Oecologia (2011) 165:1095–1107 suggesting an inﬂux of nutrients into these open sites. In Laikipia, Kenya, impala also preferred to spend time in structurally open boma sites, even during the dry season when forage resources were depleted (Augustine 2004). As a result, impala imported nutrients from the surrounding bushland areas into the boma sites, resulting in the per- sistence of these nutrient hotspots over time (Augustine 2003, 2004). There is ample evidence from savanna studies that large herbivores prefer open areas when under predation threat. Valeix et al. (2009) recently concluded that herbivores prefer open areas when lions were in the vicinity in Hwange National Park, Zimbabwe. Riginos and Grace (2008) found that savanna herbivores (except species immune to wild predators, e.g., elephants) prefer open areas with unobstructed visibility, apparently to improve predator detection. The point is that, in dense woodland areas, patches offering good visibility such as the aban- doned kraal sites in the study area stand out as safe havens for herbivores dependent on vigilance to avoid predation. Given the low density of kraal sites in the study area, it is unlikely that the presence of kraal sites signiﬁcantly altered predator–prey dynamics in the general study area. Tree recruitment constraints Our data clearly showed that woody recruitment was more constrained under fertile conditions than infertile ones, when competing with herbaceous plants. Woody plants failed to re-colonize fertile kraal sites since their aban- donment, while bush encroachment prevails in the general study area (G. Roux, unpublished data). This trend (bush encroachment) was also observed in the KNP’s granitic areas (Eckhardt et al. 2000). The results of our experiments show that the effect of herbaceous competition on tree seedlings intensiﬁed under fertile conditions and that this may lead to increased mortality rates of tree seedlings establishing under fertile conditions. This was also found in other experiments with tree seedlings and grasses (Cohn et al. 1989; van der Waal et al. 2009) and suggests that the openness of kraal sites might at least partially be explained by the suppressing effects of herbaceous competition on tree recruitment (Sankaran et al. 2008). In contrast, Reid and Ellis (1995) found that abandoned cattle bomas facilitated rather than suppressed shrub (Acacia tortilis) recruitment in Turkana, Kenya. Their results may be attributed to the much lower rainfall of about 300 mm, where perennial grasses are mostly absent, and grass competition consequently weaker, or may indicate that certain tree species are exceptions in this regard. A. tortilis is for instance often the only species colonizing nutrient hotspot areas (Belsky and Amundson 1986; Blackmore et al. 1990). In our study area, A. tortilis 1105 was scarce, and in the study area of Augustine et al. (2003), Laikipia, Kenya, it did not occur. A contributing factor limiting tree recruitment may have been tree germination constraints imposed by soil characteristics, which were not considered in this study. Roem et al. (2002), for example, found that the germination of several heathland species was affected by nutrient availability, although this effect was subsidiary to soil acidity. It is unlikely that seed dispersal limited tree recruitment, as mature trees occurred in close proximity to kraal sites. An alternative explanation for the low tree recruitment rates in fertile patches (e.g., kraals) may also be that fertile areas attract more herbivores, which translate into higher woody seedling browsing and trampling rates (Augustine and McNaughton 2004). For example, the dung accumu- lation data suggest that impala occupied kraal sites on average 20 times more often than the control sites. Since impala are known seedling predators (Prins and van der Jeugd 1993; Sharam et al. 2006), this could translate into higher seedling predation rates in fertile areas. However, the data of the ﬁeld fertilization and exclosure experiment, conducted in an area where impala were common, did not support this contention, although we acknowledge that the short study period prevent ﬁrm conclusions in this regard. Our results are consistent with those of Augustine and co- workers who concluded, after a 2-year herbivore exclusion experiment, that tree seedling predation by large herbi- vores, including impala, was not higher on fertile bomas, in spite of signiﬁcantly higher animal densities on these sites (Augustine et al. 2003; Augustine and McNaughton 2004). In a forest system in Poland, Kuijper et al. (2010) also did not ﬁnd an interaction between soil fertility and predation by ungulates on tree seedlings. In fact, Goheen et al. (2004) found that large herbivores facilitated tree seedling survival in a Kenyan savanna by suppressing rodent and inverte- brate predation of seedlings, and Fornara and du Toit (2008) found higher Acacia sapling densities in a high- herbivory area compared to a low-herbivory area in the KNP. Herbivores occurring at high densities can, however, have strong direct effects on tree cover by suppressing the recruitment of saplings into larger size classes (e.g., savanna, Augustine and McNaughton 2004; Fornara and Du Toit 2008; temperate forest, Kuijper et al. 2010). Conclusions Our results show that, by inﬂuencing local soil nutrient availability, ﬁrst livestock and probably wild herbivores after the livestock were removed, indirectly exerted long- lasting effects on the habitat quality and vegetation struc- ture of kraal sites through their effect on soil nutrient availability. Although kraals cover only a small fraction, the maintenance of abandoned kraals by wild herbivores 123