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Aim: To investigate species and clade biome occupancy patterns of Australian Acacia to test for within‐biome diversification, which indicate biome conservatism. Location: Australia. Taxon: Acacia (Fabaceae). Methods: Species distributions were predicted for 481 Australian Acacia using the Thornley Transport Resistance Species Distribution Model and mapped across four biome typologies. Within Acacia 19 clades were identified. The number of biomes occupied and niche size was quantified for every species and clade using the range area projected by the distribution model. Relationships between clade species richness, niche size and biomes occupied were tested using phylogenetic least squares regression models. Results: Only 9% of the Acacia 481 species and no clades were biome specialists. There were most specialist taxa in the Crisp Biome classification (8.7%), followed by WWF Biomes (6.2%), González–Orozco Biomes (5.0%) then Functional Biomes (1.2%). On average Acacia species occupied four WWF Biomes, seven Functional Biomes, three Crisp Biomes and three González–Orozco Biomes (out of 7, 13, 5 or 6 biomes respectively). Clades were also distributed across multiple biomes (2–13) with a significant positive relationship between clade species richness and the number of biomes occupied for all biome typologies. Species richness had positive linear relationships with biome area for all biome concepts except the González–Orozco Biomes. Larger clades had larger niche sizes. Main conclusions: Acacia diversification occurred across biome boundaries and was not associated with biome specialization. Species and clades mainly occurred in multiple biomes, and there were typically few biome specialists. Diversification in Acacia appears to be decoupled from biome conservatism, associated with expanding niche size across biome boundaries. Major ecological–environmental units such as biomes may constrain adaptive radiation processes via biome conservatism in many groups, but this study leads us to hypothesize that for some lineages biome boundaries are permeable. [ABSTRACT FROM AUTHOR]

Aim The structure of savannas ranges from scrub to woodland over broad geographical gradients. Here we examine the hypothesis that water availability is a major determinant of these structural differences by ascertaining the relationship between water availability and tree growth across a steep moisture gradient. Location The study site is a sub-tropical savanna, with a mean annual precipitation of 400 mm, located in the Phugwane river basin, Kruger National Park, Limpopo Province, South Africa. Methods We determined plant moisture stress using xylem pressure potentials, stem growth using dendrometer bands, and estimated the water sources available to plants using stable hydrogen and oxygen isotope ratios. The primary objective was to understand tree growth relative to available water along an environmental gradient. Results We found that Philenoptera violacea trees growing close to the stream have lower water stress (least negative xylem pressure potentials) and higher cumulative growth than those growing away from the stream. The stem growth of P. violacea was characterized by steady incremental growth and could not be related to antecedent rainfall. Colophospermum mopane trees experienced higher water stress, yet trees growing adjacent to the stream achieved the highest cumulative growth rates over the study period. The growth of C. mopane could be clearly linked to antecedent rainfall, and most growth was achieved during short growth pulses that followed rainfall events. δ18O values become progressively more enriched in the heavier isotope with distance from the stream, suggesting that access to groundwater decreases with distance from the stream side. The isotopic data suggest that P. violacea has access to groundwater, but that C. mopane does not. Main conclusions Our results show that water stress increases with distance from the stream side as a result of reduced access to groundwater. Trees without access to deep water adopt an opportunistic growth strategy. [ABSTRACT FROM AUTHOR]