Can savannas become forests? A coupled analysis of nutrient stocks and fire thresholds in central Brazil

Sharp ecotones spark sharp ideas: comment on "Structural, physiognomic and above-ground biomass variation in savanna–forest transition zones on three continents – how different are co-occurring savanna and forest formations?" by Veenendaal et al. (2015)

“…The same process has also been shown in savannas (Kauffman et al, 1994). In the absence of fire, soil fertility in forests is maintained by efficient nutrient recycling (Vitousek and Sanford, 1986;Silva et al, 2013). Indeed, in many parts of the tropics, as confirmed by the results of Veenendaal et al (2015), forest soils are more fertile than savanna soils (Bond, 2010;Veldman and Putz, 2011;Wood and Bowman, 2012;Dantas et al, 2013;Silva et al, 2013;Lehmann et al, 2014).…”

“…In the absence of fire, soil fertility in forests is maintained by efficient nutrient recycling (Vitousek and Sanford, 1986;Silva et al, 2013). Indeed, in many parts of the tropics, as confirmed by the results of Veenendaal et al (2015), forest soils are more fertile than savanna soils (Bond, 2010;Veldman and Putz, 2011;Wood and Bowman, 2012;Dantas et al, 2013;Silva et al, 2013;Lehmann et al, 2014). When forests expand, their trees have a positive effect on the nutrient availability of the relatively poor soils of savannas (Silva et al, 2008;Silva and Anand, 2011;Paiva et al, 2015).…”

“…The dashed arrow is hypothetical, but note that the positive feedback loop does not depend on it. The model is based on previous studies (Jackson, 1968;Bond, 2010;Wood and Bowman, 2012;Dantas et al, 2013;Silva et al, 2013;Franco et al, 2014;Bowman et al, 2015;Gray and Bond, 2015;Paiva et al, 2015).…”

“…Savanna trees, on the one hand, allocate many resources to fire resistance, for instance by developing thick barks (Keeley et al, 2011). Communities of savanna trees are thus generally not able to attain closed canopies (Silva et al, 2013). This strategy allows coexistence with flammable herbaceous vegetation, stimulating the occurrence of frequent fires.…”

“…This strategy allows coexistence with flammable herbaceous vegetation, stimulating the occurrence of frequent fires. Forest trees, on the other hand, allocate more resources to leaves and, therefore, require about 3 times less nutrients to reach canopy closure than savanna trees (Silva et al, 2013). Although forest trees are less resistant to fire, their ability to close the canopy allows them to suppress fire.…”