Early flowering plants are thought to have been woody species restricted to warm habitats. This lineage has since radiated into almost every climate, with manifold growth forms. As angiosperms spread and climate changed, they evolved mechanisms to cope with episodic freezing. To explore the evolution of traits underpinning the ability to persist in freezing conditions, we assembled a large species-level database of growth habit (woody or herbaceous; 49,064 species), as well as leaf phenology (evergreen or deciduous), diameter of hydraulic conduits (that is, xylem vessels and tracheids) and climate occupancies (exposure to freezing). To model the evolution of species' traits and climate occupancies, we combined these data with an unparalleled dated molecular phylogeny (32,223 species) for land plants. Here we show that woody clades successfully moved into freezing-prone environments by either possessing transport networks of small safe conduits and/or shutting down hydraulic function by dropping leaves during freezing. Herbaceous species largely avoided freezing periods by senescing cheaply constructed aboveground tissue. Growth habit has long been considered labile, but we find that growth habit was less labile than climate occupancy. Additionally, freezing environments were largely filled by lineages that had already become herbs or, when remaining woody, already had small conduits (that is, the trait evolved before the climate occupancy). By contrast, most deciduous woody lineages had an evolutionary shift to seasonally shedding their leaves only after exposure to freezing (that is, the climate occupancy evolved before the trait). For angiosperms to inhabit novel cold environments they had to gain new structural and functional trait solutions; our results suggest that many of these solutions were probably acquired before their foray into the cold.

The angiosperm order Malpighiales includes ∼16,000 species and constitutes up to 40% of the understory tree diversity in tropical rain forests. Despite remarkable progress in angiosperm systematics during the last 20 y, relationships within Malpighiales remain poorly resolved, possibly owing to its rapid rise during the mid-Cretaceous. Using phylogenomic approaches, including analyses of 82 plastid genes from 58 species, we identified 12 additional clades in Malpighiales and substantially increased resolution along the backbone. This greatly improved phylogeny revealed a dynamic history of shifts in net diversification rates across Malpighiales, with bursts of diversification noted in the Barbados cherries (Malpighiaceae), cocas (Erythroxylaceae), and passion flowers (Passifloraceae). We found that commonly used a priori approaches for partitioning concatenated data in maximum likelihood analyses, by gene or by codon position, performed poorly relative to the use of partitions identified a posteriori using a Bayesian mixture model. We also found better branch support in trees inferred from a taxon-rich, data-sparse matrix, which deeply sampled only the phylogenetically critical placeholders, than in trees inferred from a taxon-sparse matrix with little missing data. Although this matrix has more missing data, our a posteriori partitioning strategy reduced the possibility of producing multiple distinct but equally optimal topologies and increased phylogenetic decisiveness, compared with the strategy of partitioning by gene. These approaches are likely to help improve phylogenetic resolution in other poorly resolved major clades of angiosperms and to be more broadly useful in studies across the Tree of Life. M alpighiales are one of the most surprising clades discovered in broad molecular phylogenetic studies of the flowering plants (1-3). The order contains ∼16,000 species and 42 families (2, 3) that exhibit remarkable morphological and ecological diversity. A few examples include cactus-like succulents (Euphorbiaceae), epiphytes (Clusiaceae), holoparasites (Rafflesiaceae), submerged aquatics (Podostemaceae), and windpollinated trees (temperate Salicaceae). The order is ecologically important: species in Malpighiales constitute up to 40% of the understory tree diversity in tropical rain forests worldwide (4). They also include many economically important species, such as Barbados nut (Jatropha curcas L., Euphorbiaceae), cassava (Manihot esculenta Crantz, Euphorbiaceae), castor bean (Ricinus communis L., Euphorbiaceae), coca (Erythroxylum coca Lam., Erythroxylaceae), flax (Linum usitatissimum L., Linaceae), the poplars (Populus spp., Salicaceae), and the rubber tree (Hevea brasiliensis Müll. Arg., Euphorbiaceae). Partially for this reason, genomic resources for Malpighiales are growing at a rapid pace and include whole-genome sequencing projects completed or near completion for Barbados nut (5), cassava, castor bean (6), flax, and poplar (7). Thus, a resolved phylogeny of Malpighiales is critical not only for evol...

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