Wood as element of fluvial ecosystems: assessing the amounts, predictors and dynamics of in-stream wood in Brazilian streams

Abstract

Supplied as branches, trunks and roots to rivers and streams, wood is a natural part of aquatic systems, integrating them for million years. It is a key element, as important as sediments and riparian vegetation for the river systems function. It affects the hydraulic, morphological and geomorphological aspects of the channel, stores carbon, structures the habitat and benefits the aquatic biota. The distribution and amounts of wood vary along river basins, geographic and bioclimatic regions, biomes and the gradient of native forests. They also vary depending on the history of human activities in the basin and the occurrence of episodic events. There is a great knowledge gap with regard to in-stream wood in the tropics, since research have historically focused on temperate forest regions. There is little to no information regarding wood in rivers or streams in the Amazon, the largest tropical rainforest in the world, nor in the Cerrado, the Brazilian savanna and the second largest biome in South America. The deforestation pressure currently seen in these biomes further increases the need to gather information about in-stream wood before losing or significantly depleting its natural regime. In face of this, the objective of this thesis was to describe the wood in streams of Amazon and Cerrado, quantifying its stocks, identifying its main predictors, and checking its rates of recruitment, retention and transport. In the first article, we verified that the Amazon and Cerrado wood stocks are similar and comparable to those of temperate biomes. However, the tropical stock is highly dominated by smaller branches, such large logs are rarely found. In the second article, we observed that the main factor controlling the amount of wood is the piece size in relation to the channel, accompanied by the bankfull discharge, the stream power, the channel width and depth and the riparian vegetation. Basically, the wood present in the Amazon and Cerrado streams is the result of what was provided by the local riparian forest and how much of this was retained. In the third article, we found that most of the wood in Cerrado streams is retained from one rainy season to another, and what is transported is immediately replaced by the riparian forest. Hydrological and morphological differences as well as episodic events lead to variations in wood stock between streams. The piece travelled distance depends on its size, orientation and location in the channel, being result of the balance between drag and resisting forces. This thesis provided unprecedented information by describing the wood of Amazon and Cerrado streams and pointed out the critical factors to maintain the dynamics of it in tropical streams. As riparian forests are the primary source of wood to streams, we concluded that their degradation or removal leads to enhanced transport rates and consequent depleted wood stocks due to the decrease in piece size and changes in the hydrological regime. Therefore, it is urgent and essential to preserve riparian forests for the maintenance of fluvial wood processes.