Fire alters ecosystem carbon and nutrients but not plant nutrient stoichiometry or composition in tropical savanna

Abstract

Fire and nutrients interact to influence the global distribution and dynamics of the savanna biome, but the results of these interactions are both complex and poorly known. A critical but unresolved question is whether short-term losses of carbon and nutrients caused by fire can trigger long-term and potentially compensatory responses in the nutrient stoichiometry of plants, or in the abundance of dinitrogen-fixing trees. There is disagreement in the literature about the potential role of fire on savanna nutrients, and, in turn, on plant stoichiometry and composition. A major limitation has been the lack of fire manipulations over time scales sufficiently long for these interactions to emerge. We use a 58-year, replicated, large-scale, fire manipulation experiment in Kruger National Park (South Africa) in savanna to quantify the effect of fire on (1) distributions of carbon, nitrogen, and phosphorus at the ecosystem scale; (2) carbon: nitrogen: phosphorus stoichiometry of above- and belowground tissues of plant species; and (3) abundance of plant functional groups including nitrogen fixers. Our results show dramatic effects of fire on the relative distribution of nutrients in soils, but that individual plant stoichiometry and plant community composition remained unexpectedly resilient. Moreover, measures of nutrients and carbon stable isotopes allowed us to discount the role of tree cover change in favor of the turnover of herbaceous biomass as the primary mechanism that mediates a transition from low to high 'soil carbon and nutrients in the absence of fire. We conclude that, in contrast to extra-tropical grasslands or closed-canopy forests, vegetation in the savanna biome may be uniquely adapted to nutrient losses caused by recurring fire.

Soil or fire: what causes treeless sedgelands in Tasmanian wet forests?

“…The nexus between fire, vegetation and soil fertility must be established using fire-exclusion experiments (Pellegrini et al 2015), a chronosequence approach (caveats of Johnson and Miyanishi 2008 notwithstanding) and natural experiments (Ondei et al 2017). Tree growth, whether determined using dendrochronological techniques (Bowman et al 2013) or via classical field and pot-trial experimentation, must be related to edaphic factors.…”

“…In many, but not all (Beckage et al 2009;Staver and Levin 2012), FMASS models, fire-vegetation-soil (FVS) interactions explain edaphic differences between forest and savanna (Jackson 1968;Wood and Bowman 2012;Staal and Flores 2015;Pellegrini 2016), underpinning hysteresis in forest -savanna boundary dynamics (Kellman 1984;Staal and Flores 2015;Pellegrini 2016). Severe fire or an abrupt increase in fire frequency can result in rapid forest loss and reduced post-fire soil fertility due to loss of the organic layer, volatilisation of nutrients, long distance transport of ash, leaching and soil erosion (Pivello and Coutinho 1992;Pellegrini et al 2015;Kettridge et al 2015;Bowman 2017). Conversely, forest recovery may be much slower due to seed dispersal limitation and/or reduced tree growth rates due to recurrent soil nutrient loss and gradual recovery of nutrient capital (Kellman 1984;Pellegrini 2016).…”

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

“…Therefore, it seems reasonable to expect that savanna expansion implies on the loss of soil fertility, the expansion of savanna trees, and of flammability across the landscape. For instance, forest tree encroachment in Neotropical and African savannas leads to soil fertilization (Moreira 2000;Coetsee et al 2010;Silva et al 2013;Pellegrini et al 2014;Pellegrini et al 2015). This same pattern of soil fertilization was found when forests expanded over savannas in the past (Silva et al 2008;Silva and Anand 2011).…”

“…This positive feedback traps the system in a low tree cover state (Grady and Hoffmann 2012). However, when fire is excluded from the system, forest trees are able to recruit (Moreira 2000;Higgins et al 2007;Pellegrini et al 2015), promoting canopy closure and suppressing fire (Murphy and Bowman 2012;Silva et al 2013). Most studies consider that the availability of resources across the landscape, such as soil fertility, determines the rate of forest growth and the chance that a system can be trapped by fire in a savanna state Murphy and Bowman 2012;Dantas et al 2016).…”

“…Most studies on tropical vegetation dynamics have shown evidence that forests and savannas can be alternative stable states, maintained by the interplay of fire, tree adaptations, and resources such as rainfall and soil fertility Hoffmann et al 2009;Silva et al 2013;van Nes et al 2014;Dantas et al 2016). Transitions between biomes had only been assessed with palaeoecology (Mayle and Power 2008), or during forest encroachment in savannas protected from fire (Moreira 2000;Coetsee et al 2010;Pellegrini et al 2014;Pellegrini et al 2015). In addition to the long-term forest savannization shown in Colombia (Cavelier et al 1998), this may be the first study ever to demonstrate forestsavanna transition using a known landscape fire history (Appendices 4.1 and 4.2).…”