Wood stock in neotropical streams: Quantifying and comparing instream wood among biomes and regions

Abstract

Instream wood plays important chemical, physical and ecological functions in aquatic systems, benefiting biota directly and indirectly. However, human activities along river corridors have disrupted wood recruitment and retention, usually leading to reductions in the amount of instream wood. In the tropics, where wood is believed to be more transient, the expansion of agriculture and infrastructure might be reducing instream wood stock even more than in the better studied temperate streams. However, research is needed to augment the small amount of information about wood in different biomes and ecosystems of neotropical streams. Here we present the first extensive assessment of instream wood loads and size distributions in streams of the wet-tropical Amazon and semi-humid-tropical Cerrado (the Brazilian savanna). We also compare neotropical wood stocks with those in temperate streams, first comparing against data from the literature, and then from a comparable dataset from temperate biomes in the USA. Contrary to our expectations, Amazon and Cerrado streams carried similar wood loads, which were lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in the USA. Our results indicate that the field survey methods and the wood metric adopted are highly important when comparing different datasets. But when properly compared, we found that most of the wood in temperate streams is made-up of a small number of large pieces, whereas wood in neotropical streams is made up of a larger number of small pieces that produce similar total volumes. The character of wood volumes among biomes is linked more to the delivery, transport and decomposition mechanisms than to the total number of pieces. Future studies should further investigate the potential instream wood drivers in neotropical catchments in order to better understand the differences and similarities here detected between biomes and climatic regions.

Wood predictors in neotropical streams: Assessing the effects of regional and local controls in Amazon and Cerrado catchments

“…Despite that, the predominantly small size of LW in the study streams (Saraiva et al, 2022) indicates that they can be easily transported. Thus, if LW pieces are not arriving from upstream it is probably because they are being degraded along the way, as expected for small-sized instream wood (Haga et al, 2002;Lienkaemper & Swanson, 1987;Merten et al, 2013).…”

“…Paula et al (2011) found that LW abundance in Southeast Brazilian streams was similar to that in temperate secondary forested streams in Germany, and that LW volume was similar or greater than in temperate old-growth forested streams in New Zealand and Japan, respectively. Likewise, Saraiva et al (2022) reported that amounts of LW in streams in Brazilian tropical forests and savannas were similar to those in temperate biomes in the United States. Both studies suggested that the differences between tropical and temperate wood stock were more related to the distribution of wood size than the total number of pieces of instream wood.…”

“…Map of the study sites in six study regions across Brazilian biomes. Adapted from Saraiva et al (2022). [Color figure can be viewed at wileyonlinelibrary.com]…”

“…To our knowledge, the wood regime has never been studied in rivers of the Amazon, the largest tropical forest and hydrographic basin in the world. Only a few studies have inventoried large wood on Amazon floodplains (Chao et al, 2008; Martius, 1997; Silva et al, 2016) and only one (Saraiva et al, 2022) quantified large wood (LW) in the streams themselves. Saraiva et al (2022) is also the only study of instream wood in the Cerrado (the Brazilian Savanna), the second largest biome in South America.…”

“…Only a few studies have inventoried large wood on Amazon floodplains (Chao et al, 2008; Martius, 1997; Silva et al, 2016) and only one (Saraiva et al, 2022) quantified large wood (LW) in the streams themselves. Saraiva et al (2022) is also the only study of instream wood in the Cerrado (the Brazilian Savanna), the second largest biome in South America. Further efforts to quantify wood naturally occurring in these neotropical streams are threatened by the recent high rates of deforestation in these two neotropical biomes in recent years, triggered mainly by expansion in agriculture and livestock grazing (Parente et al, 2021; Pereira et al, 2020; Silva‐Junior et al, 2021; Trigueiro et al, 2020).…”