Nutrient limitation in tropical savannas across multiple scales and mechanisms

Abstract

Nutrients have been hypothesized to influence the distribution of the savanna biome through two possible mechanisms. Low nutrient availability may restrict growth rates of trees, thereby allowing for intermittent fires to maintain low tree cover; alternatively, nutrient deficiency may even place an absolute constraint on the ability of forests to form, independent of fire. However, we have little understanding of the scales at which nutrient limitation operates, what nutrients are limiting, and the mechanisms that influence how nutrient limitation regulates savanna–forest transitions. Here, I review literature, synthesize existing data, and present a simple calculation of nutrient demand to evaluate how nutrient limitation may regulate the distribution of the savanna biome. The literature primarily supports the hypothesis that nutrients may interact dynamically with fire to restrict the transition of savanna into forest. A compilation of indirect metrics of nutrient limitation suggest that nitrogen and phosphorus are both in short supply and may limit plants. Nutrient demand calculations provided a number of insights. First, trees required high rates of nitrogen and phosphorus supply relative to empirically determined inputs. Second, nutrient demand increased as landscapes approached the transition point between savanna and forest. Third, the potential for fire‐driven nutrient losses remained high throughout transitions, which may exaggerate limitation and could be a key feedback stabilizing the savanna biome. Fourth, nutrient limitation varied between functional groups, with fast‐growing forest species having substantially greater nutrient demand and a higher susceptibility to fire‐driven nutrient losses. Finally, African savanna trees required substantially larger amounts of nutrients supplied at greater rates, although this varied across plant functional groups. In summary, the ability of nutrients to control transitions emerges at individual and landscape scales, and is regulated through different mechanisms based on spatial (differences in underlying geology), temporal (stage in biome transition) and biological (species traits and community composition) variability.

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

“…For instance, Langan et al (2017) found that dynamic vegetation modelling experiments, designed to evaluate whether interactions among plant rooting depth, fire and precipitation, could predict the occurrence of savanna or forest biome states in tropical South America. Alternatively, if fire disturbance controls forest -savanna mosaics via feedbacks with soil conditions, then edaphic differences are a consequence, rather than the cause, of abrupt boundaries (Staal and Flores 2015;Pellegrini 2016), and slight shifts in fire regime could change the distribution of forest and savanna, making the mosaic meta-stable. This latter view has been formalised by the application of a fire-mediated alternative stable states (FMASS) model that has been developed for flammable landscape types across boreal (e.g.…”

“…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).…”

“…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

“…It has been shown that where annual rainfall is below ~1700 mm in South America, savannas are more stable than forests, and therefore more likely to expand . In tropical landscapes, yet particularly in South America, savannas have relatively less fertile and sandier soils, compared to forests (Jackson 1968;Bond 2010;Veldman and Putz 2011;Wood and Bowman 2012;Dantas et al 2013;Lehmann et al 2014;Gray and Bond 2015;Paiva et al 2015;Veenendaal et al 2015;Pellegrini 2016). Savanna and forest tree communities are also formed by different species, with contrasting adaptations to fire, and to resource availability such as root biomass Hoffmann et al 2009;Silva et al 2013;Dantas et al 2016).…”

“…The reason is that resource availability determines forest recovery rate and hence the chance of being trapped by fire (Grady and Hoffmann 2012;Hoffmann et al 2012;Murphy and Bowman 2012). An alternative view is that soils can change and be changed by plants and fire, in a plant-soil-fire feedback mechanism that can drive biome transitions (Silva et al 2013;Jackson 1968;Bond 2010;Wood and Bowman 2012;Paiva et al 2015;Pellegrini 2016). Forest tree species contribute to enrich the soil with high quality litter (Paiva et al 2015), which favours the recruitment of forest trees with higher nutrient demands (Bond 2010;Hoffmann et al 2012;Paiva et al 2015;Pellegrini 2016).…”