Evaluating Stasis in Metasequoia (Cupressaceae): Testing the Relationship between Leaf Traits and Climate

Premise of research. Plant response to climate through time is commonly investigated with leaf physiognomy, but anatomical response has not been well studied. Here, we use Metasequoia (Cupressaceae) to investigate leaf anatomical response to climate over geologic time. We establish the link between leaf anatomy and climate for extant Metasequoia and apply the results to the fossil Metasequoia milleri to examine climatic niche shift through time. Methodology. Leaves of extant Metasequoia glyptostroboides collected across climate gradients in North America, Asia, and Europe and leaves of extinct M. milleri from the Eocene Princeton Chert locality were studied. Leaf anatomical traits-cross-sectional area, resin canal area, vascular bundle area, and leaf thickness and width-of extant and extinct Metasequoia were measured, analyzed using principal components analysis (PCA), and tested for relationships with 19 bioclimatic variables in a canonical correspondence analysis (CCA). Fossil climate variables estimated from CCA results were compared with independent paleoclimate proxy estimates. Pivotal results. All measured anatomical traits statistically differ between extinct and extant Metasequoia, and the two species do not overlap in leaf anatomical morphospace. Measured traits of extant Metasequoia were found to correlate with several climate variables grouped into six climate groupings. Leaf width and cross-sectional area correlated with high cold-season precipitation, vascular bundle area with high warm-season precipitation, leaf thickness with mild cold-season temperatures and high mean annual temperatures, and resin canal area with daily temperature fluctuations and mild cold-season temperatures. Estimated paleoclimates based on the leaf anatomy-climate model were similar to independent proxy estimates. Conclusions. A relationship between leaf traits and climate was found that supports different leaf anatomical morphospaces and climatic niches for both species of Metasequoia that were tested, despite previous hypotheses of static morphology within Metasequoia. Testing interactions between climate and anatomy could improve paleoenvironmental inferences from fossil conifers.