Phylogenetic analyses
We constructed a phylogenetic tree of all identified species (n=144) with the V.PhyloMaker package (Jin & Qian 2019) in R, which generates phylogenies for vascular plants using two mega-trees as a backbone (Zanne et al . 2014; Smith & Brown 2018). Trait variation was mapped onto the phylogeny using the Phytools package (Revell 2012), which was also used to test for phylogenetic signals in heat tolerance and other leaf traits. Phylogenetic signals were evaluated with Blomberg’s K parameter (Blomberg et al . 2003) and with Pagel’s λ (Pagel, 1999). In the absence of significant phylogenetic signal in heat tolerance parameters (see below), regression of these parameters against other leaf traits were performed with ordinary least squares, which is the appropriate method regardless of phylogenetic patterns in the traits themselves (Revell 2010).