Oecologia (2011) 165:1095–1107 piles were recorded bi-monthly and marked as described for kraal sites. Woody recruitment To test the hypothesis that herbaceous competition decreases woody seedling recruitment, we conducted two experiments using the tree species C. mopane. This species was chosen because it dominates large parts of the study area and has been implicated as a bush encroacher (Smit 2001; MacGregor and O’Connor 2002). We therefore expected it to be a likely candidate to invade abandoned kraal sites within its distribution range. Also, re-measure- ment of woody transects originally conducted in 1980 in the study area (Witkowski and O’Connor 1996) revealed that C. mopane had signiﬁcantly increased in density and expanded its distribution range in the study area (G. Roux, unpublished data). In a ﬁeld fertilization and exclosure experiment, we tested the effects of soil fertility and seedling predation by large herbivores on the survival rate of tree seedlings co- occurring with herbaceous vegetation in the ﬁeld. Increased seedling predation by large herbivores might reduce tree recruitment in areas of high herbivore densities, such as found on nutrient hotspots (Grant and Scholes 2006). This might also explain the lower tree recruitment in fertile areas. The experiment was conducted in the APNR in an area open to wildlife. In this experiment, we transplanted pre-grown C. mopane seedlings (van der Waal et al. 2009) into ﬁeld plots (5 9 10 m) that were fertilized at increas- ing rates: 0 (control), 1.2 g N m-2, 6 g N m-2, and 30 g N m-2. The grass layer was minimally disturbed during transplantation to avoid compromising the compe- tition effect of grasses on tree seedlings. The fertilizer was spread by hand in December 2006, using a commercial NPK fertilizer (element ratio = 3N:2P:1K). The experi- ment was replicated three times (total n = 12). In December 2006, two groups, consisting of six C. mopane seedlings each, were transplanted into each plot (total n = 144). The one group was protected against large her- bivore use with exclosures, while the second group was planted in the open. The exclosures (4 m in diameter) were constructed from standard fence poles, with 1.8-m-high wire netting and effectively excluded herbivores the size of hares and larger. Seedlings were transplanted after 54 mm of rain was received. No extra water was given during the experiment. Four months after transplantation, the seed- lings were scored as ‘alive’ (green leaves or buds present) or ‘dead’ (green leaves and buds absent). An additional pot experiment involved a competition experiment to test the hypothesis that the effect of herba- ceous competition on tree seedlings intensiﬁes with increasing soil fertility (Sankaran et al. 2008). For this 1099 purpose, we conducted a pot experiment with the topsoil collected in kraal and control sites, for which the inorganic N and extractable P concentrations were determined as described above. For each of the kraal and control sites (2 9 9), we ﬁlled three pots with the soil collected from kraal and control sites (see above). Per site, one pot was allocated a single grass seedling (either U. mosambicensis or U. panicoides, which could not be distinguished at the seedling stage), the second pot received a single C. mopane seedling, and the third pot, a mixture of a grass and a C. mopane seedling. Grass seedlings were grown in seed- ling trays before transplantation into pots. The C. mopane seedlings were obtained from a site where a large number of seedlings germinated, probably from the same nearby mother tree. C. mopane seedlings were allowed to establish in pots before grasses were planted into the mixture treat- ments. During the experiment, rainfall was supplemented with irrigation. After 3 months, seedling height, number of leaves, stem diameter and aboveground dry mass of C. mopane seedlings and above ground grass biomass (dried at 70 C to constant weight) were measured per pot. For the C. mopane seedlings, the relative competition index (RCI) was calculated for seedling height, leaf number, stem diameter and above ground biomass following Davis et al. (1998): RCI ¼ Performancesingle Performancemixture X where X is the larger value of either single-plant perfor- mance (Performancesingle) or mixture-plant performance (Performancemixture). An RCI value of zero indicates that competitors had no effect on target plant performance and an RCI value of one indicates maximum competition intensity (competitive exclusion). Data analyses Paired t tests were used for kraal–control comparisons of soil properties, the bioassay and herbaceous biomass. Because of the colinearity between soil texture classes, only differences between the sand fractions were tested. All percentage and concentration data were arcsine-trans- formed to improve the distribution of data. The herbaceous biomass and the bioassay data were logarithmically trans- formed, except the exclosure cage biomass data. The pH data were exponentially transformed. Wilcoxon signed- rank tests were used to test for differences in woody density. The difference between kraal and control sites for dung pile densities was tested with a generalized linear model. Since most species data were overdispersed, a negative binomial distribution was assumed. A logarithmic link 123