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

Abstract

Carbon-based secondary metabolites (CBSMs) are assumed to function as 26 defences that contribute to herbivore-avoidance strategies of woody plants. Severe 27 browsing has been reported to reduce concentrations of CBSMs and increase N 28 concentrations in individual plants, causing heavily browsed plants to be characterised 29 by N-rich/C-poor tissues. We hypothesised that concentrations of condensed tannins 30 (CT) and total polyphenols (TP) should decrease, or N increase, in relation to 31 increasing intensity of browsing, rendering severely browsed plants potentially more 32 palatable (increased N:CT) and less N-limited (increased N:P) than lightly browsed 33 ones. We sampled naturally browsed trees (taller than 2 m) of four abundant species 34 in southern Kruger National Park, South Africa. Species-specific relationships 35 between N:CT, CT, TP and P concentrations and increasing browsing intensity were 36 detected, but N and N:P were consistently invariable. We developed a conceptual 37 post-hoc model to explain diverse species-specific CBSM responses on the basis of 38 relative allocation of C to total C-based defence traits (e.g., spines/thorns, 39 tough/evergreen leaves, phenolic compounds). The model suggests that species with 40 low allocation of C to C-based defence traits become C-limited (potentially more 41 palatable) at higher browsing intensity than species with high allocation of C to C-42 based defences. The model also suggests that when N availability is high, plants 43 become C-limited at higher browsing intensity than when N availability is low. 44 45

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

“…The positive relationship we observed between browsing intensity and [N] supports our hypothesis, as does the negative relationship between browsing intensity and [CT], which corroborates observations of other deciduous, spineless species that are well endowed with CTs, e.g. Combretum apiculatum (Rooke and Bergström 2007;Scogings et al 2011). However, our results contradict observations of C. mopane, which is another deciduous, spineless, CT-rich species heavily used by elephants (Wessels et al 2007;Hrabar et al 2009).…”

“…The site was also readily accessible by road. Sampling was scheduled according to observations in similar environments elsewhere, which indicated that effects of browsing on chemistry of mature leaves is readily detectible in the latter half of the wet season (Scogings et al 2011). Sampling only mature leaves at one time ensured that leaf phenology would not confound our results.…”

“…1). The method was based on one that we developed and tested across six other tree species in a similar environment (Scogings et al 2011). The method was subjective, but all estimates were made by one observer to ensure repeatability.…”

A thorny issue: Woody plant defence and growth in an East African savanna

“…Our findings lend strong support to this notion as the most heavily structurally defended species ( A. etbaica and B. pedicellaris ) performed the best in the presence of herbivores despite low investments in N‐free secondary metabolites. However, Scogings et al () predicted that the concentrations of N‐free secondary compounds would depend on browsing intensity with the highest concentrations occurring at intermediate levels of browsing for certain functional groups. For example, Scogings et al (), predict a linear increase in N‐free secondary compounds with browsing intensity, with a potentially sudden decrease at very high levels of browsing pressure in deciduous savanna species.…”

“…Chemical defences on the other have been found to be down‐regulated by herbivory in savannas, most likely due to C‐limitation imposed by the repeated removal of photosynthesising leaf material by browsing herbivores (e.g. Scogings, Hjältén, & Skarpe, , ; Scogings, Mamashela, & Zobolo, ; Wigley et al, ). It is, however, still not clear if similar plant functional types (e.g.…”