Ants, fire, and bark traits affect how African savanna trees recover following damage

Abstract

Bark damage resulting from elephant feeding is common in African savanna trees with subsequent interactions with fire, insects, and other pathogens often resulting in tree mortality. Yet, surprisingly little is known about how savanna trees respond to bark damage. We addressed this by investigating how the inner bark of marula (Sclerocarya birrea), a widespread tree species favoured by elephants, recovers after bark damage. We used a long‐term fire experiment in the Kruger National Park to measure bark recovery with and without fire. At 24 months post‐damage, mean wound closure was 98, 92, and 72%, respectively, in annual and biennial burns and fire‐exclusion treatments. Fire exclusion resulted in higher rates of ant colonization of bark wounds, and such ant colonization resulted in significantly lower bark recovery. We also investigated how ten common savanna tree species respond to bark damage and tested for relationships between bark damage, bark recovery, and bark traits while accounting for phylogeny. We found phylogenetic signal in bark dry matter content, bark N and bark P, but not in bark thickness. Bark recovery and damage was highest in species which had thick moist inner bark and low wood densities (Anacardiaceae), intermediate in species which had moderate inner bark thickness and wood densities (Fabaceae) and lowest in species which had thin inner bark and high wood densities (Combretaceae). Elephants prefer species with thick, moist inner bark, traits that also appear to result in faster recovery rates.

A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…periderm and cork) is removed using a wood chisel and then the entire layer of inner bark (i.e. secondary phloem) is removed, without damaging the underlying wood (Wigley et al 2019b). Regrowth can be calculated by measuring the regrowth on two perpendicular axes (see Fig.…”

“…Bark regrowth is a compensatory mechanism that allows woody plants to recover their resistance to fire and pathogens. Bark regrowth may be influenced by age or size and total area removed (Vermeulen et al 2012), and differs across species (Wigley et al 2019b).…”

“…(2) Bark regrowth Greater bark regrowth increases the probability of survival, whereas limited bark regrowth exposes the stem to pathogens, insect feeding and fires (Moncrieff et al 2008;Wigley et al 2019b). Limited unpublished data suggest that resistance to bark stripping and increased bark recovery is highest in firedriven systems, i.e.…”

Corrigendum to: A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…(4) Recovery from stripping Bark regrowth is a compensatory mechanism that allows woody plants to recover their resistance to fire and pathogens. Bark regrowth may be influenced by age or size and total area removed (Vermeulen et al 2012), and differs across species (Wigley et al 2019b).…”

Heterogeneity in African savanna elephant distributions and their impacts on trees in Kruger National Park, South Africa

“…. Elephants impact trees directly by stripping bark, breaking stems and branches, or simply uprooting trees entirely, all of which can make trees more susceptible to insect damage and fire-induced mortality (Jacobs & Biggs, 2002;Moncrieff et al, 2017;Wigley et al, 2019). Some tree species are at particular risk (Duffy et al, 2002;Edkins et al, 2008;Midgley et al, 2020;Shannon et al, 2008), including Sclerocarya birrea (marula), Adansonia digitata (baobab), and Acacia nigrescens (alt.…”