Differential effects of defoliation by mopane caterpillars and pruning by African elephants on the regrowth of Colophospermum mopane foliage

Abstract

Plant responses to herbivory vary depending on herbivory type, yet the comparative effects of defoliation (e.g. by insects) and pruning (e.g. by large mammals) on a single tree species are poorly documented. We investigated this in the Northern Province of South Africa by comparing the regrowth of Colophospermum mopane trees previously defoliated by caterpillars or pruned by elephants, the two main browsers of C. mopane foliage. Shoots were up to 160% and 125% longer after natural (elephant) and simulated pruning and leaves ∼25% longer in regrowth after natural pruning (n = 13-15 trees per treatment). Shoot density and chemical defences in leaves (tannin:protein ratio and total polyphenolic concentration) were, however, no different from control trees. Simulated defoliation resulted in statistically insignificant changes to regrowth in terms of leaf and shoot size (both slightly decreased) and shoot density (slightly increased). Natural (caterpillar) defoliation, however, resulted in regrowth with significantly decreased shoot and leaf size (about 50% and 20% of control lengths, respectively), as well as decreased leaf chemical defence. Shoot and leaf length were longer on trees flushing for the first time after pruning and late-season defoliation had a greater negative impact than mid-season defoliation. Despite the differences in regrowth characteristics after pruning and defoliation, mopane plants showed no apparent trade-off in investment between tolerance and resistance after either herbivory type, as neither regrowth nor chemical defence occurred at the expense of the other.

Inter- and intra-plant variations in nitrogen, tannins and shoot growth of Sclerocarya birrea browsed by elephants

“…Thus, whichever way shoot growth is viewed, either among trees or within trees, the rate of shoot growth was elevated in relation to browsing intensity and canopy position. Therefore, S. birrea trees are able to vigorously compensate (or even over-compensate) for browsing by elephants at the scale of individual organs, which is consistent with observations of other tree species browsed by elephants (Makhabu et al 2006;Hrabar et al 2009). We recognise the possibility that a non-significant result consistent with the claim of compensation could be due to a Type II statistical error (Ward 2010), but we deem this unlikely, considering the sample size of 70.…”

“…Increasing the severity of simulated elephant browsing of Colophospermum mopane trees once in the growth season led to increased investment in CBSMs, which remained elevated for 2 years (Wessels et al 2007). In contrast, severe browsing (including branch breakage) by elephants had no effect on either N or CBSMs in C. mopane and led to increased shoot growth compared with no browsing (Hrabar et al 2009). Increased shoot growth was observed in another four woody species in relation to severe browsing, compared with either no or low browsing intensity (Makhabu et al 2006).…”

“…One explanation given for positive feedback in savanna trees is preferential allocation of C to growth of new shoots rather than C-based secondary metabolites (CBSMs) such as condensed tannins (CTs), which are often assumed to function as chemical, antiherbivore defences (Fornara and du Toit 2007;Skarpe and Hester 2008;Hrabar et al 2009). Positive feedback can be achieved when the root:shoot ratio is altered such that shoot growth increases (Herms and Mattson 1992;Renton et al 2007).…”

Interactions between megaherbivores and microherbivores: elephant browsing reduces host plant quality for caterpillars

“…In a previous study (Hrabar et al 2009a) we found that mopane trees displayed enhanced shoot regrowth after browsing by elephants and we replicated the effect with manual pruning. In addition, simulated browsing of mopane plants in greenhouse experiments (Kohi et al 2009) found that intermediate levels of defoliation induced a response of elevated foliar condensed tannin concentrations, whereas high levels of defoliation (75% and 100%) did not.…”

Secondary metabolites and nutrients of woody plants in relation to browsing intensity in African savannas

“…The phenomenon has been compared to grazing lawns in the Serengeti, and the term ''browsing lawn'' has been used to describe it (McNaughton 1984;Fornara and du Toit 2007). One explanation given for positive feedback in savanna trees is preferential allocation of C to growth of new shoots rather than C-based chemical defences (Fornara and du Toit 2007;Hrabar et al 2009), which can be achieved when the root:shoot ratio is altered such that shoot growth increases (Herms and Mattson 1992;Renton et al 2007). A simultaneous increase in nutrients may occur to meet the demands of increased growth (Herms and Mattson 1992;Å gren 2008).…”