Leaf decomposition and fine fuels in floodplain forests of the Rio Negro in the Brazilian Amazon

Abstract

Abstract:Despite being inundated for up to 9 mo of the year, black-water floodplain forests in the Brazilian Amazon are susceptible to fire. Post-fire tree mortality is higher and fire spreads further in the floodplain, compared with adjacent upland forest. To understand these differences between the two forest types, we compared how leaf decomposition and fine-fuel loads change with inundation and soil texture. Litterbags containing leaves of Clitoria fairchildiana were placed on upland forest floor and submerged at two depths in a backwater of the Rio Negro. We used 80 bags per treatment and retrieved subsets every ~16 d from which the contents were cleaned, dried, weighed and discarded. Over the 81-d experiment, upland leaves decomposed two to three times faster than submerged leaves. Fine-fuel biomass (litter + root mat) was measured at 28 upland forest sites and 29 floodplain forest sites of the middle Rio Negro. Floodplain forests held about twice the fine fuel (25.9 ± 10.6 Mg ha−1) of uplands (10.9 ± 2.3 Mg ha−1). Upland soils had more sand but a carpet of fine apogeotropic tree roots was more common and thicker in floodplains. We infer that slow decomposition of submerged leaves leads to high tree mortality from fire in black-water floodplains by (1) increasing fire intensity due to high fine-litter fuel load and (2) making tree roots more vulnerable to burning because they form a peat-like mat to absorb nutrients from the thick litter.

Contrasting fire damage and fire susceptibility between seasonally flooded forest and upland forest in the Central Amazon using portable profiling LiDAR

“…Both are a consequence of slow leaf decomposition under water (Dos Santos & Nelson, 2013;Kauffmann, Uhl, & Cummings, 1988). During the late dry season, the now exposed litter and fine root mat of the black water flood forest become dry and flammable after just nine days without rain .…”

Resilience of Amazonian forests : the roles of fire, flooding and climate

“…Another difference is related to the fine root system. Black water forests have a root mat to avoid nutrient leaching (Stark and Jordan 1978;dos Santos and Nelson 2013), whereas in white water floodplains root mats are typically absent (Meyer et al 2010). In addition, Amazonian floodplains also include peatlands (Lahteenoja et al 2009(Lahteenoja et al , 2013, swamps, palm forests, white-sand forests, and extensive islands of savanna (Prance 1996;Junk et al 2011).…”

“…In the Negro river basin for instance, root mats have been found in 63% of floodplain forests, but in only 11% of upland forests (dos Santos & Nelson 2013). On the other hand, root mats increase the flammability of blackwater forests in dry years .…”

“…Root-mats occur in 63% of floodplain forests, yet in only 11% of upland forests (dos Santos & Nelson 2013). In dry years, root-mats can easily spread fires that percolate through blackwater forests ignited by camp-fires or upland agricultural sites .…”

“…Floodplain forests are surprisingly flammable due to the accumulation of fuel in the form of a root-mat (dos Santos and Nelson 2013), and to a relatively more open understory . Upland forests can percolate fire with less severe impact on tree cover (Chapter 2), yet other factors such as deforestation, logging and drought can reduce tree cover and increase upland forest flammability Brando et al 2014).…”