Upper boundary on tree cover at global drylands
L. BIANCARI, M.R. AGUIAR, H. SAIZ, N. GROSS, Y. LE BAGOUSSE-PINGUET, D.J. ELDRIDGE, F.T.MAESTRE
IFEVA, Facultad de Agronomía, Universidad de Buenos Aires, CONICET, Av. San Martín 4453. Buenos Aires (1417). Argentina

In a recent Tansley Review, Holdo and Nippert (2023) provided a comprehensive analysis of existing models that explain tree-grass coexistence along precipitation gradients in savannas. They proposed three key elements that a definitive savanna model should include and predicted the upper boundary pattern of tree cover based on the ‘Sankaran curve’ depending on soil texture. We tested this boundary pattern of tree cover in drylands using global field data (98 sites from 25 countries and six continents) accounting from a wide range of environmental conditions and grazing pressures. We found that the upper boundary on tree cover in global drylands increases with mean annual precipitation before reaching a threshold of 453 mm. Increasing grazing pressure significantly decreased this upper limit. However, the upper boundary on tree cover did not stabilize but instead decreased beyond this precipitation threshold. Our findings partially support the three coexistence mechanisms proposed by Holdo and Nippert and indicate some features that may be specific for water-limited rangelands: a lower precipitation threshold for the upper boundary on tree cover, a declining tree cover beyond the threshold, and an interplay between resource- and disturbance-based mechanisms. Overall, our findings advance our understanding of tree-grass coexistence in drylands and provide new elements to develop a definitive model of such coexistence across terrestrial ecosystems.