Functional strategies of a rare ancient cycad along an environmental gradient of a tropical dry forest

Background/Question/Methods

Functional traits influence plant responses to changing environmental conditions notably via the regeneration niche shaping plant future fitness. We studied life-stage specific functional strategies related to carbon and water economies of a rare long-lived cycad (Dioon sonorense) along a soil fertility and altitudinal gradient. We expect that 1) functional traits will be more closely linked to the regeneration niche (seedlings), and 2) functional traits related to carbon and nitrogen acquisition and water use efficiency will vary with life-stage and with soil fertility. We sampled 3-6 individuals from each cycad life-stage (seedling, juvenile, adult) in a gradient of nine sites at a tropical dry forest in northwestern Mexico. We quantified 10 leaf traits: chemical (%C, %N, d13C, d15N, C/N, %N₂-fixation), physiological (photosynthetic rate (A), transpiration rate (E) and water use efficiency (WUE: A/E) in dry and wet seasons) and morphological (leaflet width). We performed ANCOVA analysis with traits as dependent variables, life-stage as a factor, and the first principal component scores (PCA, 33% explained variance) representing a gradient of soil fertility and altitude as a covariate.

Results/Conclusions

Contrary to our first expectation, the adult stage was consistently retained in the models (p<0.05), so functional traits in the rare cycad D. sonorense seem more adapted to its adult niche and not its regeneration niche. No variation among life-stages or along the environmental gradient was detected in leaf C and N content. However, C/N ratio and photosynthetic rate (dry season) were higher and transpiration rate (wet season) lower in adults. Water use efficiency (inferred from d13C) and atmospheric nitrogen fixation increased with life-stage and with declining soil fertility at higher elevation (~45% explained variance in both cases). Quantification of functional traits provides insights into cycad persistence in areas with nutrient-poor soils and seasonal drought. Since adult survival makes the highest contribution to population growth rates in long-lived species such as cycads, adaptations that favor efficiency of carbon and water utilization, and nitrogen fixation in adult plants are particularly relevant for their persistence in arid ecosystems.