Annals of Botany 119: 563–579, 2017 doi:10.1093/aob/mcw243, available online at www.aob.oxfordjournals.org Force of habit: shrubs, trees and contingent evolution of wood anatomical diversity using Croton (Euphorbiaceae) as a model system Rafael Are´valo1,2,*, Benjamin W. van Ee3, Ricarda Riina4, Paul E. Berry5 and Alex C. Wiedenhoeft1,2 1Center for Wood Anatomy Research, USDA Forest Service, Forest Products Laboratory, Madison, WI 53726, USA, 2Department of Botany, University of Wisconsin, Madison, WI 53706, USA, 3University of Puerto Rico at Mayagu¨ez Herbarium, Department of Biology, Universidad de Puerto Rico, Call Box 9000, Mayagu¨ez, 00680, Puerto Rico, 4Real Jardın Botanico, RJB-CSIC, Plaza de Murillo 2, 28014 Madrid, Spain and 5University of Michigan, Ecology and Evolutionary Biology Department and Herbarium, Ann Arbor, MI 48108, USA *For correspondence. E-mail
[email protected] Received: 7 July 2016 Returned for revision: 3 September 2016 Accepted: 5 October 2016 Published electronically: 8 January 2017 Background and Aims Wood is a major innovation of land plants, and is usually a central component of the body plan for two major plant habits: shrubs and trees. Wood anatomical syndromes vary between shrubs and trees, but no prior work has explicitly evaluated the contingent evolution of wood anatomical diversity in the context of these plant habits. Methods Phylogenetic comparative methods were used to test for contingent evolution of habit, habitat and wood anatomy in the mega-diverse genus Croton (Euphorbiaceae), across the largest and most complete molecular phy- logeny of the genus to date. Key Results Plant habit and habitat are highly correlated, but most wood anatomical features correlate more strongly with habit.