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Acacia s.l. farnesiana, which originates from Mesoamerica, is the most widely distributed Acacia s.l. species across the tropics. It is assumed that the plant was transferred across the Atlantic to southern Europe by Spanish explorers, and then spread across the Old World tropics through a combination of chance long-distance and human-mediated dispersal. Our study uses genetic analysis and information from historical sources to test the relative roles of chance and human-mediated dispersal in its distribution. The results confirm the Mesoamerican origins of the plant and show three patterns of human-mediated dispersal. Samples from Spain showed greater genetic diversity than those from other Old World tropics, suggesting more instances of transatlantic introductions from the Americas to that country than to other parts of Africa and Asia. Individuals from the Philippines matched a population from South Central Mexico and were likely to have been direct, trans-Pacific introductions. Australian samples were genetically unique, indicating that the arrival of the species in the continent was independent of these European colonial activities. This suggests the possibility of pre-European human-mediated dispersal across the Pacific Ocean. These significant findings raise new questions for biogeographic studies that assume chance or transoceanic dispersal for disjunct plant distributions.

Context: The current fire regime threatens black pine (Pinus nigra Arn.) persistence in the Mediterranean Basin, which recommends larger-scale fuel treatments. Prescribed burning is an option for stand protection but its use in young stands (which are particularly at risk) is hindered by the scarce knowledge on post-fire tree survival. Aims: The objectives were to characterize bark thickness as a fire-resistance trait in P. nigra, and to describe how post-fire tree survival responds to tree size and fire effects in a 16-year old plantation. Methods: Bark thickness was related to diameter at breast height and height in the stem. Metrics describing tree size and stem and crown damage were measured 1-year after prescribed burning in 259 trees. Tree survival was modelled with logistic regression and CART analysis. Results: Bark thickness increased linearly with dbh and decreased with height in the stem. Tree survival was primarily a function of crown injury. Stem damage was an influent factor in small trees. Conclusion: Due to thinner bark and lower tolerance to crown damage, young P. nigra trees are less fire-resistant than other Mediterranean pines, e.g. P. pinaster. Prescribed fire should not be attempted if dbh <10 cm. Mechanical clearing is the treatment of choice in young stands with a significant shrub layer.