From frond to fern flora: An analysis of the Hawaiian pteridophyte trait database

Background/Question/Methods As the most isolated island chain in the world, Hawaii is home to 188 native pteridophytes, including 145 endemic and 33 naturalized alien taxa. This flora is distributed across steep gradients in elevation, water and light, exhibits extraordinary diversity in morphology and growth form, ranging from terrestrial and epipetric to arboreal and epiphytic habits. Yet surprisingly little is known of pteridophyte adaptation to environment. As part of our initiative to elucidate mechanisms of fern structural and functional adaptations, we created a trait database based on the most recent and comprehensive survey of Hawaii’s ferns and fern allies (Palmer 2003). We hypothesized relationships among plant traits and environment due to processes of correlated evolution, community assembly and/or ecological filtering. We also hypothesized that differences in distribution and commonness are linked to variation in frond morphology.

Results/Conclusions Our results demonstrate strong trait-trait and trait-environment relationships that help explain evolutionary patterns of Hawaiian fern diversity and biogeography. We found evidence of flora-wide allometric relationships between the sizes of fronds, frond components and whole ferns.  There are also significant differences in morphology between native and alien taxa, with aliens exhibiting a wider range of sizes and achieving greater frond sizes. 

In addition, we found strong differences in morphology and species abundance across growth forms and habitats varying in water and light availability. Disturbingly, alien species have greater abundances on average than native species and are more widely distributed across the Hawaiian Islands. Native species are more commonly distributed at higher elevations, possibly due to land use and species invasions at lower altitude. Taken together, these findings point to adaptive trajectories in Hawaiian fern evolution and provide a framework for future tests of structure-function relationships.  Further, our findings raise the question of how differences in form and physiology will affect native species persistence in more restricted habitats given alien species encroachment.

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Palmer DD. 2003. Hawaii’s Ferns and Fern Allies. University of Hawaii Press, Honolulu.