Juggling carbon: allocation patterns of a dominant tree in a fire-prone savanna

Abstract

In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving fire-induced stem death (i.e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after fire is a key component of recent models of tree-grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the first year after a burn. The concentration of starch in the roots of the burnt plants (0.08 +/- 0.01 g g(-1)) was half that of the unburnt plant (0.16 +/- 0.01 g g(-1)) at the end of the first growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4-5 months after topkill. Shoot growth of resprouting plants was restricted to the first few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. (14)C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long fire-free intervals to grow into adults. This paper provides some of the first empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults.

The effects of repeated cutting on coppice response of Terminalia sericea

“…Results from the study agree with other studies in which TNC reserves in woody plants have been shown to decrease as regrowth occurred after cutting, burning and defoliation (Miyanishi and Ellman 1986;Bowen and Pate 1993;Canadell and Lopez-Soria 1998;Landhausser and Lieffers 2002;Schutz et al 2009;McPherson and Williams 1998;Luostarinen and Kauppi 2005). Concentrations of TNC were also highest in trees cut less frequently compared to trees harvested more frequently in Gliricidia sepium (Jacq.)…”

Nonstructural Carbon in Woody Plants

“…Similarly, savanna trees, which persist through repeated top kill by resprouting, also store large amounts of NSC (e.g., sugars plus starch = 28-30% mass) belowground in specialized organs (i.e., lignotubers) (49,144). Observations of δ 13 C patterns, 14 C labeling studies, and defoliation experiments demonstrate that these belowground NSC reserves support coppicing after fire damage as well as bolting of the main stem to escape the fire kill zone generated by the grass layer (117,144). Finally, lignotuber NSC reserves are recharged by the initial coppice shoots over the following growing season and after bolting, and thus increase with plant age.…”

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

“…Biomass allocation to stems (Clarke et al 2013) and rapid stem growth rates in terms of height (Higgins et al 2000) and diameter (Gignoux et al 1997) are the plant trait parameters found to be important in resisting fires. Savanna trees resprout by utilising stored root carbohydrate reserves (Schutz et al 2009) substantially derived from leaf photosynthesis (Sakai and Sakai 1998). Hence, investment in leaves or leaf biomass provides an indirect measure of sapling success in savannas.…”