Is there a browse trap? Dynamics of herbivore impacts on trees and grasses in an African savanna

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1 Journal of Ecology 2014, 102, doi: / Is there a browse trap? Dynamics of herbivore impacts on trees and grasses in an African savanna Ann Carla Staver 1 * and William J. Bond 2 1 Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY 10027, USA; and 2 Botany Department, University of Cape Town, Private Bag X1, Rondebosch 7701, South Africa Summary 1. Despite widespread acknowledgement that large mammal herbivory can strongly affect vegetation structure in savanna, we still lack a theoretical and practical understanding of savanna dynamics in response to herbivory. 2. Like fire, browsing may impose height-structured recruitment limitations on trees (i.e. a browse trap ), but the demographics of herbivore effects have rarely been considered explicitly. Evidence that cohorts of trees in savannas may establish during herbivore population crashes and persist long term in savanna landscapes is anecdotal. 3. Here, we use an experimental approach in Hluhluwe imfolozi Park in South Africa, examining the response of grass biomass and tree populations to 10 years of graduated herbivore exclusion, and their subsequent response when exclosures were removed. 4. We found that grazer exclusion increased grass biomass and that, despite presumable increases in fire intensity and grass competition, herbivore especially mesoherbivore, including impala and nyala exclusion resulted in increases in tree size. After herbivore reintroduction, grazers reduced grass biomass over short time-scales, but tree release from browsing persisted, regardless of tree size. 5. Synthesis. This work provides the first experimental evidence that release from browsing trumps grazer grass fire interactions to result in increases in tree size that persist even after browser reintroduction. Escape from the browse trap may be incremental and not strictly episodic, but, over longer time-scales, reductions in browsing pressure may lead to tree establishment events in savanna that persist even during periods of intense browsing. Explicitly considering the temporal demographic effects of browsing will be the key for a much-needed evaluation of the potential global extent of herbivore impacts in savanna. Key-words: browse trap, browsing, demographic variability, elephant, grass, herbivory, large mammal, plant herbivore interactions, savanna, tree Introduction The dynamics of mammal herbivore impacts on trees in savannas remain the most poorly understood driver of savanna vegetation structure, despite extensive localized evidence that the impacts of herbivores can be substantial (Prins & van der Jeugd 1993; Barnes 2001; Augustine & McNaughton 2004; C^ote et al. 2004; Sharam, Sinclair & Turkington 2006; Fornara & Du Toit 2007; Asner et al. 2009; Holdo et al. 2009; Staver et al. 2009; Moncrieff et al. 2011). Savannas are understood to be systems where top-down impacts can have major effects on both tree distributions and structure (Bond *Correspondence author. acs2003@columbia.edu 2008). Impacts of fire have received extensive attention in terms of underlying dynamics (Higgins, Bond & Trollope 2000), demographics (Trollope & Tainton 1986; Hoffmann 1999; Hanan et al. 2008; Hoffmann et al. 2009; Werner & Prior 2013) and global impact (Bond, Woodward & Midgley 2005; Lehmann et al. 2011; Staver, Archibald & Levin 2011a,b). As a result, a fundamental theoretical and practical understanding of the role of fire in savanna systems is well advanced. By contrast, an equivalent understanding of the dynamics and demographics of herbivore impacts in savanna is mostly lacking. In many respects, a comprehensive understanding of herbivory presents challenges that fire does not. First, herbivory is comprised of both grazing and browsing; grazing is expected to increase tree growth rates, either because of 2014 The Authors. Journal of Ecology 2014 British Ecological Society

2 596 A. C. Staver & W. J. Bond decreased grass competition or decreased fire frequency (Knapp et al. 1999; Archibald et al. 2005; Holdo et al. 2009), while browsing is expected to directly decrease tree growth (Barnes 2001; Sharam, Sinclair & Turkington 2006; Staver et al. 2009). In addition, herbivore communities are varied and often diverse (Olff, Ritchie & Prins 2002), and dominant herbivore impacts may depend on what herbivores are present (Prins & van der Jeugd 1993; Barnes 2001; Wilson & Kerley 2003; Augustine & McNaughton 2004; C^ote et al. 2004; Sharam, Sinclair & Turkington 2006; Fornara & Du Toit 2007; Asner et al. 2009; Holdo et al. 2009; Staver et al. 2009; Midgley, Lawes & Chamaille-Jammes 2010; Moncrieff et al. 2011). A focus on elephant impacts in the modelling literature especially (Dublin, Sinclair & McGlade 1990; Baxter & Getz 2005; Bond 2008) may not be relevant to understanding the effects of a diverse browser community in Africa and especially elsewhere. However, a predictive synthesis should be possible. Here, we focus especially on the demographic structure of herbivore impacts on the tree layer in an African savanna. In the case of fire, height structure partly mediated by stem diameter and bark thickness is fundamentally important in determining how trees respond to persistent disturbances (Trollope & Tainton 1986; Hoffmann 1999; Higgins, Bond & Trollope 2000; Prior, Williams & Bowman 2010; Wakeling, Cramer & Bond 2010); the vertical zone of influence of savanna fires is known as the fire trap, above which fire minimally affects trees (Bell 1984). Two lines of evidence suggest that a similar model a browse trap may be useful in conceptualizing browsing effects as well. First of all, the frequent occurrence of browse lines in areas with high browser density suggests that herbivore effects are height structured and that trees below some threshold size are suppressed by herbivores (Trollope & Tainton 1986; Hoffmann 1999; Bond & Loffell 2001; Palmer & Truscott 2003; Hoffmann et al. 2009; Moncrieff et al. 2011). Secondly, historical analyses, including pollen analyses and long-term tree ring studies, have suggested that cohorts of trees in savannas may arise after historical herbivore populations crashes (Prins & van der Jeugd 1993; Bond, Woodward & Midgley 2005; Gillson 2006; Holdo et al. 2009; Lehmann et al. 2011; Staver, Archibald & Levin 2011a,b; Staver, Bond & February 2011); the strong implication of these apparent release episodes is that trees reach some size at which they become substantially less vulnerable to the negative effects of browsing. Here, we take advantage of a series of graduated herbivore exclosures in Hluhluwe imfolozi Park in South Africa (Knapp et al. 1999; Archibald et al. 2005; Holdo et al. 2009; Staver et al. 2009), which ran for 10 years and were subsequently removed, to examine (i) the effects of a diverse assemblage of browsers on tree population structure and growth within savanna and (ii) the extent to which the effects of herbivore removal and, by proxy, herbivore population crashes are reversible or can result in long-term, persistent changes in savanna vegetation structure. These perspectives provide a direct experimental evaluation albeit over shorter, experimental time-scales of whether browsing impacts on trees in savanna follow a browse trap model. The data provide a basis for developing a theoretical framework for analysing the effects of browsing in particular and, more broadly, mammal herbivory effects on the structure of savanna ecosystems. Materials and methods STUDY SITE Hluhluwe imfolozi Park (900 km 2 ;28 o o 26 0 S; 31 o o 09 0 E), in KwaZulu Natal, South Africa, spans a diversity of savannas from mesic systems at the boundary with forest to semi-arid ones. Rainfall is linked to elevation, resulting in a gradient between higher elevation Hluhluwe Game Reserve (975 mm mean annual rainfall) and lower elevation imfolozi GR (<600 mm MAR). Fire frequency in the park increases with rainfall; areas of high fire frequency have burned more than 10 times between 1956 and 1996, while areas of low fire frequency burned as little as once during the same period (Barnes 2001; Balfour & Howison 2002; Sharam, Sinclair & Turkington 2006; Staver et al. 2009). The park is home to a full complement of large mammals indigenous to south-eastern Africa. Large mammal herbivore densities within and between reserves vary depending on herbivore habitat preferences. Based on 2004 census data, impala (Aepyceros melampus) are the most numerous herbivore in the park, occurring with higher densities in semi-arid imfolozi (36 km 2 ) than in mesic Hluhluwe (24 km 2 ); white rhino (Ceratotherium simum) make up the largest biomass in the park, occurring with higher densities in imfolozi (2.5 km 2 ) than Hluhluwe (1.8 km 2 ). Elephants move freely between parks and have biomass roughly equivalent to impala (0.36 km 2 ; 10.9 kg ha 1 ). Herbivore densities have fluctuated substantially during the past century in Hluhluwe imfolozi Park, largely due to hunting, the rinderpest epidemic and culling campaigns during the early part of the 20th century associated with efforts to eradicate the nagana livestock disease. Herbivore densities have been more stable in the last two decades with high herbivore biomass (~ kg km 2 in 2004) [see Cromsigt, Prins & Olff (2009) and Staver et al. (2009) for more information on herbivore populations]. EXPERIMENTAL SET-UP Experimental herbivore exclosures were established at 10 sites, located throughout Hluhluwe imfolozi Park in a range of savanna types, from grazing lawns with high herbivore pressure and low fire frequency to bunchgrass savannas with low herbivore pressure and high fire frequency [see Staver et al. (2009) for additional detail]. At each site, three treatment plots of 40 m 9 40 m were established in 1999: a control (all herbivores = + all ) allowed access to all herbivores, a cable set 70 cm above the ground excluded rhinos (rhino fence = rhino ) and a standard game fence 2.5 m high that excluded hares and all larger herbivores (total exclosure = all ). Two additional treatments were established at each of the five Hluhluwe sites in 2000: double cables set at 70 and 120 cm above the ground excluded zebras and larger herbivores (zebra fence = zebra ) and an inverted standard game fence (i.e. the small mesh at the top, instead of the bottom) 2.5 m high that excluded impala and larger herbivores (impala fence = impala ). The effectiveness of herbivore exclusion treatments was validated via monthly dung counts over the full course of the experiment, showing that exclosures were largely effective (Table 1 and Fig. 1) and gradually excluded herbivore species. The rhino fence resulted in

3 Evaluating the browse trap 597 Table 1. Results of linear mixed model analyses of the effects of herbivore exclusion, herbivore reintroduction, and fire on grass biomass, tree height, and tree population structure across Hluhluwe and imfolozi Game Reserves Factor F d.f. P AIC Dung abundance Region Exclusion <0.001 Reintroduction* Fire <0.001 Grass biomass Region Exclusion <0.001 Reintroduction <0.001 Fire Plot-level mean tree height Region Exclusion <0.001 Reintroduction Fire <0.001 Proportion with height 1m Region < Exclusion: +/ impala <0.001 Reintroduction Fire <0.001 Proportion with 1 m < ht 2m Region < Exclusion: +/ impala <0.001 Reintroduction Fire <0.001 Proportion with height > 2m Exclusion: +/ impala Reintroduction Fire *Dung counts are not numerically comparable before and after herbivore reintroduction, because the temporal density of sampling differed substantially. Model did not converge when all exclusion treatments were included. On the basis of tree height analyses and Fig. 4, we reduced the treatment levels to two: either impala were present (in +all, rhino and zebra treatments) or they were absent (in impala and all treatments). Model did not converge even with fewer exclusion levels, probably because of insufficient replication, but did when the Hluhluwe v. imfolozi GR contrast was excluded from the analysis. major decreases in use by large (elephant, giraffe and buffalo) and less agile (zebra) herbivores; the zebra fence reduced use by slightly smaller herbivores (wildebeest); the impala fence effectively reduced use by medium-bodied browsers especially (impala and nyala but also warthog). Only the total exclosure ( all ) completely excluded herbivores of any type, however, and reduced herbivore visitation to zero or nearly zero (see Fig. 1). All sites were burned during the dry seasons after the 2000, 2002, 2004 and 2008 censuses. To minimize the attraction of a small burnt patch to herbivores, burns were timed to coincide with management burns in the blocks (500 ha+) containing sites. Sites and treatments that sustained high grass biomass burnt readily, while heavily grazed areas often did not burn. At all sites, the complete exclosure treatment generated sufficient grass biomass to burn the whole plot. Thus, complete herbivore exclusion was always associated with exposure of saplings to fire. Experimental treatments remained in place from 1999/2000 through to the end of 2009, when exclosures were removed (barring those at one site in Hluhluwe and one site in imfolozi). Trees were mapped in 2000 onto a permanent 2 m 9 2 m grid that covered half of each plot (i.e. an area of 40 m 9 20 m) and their heights measured every year (except those in impala and zebra plots in 2000 and 2005); new trees were recorded during each census and tracked thereafter. Tree species and heights were re-recorded in years after exclosure removal; because the permanent grid was removed with exclosures, relocating individual trees was impossible, but plots were relocated exactly and censused. Grass biomass was estimated at each point on the permanent grid using a disc pasture meter with regionspecific calibrations from grass biomass estimated from clipping (Waldram, Bond & Stock 2008) from 2000 to 2009 and as nearly as possible in DATA ANALYSIS All statistics were performed in R using the base statistical package and the package nlme (Pinheiro et al. 2012). Because exclosure treatments were nested within sites and each plot was sampled repeatedly, we analysed data using mixed-effects modelling, with region (i.e. Hluhluwe v. imfolozi), exclosure treatment, exclosure removal and fire presence/absence as fixed effects and year, site and plot within site as random effects. Results Grass biomass increased strongly with herbivore exclusion in both mesic Hluhluwe and semi-arid imfolozi (Fig. 2 and Table 1). In Hluhluwe, the exclusion of rhinos and of impala appeared to result in the biggest responses, while in imfolozi, rhino exclusion had insignificant effects on grass biomass. In neither reserve did fire have any significant effect on grass biomass 1 year after the fire (Table 1), indicating that the recovery of grass biomass following a fire is rapid. Similarly, after reintroduction of herbivores with the removal of exclosures, the effects of exclusion were completely reversed; by 2012, exclusion plots had grass biomass identical to that of controls (Fig. 2 and Table 1). Thus, the grass layer appears to

4 598 A. C. Staver & W. J. Bond Dung piles Dung piles all rhino NA zebra Impala, 60 kg Warthog, 75 kg Nyala, 100 kg Zebra, 250 kg Wildebeest, 260 kg Buffalo, 650 kg Giraffe, 1200 kg Elephant, 2800 kg NA impala all Grass biomass (kg ha 1 ) Grass biomass (kg ha 1 ) all rhino zebra impala all Fig. 1. Dung abundance response (per year, log scale) to herbivore exclusion, in mesic Hluhluwe and semi-arid imfolozi GRs (a and b, respectively). Herbivore species denoted with solid shapes and light grey lines (warthog, zebra, wildebeest and buffalo) are predominantly grazers, those with hollow shapes and black short dashes (nyala, giraffe) are predominantly browsers, and those with hollow squares and dark grey long dashes (impala, elephant) are mixed feeders. We added one to all dung counts to facilitate plotting on a log scale. respond reversibly and on short time-scales to the effects both of fire and herbivory. Tree height (Fig. 3) and tree population structure (Fig. 4) likewise responded strongly to herbivore exclusion (Table 1). In Hluhluwe, impala appeared to be the key herbivores whose exclusion released trees; responses in imfolozi were also consistent with the importance of impala, although the full range of treatments was not applied there. Fire clearly negatively affected tree height and tree population structure one year after the fire (Fig. 4 and Table 1), but the effects of fire did not increase as grazer exclusion increased grass biomass. By contrast, mixed-effects models of tree density on exclosure treatment, reintroduction and fire did not converge, suggesting that tree density was not consistently affected either by herbivore exclusion or fire. The release of trees with herbivore exclusion was not reversed when herbivores were reintroduced following exclosure removal (Figs 3 and 4; Table 1). However, mean tree height did not continue to increase either, and tree size class distributions were static (Fig. 4). This suggests that trees did not experience release from herbivory at some large size; had large trees escaped herbivory and continued to grow, mean tree height would have continued to increase or, at least, continued growth of large trees would have been countered by decreases in small tree size. Although elephant damage was visually very apparent, only two plots located at the same site, Nombali experienced any substantial degree of elephant damage after exclosures were removed; the remainder of plots experienced elephant damage ranging from mild to severe to 10% or fewer of trees (Fig. 5a). Even in the two plots that experienced substantial elephant damage, however, the release of trees into larger size classes was not reversed when herbivores were reintroduced (Fig. 5b), suggesting that elephants, though certainly capable of reversing the effects of tree release from browsing pressures, do not necessarily do so, at least over short time-scales and at the densities in which they occur in Hluhluwe imfolozi Park [see also (Guldemond & Van Aarde 2008)]. Discussion Fig. 2. Grass biomass response to 10 years of herbivore exclusion and 3 subsequent years of no fences, in mesic Hluhluwe and semiarid imfolozi GRs (a and b, respectively). These results suggest (i) that release from grazing can have strong effects on grass biomass, but reduction in grass biomass following grazer reintroduction is rapid, (ii) that release from browsing especially by mesoherbivores, including impala and nyala strongly promotes tree growth and (iii) that the effects of release from browsing on trees are stable, at least over relatively short time-scales following browser reintroduction. Large-bodied grazers did affect grass biomass, especially in mesic Hluhluwe, where the formation of grazing lawns is thought to be a direct result of megaherbivory (Waldram, Bond & Stock 2008) [see also (Knapp et al. 1999)]. By contrast, suppression of tree growth occurred primarily as a result of browsing by small mesoherbivores such as impala

5 Evaluating the browse trap 599 Mean tree height (cm) Mean tree height (cm) Initial conditions (2000) and nyala (Sharam, Sinclair & Turkington 2006; O Kane et al. 2012), but, at least over relatively short time-scales, these small-bodied browsers were not able to reverse gains in tree height that resulted from a release from browsing pressure (Prins & van der Jeugd 1993; Barnes 2001; Holdo et al. 2009); not even intensive effects of elephants could reverse the effects of a decade of release from smaller herbivores. Nonetheless, the reintroduction of herbivores did prevent further tree growth, suggesting that size-structured escape from browsing may not occur until trees are very large (in the presence of giraffe (Moncrieff et al. 2011) documented release from herbivory only above about 6 7 m in height), such that escape from browser effects is a long time-scale process. TREE LAYER DYNAMICS IN SAVANNA + all rhino zebra impala all Fig. 3. Plot-level mean tree height response to 10 years of herbivore exclusion and 3 subsequent years of no fences, in mesic Hluhluwe and semi-arid imfolozi GRs (a and b, respectively). The experimental response of tree growth to herbivory, herbivore exclusion and fire provides insights into how tree cover emerges in real savanna landscapes. First of all, the primary effect of herbivore exclusion on tree growth and population structure resulted from browser exclusion. Grazer exclusion clearly affected grass biomass, which should, in theory, intensify both tree grass competition and fire effects on trees (Holdo et al. 2009; Riginos 2009; Cramer, van Cauter & Bond 2010; February et al. 2013). However, even though fire grazer interactions have been documented in the area (Archibald et al. 2005), the effects of browser release on tree performance were far more substantial than the effects of grazers on tree grass interactions. The potential for direct browser impacts on tree emergence in savanna suggests that these deserve substantially more widespread attention. Results also provide some insights into the potential longterm temporal dynamics of tree emergence in this savanna. Extant browsing regimes, either alone (in semi-arid imfolozi) or in combination with extant fire regimes (in mesic Hluhluwe), completely prevented large tree establishment. This is at odds with the fact that the park is characterized by savanna woodland and even thicket in some places. However, when both mesoherbivore browsing (in impala or all treatments) and fire were absent (from 2004 to 2008), recruitment was substantial (Staver et al. 2009). This suggests that, when extant adult trees in the landscape established, the consumer environment was much different than it is now (Staver, Bond & February 2011), either because herbivore populations and fire management were historically radically different than now (Staver, Bond & February 2011) or because these vary naturally (Young 1994; Holdo et al. 2009), such that adult tree establishment will continue to occur episodically. In the absence of longer-term data, determining whether savannas in their current form represent a historical anomaly, are subject to continuous change or can be expected to persist into the future will be impossible. Although the combination of frequent fires and herbivory appeared sufficient to prevent tree growth in mesic areas, tree size class distributions were undoubtedly more dynamic in response to both fire and herbivory than they were in semiarid areas. This is consistent with the observation that bush encroachment in South Africa, partly driven by increases in [CO 2 ], appears to be proceeding more quickly in mesic than in arid savannas (Buitenwerf et al. 2011). Even minimal variation in interfire interval might be sufficient to allow trees to recruit in mesic Hluhluwe, especially in areas where suppression of tree growth by browsing is mild. Indeed, although this trend remains undocumented, bush encroachment appears to be rampant in the mesic end of Hluhluwe imfolozi Park in the last decade, possibly tied in with changes in fire management and reductions in mesoherbivore populations. THE BROWSE TRAP The idea of a disturbance trap is one that is well-established in the case of fire, whose effects in savannas are highly life stage specific (Bell 1984). Fire limits tree cover in savanna by preventing the recruitment of saplings into trees, but savanna saplings are usually robust resprouters (Bond & Midgley 2001; Hoffmann et al. 2009). Trees >2 3 m in height are rarely affected by fire (Hoffmann & Solbrig 2003; Prior, Williams & Bowman 2010), such that fire affects the establishment, rather than the mortality, of savanna trees. This phenomenon is repeatable and shows up clearly in this study in the fact that population structure under 2 m in height is

6 600 A. C. Staver & W. J. Bond (c) Proportion of trees <1 m tall Proportion of trees 1 2 m tall Proportion of trees >2 m tall (d) (e) (f) Proportion of trees <1 m tall Proportion of trees 1 2 m tall Proportion of trees >2 m tall + all rhino zebra impala all Initial conditions (2000) Initial conditions (2000) Initial conditions (2000) Fig. 4. Response of size class distribution to herbivore exclusion and subsequent reintroduction (fence removal is denoted by the vertical grey line). In mesic Hluhluwe and semi-arid imfolozi GRs, respectively, the proportion of trees with height 1 m (a and d) generally decreases through time, while the proportion of trees with height >1 m and 2 m (b and e) and with height >2 m (c and f) increases. Fig. 5. Rates of elephant damage after treatment removal (in 2012) across all sites and treatments, and the temporal response of tree size class distributions in the two plots most heavily impacted by elephants. In, sites are listed in the same order for each treatment; the two most heavily impacted come from the same site. Fences were removed in 2009, 3 years prior to sampling, and rates of damage represent totals across all 3 years. much more sensitive to fire than population structure over 2 m (although fires were sometimes intense and did affect large trees as well). This means that, from a theoretical perspective, the effects of fire in savanna systems lend themselves to stage-structured modelling: saplings are affected by fire, but trees are not (Higgins, Bond & Trollope 2000; Hanan et al. 2008; Staver & Levin 2012). An analogous dynamic understanding of the effects of herbivory on savanna systems has so far been elusive, although herbivory and fire are in several respects analogous processes (Bond & Keeley 2005). Large tree recruitment may heavily depend on episodic reductions in browsing pressure allowing the release of large numbers of juvenile trees in savanna landscapes (Prins & van der Jeugd 1993; Holdo et al. 2009; Staver et al. 2009). In such circumstances, a browse trap may be a useful way of thinking about the effects of browsing in savannas. However, it is also clear that a dynamic model for the browse trap must be very different than that for the fire trap. First, the temporal dynamics of the two are different, in that fire constitutes an episodic if chronic disturbance in savannas, while herbivory is relatively continuous at the population level; in the case of herbivory, it is likely release from disturbance, rather than the disturbance itself, that is episodic (Young 1994; Holdo et al. 2009).

7 Evaluating the browse trap 601 However, the results of herbivore exclusion (release) followed by herbivore reintroduction in this study suggest another major difference: browser reintroduction did not readily reverse gains in tree height that resulted from browsing release. Although the time-scale considered was relatively short, this does suggest that fire decreases the size of trees within the fire trap much more readily than browsing does within the browse trap. We did not explicitly consider the mechanism that might drive this response, but it is consistent with browsing that removes buds, foliage and twigs up to a maximum size depending on type of browser (Wilson & Kerley 2003), thereby preventing tree growth but not reducing tree size (Midgley, Lawes & Chamaille-Jammes 2010). However, regardless of whether increases in tree size from browser release are actually irreversible or are only relatively robust, tree establishment may thus proceed episodically as in the case of rinderpest or nagana outbreaks (Prins & van der Jeugd 1993) or even incrementally, since smaller release episodes may accumulate if they occur repeatedly and are not readily reversed. Theoretically, browsing by medium-sized herbivores should be formalized as affecting the establishment rate but not the mortality of trees and/or as potentially reducing their growth to zero but not less than zero. In considering adult tree establishment, including explicit stage structure may be much less critical in browsing models than it is in fire models. However, these data examine primarily trees that are much smaller than potential escape height from browsers; studies considering larger trees have suggested that the effects of browsing are stage structured (Bond & Loffell 2001; Moncrieff et al. 2011). Stage structure may also nonetheless be of critical importance in models that consider reproduction or even the effects of browsers on carbon storage in savanna systems (Holdo et al. 2009; Moncrieff et al. 2011; Tanentzap & Coomes 2011). These key assumptions are not expected to hold true for browsing that removes more than just buds and foliage, most notably for browsing by elephants, which can decrease tree size substantially when densities are high (Dublin, Sinclair & McGlade 1990; Barnes 2001; Baxter & Getz 2005). However, our results also suggest a note of caution in focusing so exclusively on the effects of elephants on the tree layer in savanna: in the continuous presence of high densities of impala and nyala, large trees simply do not establish. They limit large tree densities as much as or more than elephants, even if their effects are less visible. Here, elephants had little effect on tree growth during herbivore exclusion and (at least in relatively open woodland and over short time-scales) could not reverse gains in tree size resulting from the exclusion of smaller browsers. A comprehensive analysis of the effects of herbivores on savanna structure needs to move beyond elephants as the key browser. Herbivory especially browsing has clear local impacts on the tree layer in savanna systems, especially in interaction with fire. Fire has a demonstrable continental-scale impact on the distribution of savanna (Bond, Woodward & Midgley 2005; Lehmann et al. 2011; Staver, Archibald & Levin 2011a,b), but the potentially comparable impact of herbivory on savanna distributions has never been evaluated, in large part because an appropriate global herbivory data set is lacking. At a minimum, browsing impacts on tree growth may impact the potential global role of fire (Wakeling, Staver & Bond 2011). However, browsing also plays a role independent of that of fire, especially in more arid systems, where browsing is known to exert strong adaptive pressures on trees (Staver et al. 2012). These effects may be as fundamental as those of fire; herbivory is thought to introduce bistability in ecosystem structure (McNaughton 1984; Fornara & Du Toit 2007) and radically affect ecosystem carbon stocks (Holdo et al. 2009; Tanentzap & Coomes 2011). The global extent of these impacts merits direct evaluation. This analysis faces challenges: a diversity of herbivores that shapes and is shaped by the environment (Olff, Ritchie & Prins 2002), impacts that depend on herbivore densities (Guldemond & Van Aarde 2008) and the importance of temporal variability in herbivore pressure (Prins & van der Jeugd 1993; Holdo et al. 2009). Here, we provide some key insights into the demography and dynamics of herbivore impacts in savanna ecosystems that contribute to making such a synthesis possible. Acknowledgements We thank Hluhluwe imfolozi Park and Ezemvelo KZN Wildlife for their support and Phumlani Zwane, Sue Janse van Rensburg, An van Cauter, Julia Wakeling, Matt Waldram, Krissie Clarke, Zanele Chonco, Mendi Shelembe, Sipho Zulu, Eric Khumalo, Vincent Mkhwanazi and Dumisani Mnomezulu for logistical fieldwork support. This work forms a part of the Zululand Grass Project, the Zululand Tree Project and the Biome Boundaries Project. Funding was provided by the Andrew W. Mellon Foundation and the National Research Foundation of South Africa. References Archibald, S., Bond, W., Stock, W. & Fairbanks, D. (2005) Shaping the landscape: fire-grazer interactions in an African savanna. Ecological Applications, 15, Asner, G.P., Levick, S.R., Kennedy-Bowdoin, T., Knapp, D.E., Emerson, R., Jacobson, J., Colgan, M.S. & Martin, R.E. (2009) Large-scale impacts of herbivores on the structural diversity of African savannas. 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