Plant functional traits are viewed as key to predicting important ecosystem and community properties across resource gradients within and among biogeographic regions. Vegetation dynamics and ecosystem processes, such as aboveground net primary productivity (ANPP), are increasingly being modeled as a function of the quantitative traits of species, which are used as proxies for photosynthetic rates and nutrient and water-use efficiency. These approaches rely on an assumption that a certain trait value consistently confers a specific function or response under given environmental conditions. Here, we provide a critical test of this idea and evaluate whether the functional traits that drive the well-known relationship between precipitation and ANPP differ between systems with distinct biogeographic histories and species assemblages. Specifically, we compared grasslands spanning a broad precipitation gradient (∼200-1,000 mm/y) in North America and South Africa that differ in the relative representation and abundance of grass phylogenetic lineages. We found no significant difference between the regions in the positive relationship between annual precipitation and ANPP, yet the trait values underlying this relationship differed dramatically. Our results challenge the trait-based approach to predicting ecosystem function by demonstrating that different combinations of functional traits can act to maximize ANPP in a given environmental setting. Further, we show the importance of incorporating biogeographic and phylogenetic history in predicting community and ecosystem properties using traits.functional traits | grasslands | phylogenetics | ecosystem function | biogeography N et primary productivity (NPP), or the biomass produced by the conversion of CO 2 via photosynthesis minus that lost to respiration, is a fundamental link between the atmosphere and the biosphere. Understanding and predicting the drivers of terrestrial NPP is of the utmost importance, especially for grasslands, which cover more than 40% of terrestrial land surface, constitute upwards of 30% of terrestrial gross primary productivity (1, 2), and are responsible for important ecosystem services such as carbon sequestration and forage production. Precipitation is the primary driver of variation in aboveground NPP (ANPP) in grasslands across broad spatial scales (3-6), and a consistent relationship has been observed between ANPP and precipitation across biogeographic regions (7). However, our understanding of the factors that underlie this emergent pattern remain limited.Recently, ecosystem processes, including ANPP, have been modeled as a function of the quantitative traits of organisms that reflect adaptations to environmental variation, and are often proxies for photosynthetic rates, and nutrient and water-use efficiency (8-11). Although these approaches are promising, they rely on the assumption that a certain trait value confers a specific function or response in a given environment, regardless of differences among species in other traits or interacti...

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Although livestock exclusion has the potential to improve C sequestration, a sufficient resting period for 1-2 years followed by three consecutive grazing years at light stocking rate would be ideal for sustainable livestock production in this arid region of South Africa.

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