rivers. The study is based on the analysis of a time series of Landsat imagery (1984–2016) and field evidence. Results show that logjam-induced floods are a major driver of forest disturbance along the Andean piedmont in the Bolivian Amazon. New logjams form on an almost yearly basis, always further upriver, until an avulsion takes place. Logjam-induced floods are characterized here by the sudden deposition of a thick sand layer and the death of forest in a V-shaped area. The Bolivian Amazon offers a unique opportunity for further research on how large wood affects river behavior in lowland tropical settings and how large and frequent forest disturbance events resulting from river logjams affect forest biodiversity and community successions.

Highlights

Understanding the spatial and temporal frequency of disturbance events is of great relevance to forest ecologists due to the importance of disturbance in shaping forest ecological processes (Asner, 2013)
The analysis of time series of Landsat imagery shows that, in a vast area of lowland forest running parallel to the eastern Andean piedmont in Bolivia, logjam-induced floods are a major driver of forest disturbance
The study focuses on rivers that cause logjam-induced floods in an area of lowland forest that stretches parallel to the Andean piedmont in Bolivia

Summary

Introduction

Understanding the spatial and temporal frequency of disturbance events is of great relevance to forest ecologists due to the importance of disturbance in shaping forest ecological processes (Asner, 2013). Forest disturbance and recovery cycles affect forest tree species distribution, community composition, ecosystem processes, biodiversity patterns, nutrient cycles, and the carbon balance (Chambers et al, 2013; Collins et al, 2012; Lewis et al, 2004; Phillips et al, 2004; White and Jentsch, 2001). Disturbance events are commonly classified along a continuum that goes from small-scale or frequent events, such as a tree fall that creates an open space, to large-scale and more rare events, such as large fires (White and Jentsch, 2001). The study of the geography of forest disturbance is important in Amazonia, as it is home to about 10 % of the world’s biodiversity (Lewinsohn and Prado, 2005) and is among the most important terrestrial carbon sinks (Pan et al, 2011). At least in western Amazonia, studies have shown that river activity can be an important driver of forest disturbance and landscape reshaping via lateral erosion, overbank deposition, crevasse formation, and avulsions (Aalto et al, 2002; Kalliola et al, 1992; Lombardo, 2016; Salo et al, 1986)

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