Establishment and early persistence of tree seedlings in an annually burned savanna

Abstract

Summary 1.In severely disturbed habitats, the onset of resprouting as a persistence strategy might be problematic for tree species which do not accumulate sufficient reserves before the first disturbance event. This is due to the trade-off between the growth of reserves required to recover after disturbance and that of photosynthetic tissues. 2. In humid savannas, fire prevents trees from invading the whole landscape and nearby gallery forests have a completely different floristic composition. We test if the variations of survival during the first years of a young tree's life can explain the exclusion of forest species and the dominance pattern within savanna species. 3. Every six months for four years, we censused all seedlings and resprouts in 1 ha area of an annually burned savanna, to estimate their seasonal survival rates. We used capture-recapture statistical models to control for the probability of missing seedlings in the tall grass. 4. There were two main distinct patterns of survival among seedlings: 'fire-responding' species showed a 20-80% decrease in survival during the dry season, interpreted as mainly due to fire; 'drought-responding' species showed 20-80% variations in survival positively correlated to early-growing-season rainfall. 5. Yearly averaged survival probabilities of seedlings ranged between 0.10 and 0.63, reaching 0.850-0.996 for > 3-year-old resprouts of savanna species. Forest species showed no increase in survival with age. 6. A 4-year-survival-probability analysis showed that forest species were excluded from the savanna at the seedling stage. No parameter of the early survival curve related to the abundance of savanna species at the adult stage. 7. Synthesis . Savanna tree species follow two mutually exclusive main patterns of early survival probably related to fire and early-wet-season drought. The exclusion of forest species is consistent with a build up of reserves that is too slow due to the growth-resistance trade-off. We conclude from these findings that the use of resprouting as a persistence strategy is heavily constrained by disturbance frequency and imposes strong trade-offs on plant growth strategy.

How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness

“…However, many studies from both Africa and the Brazilian cerrado have also found that fire excludes faster growing forest species from savanna (Hoffmann and Solbrig 2003, Gignoux et al 2009), which is not expected if height growth rate is the key factor for survival in fire-prone savanna. Even though forest species grow more quickly than savanna species, in South America they are excluded from savanna (cerrado) by virtue of having thin bark (;20% of the relative bark thickness of savanna species) Franco 2003, Hoffmann et al 2009).…”

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

“…Due to low light availability under canopy shade, saplings invest heavily in shoot growth and not in root reserves, which makes them more vulnerable to drought (Barnes 2001). For this reason, fire is also a considerable threat, particularly because fire is a phenomenon frequent in humid savannas (Bond 2008, Gignoux et al 2009). Hence, the benefits of shade on sapling growth in Schotia brachypetala and A. nigrescens may not apply to long-term survival and growth.…”

“…Grass competition, identified as the greatest hurdle for sapling survival by Grellier et al (2012) and Ward and Esler (2011), also did not have an effect on sapling survival. Perhaps, in humid savannas, other environmental factors such as temperature, fire and herbivory play a more influential role in savanna sapling survival (Shaw et al 2002, Hoffmann and Solbrig 2003, Sharam et al 2006, Chidumayo 2008, Gignoux et al 2009). …”

“…Hoffmann (1996), Gignoux et al (2009), Rolhauser et al (2011) and Bond et al (2012) have made some key observations on tree sapling establishment in humid savannas. Very few studies conducted on savannas have concentrated on multiple species to make general interpretations of the seedling dynamics across environmental gradients (see Hoffmann andFranco 2003, Tomlinson et al 2012).…”

“…In many savannas, seeds germinate with the onset of rainy season after being stimulated by dry season fires (Gashaw and Michelsen 2002). However, an early wet season drought after germination can cause heavy mortality of tree saplings (Harrington 1991, Hoffmann 1996, Gignoux et al 2009). Therefore, frequent rainfall is more important than overall mean annual precipitation in estimating tree sapling establishment (Hoffmann 1996, Wilson and Witkowski 1998, Kraaij and Ward 2006.…”