The monocotyledonous underground: global climatic and phylogenetic patterns of geophyte diversity

Premise: Geophytes-plants that typically possess a bulb, corm, tuber, and/or rhizome-have long captured the attention of hobbyists and researchers. However, despite the economic and evolutionary importance of these traits, the potential drivers of their morphological diversity remain unknown. Using a comprehensive phylogeny of monocots, we test for correlations between climate and geophyte growth form to better understand why we observe such a diversity of underground traits in geophytes. Understanding the evolutionary factors promoting independent origins of these potentially adaptive organs will lend insights into how plants adapt to environmental hardships.

Methods: Using a comprehensive phylogeny incorporated with global occurrence and climate data for the monocots, we investigated whether climatic patterns could explain differences between geophytes and non-geophytes, as well as differences among bulbous, cormous, tuberous, rhizomatous, and non-geophytic taxa. We used phylogenetically-informed ANOVAs, MANOVAs, and PCAs to test differences in climatic variables between the different growth forms.

Results: Geophytes inhabit cooler, drier, and more thermally variable climates compared to non-geophytes. Although some underground traits (i.e., bulb, corm, and tuber) appear to inhabit particular niches, a result supported by strong phylogenetic signal, our data has limited evidence for an overall role of climate in the evolution of these traits. However, temperature may be a driving force in rhizome evolution, as well as the evolution of taxa which we considered here as non-geophytic (e.g., trees, epiphytes, etc.).

Conclusions: While precipitation patterns have played a role in the evolution of geophytism, our results suggest that temperature should be more strongly considered as a contributing factor promoting the evolution of belowground bud placement, specifically in rhizomatous and non-geophytic taxa. Bulbous, cormous, and tuberous taxa need closer examination of other mechanisms, such as anatomical constraints or genetic controls, in order to begin to understand the causes behind the evolution of their underground morphology.