Facultative and Obligate Trees in a Mesic Savanna: Fire Effects on Savanna Structure Imply Contrasting Strategies of Eco-Taxonomic Groups

Abstract

Fire is a major determinant of savanna tree communities and, as such, manipulation of fire frequency is an important management tool. Resolving the effects of fire management on tree size class distributions can help managers predict and plan for short-term ecological and economic outcomes, reveal different strategies by which woody plants cope with frequent fire, and help us predict vegetation changes under future fire scenarios. Savanna structure and size class distribution are strongly influenced by the ability of suppressed tree resprouts to escape stem death by frequent fire. A widespread assumption is that resprouts have an imperative to escape fire to reach sexual maturity in the canopy and thereby ensure long-term species viability. We use a census of Australian mesic savanna tree communities subjected to annual, triennial, and fire exclusion (unburnt) fire treatments to ask how fire frequency affects size class distributions within and between eco-taxonomic groups of species. Total tree densities did not significantly differ, but were highest in the triennial (7,610 ± se 1,162 trees ha−1) and unburnt fire treatments (7,051 ± se 578 trees ha−1) and lowest in the annual fire treatment (6,168 ± se 523 trees ha−1). This was caused by increased sapling densities in the triennial and unburnt fire treatments, predominantly of Acacia and pantropical genera. Eucalypts (Eucalyptus and Corymbia spp.) dominated the canopy across all fire treatments indicating relatively greater success in recruiting to larger sizes than other species groups. However, in the sub-canopy size classes eucalypts co-dominated with, and in some size classes were outnumbered by, pantropicals and Acacia, regardless of fire treatment. We hypothesize that such results are caused by fundamental differences in woody plant strategies, in particular sexual reproduction, that have not been widely recognized in Australian savannas.

Does rapid utilization of elevated nutrient availability allow eucalypts to dominate in the tropical savannas of Australia?

“…That is, seedlings of the two dominant tree species of northern Australia's mesic savannas ( Eucalyptus tetrodonta and Eucalyptus miniata ) are highly responsive to nutrient addition and capitalize on the seasonal increase in available soil nutrients during the early wet/growing season when there is a lack of competition for nutrients from non‐eucalypts (if they have been impacted by dry season fires) and the grass layer (which is yet to re‐establish after fire). This may explain why low‐intensity fires appear to promote the growth of Eucalypts in field conditions (Freeman, Murphy, Richards, Vesk, & Cook, 2018; Prior et al, 2006; Werner, 2005; Werner & Prior, 2013). However, the unresponsiveness of Erythrophleum chlorostachys seedlings to nutrient addition may be explained by their nitrogen‐fixing ability, which would lesson their reliance on soil‐stored nutrients for growth, such that carbon availability may be the main growth‐limiting factor in this species (Coskun, Britto, & Kronzucker, 2016).…”

“…Erythrophleum chlorostachys is considered to be a slow‐growing species in these savannas and, in some cases, may never escape the fire trap. However, they are able to persist through fire events probably because of their thicker bark and successfully reproduce within the flame zone (unlike Eucalypts) (Freeman et al, 2018; Lawes, Richards, et al, 2011).…”

Patchy Fires Promote Regeneration of Longleaf Pine (Pinus palustris Mill.) in Pine Savannas

“…These fires are thought to have historically occurred primarily at 1-3 year intervals, as interpreted from requirements to maintain savanna structure and associated biodiversity [43][44][45] and dendrochronological evidence [46][47][48]. Whereas most tree species in savanna systems worldwide are capable of surviving fire by resprouting [5,49], longleaf pine seedlings exhibit very restricted resprouting capability [39], requiring the entire vascular cambium and apical meristem to survive fires. Once longleaf pine seedlings reach the lifecycle stage known as the grass-stage (in which the plant resembles a grass tussock), fire tolerance provides a competitive advantage over broadleaf trees and shrubs, which are typically top-killed or completely killed in this fire-dominated landscape [32,50,51].…”