Geographic patterns and climatic drivers of the mean genus age of liverworts in North America.

Phylogenetic niche conservatism posits that species tend to retain ancestral ecological traits and distributions, which has been broadly tested for lineages originating in tropical climates but has been rarely tested for lineages that originated and diversified in temperate climates. Liverworts are thought to originate in temperate climates. Mean lineage age reflects evolutionary history of biological communities. Here, using regional liverwort floras across a long latitudinal gradient from tropical to arctic climates in North America, we test the age-component of the temperate niche conservatism hypothesis. Mean genus age (MGA) was estimated for each of 76 regional floras of liverworts. We related MGA to climatic variables for North America as a whole and for its eastern and western parts separately, and used variation partitioning analysis to assess the relative importance of temperature- versus precipitation-related variables and of climate extremes versus seasonality on MGA. We found that older genera of liverworts tend to concentrate in humid regions of intermediate temperatures in the range of 10 °C-20 °C, from which liverworts have adapted to and diversified into more arid, colder, and hotter regions, supporting the temperate niche conservatism hypothesis. We also found that across North America the MGA of liverwort assemblages is more strongly affected by precipitation-related variables than by temperature-related variables, and is more strongly affected by climate extremes than by climate seasonality. Geographic patterns of the MGA of liverworts are consistent with the temperate niche conservatism hypothesis, rather than the tropical niche conservatism hypothesis, the latter of which is broadly supported by angiosperms.