Vertical zonation of browse quality in tree canopies exposed to a size-structured guild of African browsing ungulates

We investigated whether the food quality of tree foliage for African savanna browsers varies across the feeding height range of the guild. This was to address the question of why giraffes (Giraffa camelopardalis) generally feed at a higher level in the canopy than is accessible to all other browsers. We defined a giraffe browse unit (GBU) as the length of twig corresponding to the average "bite" taken by giraffes from two staple browse plants: Acacia nigrescens and Boscia albitrunca. We sampled at three study sites in South Africa in the late dry season, at each site clipping GBUs at three heights above ground: 0.5 m, 1.5 m and 2.5 m; these representing the levels typically browsed by small, medium and large-bodied browsing ungulates respectively. For each GBU we measured leaf dry mass, total N, neutral detergent fibre and condensed tannin, using near-infrared spectroscopy calibrated by conventional laboratory analyses. We found no differences between height levels with regard to leaf chemistry concentrations, but leaf biomass per GBU was significantly higher at the 1.5-m and 2.5-m levels than at the 0.5-m level. The larger browsers thus gain a bite-size advantage by browsing above the reach of the smaller species. A likely reason for the reduced leaf biomass per GBU at the low browsing level is the tendency for small browsers to pluck individual leaves from shoots, while large browsers prune off whole shoots. We contend that our findings are analogous to those from parallel studies on the grazing guild, and are consistent with the hypothesis that the smaller members of ungulate guilds competitively displace the larger ones from shared feeding sites when resources become restricted. A prediction of this hypothesis is that the smaller members of each guild drive the grazing succession from behind and maintain browsing height stratification from below.