Defoliation depletes the carbohydrate reserves of resprouting Acacia saplings in an African savanna

Abstract

Over the past century there has been a global trend towards tree expansion and densification in rangelands and savannas. This phenomenon is commonly referred to as bush encroachment. In South Africa Acacia karroo is one of the key species responsible for bush encroachment. It has been suggested that the combination of fire and browsing might limit bush encroachment by A. karroo more effectively than either browsing or fire alone. We hypothesized that these repeated disturbances progressively deplete root carbohydrates and compromise resprouting ability. This was tested by burning and then manually defoliating A. karroo once a month for 1 year. Manual defoliation did not inhibit the rapid shoot elongation after topkill of A. karroo saplings. During this initial phase, the growth of the new shoots of A. karroo was dependent more on mobilised root reserves than on photoassimilates from the new shoots. Frequent manual defoliation of resprouting A. karroo saplings prevented the replenishment of starch reserves. We suggest a mechanism for how the interaction of browsing and fire can suppress and perhaps reverse bush encroachment in African savannas. Saplings that have reduced starch reserves at the end of dry season due to browsing will struggle to resprout if they are burnt. Even if they do not die, they will be less able to escape fire damage in the next fire than if they had been able to resprout unimpeded.

Ecology of plant resprouting: populations to community responses in fire-prone ecosystems

“…Evidence suggests resprouters are able to persist through disturbance by a combination of unique bud anatomy (for example in the eucalypts; Burrows 2002) and location, greater protection of these buds by differential stem and bark growth rates (e.g. Lawes et al 2011a) and bark thickness (Lawes et al 2011b, this issue), and by how these buds are resourced (this issue: Goorman et al 2011;Paula and Ojeda 2011;Schutz et al 2011). In this editorial, we present the first attempt at a unified framework or classification scheme (Table 1) based on the Buds-ProtectionResources approach (Clarke et al 2010) of characterizing sprouting responses.…”

Do Wildfires Promote Woody Species Invasion in a Fire-Adapted Ecosystem? Post-fire Resprouting of Native and Non-native Woody Plants in Central Argentina

“…However, resprouting under repeated disturbances depends not only on the availability of buds but also below-ground reserves. Schutz et al (2011) showed that initial growth of resprouts depended of reserve mobilization from roots and that such process could be depleted under chronic defoliation, compromising resprouting ability.…”

Resprouting ability and mobile carbohydrate reserves in an oak shrubland decline with increasing elevation on the eastern edge of the Qinghai–Tibet Plateau

“…A possible bud-bank limitation has also been proposed to explain the progressive decrease in the aboveground regrowth across frequent clipping treatments in Erica (Paula and Ojeda, 2009). The answer of the question of whether NSC limitation or bud limitation is more important for resprouting depends on the environmental disturbance context (Schutz et al, 2009(Schutz et al, , 2011. It is found that NSC limitation is more important than bud limitation in fire-prone trees such as oak (Schutz et al, 2009(Schutz et al, , 2011.…”

“…Individual-based resprouting success was found to depend upon carbohydrate and nutrient storage (Iwasa and Kubo, 1997; de Jong and van der Meijden, 2000), although some studies have questioned their significance (Taylor and Pharis, 1982;Erdmann et al, 1993;Garcia et al, 2001;Cruz et al, 2003). Resprouting after loss of practically all aboveground biomass requires resources reserves to support expansion of the first leaf of new resprouts (Pate et al, 1990;Canadell and López-Soria, 1998;Midgley, 2001, 2003;El Omari et al, 2003;Carpenter et al, 2008;Schutz et al, 2011). There is evidence that carbohydrate reserves are consumed and used during the resprouting process in various plant species following serious damage to the aboveground portions of the plant (Kays and Canham, 1991;von Fircks and SennerbyForsse, 1998;Bellingham and Sparrow, 2000;Del Tredici, 2001;Luostarinen and Kauppi, 2005;Paula and Ojeda, 2009;Schutz et al, 2009).…”